Patent Application
20150075277
The present invention relates to a method for thermally determining mass flow of a gaseous medium and a thermal mass flow meter.
Methods for thermally determining mass flow of a gas and corresponding thermal mass flow meters are known per se. They operate by heating a first temperature sensor flowed around by the medium, in order to produce, respectively to maintain, a temperature difference relative to a second temperature sensor flowed around by the medium.
The greater the mass flow, the more heat is transported away by the medium, and the more heating power is required, in order, for example, to maintain a predetermined temperature difference. As a result, the so called power coefficient, thus the ratio between the heating power Q and the achieved temperature difference, is a measure for the mass flow.
For exact determination of the mass flow, the power coefficient is used to ascertain a heat transfer coefficient (htc), based on which then via the Nusselt-, Prandtl- and Reynolds number, the mass flow is determined. Details concerning this are disclosed especially in European patent EP 0 624 242 B1. Offenlegungsschrift DE 10 2005 057 688 A1 discloses a method for determining mass flow using a Mach number correction.
Both aforementioned documents of the state of the art describe good methods for sufficiently low flow velocities, respectively for symmetric arrangements of temperature sensors.
Especially when asymmetries occur between the temperature sensors at higher flow velocities, the so called recovery effect must be taken into consideration. The recovery effect brings about a warming of the temperature sensor due to stopping of the gas stream on the probe surface. Especially in the case of asymmetric flowmeters, the recovery effect negatively influences the accuracy of measurement considerably. It is, consequently, an object of the present invention to provide a method for thermally determining the mass flow of a gas and to provide a thermal mass flow meter, whereby the recovery effect is corrected.
The object is achieved according to the invention by the method as defined in claim 1 and the thermal flow meter as defined in claim 13.
The method of the invention for determining mass flow of a gaseous medium by means of a thermal mass flow meter, which has at least a first temperature sensor, which can be flowed around by the medium and which has at least a second temperature sensor, which can be flowed around by the medium, wherein the first temperature sensor is spaced from the second temperature sensor, wherein the first temperature sensor is heated with a heating power Q, wherein heat transfer from the first temperature sensor into the medium depends on the mass flow of the medium, wherein the mass flow of the medium is determined by means of a power coefficient PC=Q/ΔT as a function of a heating power Q and a temperature difference ΔTm=T1−T2 between the measured values of the first temperature sensor and the second temperature sensor, characterized in that a corrected power coefficient PCcorr is determined from corrected values of the temperature difference ΔT, wherein at least one correction occurs by means of at least one recovery correction term Ki.
In a further development of the invention, the recovery correction term Ki has the form Ki=Δx·u2/(2·cp), wherein u is a value for the flow velocity of the medium, wherein cp is a value for the heat capacity of the medium, wherein Δx is an element of the set {Δ1; Δ2; Δ12}, wherein Δ1:=e1−cr, Δ2:=e2−cr and Δ12:=e1−e2=Δ1−Δ2, wherein e1 and e2 are the recovery factors of the first, respectively second, temperature sensor, and wherein cr is a constant reference value, wherein cr≦1, especially cr=1.
In a further development of the invention, the recovery correction term Ki is subtracted from a temperature difference ΔT or added to the temperature difference ΔT, in order to obtain a corrected temperature difference ΔTcorr, which enters as divisor, respectively factor, into the determining of a corrected power coefficient PCcorr.
In a further development of the invention, the divisor Di, respectively the factor Fi, is normalized with the temperature difference ΔT, so that the divisor Di, respectively factor Fi, has the form [1±(Ki/ΔT)].
In a further development of the invention, the first and second temperature sensors have different recovery factors e1, e2, wherein, especially, the first and second temperature sensors have different flow effective forms.
In a further development of the invention, at least a first corrected temperature difference ΔTcorr
In a further development of the invention, by means of the first corrected temperature difference, a first corrected power coefficient is ascertained, wherein especially the following holds: PCcorr
In a further development of the invention, at least a second corrected temperature difference ΔTcorr
In a further development of the invention, based on the correction term K2 dependent on only one recovery factor and added to the first corrected temperature difference ΔTcorr
In a further development of the invention, the value for the flow velocity is derived from a value for the mass flow as well as from values for the pressure and the temperature of the medium, wherein the pressure value is provided as a measured value or is predetermined.
In a further development of the invention, based on the corrected power coefficient, a heat transfer coefficient (htc) is ascertained, wherein based on the corrected ascertained heat transfer coefficient (htc), current values for the Nusselt number (Nu) and the Prandtl number (Pr) are determined, with whose help a current value for the Reynolds number (Re) is ascertained, wherein, based on the Reynolds number, values for the mass flow and the flow velocity are ascertained.
In a further development of the invention, a renewed correction for the recovery effect is obtained with the ascertained value for the flow velocity, wherein with the resulting power coefficient a renewed ascertaining of mass flow and flow velocity is performed, and wherein the method is repeated iteratively, until the values for mass flow and flow velocity sufficiently converge, in order to satisfy a given requirement and accuracy of measurement.
The flow meter of the invention for thermally determining the mass flow of a gas in a line, especially according to one of the above methods, includes at least a first temperature sensor, which can be flowed around by the medium; at least a second temperature sensor, which can be flowed around by the medium; wherein the first temperature sensor is spaced from the second temperature sensor, wherein the first temperature sensor has a heating element; wherein the flow meter further includes an operating and evaluating circuit, in order to feed the heating element with a power Q, to register the temperatures of the first temperature sensor and the second temperature sensor T1, T2, based on the power and a difference ΔT between the measured temperatures T1, T2 to ascertain a power coefficient, from the power coefficient to ascertain a heat transfer coefficient (htc), wherein based on the ascertained heat transfer coefficient (htc) current values for the Nusselt number (Nu), and the Prandtl number (Pr) are determined, with whose help a current value for the Reynolds number (Re) is ascertained, wherein based on the Reynolds number value the mass flow and the flow velocity are ascertained, characterized in that the operating and evaluating circuit is adapted to ascertain a power coefficient PCkorr, which is corrected as regards the recovery effect.
In a further development of the invention, the operating and evaluating circuit includes a microprocessor and at least one data memory, in which the at least one recovery factor or a difference between two recovery factors is stored.
In a further development of the invention, the first and second temperature sensors have different flow effective forms.
In a further development of the invention, the operating and evaluating circuit includes at least one data memory, in which relationships between the heat transfer coefficient (htc) and the Nusselt number (Nu), respectively between the Nusselt number (Nu), the Prandtl number (Pr) and the Reynolds number (Re), are stored tabularly and/or functionally.
The invention will now be described in greater detail based on the example of an embodiment illustrated in the drawing, the figures of which show as follows:
The thermal flowmeter 1 shown in
The flow meter further includes an electronics module 20 with a microcontroller 21, which contains a data memory 23, as well as an operating and measuring circuit 24 for heating the first temperature sensor 11 and for measuring the temperatures T1 and T2 of the first and second temperature sensors 11, 12, wherein the operating and measuring circuit 24 is controlled by the microcontroller.
Fundamentally, the thermal ascertaining of a mass flow occurs by supplying the first temperature sensor with a heating power Q, which is so controlled that the difference between the temperatures T1 and T2 of the two temperature sensors corresponds to a desired value. From the power coefficient resulting therefrom, then a heat transfer coefficient htc is ascertained, based on which then via ascertaining of the Nusselt number (Nu), and the relationship between the Nusselt number (Nu), the Prandtl number (Pr) and the Reynolds number (Re), the mass flow and the velocity are ascertained. This method is described in European Patent EP 0 624 242 B1 and is used for putting the present invention into practice. The required relationships between PC, htc, Nu, Pr and Re are stored tabularly and/or functionally in the data memory 23, respectively furnished in an additional data memory.
According to the invention, there occurs supplementally at least one correction for the recovery effect. The basis for this is knowledge of the recovery factors e1, e2 and the difference between the recovery factors Δ12=e1−e2.
The difference between the recovery factors Δ12=e1−e2 can be determined, for example, by exposing an unheated thermal flowmeter with a heating power of Q=0 to a defined mass flow of a gas under different velocities u. From the resulting temperature differences ΔT=T1−T2=Δ12·u2/(2·cp), Δ12 can be determined, without having to know the individual values of the recovery factors e1, e2.
The determining of a recovery factor of a temperature sensor can basically occur by flowing a defined mass flow of a gas of known temperature and different flow velocities around it or by difference measurement against another temperature sensor with known recovery factor. A method for determining a recovery factor of a temperature probe is disclosed by Stephan Ihle in his bachelor thesis at the Technical University Wien bearing the title “Experimentally Determining the Recovery Factor of a Temperature Probe”.
The ascertained values of Δ12Δ1 are stored in the data memory 23 and are available there for evaluation purposes.
In the case of the mentioned geometries, the recovery factors of both temperature sensors lie between 0.9 and 1, wherein the recovery factor of the first temperature sensor is usually the smaller of the two.
Furthermore, the data memory contains at least one value for the heat capacity cp of a gas whose mass flow is to be measured.
A further essential parameter is the pressure, with which the gas is flowing past the temperature sensors, for therefrom depends in the case of given mass flow the flow velocity.
In arrangements with constant pressure, the pressure can be predetermined and stored as a fixed parameter. In the case of arrangements with fluctuating gas pressure, in contrast, a pressure measurement value of an external pressure sensor is supplied via a signal input 25, to the extent that the flow meter has no integrated pressure meter.
In step 100, first of all, conventionally a power coefficient PCm is determined based on the measured values for the power Q and the temperature difference ΔT.
Then, there follows in step 110 a correction for asymmetry in the recovery factors.
In step 120, then a second correction is performed, in order to compensate for the recovery effect of the first temperature sensor per se.
In step 130, there occurs, finally, the ascertaining of the heat transfer coefficient htc from the power coefficient corrected for the recovery effect. This step and the following steps for ascertaining the mass flow and the flow velocity occur according to the state of the art.
In step 110, the flow velocity is explored as a parameter. In such case, usually, a comparatively current value, for example, the last available measured value, is used as starting value for the correction in an iterative method, in the case of which the correction for the recovery effect is performed anew with the flow velocity ascertained at the end of a trial, until the resulting values for mass flow and flow velocity become sufficiently stable, in order to satisfy a predetermined criterion for the accuracy of measurement, for example, (un−un-1)/un<3%, especially <1%, and preferably <0.5%.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2012 007 880.3 | Apr 2012 | DE | national |
| 10 2012 106 657.4 | Jul 2012 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2013/055627 | 3/19/2013 | WO | 00 |
