Patent Application
20250121145
This application claims the benefit of priority under 35 U.S.C. § 119 of German Application 10 2023 128 115.1, filed Oct. 13, 2023, the entire contents of which are incorporated herein by reference.
The invention relates to a sensor arrangement and a measuring process for measuring the respective concentration of three gas components of a gas sample.
The task of measuring the respective concentration of several gas components in a gas sample occurs, for example, during artificial ventilation (artificial respiration) of a patient. A ventilator delivers a breathable gas mixture to a patient-side coupling unit, for example to a breathing mask or a tube, wherein the patient-side coupling unit is connected to the patient. The breathable gas mixture contains oxygen and, in one arrangement at least one anesthetic agent (anesthetic gas).
A process and device for measuring the oxygen concentration and the anesthetic concentration is described in US 2023 0114548 A1. A magnetic field with an oscillating magnetic field strength is generated in a measuring chamber. The thermal conductivity and the magnetically modulated thermal conductivity of the breathable gas mixture are measured. Use is made of the fact that oxygen is a paramagnetic gas.
It is an object of the invention to provide a sensor arrangement and a measuring process which are capable of measuring the respective concentration of three gas constituents of a gas sample, namely with a higher reliability than known sensor arrangements and measuring processes.
The problem is solved by a sensor arrangement with features according to the invention and by a measuring process with features according to the invention. Advantageous embodiments of the sensor arrangement according to the invention are, where appropriate, also advantageous embodiments of the measuring process according to the invention and vice versa.
The sensor arrangement according to the invention and the measuring process according to the invention are capable of measuring the respective concentration (the respective proportion/share) of three different gas components of a gas sample. Preferably, “the concentration” is understood to mean the respective proportion in % by volume (vol.-%), the concentration can also be the proportion in % by weight, i.e. the quantity share. One gas component of the gas sample is a paramagnetic gas, in particular oxygen.
Remark: A paramagnetic gas has the following properties: The atoms or molecules of the gas comprises permanent magnetic properties, i.e. a magnetic moment, and automatically align in a magnetic field.
The respective density and the respective thermal conductivity of each one of the three gas components are predetermined in a computer-evaluable form. For example, the sensor arrangement has at least temporary read access to a data memory in which the densities and thermal conductivities are stored.
The sensor arrangement comprises a measuring chamber, a thermal conductivity sensor and a density sensor. The measuring chamber is configured to accommodate a gas sample. The thermal conductivity sensor is configured to measure the thermal conductivity of the gas sample in the measuring chamber. The density sensor is configured to measure the density of the gas sample. The measuring process according to the invention is carried out using such a sensor arrangement and comprises the steps of measuring the thermal conductivity and the density of the gas sample.
Note: The formulation that a sensor measures a physical quantity means the following: The sensor directly measures the physical quantity or at least one other physical quantity, wherein the or each other physical quantity correlates with the physical quantity to be measured. The or each other physical quantity is therefore an indicator of the physical quantity to be measured.
The measuring process according to the invention further comprises the following steps, and the sensor arrangement according to the invention is configured to perform the following steps:
The “magnetically modulated thermal conductivity” of the gas sample is understood to be the proportion (share) of the thermal conductivity that oscillates depending on the magnetic field strength. This magnetically modulated thermal conductivity depends essentially, but usually not exclusively, on the concentration of the paramagnetic gas in the gas sample. It is possible that the thermal conductivity measured as an indicator of the (total) thermal conductivity of the gas sample is the thermal conductivity of the gas sample in the measuring chamber. It is possible that the same thermal conductivity sensor measures both the thermal conductivity and the magnetically modulated thermal conductivity, i.e. provides two different signals that can be independent of each other. It is also possible that two different thermal conductivity sensors are used.
The three gas component concentrations are determined using the following information:
According to the invention, three different properties of the gas sample are measured, namely the thermal conductivity, the magnetically modulated thermal conductivity and the density. As a rule, these three properties of the gas sample depend on the sought concentrations of the three gas components in the gas sample, but do not depend on each other. In particular, these three properties generally change independently of each other. With the help of these three independent properties of the gas sample, which properties have been determined by measurements and are therefore known, three unknowns can be determined, namely the three concentrations of the three gas components sought.
The invention does not require the gas sample to consist only of these three gas components. Rather, the gas sample can comprise at least one further gas component. In order to determine its concentration, in many cases the fact that the sum of the concentrations of the gas components, measured in vol.-% or in wt.-%, is 100% can be utilized.
In some applications, in particular the concentration of the paramagnetic gas is required, wherein the paramagnetic gas is oxygen in particular. Some known sensor arrangements and measuring processes are also capable of measuring the oxygen concentration in a gas sample, but have a relatively high cross-sensitivity to other components of the gas sample. Such a cross-sensitivity may lead to wrong measurement results and is therefore undesirable. The sensor arrangement according to the invention and the measuring process according to the invention, on the other hand, are significantly less cross-sensitive, ideally not cross-sensitive at all, to other gas components in the gas sample.
Thanks to the invention, in many cases it is not necessary to provide a reference gas with a known composition and use it for the measurement, while the sensor arrangement and the measurement process measure the gas component concentrations.
Electrochemical sensors have become well known. In an electrochemical sensor, a chemical process takes place that depends on the concentration of oxygen and/or another gas component in the gas sample. A detection variable sensor of the electrochemical sensor measures an electrical detection variable that correlates with the oxygen concentration being sought. In some cases, such electrochemical sensors have the disadvantage that they require a chemical that is consumed over time and therefore needs to be replenished from time to time. In addition, a chemical can be altered by environmental influences, in particular by ambient heat, or can evaporate. The invention does not require a chemical and therefore avoids these disadvantages.
In one application, the paramagnetic gas component is oxygen. In this application, the two other gas components, whose concentrations are measured according to the invention, are an anesthetic agent (an anesthetic gas) or carbon dioxide and argon. The noble gas argon occurs in the ambient air at a known concentration. However, the concentration of argon in the gas sample is in many cases higher than the known concentration of argon in the ambient air. This situation occurs in particular in the following application: The gas sample comes from a breathable gas mixture that is used to artificially ventilate a patient. The gas mixture is produced using ambient air, but has a higher oxygen content than ambient air. A so-called concentrator is used to produce the gas mixture from ambient air. If a gas mixture with a higher oxygen content than ambient air is produced using a concentrator, the gas mixture produced usually also has a higher concentration of argon than the ambient air. This higher concentration is measured in the application of the invention just described.
Preferably, the sensor arrangement is capable of simultaneously measuring both the thermal conductivity and the magnetically modulated thermal conductivity of a gas sample in the same measuring chamber. In one embodiment, the sensor arrangement comprises a heating element and is capable of heating the heating element, in particular by applying an electrical voltage to the heating element and causing an electrical current to flow through an electrical heating wire of the heating element. When the heating element is heated, it supplies thermal energy to a gas sample in the measuring chamber. Preferably, this heating element does not react chemically with the gas sample in the measuring chamber.
According to the invention, a magnetic field with an oscillating magnetic field strength is applied to the measuring chamber. The thermal conductivity and the magnetically modulated thermal conductivity of the gas sample are measured in the measuring chamber. Preferably, an electrical detection variable is measured. This electrical detection variable correlates with the thermal conductivity of the gas sample in the measuring chamber. Electrical or electronic filtering of the electrical detection variable is carried out. This filtering provides two signals, namely an oscillating signal and another signal. The oscillating signal oscillates depending on the oscillating magnetic field strength. The time course of the other signal ideally does not depend on the oscillating magnetic field strength. The oscillating signal is used as the or as an indicator of the magnetically modulated thermal conductivity. The oscillating signal thus correlates with the concentration of the paramagnetic gas in the gas sample. The other signal is used as the or an indicator of the thermal conductivity of the gas sample. It correlates with the thermal conductivity of the gas sample.
In many cases, this design results in a particularly compact sensor arrangement. Two independent properties of the gas sample are measured in the same measuring chamber using the same magnetic field generator and the same heated heating element, namely the thermal conductivity and the magnetically modulated thermal conductivity.
Various configurations are possible as to how the density of the gas sample is measured. In one embodiment, a mechanical or electronic flexural resonator (flexural oscillator) is used. This flexural resonator comprises an oscillating body. In the case of a mechanical flexural resonator, this resonator is or comprises a movably mounted fluid guide unit. A “fluid guide unit” is a component that guides a fluid along a trajectory, wherein this trajectory is predetermined by the configuration and arrangement of the component. A corrugated hose, a smooth hose and a tube are examples of a fluid guide unit. The fluid guide unit does not necessarily include a conveying unit. In the case of an electronic flexural resonator, this oscillating body is preferably a quartz crystal.
The gas sample is passed through the fluid guide unit of the flexural resonator or past the quartz crystal. The flexural resonator is configured to cause the oscillating body (fluid guide unit or quartz crystal) to oscillate while a part of the gas sample is in the fluid guide unit or is guided past the oscillating body or surrounds the oscillating body. The natural frequency of the oscillating body is measured. This measured natural frequency correlates with the mass of the gas sample.
In the case of a mechanical flexural resonator, the natural frequency correlates with the mass of the filled fluid guide unit. As a rule, this mass is the sum of the mass of the empty fluid guide unit and the sought mass of the gas sample in the fluid guide unit. The mass of the empty fluid guide unit is known by the configuration of the flexural resonator and does not change. The volume of the fluid guide unit is also known from the configuration and remains constant during a measurement. The sought density of the gas sample can be derived from the measured natural frequency and the mass and volume of the empty fluid guide unit.
The principle of such a flexural resonator has been known for a long time, and reliable flexural resonators for density measurement are available.
In one embodiment, the concentration of at least a fourth gas component of the gas sample is measured directly. The respective concentration of the or each fourth gas component of the gas sample is used for the step of determining the respective concentration of the three gas components. In many cases, this embodiment increases the reliability with which the three concentrations of the three gas components are determined.
In one implementation of this embodiment, the concentration of the or at least a fourth gas component is measured by a photo-electric sensor using the following well-known principle:
Many gases exhibit a characteristic spectral course of absorption, i.e. a characteristic dependence of the degree of absorption on the wavelength of the electromagnetic radiation or sound that penetrates this gas. As a rule, the concentration of the fourth gas component can be measured independently of the sought concentrations of the three gas components.
According to the invention, three properties of the gas sample are measured in order to determine values for three unknowns, the three unknowns being the three sought concentrations of the three gas components in the gas sample. In one embodiment, three functional relationships, three difference indicators and a target function (objective function) are predetermined. Preferably, the sensor arrangement according to the invention has at least temporary read access to a data memory in which the target function, the difference indicators and the functional relationships are stored in a form that can be analyzed by a computer.
The first functional relationship describes the thermal conductivity of the gas sample as a function of the three gas component concentrations sought. The second functional relationship describes the magnetically modulated thermal conductivity of the gas sample as a function of the three gas component concentrations. The third functional relationship describes the density of the gas sample as a function of the three gas component concentrations. Therefore, all three functional relationships depend on the three unknown and sought gas component concentrations.
The first difference indicator describes an indicator for a difference between the measured thermal conductivity of the gas sample and the thermal conductivity, which is derived using the first functional relationship. The second difference indicator describes an indicator for a difference between the measured magnetically modulated thermal conductivity of the gas sample and the magnetically modulated thermal conductivity, which is derived with the aid of the second functional relationship. The third difference indicator describes an indicator of a difference between the measured density of the gas sample and the density derived using the third functional relationship. Preferably, each difference indicator is the square of the respective difference, alternatively the magnitude of the difference or another function which is the greater the greater the magnitude of the difference.
The target function is an aggregation of these three difference indicators. Preferably, the target function is a weighted average of the three difference indicators. For example, the weighting factors compensate for different orders of magnitude of the three properties thermal conductivity, magnetically modulated thermal conductivity and density. It is also possible that additionally or in lieu the weighting factors take into account different accuracy requirements for the gas component concentrations sought.
The three measured values for the gas sample, i.e. the measured thermal conductivity, the magnetically modulated thermal conductivity and the density of the gas sample, are inserted into the target function. After insertion, the target function depends on three unknowns, namely the three gas component concentrations sought. The target function is numerically minimized. This means that different value triplets for the three gas component concentrations are inserted into the target function on a trial basis. Each value triplet assigns a value to each of the three concentrations. The value triplet or a value triplet that leads to a minimum value of the target function provides the three gas component concentrations sought. Ideally, the minimizing value triplet provides the value zero for the target function. In practice, there is always a gap (difference) between the measured property and the property predicted according to the functional relationship. Preferably an iterative minimization procedure is applied.
It is possible that the target function has several minima.
It is possible that the concentration of a fourth gas component also occurs in at least one functional relationship. In one implementation, a sensor directly measures the concentration of this fourth gas component, for example using the photoelectric measuring principle described above, comprising a radiation source and a detector. In another implementation, a standard value is predetermined for the concentration of the fourth gas component. It is possible that there is a further, i.e. a fifth gas component whose concentration is measured or for which a standard value is predetermined.
The invention further relates to a ventilation arrangement (ventilator system) and a ventilation process for artificially ventilating a patient. The ventilation arrangement comprises
The ventilation process (procedure) is carried out using such a ventilation arrangement.
The patient is connected to the patient-side coupling unit or can at least temporarily be connected to a patient-side coupling unit. In particular, the patient-side coupling unit comprises a breathing mask and/or a tube and/or a catheter. The ventilation arrangement is capable of conducting and/or conveying a breathable gas mixture from the medical device to the patient-side coupling unit. The gas mixture contains oxygen and, in one embodiment, at least one anesthetic and/or a drug. It is possible that a ventilation circuit is established. The gas mixture exhaled by the patient flows in the ventilation circuit back to the medical device.
The concentration of at least one gas component of the breathable gas mixture, in particular the concentration of oxygen, should lie within a predetermined target range. It is possible that this target range is variable over time. It is also possible that a respective target range is predetermined for at least two gas components.
The ventilation arrangement is configured to perform the following steps, and the ventilation process comprises the following steps:
The ventilation control unit performs closed-loop control or open-loop control. The controlled variable is the actual concentration of at least one gas component in the gas mixture that is fed to the patient-side coupling unit. The control target (control gain) for the closed-loop control is that the control variable, i.e. the actual gas component concentration, is within the predetermined target range. For the closed-loop control, the ventilation control unit uses the gas component concentration in the branched-off gas sample, wherein the sensor arrangement has measured this concentration.
The invention is described below by means of embodiment examples. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.
In the drawings:
Referring to the drawings, in the embodiment examples, the sensor arrangement according to the invention is used for the artificial ventilation of a patient. The patient is supplied with a breathable gas mixture comprising oxygen and at least one anesthetic agent (anesthetic). The patient is therefore anaesthetized or at least sedated.
In the ventilation circuit 40, a breathable gas mixture Gg is supplied to the patient P. In the embodiment example, this gas mixture Gg comprises oxygen (O2), at least one anesthetic agent and optionally a carrier gas for an anesthetic agent, e.g. nitrous oxide (N2O) and optionally also a drug. An anesthetic dispenser 31 generates a fluid flow 28 with vaporous anesthetic and a carrier gas. The fluid flow 28 is fed into the ventilation circuit 40. In addition, a fluid flow 27 of fresh air or other fresh gas is fed in at least intermittently. At least as long as there is no excessive overpressure in the ventilation circuit 40, the gas mixture exhaled by the patient P remains in the ventilation circuit 40 so that no anesthetic agent enters the environment.
The gas mixture At, which is exhaled by the patient P and flows through the ventilation circuit 40, inevitably contains carbon dioxide (CO2). A carbon dioxide absorber (CO2 absorber) 25a is configured to absorb carbon dioxide from the ventilation circuit 40. In addition, the breathable gas mixture Gg may contain other components that also occur in the ambient air, albeit possibly at a different concentration than in ambient air.
The invention can also be used in an application in which no ventilation circuit is implemented. The gas mixture At exhaled by the patient P passes through an expiratory line 33 into the environment.
A patient-side coupling unit 21, shown only schematically, for example a mouthpiece or a breathing mask or a tubus, connects the patient P to the ventilation circuit 40. A tube close to the patient connects the patient-side coupling unit 21 to the base part of a Y-piece 22. One limb of the Y-piece 22 is connected to an inspiration line 32 for inhalation (inspiration), the other limb to an expiration line 33 for exhalation (expiration).
A fluid delivery unit 24a sucks in gas and generates a permanent flow of breathable gas mixture Gg through the inspiration line 32 to the Y-piece 22 and to the patient-side coupling unit 21 and thus to the patient P. The ventilation circuit 40 is kept going by the fluid delivery unit 24a and optionally by a breathing bag 26, which can be operated manually. The fluid delivery unit 24a can be implemented in particular as a blower, a pump or a piston-cylinder unit.
A non-return valve (check valve) 23a allows a gas flow through the inspiration line 32 towards the Y-piece 22 and blocks a gas flow in the opposite direction. A non-return valve 23b allows a gas flow to pass through the expiration line 33 away from the Y-piece 22 and blocks a gas flow in the opposite direction.
In the embodiment example, the fluid conveying unit 24a continuously conveys the gas mixture Gg. A controllable valve 24b, preferably a proportional valve, allows the flow of the gas mixture generated by the fluid conveying unit 24a to pass or blocks it completely or entirely, depending on its position, thereby contributing to the generation of the individual ventilation strokes and determining the amplitudes and frequencies of these ventilation strokes.
Additionally, a PEEP valve 24c (PEEP=“positive end-expiratory pressure”) ensures that a sufficiently high air pressure is maintained in the lungs of patient P even at the end of exhalation or if the ventilation circuit 40 is briefly opened or interrupted. This reduces the risk of patient P's lungs collapsing due to insufficient pressure.
A pressure relief valve 29 is configured to reduce excess pressure in the ventilation circuit 40 by causing gas to escape from the ventilation circuit 40 if the pressure is too high. Preferably, the released gas is discharged into a transport line for anesthetic gas and reaches a conditioner not shown.
A signal-processing ventilation control unit 35, shown only schematically, receives a signal from an optional pressure sensor 58, which measures the air pressure Pamb in the environment of the anesthesia machine 1 and thus of the ventilation circuit 40, and a signal from an optional temperature sensor 39, which measures the ambient temperature Tempamb. In addition, the ventilation control unit 35 receives measured values from a pressure sensor 36, which measures the current pressure in the ventilation circuit 40, for example the ventilation pressure applied to the patient P (pressure in airway, Paw), in one embodiment as a differential pressure relative to the ambient pressure Pamb. The measuring position of the pressure sensor 36 can be in the inspiration line 32 or in or, as shown, on or near the Y-piece 22. Preferably, the measuring position of the pressure sensor 36 is located on the tube close to the patient. The ventilation control unit 35 controls the fluid delivery unit 24a, the valve 24b, the anesthetic dispenser 31 and other components of the ventilation circuit 40 in order to realize a desired artificial ventilation and anesthesia of the patient P.
It is often desired that the gas mixture Gg, which is supplied to the patient P, fulfills a certain given property. In particular, the concentration of a component of the gas mixture Gg should be within a predetermined target range. In particular, the proportion of pure oxygen (O2) should often be within a predetermined target range, for example between 40 vol.-% and 50 vol.-% or between 25 vol.-% and 30 vol.-%. Or the proportion of anesthetic should be within a predetermined range. In the following, the term “concentration” of a gas component in a gas mixture is used. Synonymous terms are “proportion” or “share”.
For this purpose, a sample of gas (hereinafter: a gas sample Gp) is branched off (diverted) from the gas mixture Gg, At in the ventilation circuit 40 via a gas sample fluid guide unit in the form of a sampling tube 52, analyzed and fed back into the ventilation circuit 40 via a discharge tube 56. The sampling tube 52 starts at a branching point 34 between the patient-side coupling unit 21 and the Y-piece 22. Because the branching point 34 is positioned at this point, the gas sample Gp contains a sample from the breathable gas mixture Gg or from the exhaled gas mixture At, depending on the time of branching-off. At times a gas mixture Gg from the inspiration line 22 and at times a gas mixture At for the expiration line 33 enter the sampling tube 52. At the branch point 34 there is an optional valve, not shown, which in a closed position separates the sampling tube 52 from the ventilation circuit 40 and which can be controlled by the ventilation control unit 35. When the valve is fully open or left open or omitted, the sampling tube 52 is in an unrestricted fluid connection with the ventilation circuit 40. The discharge tube 56 leads to a confluence point 37 upstream of the carbon dioxide absorber 25a.
The sampling tube 52 directs the gas sample Gp to a sensor arrangement 50. This sensor arrangement 50 is spatially remote from the patient-side coupling unit 21 and from the lines 32 and 33 and preferably belongs to the schematically shown ventilator 1. In
In the embodiment example, the sensor arrangement 50 comprises a pump 55, which sucks the gas sample Gp out of the ventilation circuit 40 and sucks it in through the sampling tube 52. Preferably, the pump 55 generates a permanent negative pressure on the side pointing towards the sampling tube 52 and a permanent positive pressure on the side pointing towards the discharge tube 56. The pump 55 is capable of generating a volume flow which is preferably constant over time and amounts to 200 ml/min, for example. A water trap 51 is arranged at the inlet of the sensor arrangement 50, wherein moisture is condensed and collected in the water trap 51. This reduces the water content in the gas sample Gp.
A sensor 54 is configured to generate signals, wherein the signals correlate with the measured respective concentration of CO2 and of N2O in the extracted gas sample Gp. Preferably, this sensor 54 comprises an infrared measuring head which utilizes the dipole moment of molecules in the gas sample Gp and quantitatively evaluates the absorption of infrared-active gases in order to determine the respective concentration. Preferably, the sensor 54 comprises a radiation source that emits electromagnetic radiation and a detector that measures the intensity of impinging electromagnetic radiation and generates a corresponding signal. A sensor 53 is configured to generate a signal that correlates, among other things, with the concentration of O2 or another paramagnetic component of the gas sample Gp. The sensor 53 is described in more detail below with reference to
In addition, the sensor arrangement 50 comprises a pressure sensor 57, which measures the pressure Pcell of the gas sample Gp at the inlet of the sensor arrangement 50. This pressure Pcell is variable over time because the pressure in the ventilation circuit 40 oscillates and because the sampling tube 52 is in a fluid connection with the ventilation circuit 40 if there is no valve at the branch point 34 or as long as the optional valve at the branch point 34 is open. If the valve is missing or open, the pressure in the ventilation circuit 40 propagates at approximately the sound velocity to the sensor arrangement 50.
A density sensor 59, 60 measures the density of the gas sample Gp flowing through the sensor arrangement 50. This density sensor 59, 60 is described below with reference to
The sensor arrangement 50, the water trap 51, the signal processing unit 30, and the tubes 52 and 56 belong together to a measurement system 100, which sucks in the gas sample Gp from the ventilation circuit 40, examines (analyzes) it and feeds it back into the ventilation circuit 40. The measurement system 100 is part of the ventilation arrangement 200.
Two pole pieces (pole shoes) 6, 7 and two field coils 4, 5 generate a magnetic field. An air gap 3 occurs between the two pole pieces 6, 7, which acts as a measuring chamber 2. This measuring chamber 2 is bounded by the two pole pieces 6, 7 and a wall 9. An inlet 10 and an outlet 11 are recessed into the wall 9.
A thermocouple 8 is attached to two support wires 15, 16 at two connection points 12 and 14, wherein the two support wires 15, 16 are passed through the lower pole piece 7 and are in thermal contact with the lower pole piece 7. The thermocouple 8 comprises two wires 17, 18, which are connected to each other at a connection point 13. A voltage source 20 applies an alternating voltage to the two support wires 15, 16 and thus to the thermocouple 8. This heats the thermocouple 8 to a working temperature that is higher than the temperature of the gas sample Gp in the measuring chamber 2. An electrical voltage U, which is applied to a measuring resistor 19, is measured. This electrical voltage U contains an AC voltage component and a DC voltage component.
The current working temperature of the thermocouple 8 is measured at the connection point 13. This temperature depends on the one hand on the voltage U that occurs between the support wires 15, 16 and on the other hand on the thermal conductivity 2 of the gas sample Gp in the measuring chamber 2. Closed-loop control is performed with the control objective of keeping the working temperature of the thermocouple 8 to a constant value. The control variable is the time-varying voltage U, which the voltage source 20 applies to the support wires 15, 16. Because the temperature of the thermocouple 8 ideally remains constant, an electrical detection variable of the thermocouple 8 correlates with the thermal conductivity 2 of the gas sample Gp in the measuring chamber 2.
One way of measuring the detection variable is as follows: The time-varying electrical power supplied to the thermocouple 8 is measured as the detection variable. The two support wires 15 and 16 are electrically connected to each other by the electrical measuring resistor 19. The electrical voltage U that drops across this measuring resistor 19 is measured. The strength of the electric current flowing through the thermocouple 8 is also measured. The electrical power is known to depend on the voltage and the current. It is also possible to use the electrical voltage U as a detection variable.
A voltage source 43 is connected to one field coil 5 via a power amplifier 42, and the other field coil 4 is connected to electrical ground. The voltage source 43 applies a sinusoidal electrical voltage. This voltage generates a sinusoidally varying magnetic field in the measuring chamber 2. The thermocouple 8, which is heated to a constant temperature, emits a quantity of heat per time unit to the gas sample Gp in the measuring chamber 2. This amount of heat emitted per time unit correlates with the measured electrical power supplied to the thermocouple 8. Because the field strength of the magnetic field in the measuring chamber 2 oscillates periodically, the amount of heat emitted per time unit also oscillates-provided that a paramagnetic gas is present in the measuring chamber 2 as part of the gas sample Gp. The detection variable correlates with the thermal conductivity 2 of the gas sample Gp. Using the detection variable, a signal that oscillates depending on the oscillation of the magnetic field, for example an AC voltage, and a non-oscillating or less oscillating signal, for example a DC voltage, are generated. The oscillating signal correlates with the thermal conductivity of the paramagnetic part of the gas sample Gp and thus with the O2 concentration. This thermal conductivity is also referred to as the magnetically modulated thermal conductivity 22f. The other signal correlates with the (total) thermal conductivity 2 of the entire gas sample Gp.
An electromagnet 62 with an electric coil 63 generates a time-varying, preferably oscillating, magnetic field in the air gap 3, see
The heating unit 66 heats the measuring element 65 to a working temperature that is higher than the temperature of a gas sample Gp in the measuring chamber 2. The heated measuring element 65 measures the temperature at the measuring position 77. In the implementation shown, the measuring element 65 measures a thermoelectric voltage and utilizes the Seebeck effect. The thermal conductivity of the gas sample Gp at the measuring position 77 changes synchronously with the time-varying magnetic field generated by the electromagnet 62—provided that the gas sample Gp at the measuring position 77 contains a sufficiently high concentration of a paramagnetic gas. A higher thermal conductivity means that the thermal energy is better dissipated. This in turn results in a lower temperature, which in turn results in a lower thermoelectric voltage.
The heating unit 66 is connected to the DC voltage source 72 via the amplifier 70 and the voltage divider 71. The output signal of the heat conduction measuring element 65 is routed via the low-pass filter 73 and the high-pass filter 74. A signal 75 is present at the output of the high-pass filter 74, which signal oscillates depending on the oscillation of the magnetic field. The signal 75 correlates with the magnetically modulated thermal conductivity λ2f and thus with the concentration of the paramagnetic gas, for example with the concentration of oxygen, in the gas sample Gp. A signal 76 is present at the output of the low-pass filter 73, which signal does not oscillate at all or at least not synchronously with the magnetic field oscillation. The signal 76 correlates with the thermal conductivity 2 of the entire gas sample Gp.
Because the magnetic field strength oscillates, the amount of heat supplied per time unit comprises a superposition of a proportion that remains constant over time and a proportion that fluctuates periodically. The periodically fluctuating component correlates with the thermal conductivity and thus with the concentration of oxygen in the measuring chamber 2. The constant component, i.e. the component that does not oscillate depending on the strength of the magnetic field, correlates with the thermal conductivity of the entire gas sample Gp in the measuring chamber 2. The thermal conductivity of the oxygen and the respective thermal conductivity of each other component of the gas sample Gp contribute to this thermal conductivity 2 of the gas sample Gp. Both components are measured. The sensor 53 therefore provides two signals:
Optionally, a measured value for the concentration of a component and/or for the thermal conductivity 2 is displayed on a display device 44, see
According to the invention, the density of the gas sample Gp is also measured.
The principle of density measurement described below has been known for a long time, but not for the application described here. In the embodiment example, the density sensor 59, 60 measures the density p of the gas sample Gp flowing through the sensor arrangement 50.
First, the mechanical flexural resonator 59 is described with reference to
In one embodiment, the tube 80 can be temporarily closed for the purpose of a measurement. In another embodiment, the gas sample Gp also flows through the tube 80 during the measurement.
A schematically shown actuator excites this flexural resonator to undamped oscillations. In the implementation shown, the actuator is implemented by two transducers 82.1, 82.2. Each transducer 82.1, 82.2 comprises a coil 83.1, 83.2 and a magnet 84.1, 84.2. In one mode, an alternating electrical voltage is applied to each transducer 82.1, 82.2, the transducer 82.1, 82.2 generates mechanical oscillations, and these mechanical oscillations are transmitted to the tube 80, preferably by a mechanical connection between the magnet 84.1, 84.2 and the tube 80.
The two legs 80.1, 80.2 span a plane. The two directions of the oscillations generated by the excitation are orthogonal to this plane. An area B of the tube 80 is set into oscillation. The actuator 84 is then switched off. Area B then continues to oscillate, namely at its natural frequency after a transient phase.
The area B of the tube 80 and the part of the gas sample Gp that is located inside this area B take part in the oscillation. This oscillating system has a mass, wherein this mass is the sum of the mass of area B and the mass of the gas sample part in area B. The mass of area B is predetermined by the configuration and remains constant. The volume that area B provides for the gas sample Gp also remains constant. As a rule, the mass of that part of the gas sample Gp that is located in area B can be regarded as constant during a measurement. The density p sought is then proportional to the mass of the gas sample part.
The two converters 82.1 and 82.2 are then switched off. As a result, the system just described oscillates at a natural frequency t. This natural frequency t depends on the mass of the system and is measured. In the implementation shown, the two converters 82.1, 82.2 function in a different mode after switching off and convert the mechanical oscillations of area B into an alternating voltage. The frequency of this alternating voltage is equal to the natural frequency t of area B. An amplifier 85 amplifies this alternating voltage. An evaluation unit 86 receives a signal from the amplifier 85 and supplies the required density p.
The sought density p is related to the measured natural frequency t as follows:
A and B are two constants of the density sensor (flexural resonator) 59. In a previous calibration, these two constants A and B are determined empirically, for example by means of a regression analysis. For this purpose, at least two different gases with known and differing densities are sent through the density sensor 59 and the respective resulting natural frequency is measured.
One such flexural resonator 60 is described, for example, in Th. Voglhuber-Brunnmeier et al: Fluid Sensing Using Quartz Tuning Forks-Measurement Technology and Applications, Sensors, Vol. 19 (2019) No. 10 (which is hereby incorporated by reference).
In one embodiment, the following context is used:
Again, A, B and C are device-specific constants that are determined in advance by calibration. Because three constants have to be determined, at least three different gases with known and differing densities are used for calibration.
As already explained, the sensor arrangement 50 is intended to measure the respective concentration (proportion) in vol.-% of oxygen and anesthetic agent (anesthetic gas) in the tapped gas sample Gp. Other components may be present in this gas sample Gp, which can falsify this measurement without suitable countermeasures. In particular, the noble gas argon (Ar) in the gas sample Gp can falsify the measurement. It is well-known that the proportion of argon in the ambient air is approximately 0.93% by volume. However, the concentration of argon in the gas sample Gp can be significantly higher than that of argon in the ambient air. This situation occurs in particular if the concentration of oxygen in the ventilation circuit 40 and therefore in the gas sample Gp is greater than the concentration of oxygen in the ambient air, and in particular if the gas mixture Gg is generated from ambient air and the oxygen concentration has been increased with the aid of a so-called oxygen concentrator.
In some situations, information about the measured argon concentration is also used by another sensor or other component of the anesthesia device 1. For example, the measured argon concentration is compared with an upper limit. As soon as this upper limit for the argon concentration is reached, the ventilation circuit 40 is flushed.
In the embodiment example, the gas sample Gp contains n different components, including nitrogen (N2), pure oxygen (O2), carbon dioxide (CO2), an anesthetic gas (A gas), nitrous oxide (N2O), water (H2O) and argon (Ar). The respective density ρ(xi) (i=1, . . . , n) and the respective thermal conductivity λ(xi) of these n components are known and are predetermined. Preferably, the sensor arrangement 50 has at least temporary read access to a data memory in which the densities and thermal conductivities are stored. The following table shows examples of the respective density and the respective thermal conductivity of components that occur or can occur in the ventilation circuit 40 and thus in the gas sample Gp, wherein the densities and the thermal conductivities relate to a reference ambient temperature of 15 degrees C. and a reference ambient pressure of 1 bar.
The gas components have different densities. It can be seen that the anesthetic isoflurane mentioned as an example has a density several times greater than the other gas components.
As explained above, an optional pressure sensor 36 measures the pressure in the ventilation circuit 40 and preferably provides the airway pressure PAW. The optional pressure sensor 58 measures the ambient pressure Pamb, the optional temperature sensor 39 measures the ambient temperature Tempamb, and an optional pressure sensor 57 measures the pressure Pcell of the gas sample Gp at the inlet of the sensor arrangement 50. The values listed in Table 1 refer to a specific reference ambient temperature and a specific reference ambient pressure. Optionally, the signal processing unit 30 uses signals from the sensors 36, 58 and/or 39 to derive the actual thermal conductivity and the actual density of the possible gas components at the actual ambient temperature Tempamb and the actual ambient pressure Pamb.
The respective concentration ξ(xi), i.e. the proportion in % by volume, of each component xi (i=1, . . . , n) is sought.
In many cases, with sufficient accuracy it is valid that the density p of the gas sample Gp is a weighted average of the respective densities ρ(xi) of the n components, i.e. the following applies:
The sought concentrations ξ(x1), . . . , ξ(xn) of the gas components x1, . . . , xn act as the weight factors.
With reference to
The sought concentrations ξ(x1), . . . , ξ(xn) of the gas components x1, . . . , xn act as the weight factors.
It is also possible to use the following functional dependency:
The factors G(i,k) (i=1, . . . , n; k=1, . . . , n) can be determined empirically in advance.
The sensor 53 is also able to measure the magnetically modulated thermal conductivity λ2f of the gas sample Gp, for example by using the periodic component of the detection variable of the embodiment according to
The functional dependency f is predetermined. Tempamb denotes the ambient temperature, which is measured by the temperature sensor 39, for example. As a rule, the temperature of the gas sample Gp does not differ significantly from the ambient temperature Tempamb. The O2 concentration is approximately linear. The pressure Pcell is measured by sensor 57, for example.
The sensor 54 measures the CO2 concentration ξ(CO2) and the N2O concentration ξ(N2O). The pressure sensor 57 measures the pressure Pcell. A capacitive sensor (not shown) measures the H2O concentration ξ(H2O), i.e. the concentration of water vapor in the gas sample Gp. The density sensor 59, 60, which was described above with reference to
The CO2 concentration ξ(CO2), the N2O concentration ξ(N2O) and the water vapor concentration ξ(H2O) are measured directly. This leaves three unknown components, namely the O2 concentration ξ(O2), the anesthetic concentration ξ(A gas) and the argon concentration ξ(Ar). To determine these three unknowns, the three equations (3), (4) and (6) are used. Taken together, a system of equations with three unknowns, namely ξ(O2), ξ(A gas) and ξ(Ar), and three equations, namely (3), (4) and (6), is used.
In one implementation, three values ξ(x1), ξ(x2) and ξ(x3) are calculated for the three unknowns ξ(O2), ξ(A-gas) and ξ(Ar) in such a way that a target function Z with
is minimized. Here, ρmeas denotes the measured density, λmeas the measured thermal conductivity and λ,2fmeas the measured magnetically modulated thermal conductivity. ρ[ξ(x1), ξ(x2), ξ(x3)] denotes the density that occurs according to equation (3), λ[ξ(x1), ξ(x2), ξ(x3)] is the thermal conductivity according to equation (4) or (5) and λ2f [ξ(x1), ξ(x2), ξ(x3)] is the magnetically modulated thermal conductivity according to equation (6). The three factors α1, α2, α3 are selected so that all three summands of the target function Z have the same order of magnitude and/or different accuracy requirements for the measurement of the three concentrations ξ(x1), ξ(x2), ξ(x3) are met. The sum of the three factors α1, α2, α3 is 1.
Obviously, the sum of the gas component parts is 1, so the following applies
Equation (8) can be used to derive the N2 concentration ξ(N2).
While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2023 128 115.1 | Oct 2023 | DE | national |
