Ultrasonic Flow Sensor and Thermal Energy Sensor with Non-Invasive Identification of No-Flow and Improved Accuracy

Abstract

An ultrasonic flow sensor is configured to determine the time-of-flight of ultrasonic waves and calculate a change in the speed of sound on the basis of the time-of-flight, to calculate the expected change in speed of sound as a function of the detected temperature of the fluid, to determine if the expected change in speed of sound corresponds to the change in the speed of sound calculated on the basis of the time-of-flight; and to identify a no-flow state, in which there is no flow of the fluid when the expected change in the speed of sound corresponds to the change in the speed of sound calculated on the basis of the time-of-flight, and a temperature difference between the surroundings and the fluid is below a predefined level which is pre-set at a fixed level between 0.01 degree Celsius and 0.5 degree Celsius.

Description

FIELD OF INVENTION

The present invention relates to ultrasonic flow sensors in general and in particular to clamp-on ultrasonic flow sensors. The present invention also relates to thermal energy meters using an ultrasonic flow sensor and to clamp-on ultrasonic thermal energy meters.

BACKGROUND

Flow measurement is widely used for measuring flow in industry, buildings and utility grids. Flow can be detected using various types of flow sensors. The prior art flow sensors include mechanical flow sensors and ultrasonic flow sensors. Ultrasonic flow sensors are mainly used in two versions, namely delta-time-of-flight for measuring on pure fluids (water, gas, industry liquids, etc.) and Doppler effect for measuring fluids containing many particles (slurry, liquids with air bubbles, etc.).

In state-of-the-art ultrasonic flow measurement, there are known limitations which limit the use of the technology. One of these limitations is that it is difficult to detect no-flow (the fluid stands still in the pipe), which is needed to identify the off-set of the sensor.

Even though prior art ultrasonic flow sensors normally are reliable and accurate, it would be advantageous to improve the measurement accuracy. Moreover, it would be an advantage to provide an ultrasonic thermal energy meter having an improved accuracy.

BRIEF DESCRIPTION

It is an object of the present disclosure is to provide methods and ultrasonic flow sensors that can provide a detection of no-flow. It is also an object of the present disclosure to provide methods and ultrasonic flow sensors that are capable of providing a higher accuracy than the solutions known in the prior art. It is also an object of the present disclosure to provide ultrasonic thermal energy meters that have a higher accuracy than known ultrasonic thermal energy meters.

A flow sensor according to an embodiment is an ultrasonic flow sensor configured to measure the flow of a fluid flowing through a tubular structure, said flow sensor comprising:

• • a first detection unit arranged to transmit and receive ultrasonic waves using at least one ultrasonic transducer;
  • a temperature sensor arranged and configured to detect the temperature of the fluid;
  • a temperature sensor arranged and configured to detect the temperature of the surroundings (the ambient temperature); and
  • a data processor configured to receive data detected by the at least one ultrasonic transducer and the temperature sensors,wherein the flow sensor is configured to:
  • determine the time-of-flight of the ultrasonic waves and calculate a change in the speed of sound on the basis of the time-of-flight;
  • calculate the expected change in the speed of sound as a function of the detected temperature of the fluid;
  • determine if the expected change in the speed of sound corresponds to the change in the speed of sound calculated on the basis of the time-of-flight; and
  • identify a no-flow state, in which there is no flow of the fluid when the following criteria are met:
  • A) the expected change in the speed of sound corresponds to the change in the speed of sound calculated on the basis of the time-of-flight, and
  • B) the temperature difference between the surroundings and the fluid is below a predefined level which is pre-set at a fixed level between 0.01 degree Celsius and 0.5 degree Celsius.

Hereby, it is possible to provide a detection of no-flow. Accordingly, the off-set of the flow sensor can be identified.

The tubular structure may be a pipe or another structure, through which the fluid is flowing. In an embodiment, the tubular structure is a pipe. In an embodiment, the tubular structure is a hose. In an embodiment, the tubular structure is a container. In an embodiment, the tubular structure is a box.

A flow sensor according to the present disclosure is a flow sensor configured to measure the flow of a fluid. In an embodiment, the fluid is a liquid. In an embodiment, the fluid is a water-containing liquid. In an embodiment, the fluid is a gas.

The data processor may be a micro-processor.

In an embodiment, the no-flow state is used to calibrate the ultrasonic flow measurement calculation(s) of the flow sensor, to ensure stability and correct ultrasonic flow measurement of the flow sensor.

In an embodiment, the ultrasonic flow sensor is configured to calculate a corrected value of the change in the density of the fluid on the basis of the change in speed of sound calculated on the basis of the time-of-flight, if the expected speed of sound does not correspond to the change in the speed of sound calculated on the basis of the time-of-flight. Hereby, it is possible to provide an ultrasonic flow sensor that is capable of providing a higher accuracy than the solutions known in the prior art.

In an embodiment, the ultrasonic flow sensor is configured to calculate a corrected value of the specific heat capacity of the fluid on the basis of the corrected value of the density, if the expected change in the speed of sound does not correspond to the change in the speed of sound calculated on the basis of the time-of-flight. Hereby, it is possible to provide an ultrasonic flow sensor that is capable of providing a higher accuracy than the solutions known in the prior art.

In an embodiment, the ultrasonic flow sensor is configured to calculate a corrected value of the flow of the fluid on the basis of the change in the speed of sound calculated on the basis of the time-of-flight, if the expected change in the speed of sound does not correspond to the change in the speed of sound calculated on the basis of the time-of-flight. Hereby, it is possible to provide an ultrasonic flow sensor that is capable of providing a higher accuracy than the solutions known in the prior art.

In an embodiment, the first detection unit is configured to detect flows above a predefined lower flow level representing the lower flow that can be measured using the first detection unit, wherein the flow sensor comprises a second detection unit that comprises:

• • a first temperature sensor arranged and configured to detect the temperature of the surroundings (the ambient temperature);
  • a data processor connected to the temperature sensors,wherein the second detection unit is configured to estimate the flow below the lower flow level on the basis of the temperature difference between the surroundings and a fluid, wherein the temperature difference is measured by the first temperature sensor and the second temperature sensor, wherein the second detection unit is configured to estimate the flow below the lower flow level on the basis of one or more measurements made in a flow-calibration-area, in which flow-calibration-area the flow sensor can detect the flow that depends on the temperature difference, wherein the one or more measurements made in the flow-calibration-area are used to determine one or more parameters required to determine how the flow depends on the temperature difference in the flow-calibration-area and in the flow area below the flow-calibration-area.

Hereby, it is possible to provide a sensor that can detect flows in a larger flow range than the prior art flow sensors. A flow sensor according to the present disclosure can in particular detect flows below the lower flow level.

In an embodiment, the second detection unit is configured to estimate the flow below the lower flow level on the basis of a single measurement and predefined data that includes the density and the specific heat capacity of the fluid.

In an embodiment, the second detection unit is configured to estimate the flow below the lower flow level on the basis of two single measurements.

Hereby, it is possible to use the two measurements to fit a curve describing the relationship between the flow and the temperature difference.

This can be done because the curve has a known shape (this follows by the relationship defined by equation (1) and equation (6) as shown in and explained with reference to FIG. 8).

In an embodiment, the second detection unit is configured to estimate the flow below the lower flow level on the basis of two or more measurements made in a flow-calibration-area.

In an embodiment, the flow sensor is configured to regularly or continuously:

• • carry out the one or more measurements in a flow-calibration-area; and
  • update the one or more parameters required to determine how the flow depends on the temperature difference in the flow-calibration-area and in the flow area below the flow-calibration-area.

Hereby, it is possible to provide reliable flow measurements and on a regular basis adjust the parameters according to changes of the ambient conditions (e.g. an increased ventilation). The flow sensor is configured to automatically perform a required number of measurements in the flow-calibration-area and calculate and update the one or more parameters required to determine how the flow depends on the temperature difference in the flow-calibration-area and in the flow area below the flow-calibration-area.

In an embodiment, the term “regularly or continuously” has to be understood as once every second, in which attempts are made to provide one or more measurements in the flow-calibration-area.

In an embodiment, the term “regularly or continuously” has to be understood as once every 5 seconds, in which attempts are made to pro-vide one or more measurements in the flow-calibration-area.

In an embodiment, the term “regularly or continuously” has to be understood as once every 10 seconds, in which attempts are made to provide one or more measurements in the flow-calibration-area.

In an embodiment, the term “regularly or continuously” has to be understood as once every 30 seconds, in which attempts are made to provide one or more measurements in the flow-calibration-area.

In an embodiment, the term “regularly or continuously” has to be understood as once every minute, in which attempts are made to provide one or more measurements in the flow-calibration-area.

In an embodiment, the term “regularly or continuously” has to be understood as once every 2 minutes, in which attempts are made to pro-vide one or more measurements in the flow-calibration-area.

In an embodiment, the term “regularly or continuously” has to be understood as once every 5 minutes, in which attempts are made to pro-vide one or more measurements in the flow-calibration-area.

In an embodiment, the term “regularly or continuously” has to be understood as once every 15 minutes, in which attempts are made to provide one or more measurements in the flow-calibration-area.

In an embodiment, the term “regularly or continuously” has to be understood as once every 30 minutes, in which attempts are made to provide one or more measurements in the flow-calibration-area.

In an embodiment, the term “regularly or continuously” has to be understood as once every hour, in which attempts are made to provide one or more measurements in the flow-calibration-area.

In an embodiment, the dependency between the flow and the temperature

difference (ΔT_sf) is defined by of the following equations:

$\Delta T_{\mathrm{sf}}\left(Q\right)=\Delta T_B\left(1-e^{-C_{1}Q}\right)\mathrm{or}Q\left(\Delta T_{\mathrm{sf}}\right)=\frac{-1}{C_1}\ln \left(1-\frac{\Delta T_{\mathrm{sf}}}{\Delta T_B}\right)$

where C₁ is a constant and ΔT_B is a temperature difference corresponding to a base flow level. In FIG. 8, the base flow level Q_B is illustrated.

These equations have two unknowns: the temperature difference ΔT_B corresponding to the base flow level Q_B and the constant C₁.

Accordingly, two measurements made in the flow-calibration-area provide sufficient information to determine the dependency between the flow and the temperature difference (ΔT_sf).

In an embodiment, a (second) temperature sensor is arranged and configured to detect the temperature of the fluid by measuring a temperature at the outside of the tubular structure. Hereby it is possible to provide the flow sensor as a clamp-on type flow sensor that can be mounted on the outside of the tubular structure (e.g. a pipe). Accordingly, there is no need to bring the second temperature sensor into direct contact with the fluid.

In an embodiment, the data processor and a (second) temperature sensor are arranged inside a housing. Hereby, it is possible to provide a simple, easy mountable and robust flow sensor.

In an embodiment, the (first) temperature sensor is arranged in the housing. Hereby, all components of the flow sensor can be provided in a single housing.

In an embodiment, the (first) temperature sensor is arranged outside the housing. Hereby, it is possible to take into consideration the heat transfer caused by convection.

In an embodiment, the second detection unit comprises an intermediate temperature sensor arranged and configured to detect an intermediate temperature of a position inside the housing, wherein said position is expected to have a temperature between the ambient temperature and the temperature of the fluid. Hereby, it is possible to provide additional information and thus provide an improved estimation of the flow in the low flow range.

In an embodiment, the flow sensor is a clamp-on flow sensor configured to measure the flow of the fluid from outside the tubular structure.

In an embodiment, the flow sensor is an ultrasonic flow sensor and the first detection unit comprises at least one ultrasonic transducer arranged to transmit ultrasonic waves and at least one ultrasonic transducer arranged to receive ultrasonic waves.

In an embodiment, the flow sensor is configured to automatically calculate the distance L that the transmitted ultrasonic waves and received ultrasonic waves travel in the fluid on the basis of a detected value of the speed of sound c. Hereby, it is possible to measure the flow in a pipe without knowing the exact dimensions of the pipe. It is also possible to perform accurate measurements, even if sediments are provided at an inside surface of a pipe over time.

A thermal energy meter according to the present disclosure comprises a flow sensor as described herein.

In an embodiment, the second detection unit is integrated in the first detection unit. In an embodiment, the second detection unit and the first detection unit are provided as separated units.

In an embodiment, the second detection unit is communicatively connected to a storage or an external device containing information about how the flow depends on the temperature difference, wherein the data processor is configured to access and use said information in such a manner that the data processor can determine the flow on the basis of the temperature difference.

A method according to the present disclosure is a method for measuring the flow of a fluid flowing through a tubular structure by an ultrasonic flow sensor comprising a first detection unit provided with:

• • at least one ultrasonic transducer arranged to transmit and receive ultrasonic waves by using at least one ultrasonic transducer, wherein the ultrasonic flow sensor comprises a temperature sensor arranged and configured to detect the temperature of the fluid,wherein the method comprises the following steps:
  • determining the time-of-flight of the ultrasonic waves;
  • calculating a change in the speed of sound on the basis of the time-of-flight;
  • calculating the expected change in the speed of sound as a function of the detected temperature of the fluid; and
  • determining if the expected change in the speed of sound corresponds to the change in the speed of sound calculated on the basis of the time-of-flight;
  • identifying a no-flow state, in which there is no flow of the fluid when the following criteria are met:
  • A) the expected change in the speed of sound corresponds to the change in the speed of sound calculated on the basis of the time-of-flight, and
  • B) the temperature difference between the surroundings and the fluid is below a predefined level which is pre-set at a fixed level between 0.01 degree Celsius and 0.5 degree Celsius.

Hereby, it is possible to provide a detection of no-flow. Accordingly, the off-set can be identified.

In an embodiment, the no-flow state is used to calibrate the ultrasonics flow measurement calculation of the said flow sensor, to ensure stability and correct ultrasonic flow measurement of the flow sensor.

In an embodiment, the ultrasonic flow sensor is configured to calculate a corrected value of the change in the density of the fluid on the basis of the change in the speed of sound calculated on the basis of the time-of-flight, if the expected speed of sound does not correspond to the change in the speed of sound calculated on the basis of the time-of-flight. Hereby, it is possible to improve the flow measurement accuracy.

Hereby, it is possible to determine a corrected value of the density of the fluid and hereby provide more accurate measurements.

In an embodiment, the method comprises the step of calculating a corrected value of the specific heat capacity of the fluid on the basis of the corrected value of the density, if the expected change in the speed of sound does not correspond to the change in the speed of sound calculated on the basis of the time-of-flight. Hereby, it is possible to provide measurements with an improved accuracy.

In an embodiment, the method comprises the step of calculating a corrected value of the flow of the fluid on the basis of the change in the speed of sound calculated on the basis of the time-of-flight, if the expected change in the speed of sound does not correspond to the change in the speed of sound calculated on the basis of the time-of-flight. Hereby, it is possible to improve the flow measurement accuracy.

In an embodiment, the first detection unit is configured to detect flows above a predefined lower flow level representing the lowest flow that can be measured using the first detection unit, wherein the method comprises the steps of applying a second detection unit to:

• • detect the temperature of the surroundings (the ambient temperature) by a temperature sensor;
  • detect the temperature of the fluid by a temperature sensor arranged and configured to detect the temperature of the fluid;
  • estimate the flow below the lower flow level on the basis of the temperature difference between the surroundings and a fluid measured by the temperature sensors,wherein the method comprises the following steps:
  • a) performing one or more flow measurements by the first detection unit in a flow-calibration-area, in which flow-calibration-area the flow sensor can detect the flow that depends on the temperature difference;
  • b) applying the one or more measurements made in the flow-calibration-area to determine one or more parameters required to determine how the flow depends on the temperature difference in the flow-calibration-area and in the flow area below the flow-calibration-area; and
  • c) estimating the flow below the lower flow level on the basis of the one or more measurements made in the flow-calibration-area.

Hereby, it is possible to detect lower flows than in the prior art.

In an embodiment, the method comprises the step of performing two or more flow measurements in the flow-calibration-area.

In an embodiment, the method comprises the steps of regularly or continuously:

• • carrying out the one or more measurements in a flow-calibration-area; and
  • updating the one or more parameters required to determine how the flow depends on the temperature difference in the flow-calibration-area and in the flow area below the flow-calibration-area.

In an embodiment, the dependency between the flow and the temperature difference (ΔT_sf) is defined by one of the following equations:

$\Delta T_{\mathrm{sf}}\left(Q\right)=\Delta T_B\left(1-e^{-C_{1}Q}\right)\mathrm{or}Q\left(\Delta T_{\mathrm{sf}}\right)=\frac{-1}{C_1}\ln \left(1-\frac{\Delta T_{\mathrm{sf}}}{\Delta T_B}\right)$

where C₁ is a constant and ΔT_B is a temperature difference corresponding to a base flow level.

In an embodiment, the temperature of the fluid is measured by a temperature sensor arranged at the outside of the tubular structure.

In an embodiment, the method comprises the step of detecting an intermediate temperature by an intermediate temperature sensor arranged in a position inside a housing, wherein the housing houses the temperature sensor that is used to detect the temperature of the fluid and the intermediate temperature sensor, wherein the intermediate temperature is expected to have a value between the ambient temperature and the temperature of the fluid.

In an embodiment, the method comprises the step(s) of measuring the density and/or the estimated inhomogeneity of the fluid prior to measuring the flow.

Hereby, it is possible to improve the flow measurements and take into account the density and/or inhomogeneity of the fluid.

In an embodiment, the method comprises the step of calculating a corrected value of the specific heat capacity of the fluid if the detected value of the speed of sound c does not correspond to the expected speed of sound c as a function of the detected temperature of the fluid. Hereby, it is possible to apply the flow sensor to provide a heat energy meter having an improved accuracy. Using a corrected value of the specific heat capacity of the fluid will ensure that the heat energy meter delivers the most accurate measurements.

In an embodiment, the method is carried out by using a clamp-on flow sensor configured to measure the flow of the fluid from outside the tubular structure.

In an embodiment, the method comprises the step of automatically calculating the distance L that the transmitted ultrasonic waves and received ultrasonic waves travel in the fluid on the basis of a detected value of the speed of sound c (and optionally the measured time of flight). Hereby, it is possible to measure the flow in a pipe without knowing the exact dimensions of the pipe. It is also possible to perform accurate measurements, even if sediments are provided at an inside surface of a pipe over time.

A method for measuring the thermal energy of a fluid applies a method according to the present disclosure to detect the flow of the fluid.

A flow sensor according to the present disclosure is a flow sensor configured to measure the flow of a fluid. In an embodiment, the fluid is a liquid. In an embodiment, the fluid is a water-containing liquid. In an embodiment, the fluid is a gas.

Typically, the fluid is flowing through a tubular structure. In an embodiment, the tubular structure is a pipe. In an embodiment, the tubular structure is a hose. In an embodiment, the tubular structure is a container. In an embodiment, the tubular structure is a box.

In an embodiment, the first detection unit may be a structure of a positive displacement meter that requires fluid to mechanically displace components of the mechanical flow detection unit in order to provide flow measurements. In an embodiment, the first detection unit is a turbine. In an embodiment, the first detection unit is an impeller.

In an embodiment, the first detection unit may a structure of an ultrasonic flow sensor. In an embodiment, the first detection unit comprises one or more ultrasonic transducers. In an embodiment, the first detection unit comprises one or more ultrasonic transmitters and one or more ultrasonic receivers.

The data processor may be a micro-processor.

In an embodiment, the second detection unit contains a storage containing information about how the flow depends on the temperature difference, wherein the data processor is configured to access and use said information in such a manner that the data processor can determine the flow on the basis of the temperature difference. In the flow range below the lower flow level, the second detection unit can detect the flow on the basis of the temperature difference value. This can be accomplished, when the relationship between the flow and the temperature difference is known and stored in the storage.

The expected speed of sound depends on the detected temperature of the fluid and can be calculated using a predefined relationship between the speed of sound as a function of the temperature of the fluid. If the fluid is pure water, by way of example, the relationship between the expected speed of sound as a function of the detected temperature of the fluid would be defined as illustrated in FIG. 7.

If the fluid is different from pure water (e.g. water containing salt, sugar or another substance), a different predefined relationship between the expected speed of sound as a function of the detected temperature of the fluid can be used.

The expected speed of sound can be compared with a detected value of the speed of sound simply by detecting the speed of sound and making the comparison. The detection can be carried out using the following formula:

$\begin{array}{cc}C=\frac{L}{2}\frac{t_2+t_1}{t_2{t}_1}& \left(16\right)\end{array}$

where c is the sound of speed, L is the distance the sound signal travels and t₁ and t₂ are the transit time for the sound signal transmitted and reflected, respectively.

The corrected value of the density and the flow is calculated if the detected value of the speed of sound does not correspond to the expected speed of sound. The corrected value of the density can be calculated using the following equation (18):

$\begin{array}{cc}c=\sqrt{\frac{K}{\rho }}\Rightarrow \rho =\sqrt{\frac{K^2}{c^2}},& \left(18\right)\end{array}$

where K is the Bulk Modulus of Elasticity of the fluid and ρ is the density of the fluid.

In an embodiment, the flow sensor is configured to calculate a corrected value of the specific heat capacity of the fluid if the detected value of the speed of sound c does not correspond to the expected speed of sound c as a function of the detected temperature of the fluid. Hereby, it is possible to apply the flow sensor to provide a heat energy meter having an improved accuracy. Using a corrected value of the specific heat capacity of the fluid will ensure that the heat energy meter delivers the most accurate measurements.

In an embodiment, the fluid is a liquid. In an embodiment, the fluid is a water-containing liquid. In an embodiment, the fluid is a gas.

In an embodiment, the method comprises the following steps:

• • storing information about how the flow depends on the temperature difference;
  • using said information to determine the flow on the basis of the temperature difference.

Hereby, the stored information can be used to provide a flow estimation in a simple and reliable manner. The information may be stored in an external device. In an embodiment, the information is stored in a web-based service.

In an embodiment, the method comprises the following steps:

• • storing in the second detection unit information about how the flow depends on the temperature difference; and
  • using said information to determine the flow on the basis of the temperature difference.

Hereby, the stored information can be used to provide a flow estimation in a simple and reliable manner.

In an embodiment, the method is carried out by a flow sensor comprising a data processor, wherein the data processor and the second temperature sensor are arranged inside a housing.

In an embodiment, the method is carried out using a flow sensor, in which the first temperature sensor is arranged in the housing.

In an embodiment, the method is carried out by using a flow sensor, in which the first temperature sensor is arranged outside the housing.

In an embodiment, the method comprises the following steps:

• • performing one or more measurements on a sample of the fluid; and
  • applying the one or more measurements to calculate the density and/or estimated inhomogeneity of the fluid prior to measuring the flow.

In an embodiment, the estimated inhomogeneity of the fluid corresponds to the content of one or more substrates in the fluid. The substrate may be one or more of the following substances: sugar, salt, ethylene glycol, glycerol and propylene glycol.

BRIEF DESCRIPTION OF THE DRAWINGS

Systems and methods will become more fully understood from the detailed description given herein below. The accompanying drawings are given by way of illustration only, and thus, they are not limitative. In the accompanying drawings:

FIG. 1A shows a graph depicting the temperature difference between the surroundings and a fluid flowing through a pipe as a function of the fluid flow through the pipe;

FIG. 1B shows the low flow portion of the graph shown in FIG. 1A;

FIG. 2A shows a schematic view of a clamp-on type flow sensor according to an embodiment;

FIG. 2B shows a schematic view of another clamp-on type flow sensor according to an embodiment;

FIG. 3A shows a schematic view of a flow sensor according to an embodiment;

FIG. 3B shows a schematic view of another flow sensor according to an embodiment;

FIG. 4A shows a schematic view of a clamp-on type flow sensor according to an embodiment mounted on the outside of a pipe;

FIG. 4B shows a schematic view of another flow sensor according to an embodiment;

FIG. 5A shows a schematic view of a flow sensor according to an embodiment;

FIG. 5B shows a schematic view of another flow sensor according to an embodiment;

FIG. 6A shows a schematic view of a flow sensor according to an embodiment;

FIG. 6B shows a schematic view of another flow sensor according to an embodiment;

FIG. 7 shows a graph depicting the speed of sound in water as a function of the temperature of the water; and

FIG. 8 shows the flow as a function of the temperature difference.

DETAILED DESCRIPTION

Referring now in detail to the drawings for the purpose of illustrating embodiments of the present systems and methods, a graph 28 depicting the temperature difference ΔT_sf between the surroundings and a fluid flowing through a pipe as a function of the fluid flow Q through the pipe is illustrated in FIG. 1A.

It can be seen that the graph 28 (indicated with a solid line) extends above a lower flow level Q_A. The lower flow level Q_A represents the lowest flow that can be measured using prior art flow sensors. Below this lower flow level Q_A, the graph 28, however, has been extrapolated. This lower area 30 is indicated with a dotted ellipse.

FIG. 1B illustrates the low flow portion 30 of the graph 28 shown in FIG. 1A. While the prior art flow sensors are not capable of detecting flow below the lower flow level Q_A, the flow sensors and methods according to the present disclosure are capable of providing flow measurements below this lower flow level Q_A.

Above a base flow level Q_B the graph 28 shows that the temperature difference ΔT_sf is constant and thus independent of the flow Q.

In the flow-calibration-area B₂ between the lower flow level Q_A and the base flow level Q_B the temperature difference ΔT_sf increases as a function of the flow Q. In this flow-calibration-area B₂, a first flow sensor measurement M₁ and a second flow sensor measurement M₂ are indicated.

It is possible to use one or more of the flow sensor measurements made in the flow-calibration-area B₂ to determine the parameters required to determine how the flow Q depends on the temperature difference ΔT_sf in the flow-calibration-area B₂ and in the flow area B₁ below the flow-calibration-area B₂.

The temperature difference ΔT_sf as a function of the flow Q is given by the following equation

ΔT_sf(Q)=ΔT_B(1−e^−C1Q)   (1)

where ΔT_B is a temperature difference corresponding to the base flow level Q_B and C₁ is a constant.

By performing two measurements M₁ and M₂, it is possible to determine the two unknowns ΔT_B and C₁ from equation (1).

Therefore, it is possible to determine a flow Q_M3 in the flow area B₁, in which the flow sensor cannot provide any measurements. The flow Q_M3 can be determined on the basis of a measured temperature difference ΔT_M3 detected by the flow sensor. The flow Q_M3 can be determined by using equation (1) or the following equation defining the flow Q as a function of the detected temperature difference ΔT_sf:

$\begin{array}{cc}Q\left(\Delta T_{\mathrm{sf}}\right)=\frac{-1}{C_1}\ln \left(1-\frac{\Delta T_{\mathrm{sf}}}{\Delta T_B}\right)& \left(2\right)\end{array}$

where C₁ is a constant and ΔT_B is a temperature difference corresponding to the base flow level Q_B.

A flow sensor and method according to the present disclosure estimate flows Q below the lower flow level Q_A by measuring the temperature difference ΔT_sf between the surroundings and a fluid flowing through the pipe. The estimation is possible because one or more flow measurements M₁, M₂ made in the flow-calibration-area B₂ are used to determine the unknowns in equation (1) or equation (2). Accordingly, any flow Q in the flow area B₁ can be calculated by using equation (2).

In FIG. 1B it can be seen that a first flow Q₁ is detected on the basis of a first measured temperature difference ΔT₁. Likewise, FIG. 1B shows that a second flow Q₂ is detected on the basis of a second measured temperature difference ΔT₂.

The lower flow level Q_A corresponds to a measured temperature difference ΔT_A. Likewise, the base flow level Q_B corresponds to a higher measured temperature difference ΔT_B.

The temperature difference can be detected by using temperature sensors described herein. This is shown in and explained with reference to FIG. 2A, FIG. 2B, FIG. 3A, FIG. 3B and FIG. 4B.

In an example, in the flow-calibration-area B₂, a flow sensor according to the present disclosure used to measure water at 20° C. is applied to make a measurement point M₂ corresponding to a flow Q_M2 of 2 ml/s (which is 0.000002 m³/s) and a temperature difference ΔT_M2 of 10° C.

Relationship between the temperature difference ΔT_sf between the surroundings and the fluid and the flow Q is given by equation (2):

$\begin{array}{cc}Q\left(\Delta T_{\mathrm{sf}}\right)=\frac{-1}{C_1}\ln \left(1-\frac{\Delta T_{\mathrm{sf}}}{\Delta T_B}\right)& \left(2\right)\end{array}$

If

$C_1=5.02\frac{\min }{{\mathrm{cm}}^3}$

and dt_B=10.02° C. one can calculate the following values:

	TABLE 1
	ΔTsf [° C.]	0.980	2.224	3.652	4.446
	Flow [cm3/min]	0.020	0.050	0.116	0.572

In another example, below the lower flow level Q_A, the relationship between the temperature difference ΔT_sf and the flow Q is given by the equation (2), where C₁=4.88 and dt_B=12.54° C. one can calculate the following values:

	TABLE 2
	ΔTsf [° C.]	1.124	2.462	3.866	5.562
	Flow [cm3/min]	0.019	0.045	0.076	0.120

FIG. 2A illustrates a schematic view of a clamp-on type flow sensor 1 according to the present disclosure. The flow sensor 1 is arranged to detect the flow of a fluid 26 (e.g. a liquid) in the pipe 2. The flow sensor 1 comprises a data processor 10.

The flow sensor 1 comprises a first temperature sensor 12 arranged to detect the ambient temperature (the temperature in the surroundings of the pipe 2). The flow sensor 1 comprises a second temperature sensor 14 arranged to detect the temperature of the fluid 26. The flow sensor 1 comprises a first ultrasonic wave generator 4 and a second ultrasonic wave generator 4′. The wave generators are formed as piezo transducers 4, 4′ arranged and configured to generate ultrasonic waves, which are introduced into the fluid 26 at an angle to the direction of flow Q. The flow sensor 1 may be either a Doppler effect type flow sensor 1 or a propagation time measuring type flow sensor 1. It is indicated that both ultrasonic waves 6, 8 travel a distance ½L. Accordingly, the total distance of travel is L.

The piezo transducers 4, 4′ are operated as a transducer to detect the flow Q through a pipe by using acoustic waves 6, 8. In an embodiment, the flow sensor 1 comprises several piezo transducers 4, 4′ in order to be less dependent on the profile of the flow Q in the pipe 2. The operating frequency may depend on the application and be in the frequency range 100-200 kHz for gases and in a higher MHz frequency range for liquids.

In an embodiment, the flow sensor 1 is a Doppler effect flow sensor 1. In this embodiment, the flow sensor 1 comprises a single piezo transducer only. In this case the second piezo transducer 4′ can be omitted and the first piezo transducer 4 is used for both sending ultrasonic waves 6 and for receiving ultrasonic waves 8. In a Doppler effect type flow sensor 1, when the transmitted wave 6 is reflected by particles or bubbles in the fluid, its frequency is shifted due to the relative speed of the particle. The higher the flow speed of the liquid, the higher the frequency shift between the emitted and the reflected wave.

In an embodiment, the flow sensor 1 is a Doppler effect flow sensor 1 that comprises several piezo transducers 4, 4′. In this case one piezo transducer 4 can be used to transmit an ultrasonic wave 6, while the other piezo transducer 4′ can be used to receive the reflected ultrasonic wave 8.

In an embodiment, the flow sensor 1 is a propagation type flow sensor 1. In this embodiment, the flow sensor 1 applies two piezo transducers operating as both transmitter and receiver arranged diagonally to the direction of flow Q. Transmission of ultrasonic waves in the flowing medium causes a superposition of sound propagation speed and flow speed. The flow speed proportional to the reciprocal of the difference in the propagation times in the direction of the flow Q and in the opposite direction. The propagation type measuring method is independent of the sound propagation speed and thus also the medium. Accordingly, it possible to measure different liquids or gases with the same settings.

The temperature sensors 12, 14 and the piezo transducers 4, 4′ are connected to the data processor 10. Accordingly, the data processor 10 can process data from the temperature sensors 12, 14 and the piezo transducers 4, 4′ and hereby detect the flow based on the data.

In FIG. 2A, the second temperature sensor 14 is arranged outside the pipe 2. The second temperature sensor 14 is thermally connected to the pipe 2. Accordingly, the second temperature sensor 14 is capable of measuring the temperature of the pipe 2. The temperature of the pipe 2 will normally correspond to or be very close to the temperature of the fluid 26 in the pipe 2.

In the low flow area below the lower flow level of the flow sensor 1, the flow sensor 1 determines the flow on the basis of the temperature measurements made by the first temperature sensor 12 and the second temperature sensor 14. In fact, below the lower flow level of the flow sensor 1, the flow sensor 1 determines the flow on the basis of the temperature difference ΔT_sf defined as the difference between the temperatures detected by the first temperature sensor 12 and the second temperature sensor 14.

ΔT_sf=|T_s−T_f|  (9)

where T_s is the temperature of the surroundings measured by the first temperature sensor 12 and T_f is the temperature of the fluid 26 measured by the second temperature sensor 14.

FIG. 2B illustrates a schematic view of a clamp-on type flow sensor 1 according to the present disclosure. The flow senor 1 shown in FIG. 2B basically corresponds to the one shown in FIG. 2A. The temperature sensor 14, however, is in contact with the fluid 26 inside the pipe 2. A structure extends through the wall of the pipe 2. The temperature sensor 14 is connected to the data processor 10 via a wire extending through said structure. It is indicated that both ultrasonic waves 6, 8 travel a distance ½L. Accordingly, the total distance of travel is L.

FIG. 3A illustrates a schematic view of a heat energy meter 5 according to the present disclosure. The heat energy meter 5 comprises a flow sensor 1 according to the present disclosure. The flow sensor 1 comprises a housing 20 that is attached to a pipe 2. The flow sensor 1 is arranged and configured to detect the flow Q of the fluid 26 (e.g. a water containing liquid) in the pipe 2.

The flow sensor 1 comprises a first temperature sensor 12 arranged to detect the temperature T_s of the surroundings (e.g. the ambient temperature). The flow sensor 1 comprises a second temperature sensor 14 arranged to detect the temperature T_f of the fluid 26 in the pipe 2. The flow sensor 1 comprises a third temperature sensor 16 arranged to detect an intermediate temperature T_i that is expected to have a value between the ambient temperature T_s and the temperature T_f of the fluid 26.

The flow sensor 1 comprises a first ultrasonic wave generator 4 and a second ultrasonic wave generator 4′ formed as piezo transducers 4, 4′ that are arranged and configured to generate ultrasonic waves transmitted into the fluid 26 at an angle to the direction of flow Q. The piezo transducers 4, 4′ are used in the same manner as shown in and explained with reference to FIG. 2A and FIG. 2B.

The flow sensor 1 comprises a data processor 10 connected to the piezo transducers 4, 4′ and to the temperature sensors 12, 14, 16. Therefore, the data processor 10 can process data from the temperature sensors 12, 14 and the piezo transducers 4, 4′ and hereby detect the flow based on the data.

The third temperature sensor 16 provides temperature measurements that can be applied to provide an improved estimation of the flow below the lower flow level of the flow sensor 1. The improved estimation can be accomplished by using two temperature differences:

• • the difference ΔT_sf between the surroundings and the fluid 26:

ΔT_sf=|T_s−T_f| and   (10)

• • the temperature difference ΔT_if between the intermediate point in the housing 20 and the fluid 26:

ΔT_if=|T_i−T_f|  (11)

The heat energy meter 5 has an external temperature sensor 17 thermally connected to a pipe 3. By measuring the temperature of the fluid in the supply pipe 3 and the temperature of the fluid 26 in the return pipe 2, it is possible to calculate the consumed heat quantity (heat energy). The external temperature sensor 17 may be connected to the data processor 10 by a wired connection as shown in FIG. 3A or by a wireless connection as shown in FIG. 3B.

FIG. 3B illustrates a schematic view of another heat energy meter 5 according to the present disclosure. The heat energy meter 5 comprises a flow sensor 1 according to the present disclosure. The flow sensor 1 basically corresponds to the one shown in FIG. 3A. The first temperature sensor 12, however, is placed on the outside surface of the housing 20. The heat energy meter 5 has an external temperature sensor 17 that is attached to the outside surface of a supply pipe 3. Accordingly, the temperature sensor 17 is thermally connected to the supply pipe 3. By measuring the temperature of the fluid in the supply pipe 3 and the temperature of the fluid 26 in the return pipe 2, it is possible to calculate the consumed heat quantity (heat energy).

FIG. 4A illustrates a schematic view of a clamp-on type flow sensor 1 according to the present disclosure. The flow sensor 1 is mounted on the outside of a pipe 2. The flow sensor 1 comprises a housing 20 having a contact structure that matches the outer geometry of the pipe 2. A thermal connection structure (e.g. a metal layer) is attached to the contact structure. Hereby, the thermal connection structure reduces the thermal resistance and therefore provides an improved and effective heat transfer between the pipe 2 and the temperature sensors (not shown) of the flow sensor.

In an embodiment, the thermal connection structure is a metal foil, coated with thermal adhesive on each side. Such thermal connection structure is capable of providing a permanent bond and reducing the thermal resistance by filling micro-air voids at the interface. In an embodiment, the thermal connection structure is thermally conductive aluminum tape. The thermal connection structure may be thermally conductive double-sided structural adhesive aluminum tape.

FIG. 4B illustrates a schematic view of a flow sensor 2 according to the present disclosure. The flow sensor comprises a mechanical flow detection unit 24 that is arranged inside a pipe 3 and thus submerged into the fluid 26.

The flow sensor 1 is a positive displacement meter that requires fluid to mechanically displace components of the mechanical flow detection unit 24 in order to provide flow measurements. The mechanical flow detection unit 24 can be a turbine or impeller. The activity and rotational speed of the turbine or impeller can either be determined using a direct connection to a data processor 10 or by a detection member (not shown) arranged and configured to measure the angular velocity of the turbine or impeller. The flow sensor 1 may be a turbine flow meter, a single jet flow meter or a paddle wheel flow meter by way of example. The mechanical flow detection unit 24 constitutes a first detection unit 34. The data processor 10 and the temperature sensors 12, 14 constitute the second detection unit 36.

The flow sensor 1 comprises a first temperature sensor 12 arranged and configured to detect the temperature of the surroundings (the ambient temperature). The flow sensor 1 comprises a second temperature sensor 14 arranged and configured to detect the temperature of the fluid 26 inside the pipe 3. The second temperature sensor 14 bears against the outside portion of the wall of the pipe 3. In another embodiment, however, the second temperature sensor 14 may be arranged inside the pipe 3. In a further embodiment, the second temperature sensor 14 may be integrated into the wall of the pipe 3.

The flow sensor 1 comprises a pipe 3 provided with a first flange 18 and a second flange 18′. These flanges 18, 18′ are configured to be mechanically connected to corresponding flanges 19, 19′ of two pipes 2, 2′. In an embodiment, the flanges 18, 18′ are replaced with similar attachment structures designed to attach the flow sensor 1 to pipes 2, 2′.

In an embodiment, the distal portions of the pipes 2, 2′ are provided outer threads while the distal portions of the pipe 3 of the flow sensor are provided with corresponding inner threads allowing the pipe 3 to be screwed onto the pipes 2, 2′.

In an embodiment, the distal portions of the pipes 2, 2′ are provided inner threads while the distal portions of the pipe 3 of the flow sensor are provided with corresponding outer threads allowing the pipe 3 to be screwed onto the pipes 2, 2′.

FIG. 5A illustrates a schematic view of a flow sensor 1 according to the present disclosure. The flow sensor 1 basically corresponds to the one shown in FIG. 3A.

FIG. 5B illustrates a schematic view of a flow sensor 1 according to the present disclosure. The flow sensor 1 basically corresponds to the one shown in FIG. 3B.

In FIG. 5A and FIG. 5B, the housing 20, however, comprises a portion that bears against the pipe 2, while the second temperature sensor 14 as well as the piezo transducers 4, 4′ extend through said portion of the housing 20 in order to be directly connected to the outside portion of the pipe 2, when the flow sensor 1 is attached to the pipe 2. It is possible to apply clamping structures such as cable tie or hose clamps to clamp the flow sensor to the pipe 2.

The piezo transducers 4, 4′ constitute a first detection unit 34. The data processor 10 and the temperature sensors 12, 14, 16 constitute a second detection unit 36.

A flow sensor 1 according to the present disclosure uses the fact that the fluid 26 in most cases transports heat between the physical zones it flows through and that these physical zones have different temperatures. By detecting the temperature difference between these zones, it is possible to provide an alternative measure for the flow rate.

Accordingly, a flow sensor 1 and a method according to the present disclosure can detect flow in the low flow range, in which the prior art flow sensors cannot detect any flow.

Moreover, a flow sensor 1 and a method according to the present disclosure can provide an improved (more accurate) flow detection in general by using the temperature difference between the above-mentioned zones.

The heat transfer rate q (corresponding to E/t) from the fluid to the surroundings is defined in the following equation (12):

q=UAΔT_sf   (12)

where ΔT_sf is the temperature difference between the surroundings and the fluid 26; A is the surface area where the heat transfer takes place and U is the heat transfer coefficient.

The heat transfer coefficient U is defined in the following equation (13):

$\begin{array}{cc}U=\frac{k}{s},& \left(13\right)\end{array}$

where k is the thermal conductivity of the material through which the heat transfer takes place and s is the thickness of the material through which the heat transfer takes place.

The working principle of a Doppler Effect flow sensor 1 is shown in and briefly explained with reference to FIG. 6A. Doppler Effect flow sensors are affected by changes in the sonic velocity of the fluid 26. Accordingly, Doppler Effect flow sensors are sensitive to changes in density and temperature of the fluid 26. Therefore, many prior art Doppler Effect flow sensors are unsuitable for highly accurate measurement applications. The present disclosure, however, makes it possible to detect the temperature and speed of sound of the fluid 26 and compensate for temperature and fluid (density) changes and thus provide an improved accuracy. Likewise, the present disclosure, makes it possible to detect the density of the fluid 26 (via measurement made on a sample of the fluid 26) and compensate for temperature and/or fluid (density) changes in order to even further improve the accuracy of the flow sensor 1.

The Doppler Effect flow sensor 1 is a time-of-flight ultrasonic flow sensor that measures the time for the sound to travel between a transmitter 4 and a receiver 4′. In a typical setup, like the one illustrated in FIG. 6A, two transducers (transmitters/receivers) 4, 4′ are placed on each side of the pipe 2 through which the flow Q is to be measured. The transmitters 4, 4′ transmit pulsating ultrasonic waves 6 in a predefined frequency from one side to the other. The average fluid velocity V is proportional to the difference in frequency.

Accordingly, the fluid velocity V can be expressed as:

$\begin{array}{cc}V=\frac{t_2-t_1}{t_2{t}_1}\frac{L}{2\cos \left(\varphi \right)}& \left(14\right)\end{array}$

where t₁ is the transmission time for the transmission time downstream, t₂ is the transmission time upstream, L is the distance between the transducers and ϕ is the relative angle between the transmitted ultrasonic beam 6 and the fluid flow Q.

The flow Q can be calculated as the product between the fluid velocity V and the cross-sectional area A_pipe of the pipe 2:

Q=VA_pipe   (15)

At the same time the speed of sound c is given by the following equation:

$\begin{array}{cc}c=\frac{L}{2}\frac{t_2+t_1}{t_2{t}_1}& \left(16\right)\end{array}$

The flow sensor 1 shown in FIG. 6A comprises a first temperature sensor 12 arranged to detect the ambient temperature (the temperature in the surroundings of the pipe 2. The flow sensor 1 comprises a second temperature sensor 14 arranged to detect the temperature of the fluid 26. The flow sensor 1 comprises a data processor 10. Even though it is not shown in FIG. 6B, the temperature sensors 12, 14 and the two transducers 4, 4′ are connected to the data processor 10. Accordingly, the data processor 10 can process data and calculate the flow Q based on data from the temperature sensors 12, 14 and the two transducers 4, 4′.

The working principle of a Doppler Effect flow sensor 1 measuring the flow in a fluid containing particles 32 is shown in and briefly explained with reference to FIG. 6B.

The fluid velocity V can be calculated by using the following equation (17):

$\begin{array}{cc}V=\frac{c\left(\mathrm{fr}-\mathrm{ft}\right)}{2\mathrm{ft}\cos \left(\varphi \right)},& \left(17\right)\end{array}$

where fr is the frequency of the received wave; ft is the frequency of the transmitted wave; ϕ is the relative angle between the transmitted ultrasonic beam and the fluid flow Q and c is the velocity of sound in the fluid 26.

The flow Q can be calculated as the product between the fluid velocity V and the cross-sectional area A_pipe of the pipe 2:

Q=VA_pipe   (15)

Equations 15 and 16 can also be used when calculating the flow by using the flow sensor shown in FIG. 2A, FIG. 2B, FIG. 3A and FIG. 3B.

FIG. 7 illustrates a graph depicting the speed of sound c in water as a function of the temperature T of the water. Similar graphs can, however, be made for other liquids. In the following, water is just representing on possible fluid and water may be replaced with another liquid.

If the dimensions of the tubular structure (e.g. pipe, through which a flow Q of water is flowing, are not known, an estimation of the distance L that the sound travels in the water is needed. This problem is in particular relevant for ultrasonic clamp-on sensors. Over time, sediments may be provided at an inside surface of a pipe. This will gradually decrease the distance L. Accordingly, the systems and methods make it possible to use an estimation of the distance L under such conditions.

By determining the speed of sound c in the water, it is possible to estimate the distance L and hereby improve the accuracy of the detected speed V and flow Q of the water. Accordingly, changes in the speed of sound c in the water are highly relevant.

When the speed of sound c is detected, it is possible to calculate the distance L that the sound travels in the water.

The speed of sound c is given by the following formula:

$\begin{array}{cc}c=\sqrt{\frac{K}{\rho }}& \left(18\right)\end{array}$

where K is the Bulk Modulus of Elasticity and ρ is the density.

Since the density of water depends on the temperature T, the speed of sound c depends on the temperature T. Moreover, the speed of sound c depends on the concentration of substances (e.g. glycol) in the water.

When the inclination angle α is known, the average speed V of the water (in the tube measured by delta time of flight) can be obtained using the following equation (19):

$\begin{array}{cc}V=\frac{L}{2\cos \left(\varphi \right)}\frac{t_2-t_1}{t_2{t}_1}& \left(19\right)\end{array}$

When the speed of sound c is known. L can be calculated or estimated by using the following equation (since t₁ and t₂ are being measured).

$\begin{array}{cc}c=\frac{L}{2}\frac{t_2+t_1}{t_2{t}_1}& \left(16\right)\end{array}$

Accordingly, the flow Q can be calculated as the product between the average speed V of water and the cross-sectional area A_pipe of the pipe 2:

Q=VA_pipe   (15)

The measured fluid temperature T and the measured time-of-flight can be used to determine the density ρ and the speed of sound c by using equation (18).

If the flow sensor is calibrated in pure water at a temperature T₂ of 26° C., FIG. 7 shows that the speed of sound c(T₂) is 1500 m/s. If a lower temperature T₁ of 21.5° C. is detected, the speed of sound c(T₁) is 1485 m/s. Accordingly, by calibrating the flow sensor by using a fluid (e.g. a liquid such as water) at a known temperature T and density ρ, a simple temperature measurement is sufficient to detect the speed of sound c by using equation (18).

$\begin{array}{cc}c=\sqrt{\frac{K}{\rho }}& \left(18\right)\end{array}$

The specific heat capacity of the fluid (e.g. water) depends on the content of additional substances (e.g. sugar, salt, ethylene glycol, glycerol or propylene glycol).

When the speed of sound c is known, it is possible to calculate the specific heat capacity of the fluid (e.g. water) having additional substances on the basis of the detected density of the fluid. Hereby, it is possible to make a heat energy meter having a flow sensor according to the present disclosure more accurate.

It may be an advantage to measure content of additional substances (e.g. sugar, salt, ethylene glycol, glycerol or propylene glycol). Hereby, it would be possible to calibrate the flow sensor on the basis of the measurements.

EXAMPLE 1

If the flow sensor being used in pure water detects a flow Q of 1 liter/minute at a temperature T₂ of 26° C., FIG. 7 shows that the speed of sound c(T₂) is 1500 m/s.

When the speed of sound c (1500 m/s) is known. L can be calculated by using the following equation (since t₁ and t₂ are detected by the flow sensor).

$\begin{array}{cc}c=\frac{L}{2}\frac{t_2+t_1}{t_2{t}_1}& \left(16\right)\end{array}$

When the flow sensor is used at a later point in time, the expected speed of sound c, at the same temperature T₂ of 26° C. would be 1500 m/s. If, however, the detected speed of sound c is 1485 m/s calculated by using equation (16) and the known L, the decreased speed of sound is approximately 1%. This may be caused by a change in the density ρ of the water. If we presume that the Bulk Modulus of Elasticity K is constant, equation (18) will give us that the density ρ is increased by approximately 2% (by using equation 18).

If the flow sensor is used in a heat energy meter, it would be possible to correct the specific heat capacity of the water based on the detected density of the water. It can be concluded that the content of additional substances (e.g. sugar, salt, ethylene glycol, glycerol or propylene glycol) has increased. Accordingly, it is possible to improve the accuracy of the heat energy meter. This is relevant since the content of additional substances (e.g. sugar, salt, ethylene glycol, glycerol or propylene glycol) may vary as a function of time. If the flow sensor is configured to automatically detect changes in the density of the fluid, the flow sensor is used in a heat energy meter will be capable of providing a high accuracy even when the content of additional substances varies over time.

FIG. 8 illustrates a graph depicting the flow Q detected by a flow sensor according to the present disclosure as a function of the temperature difference ΔT_sf.

The lower flow level Q_A represents the lowest flow that can be measured using prior art flow sensors. Prior art flow sensors are not capable of detecting flow below the lower flow level Q_A, flow sensors and methods according to the present disclosure, however, are capable of providing flow measurements below this lower flow level Q_A.

Above a base flow level Q_B the graph shows that the temperature difference ΔT_sf is constant and thus independent of the flow Q.

In the flow-calibration-area B₂ between the lower flow level Q_A and the base flow level Q_B the temperature difference ΔT_sf increases as a function of the flow Q. In this flow-calibration-area B₂, a first flow sensor measurement M₁ and a second flow sensor measurement M₂ are indicated.

These flow sensor measurements M₁ and M₂ are made in the flow-calibration-area B₂ in order to determine the parameters required to determine how the flow Q depends on the temperature difference ΔT_sf in the flow-calibration-area B₂ and in the flow area B₁ below the flow-calibration-area B₂. The relationship between the flow Q and temperature difference ΔT_sf is given by equation:

$\begin{array}{cc}Q\left(\Delta T_{\mathrm{sf}}\right)=\frac{-1}{C_1}\ln \left(1-\frac{\Delta T_{\mathrm{sf}}}{\Delta T_B}\right)& \left(2\right)\end{array}$

It is possible to measure temperature differences ΔT₁, ΔT_M3 and ΔT₂ and calculate the flow Q by using equation (2).

LIST OF REFERENCE NUMERALS

• • 1 Flow sensor
  • 2, 2′, 3 Pipe
  • 4, 4′ Ultrasonic transducer (piezo transducer)
  • 5 Thermal energy meter
  • 6 Ultrasonic vibration wave
  • 8 Reflected ultrasonic vibration wave
  • 10 Data processor (e.g. a micro-processor)
  • 12 Temperature sensor
  • 14 Temperature sensor
  • 16 Temperature sensor
  • 18, 18′ Flange
  • 19, 19′ Flange
  • 20 Housing
  • 22 Thermal connection structure (e.g. a metal layer)
  • 24 Mechanical flow detection unit
  • 26 Fluid
  • 28 Graph
  • 30 Low flow area
  • 32 Particle
  • 34, 36 Detection unit
  • T_s Temperature of the surroundings
  • T_f Temperature of the fluid
  • ΔT Temperature difference
  • ΔT_sf Temperature difference between the surroundings and the fluid
  • ΔT_i, ΔT₂ Temperature difference
  • ΔT_A, ΔT_B Temperature difference
  • T₁, T₂ Temperature
  • M₁, M₂, M₃ Flow measurement
  • B₁ Flow area
  • B₂ Flow-calibration-area
  • c_p Specific heat capacity
  • k Thermal conductivity
  • U Coefficient of heat transfer
  • A Surface area
  • W Volume
  • t Time-of-flight
  • t′ Temperature compensated time-of-flight
  • Δt Delta-time-of-flight
  • t₁, t₂ Time-of-flight
  • dt₁, dt₂ Temperature difference
  • dt_A, dt_B Temperature difference
  • dt_M1, dt_M2 Temperature difference
  • dt_M3Temperature difference
  • s Thickness
  • Q Flow
  • Q₁, Q₂ Flow
  • Q_A, Q_B Flow
  • Q_M1, Q_M2Flow
  • Q_M3 Flow
  • V Fluid velocity
  • α Angle
  • L Distance

Claims

1. An ultrasonic flow sensor configured to measure a flow of a fluid flowing through a tubular structure, the flow sensor comprising: a first detection unit arranged to transmit and receive ultrasonic waves using at least one ultrasonic transducer;a first temperature sensor arranged and configured to detect a temperature (Tf) of the fluid;a second temperature sensor arranged and configured to detect a temperature (Ts) of the surroundings;a data processor configured to receive data detected by the at least one ultrasonic transducer and the first and second temperature sensors,

2. The flow sensor according to claim 1, wherein the ultrasonic flow sensor is configured to calculate one or more of: (i) a corrected value of density (ρ) of the fluid based on the change in the speed of sound calculated based on the time-of-flight (t, t1, t2), if the expected speed of sound does not correspond to the change in the speed of sound calculated based on the time-of-flight (t, t1, t2);(ii) a corrected value of specific heat capacity (cp) of the fluid on the basis of the corrected value of the density (ρ), if the expected change in the speed of sound does not correspond to the change in the speed of sound calculated based on the time-of-flight (t, t1, t2); and(iii) a corrected value of the flow of the fluid based on the change in the speed of sound calculated based on the time-of-flight (t, t1, t2), if the expected change in the speed of sound does not correspond to the change in the speed of sound calculated based on the time-of-flight (t, t1, t2).

3. The flow sensor according to claim 1, wherein the first detection unit is configured to detect flows above a predefined lower flow level (QA) representing a lower flow (QA) that can be measured using the first detection unit, wherein the flow sensor comprises a second detection unit configured to estimate the flow below the lower flow level (QA) based on the temperature difference (ΔTsf) between the surroundings and the fluid, wherein the temperature difference (ΔTsf) is measured by the first temperature sensor and the second temperature sensor, wherein the second detection unit is configured to estimate the flow below the lower flow level (QA) based on one or more measurements (M1, M2) made in a flow-calibration-area (B2) where the flow sensor can detect the flow that depends on the temperature difference (ΔTsf), wherein the one or more measurements (M1, M2) made in the flow-calibration-area (B2) are used to determine one or more parameters required to determine how the flow depends on the temperature difference (ΔTsf) in the flow-calibration-area (B2) and in a flow area (B1) below the flow-calibration-area (B2).

4. The flow sensor according to claim 3, wherein the second detection unit is configured to estimate the flow below the lower flow level (QA) based on a single measurement (M1, M2) and predefined data that includes density (ρ) and specific heat capacity (Cp) of the fluid.

5. The flow sensor according to claim 3, wherein the flow sensor is configured to regularly or continuously: carry out the one or more measurements (M1, M2) in the flow-calibration-area (B2); andupdate the one or more parameters required to determine how the flow depends on the temperature difference (ΔTsf) in the flow-calibration-area (B2) and in the flow area (B1) below the flow-calibration-area (B2).

6. The flow sensor according to claim 3, wherein a dependency between the flow and the temperature difference (ΔTsf) is defined by one of the following equations:

7. The flow sensor according to claim 1, wherein the second temperature sensor is arranged and configured to detect the temperature (Tf) of the fluid by measuring a temperature outside of the tubular structure.

8. The flow sensor according to claim 1, wherein the data processor, the first temperature sensor and the second temperature sensor are arranged inside a housing.

9. The flow sensor according to claim 1, wherein the data processor and the second temperature sensor are arranged inside the housing and the first temperature sensor is arranged outside the housing.

10. The flow sensor according to claim 1, wherein the flow sensor is a clamp-on flow sensor configured to measure the flow of the fluid from outside the tubular structure.

11. A thermal energy meter comprising a flow sensor according to claim 1.

12. A method for measuring flow of a fluid flowing through a tubular structure using an ultrasonic flow sensor comprising a first detection unit having at least one ultrasonic transducer arranged to transmit and receive ultrasonic waves, the method comprising: determining a time-of-flight (t, t1, t2) of the ultrasonic waves;calculating a change in speed of sound based on the time-of-flight (t, t1, t2);calculating an expected change in the speed of sound as a function of a detected temperature (Tf) of the fluid;determining if the expected change in the speed of sound corresponds to the change in the speed of sound calculated based on the time-of-flight (t, t1, t2); andidentifying a no-flow state, in which there is no flow of the fluid when:

13. The method according to claim 12, wherein the no-flow state is used to calibrate the ultrasonics flow measurement calculation of the flow sensor to ensure stability and correct ultrasonic flow measurement of the flow sensor.

14. The method according to claim 12, further comprising calculating one or more of: (i) a corrected value of change in density (ρ) of the fluid based on the change in the speed of sound calculated based on the time-of-flight (t, t1, t2), if the expected speed of sound does not correspond to the change in the speed of sound calculated based on the time-of-flight (t, t1, t2);(ii) a corrected value of specific heat capacity (cp) of the fluid based on the corrected value of the density (ρ), if the expected change in the speed of sound does not correspond to the change in the speed of sound calculated based on the time-of-flight (t, t1, t2); and(iii) a corrected value of the flow of the fluid based on the change in the speed of sound calculated based on the time-of-flight (t, t1, t2), if the expected change in the speed of sound does not correspond to the change in the speed of sound calculated based on the time-of-flight (t, t1, t2).

15. The method according to claim 12, wherein the first detection unit is configured to detect flows above a predefined lower flow level (QA) representing a lowest flow that can be measured using the first detection unit, and a second detection unit estimates the flow below the lower flow level (QA) based on the temperature difference (ΔTsf) between the surroundings and the fluid by: a) performing one or more flow measurements (M1, M2) in a flow-calibration-area (B2) where the flow sensor can detect the flow that depends on the temperature difference (ΔTsf);b) applying the one or more measurements (M1, M2) made in the flow-calibration-area (B2) to determine one or more parameters required to determine how the flow depends on the temperature difference (ΔTsf) in the flow-calibration-area (B2) and in a flow area (B1) below the flow-calibration-area (B2); andc) estimating the flow below the lower flow level (QA) on the basis of the one or more measurements (M1, M2) made in the flow-calibration-area (B2).

16. The method according to claim 15, further comprising regularly or continuously: carrying out the one or more measurements (M1, M2) in the flow-calibration-area (B2); andupdating the one or more parameters required to determine how the flow depends on the temperature difference (ΔTsf) in the flow-calibration-area (B2) and in the flow area (B1) below the flow-calibration-area (B2).

17. The method according to claim 15, wherein a dependency between the flow and the temperature difference (ΔTsf) is defined by one of the following equations:

18. The method according to claim 12, wherein the temperature (Tf) of the fluid is measured by a temperature sensor arranged outside of the tubular structure.

19. The method according to claim 12, further comprising measuring density and/or an estimated inhomogeneity of the fluid prior to measuring the flow.

20. The method according to claim 12, wherein the method is carried out using a clamp-on flow sensor configured to measure the flow of the fluid from outside the tubular structure.