RU2172953C2 - Device measuring concentration of carbon dioxide in exhaled air - Google Patents

Abstract

FIELD: medicine, diagnostics of some diseases. SUBSTANCE: device includes measurement chamber connected to respiratory tracts of patient which carries ultrasonic radiator and reflector of audio signal, pulse generator which first output is connected to radiator linked to first input of amplifier and channel processing output signal incorporating computation circuit. Device is fitted with thermostatting unit which input and first output are connected correspondingly to temperature-sensitive element and heater located in measurement chamber. Channel processing output signal has first amplitude detector which input is connected to first output of generator, second amplitude detector which input is connected to output of amplifier, single-shot multivibrator which input is connected to second output of generator and which output is connected to second input of amplifier. Device also has logarithmic converter which first and second signal inputs are connected correspondingly to outputs of first and second amplitude detectors, threshold unit which signal input is connected to output of logarithmic converter. First input of computation circuit is connected to output of threshold unit, its second input is connected to output of logarithmic converter, its third input is connected to second output of thermostatting unit, its fourth and fifth inputs are used correspondingly to set calibration points and to control operational conditions, first output of computation circuit is connected to control inputs of logarithmic converter and threshold unit and its second output is connected to input of unit of indicators. EFFECT: increased accuracy of determination of concentration of carbon dioxide in air exhaled by patient. 6 dwg

Description

The invention relates to medicine, in particular to devices for determining the content of CO ₂ in exhaled air, and can be used to diagnose a number of diseases, such as asthma, hyperventilation syndrome, etc.

 Known ultrasonic gas analyzer [1], based on the use of the dependence of the speed of propagation of ultrasound in a gaseous medium on the concentration of the desired component of this medium. It is intended for use including in medicine, it contains a pulse generator, a measuring chamber, in which an emitter and a receiver of ultrasonic (ultrasound) waves, an amplifier, an output signal generating channel, and a temperature unit are located opposite each other. The channel for generating the output signal includes a delay circuit, a comparator, and a pulse width to voltage converter. The test gas mixture is introduced into the measuring chamber.

 This gas analyzer works as follows. The pulse generator generates a signal supplied to the ultrasound transmitter, as a result of which the ultrasonic wave propagates in the measuring chamber. The ultrasonic receiver receives the wave transmitted through the camera and converts the ultrasonic signal into a voltage pulse. Further, this pulse is amplified and, passing through the delay circuit, enters one of the inputs of the comparator, the other input of which receives a signal from the output of the pulse generator, while the output signal supplied to the comparator from the ultrasonic wave receiver is phase shifted relative to the signal, generated by a pulse generator and an exciting ultrasonic transmitter (emitter). The phase difference depends on the speed of propagation of ultrasound in the measuring chamber.

 The difference signal generated by the comparator is converted to a constant voltage, which is a measure of the concentration of the test gas in the gas mixture.

 In order to reduce the influence of temperature on the speed of propagation of ultrasonic waves in the measuring chamber, temperature compensation is used in the analogue.

где γ - отношение удельной теплоемкости газовой смеси при постоянном давлении к удельной теплоемкости при постоянном объеме;
R - универсальная газовая постоянная;
Т - температура в градусах Кельвина;
μ - молярная масса газовой смеси.This gas analyzer does not provide high accuracy in measuring the concentration of CO ₂ in exhaled air. As is known [2], the speed of propagation of ultrasonic waves in a gas mixture substantially depends on humidity and temperature. The formula for calculating the speed of sound c is:

where γ is the ratio of the specific heat of the gas mixture at constant pressure to the specific heat at a constant volume;
R is the universal gas constant;
T is the temperature in degrees Kelvin;
μ is the molar mass of the gas mixture.

Using (1), it can be shown that a change in ambient temperature of 1 ^o C leads to a change in the propagation velocity of ultrasound, which corresponds to a change in the concentration of CO ₂ by 0.8%. The effect of humidity is also significant - a change in humidity in the room by 10% at a temperature of 20 ^o C will lead to a change in the speed of propagation of ultrasound, equivalent to a change in CO ₂ concentration of 0.26%. When analyzing exhaled air, temperature and humidity changes in the measuring chamber will occur abruptly at the moment of the beginning of inspiration and the beginning of exhalation when replacing the exhaled gas mixture with atmospheric air. For example, at the beginning of inspiration, the temperature of the analyzed gas mixture in the measuring chamber changes from the temperature of the human body (about 37 ^o C) to the air temperature in the room (about 20 ^o C). Humidity also varies significantly - from indoor air humidity to 100% relative humidity in the exhaled gas mixture. Thus, the use of this device for measuring the concentration of 002 in exhaled air is impossible, since in modern medical practice high demands are placed on the accuracy of such measurements, the absolute error of which should not exceed 0.2%.

As a prototype, a device was selected for determining the concentration of exhaled air components, based on the measurement of ultrasound absorption by the analyzed gas mixture [3]. This device allows to achieve greater accuracy in determining the concentration of CO ₂ compared with previously considered, because to a much lesser extent affected by temperature and humidity.

One of the indicators of the propagation of ultrasonic waves in a gas medium is the absorption coefficient α, defined as the reciprocal of the distance at which the amplitude of the ultrasonic wave decreases e times. The value of α depends primarily on the frequency of ultrasound and the composition of the gas mixture. Thus, by determining the absorption coefficient of ultrasound at a certain frequency in the gas mixture, one can estimate the concentration of CO ₂ .

где ω - круговая частота ультразвуковой волны;
ρ - плотность газовой смеси;
η - коэффициент сдвиговой вязкости;
ξ - коэффициент объемной вязкости;
χ - коэффициент теплопроводности;
с - скорость ультразвука;
C_v и C_p - теплопроводность среды при постоянном давлении и объеме соответственно.The absorption of ultrasonic waves in a gas mixture is due to thermal conductivity, shear and bulk viscosity of the gas medium [4]. The absorption coefficient determined by these values can be calculated by the formula:

where ω is the circular frequency of the ultrasonic wave;
ρ is the density of the gas mixture;
η is the shear viscosity coefficient;
ξ is the coefficient of bulk viscosity;
χ is the coefficient of thermal conductivity;
C is the speed of ultrasound;
C _v and C _p - thermal conductivity of the medium at constant pressure and volume, respectively.

где τ - время релаксации,
c₀ - скорость распространения ультразвука при малых частотах (τω ≪ 1),
c_∞ - скорость ультразвука при высоких частотах (τω ≫ 1).
Поэтому полный коэффициент поглощения можно определить как сумму так называемого классического (который вычисляется по (2) без учета объемной вязкости) и релаксационного коэффициента поглощения (3):
α = α_к+α_p (4)
Поглощение, связанное со вторым слагаемым, проявляется на частотах, лежащих около релаксационной частоты f_p. Каждый газ характеризуется своей f_p, определяемой структурой и весом молекул. Релаксационная частота CO₂ превышает релаксационные частоты остальных компонентов выдыхаемого воздуха (азота, кислорода и паров воды). Однако на этой частоте для выдыхаемой газовой смеси становится заметным поглощение звуковых волн кислородом, так как в результате взаимодействия с парами воды релаксационная частота кислорода сдвигается в область высоких частот. Поэтому для повышения точностных характеристик прибора становится необходимым введение средств компенсации влияния кислорода и паров воды. Влиянием азота можно пренебречь, так как его коэффициент поглощения не превышает долей см^-1 (коэффициент поглощения CO₂ составляет около 100 см^-1, кислорода ~1 см^-1).In the low-frequency range, the quantities η, ξ, and χ are practically independent of ω, and the absorption coefficient is proportional to the square of the frequency. At high frequencies, the bulk viscosity coefficient ξ begins to depend on the frequency, which is associated with relaxation processes of excitation of vibrational or rotational degrees of freedom of gas molecules. The expression for the relaxation-related part of the absorption coefficient has the form:

where τ is the relaxation time,
c ₀ is the propagation velocity of ultrasound at low frequencies (τω ≪ 1),
c _∞ is the ultrasound velocity at high frequencies (τω ≫ 1).
Therefore, the total absorption coefficient can be defined as the sum of the so-called classical (which is calculated according to (2) without taking into account bulk viscosity) and the relaxation absorption coefficient (3):
α = α _to + α _p (4)
The absorption associated with the second term appears at frequencies lying near the relaxation frequency f _p . Each gas is characterized by its f _p , determined by the structure and weight of the molecules. The relaxation frequency of CO ₂ exceeds the relaxation frequencies of the remaining components of the exhaled air (nitrogen, oxygen and water vapor). However, at this frequency, for the exhaled gas mixture, the absorption of sound waves by oxygen becomes noticeable, since as a result of interaction with water vapor, the relaxation frequency of oxygen is shifted to the high frequency region. Therefore, to increase the accuracy characteristics of the device, it becomes necessary to introduce means to compensate for the effects of oxygen and water vapor. The effect of nitrogen can be neglected, since its absorption coefficient does not exceed fractions of cm ^-1 (the absorption coefficient of CO ₂ is about 100 cm ^-1 , oxygen ~ 1 cm ^-1 ).

The error in measuring the concentration of carbon dioxide in this way will be determined by the error in measuring the absorption coefficient, which in the case of analysis of exhaled air mainly depends as well as the speed of sound on humidity and temperature. The maximum change in these values will occur during gas exchange in the chamber at the moment of the beginning of inspiration or the beginning of exhalation, and the “classical” coefficient of air absorption will change, since the process of relaxation absorption by CO ₂ molecules occurs at steady-state temperature and humidity during exhalation.

The change in the absorption coefficient depending on the temperature is mainly determined by the change in the speed of ultrasound, because these values are closely related. By evaluating the change in speed according to (1), using (2) it can be shown that the "classical" coefficient of air absorption at temperature fluctuations from 20 to 37 ^o C changes by about 10%. But, since the relaxation absorption coefficient of ultrasound by the exhaled mixture with carbon dioxide exceeds it by two orders of magnitude, as a result, the absolute error in determining the concentration of CO ₂ by temperature will not exceed 0.01%. A quantitative calculation of the error in humidity is difficult, since the analytical dependence of the absorption coefficient on humidity is not known. However, based on the experimental data given in the literature [5], it can be concluded that the error in humidity will be of the same order as in temperature. Thus, the accuracy of the method for determining the concentration of CO ₂ based on measuring the absorption coefficient of ultrasound is much higher.

 In this device, to account for the influence of components that distort the measurement result (interfering components), a two-frequency analysis circuit is used. The device comprises a measuring chamber connected to the respiratory tract of the patient, in which there is an ultrasound transducer and a sound signal reflector. On the input side, the ultrasonic emitter is connected through the pulse generator and the first amplifier with two master generators, on the output side through the second amplifier, with a computing circuit, with a control device associated with the pulse generator, and with a delay circuit connected to the control device and the computing circuit.

The known device operates as follows. The master oscillators generate signals with frequencies f ₁ and f ₂ , which are supplied to the inputs of a pulse generator, at the output of which groups of sinusoidal oscillations (sound pulses) are formed, and the oscillation frequency in each group is different.

In the first amplifier, amplification of sound pulses occurs, which are then transmitted by the emitter to the measuring chamber. The sound signal transmitted through the camera and reflected from its rear wall is received by the emitter (which is a transceiver) and is amplified by a second amplifier. Due to the delay, a signal with a frequency f _{1 is} delayed until a signal with a frequency f ₂ arrives at the input of a computational circuit in which signals with frequencies f ₁ and f ₂ are processed and a signal is formed, which represents the concentration of the initial gas component.

 The output signal of the second amplifier is also fed to a control device that controls the operation of the pulse generator and causes the formation of new pulses arriving at the emitter.

 The known device is characterized by the following disadvantages.

 1. The use of a 2-frequency method in the presence of one transceiver of the ultrasonic signal necessitates the use of a broadband emitter of an audio signal. Such emitters have a low quality factor and a low signal to noise ratio, which reduces the sensitivity of the conversion and, as a result, the accuracy of determining the concentration of the studied gas component in the air exhaled by the patient.

 2. The absence in the known device of means to maintain a constant temperature of the measuring chamber leads to the fact that condensation of water vapor on the surface of the ultrasonic transducer can be observed in the measuring chamber, since the temperature of the exhaled air is higher than the temperature of the walls of the measuring chamber. This leads to the absorption and scattering of ultrasonic waves received by the transducer and to a decrease in the level of the output signal, which ultimately also leads to a decrease in the accuracy of determining the concentration of the studied gas component in exhaled air.

 3. The lack of calibration of the device calibration gas mixtures leads to the following. The exact nature of the concentration dependence on the absorption coefficient is not known, due to the complexity of the exhaled mixture. It can be considered linear only within a certain error. In addition, an analog and digital signal that carries information about the absorption coefficient must be converted during the operation of the device, and the conversion characteristics are determined by real devices and can quite significantly differ from ideal ones. All this also leads to a decrease in accuracy.

 The problem solved by the invention is to increase the accuracy of determining the concentration of gas in the air exhaled by the patient.

 This problem is solved in that the device for measuring the concentration of carbon dioxide in the expired air, comprising a measuring chamber connected to the respiratory tract of the patient and containing an ultrasonic emitter and a sound reflector, a pulse generator, the first output of which is connected to the emitter connected to the first input of the amplifier, and the channel for processing the output signal containing the computing circuit is provided with a thermostatic control unit, the input and the first output of which are connected, respectively, to the sensors temperature and the heater located in the measuring chamber, the output signal processing channel contains a first amplitude detector, the input of which is connected to the first output of the generator, a second amplitude detector, the input of which is connected to the output of the amplifier, waiting for a multivibrator, the input of which is connected to the second output of the generator, and the output is connected to the second input of the amplifier, a logarithmic converter, the first and second signal inputs of which are connected, respectively, with the outputs of the first and second amplitude detectors c, a threshold device, the signal input of which is connected to the output of the logarithmic converter, while the first input of the computational circuit is connected to the output of the threshold device, the second input is connected to the output of the logarithmic converter, the third input is connected to the second output of the temperature control unit, the fourth and fifth inputs serve, respectively , to set calibration points and control the operating mode, the first output is connected to the control inputs of the logarithmic converter and the threshold device, and the second the output is connected to the input of the indicator block.

 The essence of the invention lies in such an organization of a device that allows you to select the periods of inspiration and expiration in the structure of the respiratory cycle, measure the total absorption coefficient of the components of the gas mixture in each period and, thus, eliminate the influence of interfering components (oxygen and water vapor), as well as in the introduction thermostating the gas volume of the measuring chamber and the calibration mode of the device with calibration gas mixtures.

The approach to solving the problem of determining the concentration of CO ₂ in exhaled air, based on the allocation of the periods of inspiration and expiration in the structure of the respiratory cycle, allows the use of a single-frequency method of operation of the ultrasonic piezoelectric transducer, which leads to an increase in its sensitivity and increase, as a result, the accuracy of determining the concentration of CO ₂ .

Thermostating of the measuring chamber also improves the accuracy of determining the concentration, since it reduces the temperature error and eliminates the condensation of water vapor on the surface of the ultrasonic emitter and reflector. The absence of temperature control would lead to the fact that the ultrasonic waves would undergo scattering on droplets of condensed water, and this, in turn, would lead to a decrease in the accuracy of determining the concentration of CO ₂ .

Calibration of the device with calibration gas mixtures also increases the accuracy of determining the concentration, since it eliminates the influence of non-ideal transfer characteristics of the converting elements of the circuit and the error due to the nonlinear nature of the dependence of the analytical concentration of CO ₂ on the absorption coefficient.

The technical and medical result that can be obtained by using the invention is to increase the reliability of the diagnosis of the patient's condition by increasing the accuracy of determining the concentration of CO ₂ in exhaled air.

 The invention is illustrated by drawings.

In FIG. 1 shows a block diagram of the inventive device;
in FIG. 2 shows the dependence of the absorption coefficient on the frequency during breathing;
in FIG. 3 a, b are timing diagrams illustrating the operation of the device:
in FIG. 3a - change in absorption coefficient in the process of respiration;
in FIG. 3b - output signal of a threshold device;
in FIG. Figure 4 shows the construction of a logarithmic converter;
in FIG. 5 shows an embodiment of a threshold device.

 The device for measuring the gas concentration in the air exhaled by the patient contains a measuring chamber 1, in which an ultrasonic emitter 2 is located, which is a transceiver of an ultrasonic signal, and the wall of chamber 1 located opposite the emitter 2 acts as a reflector of an audio signal, a pulse generator 3, the first output of which is connected to emitter 2, amplifier 4 and the first amplitude detector 5, and the second output to the input of the standby multivibrator 6, the second amplitude detector 7, the input of which is connected to the output starter 4, and the output is connected to the first input of the logarithmic converter 8, the second input of which is connected to the output of the first amplitude detector 5, and the output is connected to the input of the threshold device 9 and the second input of the computing circuit 10. The output of the threshold device 9 is connected to the first input of the computing circuit 10 The third input of the computing circuit 10 is connected to the second output of the temperature control unit 11. The first output of which is connected to the heater 12, and the input to the temperature sensor 13 located inside the measuring chamber 1. The calculator Naya circuit 10 also has an operation mode control inputs and inputs for inputting calibration values. The first output of the computing circuit is connected to the control inputs of the logarithmic converter 8 and the threshold device 9, and the second to the block of indicators 14.

The dependence of the absorption coefficient on frequency during breathing is shown in FIG. 2. Curve 1 describes the dependence of the ultrasonic absorption coefficient of atmospheric air on frequency (inspiratory phase). The absorption of ultrasound in this case is mainly determined by oxygen and water vapor and at the relaxation frequency of absorption of CO ₂ is equal to some value α ₀ . Curve 2 corresponds to the absorption in the mixture of CO ₂ and nitrogen, and the absorption at the relaxation frequency f _p is mainly determined by CO ₂ and is equal to α _co2 , and the effect of nitrogen has practically no effect. The exhalation absorption coefficient is described by curve 3, which is actually the algebraic sum of the first two curves, and the absorption coefficient of the mixture on f _p is defined as α = α ₀ + α _co2 . Thus, by measuring the absorption of the gas mixture during inhalation and exhalation at f _p , it is possible to compensate for the effect of interfering components without using two frequencies by subtracting from the value of the absorption coefficient determined on the exhalation the corresponding value of the absorption coefficient determined on the inspiration. Strictly speaking, this method of compensation has its own error, because during breathing, the concentration of oxygen on inspiration and expiration changes. However, this error can be neglected, since the absorption coefficient of carbon dioxide by f _p significantly exceeds the absorption coefficient of oxygen.

Consider the operation of the inventive device for determining the concentration of CO ₂ in exhaled air. When you turn on the inventive device, the thermostatic control unit 11 starts working. It, with the help of a heater 12, raises the temperature of the measuring chamber 1 to 37 ^° C, after which it keeps it constant. This process is monitored using a temperature sensor 13. After the walls of the measuring chamber reach a temperature of 37 ^o C, a high level signal is established at the output of the thermostating device. Computing circuit 10 starts processing the data arriving at its inputs only after the temperature of the measuring chamber is established, i.e. after setting a high level at the third entrance.

 Generator 3 periodically generates short probe pulses filled with a frequency close to relaxation, which are supplied to the emitter 2.

 The sound wave propagates in the measuring chamber 1, is reflected from the wall of the chamber 1 opposite to the emitter 2 and returns to the emitter 2, thus playing the role of a transceiver, which again converts the sound pulse into alternating voltage.

The dependence of the amplitude of the received pulse U on the absorption coefficient of the gas mixture in the measuring chamber is exponential and is expressed by the formula:
U = U ₀ • e ^{-α · l} , (4)
where l is the length of the path traveled by the ultrasonic pulse;
U ₀ is the amplitude of the emitted pulse.

В усилителе 4 происходит усиление амплитуды отраженного сигнала, поступающего с излучателя 2 с постоянным коэффициентом k. Управляющий сигнал, поступающий на усилитель от ждущего мультивибратора 6, закрывает усилитель 4 на время, в течение которого действует измерительный импульс от генератора 3, таким образом на вход амплитудного детектора 7 с выхода усилителя 4 поступает только отраженный импульс. Далее сигнал, пропорциональный экспоненте коэффициента поглощения (пропорциональный значению (5) на вдохе и (6) на выдохе), поступает на вход второго амплитудного детектора 7, где преобразуется в уровень напряжения U₁. Первый амплитудный детектор 5, подключенный к первому выходу генератора 3, преобразует в уровень напряжения U₂ амплитуду излучаемого импульса. Далее полученные сигналы поступают соответственно на первый и второй входы логарифмирующего преобразователя 8.Given the equality α = α ₀ + α _{co2, the} dependence of the amplitude of the received pulse on the absorption coefficient on inspiration and expiration is expressed by formulas (5) and (6), respectively:

In the amplifier 4 there is an amplification of the amplitude of the reflected signal coming from the emitter 2 with a constant coefficient k. The control signal supplied to the amplifier from the standby multivibrator 6 closes the amplifier 4 for the duration of the measurement pulse from the generator 3, so that only the reflected pulse arrives at the input of the amplitude detector 7 from the output of the amplifier 4. Next, a signal proportional to the exponent of the absorption coefficient (proportional to the value of (5) on inspiration and (6) on exhalation) is fed to the input of the second amplitude detector 7, where it is converted to the voltage level U ₁ . The first amplitude detector 5, connected to the first output of the generator 3, converts the amplitude of the emitted pulse into voltage level U ₂ . Further, the received signals are respectively supplied to the first and second inputs of the logarithmic converter 8.

где U₁ и U₂ - напряжение соответственно на первом и втором входах логарифмирующего устройства в момент преобразования:
A - постоянный коэффициент.The logarithmic converter 8 converts the voltage levels at its inputs into a sequence of pulses. The start of the conversion is initiated by the computational circuit 10. The duration of each pulse τ _i is equal to:

where U ₁ and U ₂ are the voltage, respectively, at the first and second inputs of the logarithmic device at the time of conversion:
A is a constant coefficient.

 One of the options for constructing a logarithmic converter is shown in FIG. 4.

- на выдохе,
где k - коэффициент усиления усилителя 4.In accordance with (7), at the output of the logarithmic device 8 after the arrival of the trigger signal from the computing circuit 10, a pulse is generated with a duration equal to:
τ _{i vd} = A • (α ₀ l-lnk) - on inspiration,

- on the exhale,
where k is the gain of the amplifier 4.

The received pulse is fed to the input of the threshold device 9 and the second input of the computational circuit 10. The threshold device (Fig. 5), also triggered by the computational circuit, generates a high level signal when the input pulse τ _i exceeds a certain threshold duration τ _n that corresponds to the threshold absorption coefficient α _n slightly exceeding α ₀ . Thus, with the help of a threshold device, phases of the respiratory cycle are distinguished, and the beginning of the inspiration corresponds to the transition of the output signal of the circuit from one to zero. The operation of the device is illustrated in FIG. 3. The change in absorption coefficient during breathing is shown in FIG. 3a. The output signal of the threshold device is shown in FIG. 3b. This signal is the control signal when calculating the concentration of CO ₂ computing circuit 10.

равной (8) или (9), который преобразуется вычислительной схемой в число в параллельном коде N_i. Постоянно в процессе работы вычислительной схемы из каждого нового отсчета логарифмирующего преобразователя отнимается значение N_ср, которое является результатом усреднения m импульсов, полученных вычислительной схемой после начала вдоха и обновляется в памяти вычислительной схемы после каждого очередного вдоха. В результате вычитания получается значение

, пропорциональное коэффициенту поглощения выдыхаемой газовой смеси на выдохе:

Далее из зависимости

, которая получена в режиме калибровки ранее и хранится в памяти вычислительной схемы, вычисляется концентрация углекислого газа.Computing circuit 10, implemented on the basis of a single-chip microcomputer, provides the device in two modes - calculating the concentration of CO ₂ and calibration. In each of the modes, the computational circuit initiates a survey of the measuring path, sending a triggering pulse to the control input of the logarithmic converter 8. After conversion, a voltage pulse with a duration of

equal to (8) or (9), which is converted by the computing circuit into a number in the parallel code N _i . Constantly during the operation of the computational circuit, the value N _{cf is} taken from each new count of the logarithmic converter, which is the result of averaging m pulses received by the computational circuit after the start of inspiration and is updated in the memory of the computational circuit after each next inspiration. Subtraction results in a value

proportional to the absorption coefficient of the exhaled gas mixture on the exhale:

Further from the dependency

, which was obtained in the calibration mode earlier and stored in the memory of the computational circuit, the concentration of carbon dioxide is calculated.

. Координаты остальных точек на этой зависимости вычисляются при помощи интерполяции в рабочем режиме. В режиме калибровки вычислительная схема не учитывает структуры дыхания.In calibration mode, when passing n calibration gas mixtures with a known CO ₂ content through the measuring chamber, the coordinates of n dependence points can be set

. The coordinates of the remaining points on this dependence are calculated using interpolation in the operating mode. In calibration mode, the computational circuit does not take into account the structure of respiration.

In FIG. 4 shows one of the options for constructing a logarithmic device 8. A triggering pulse from a computational circuit charges the capacitor to a constant level. From the moment the pulse ceases, the capacitor begins to discharge with the RC time constant. The first comparator goes from zero to one at a time when the voltage at its inverse input drops below the level of U ₂ , and the second comparator drops to zero as soon as the voltage on the capacitance falls below the input voltage U ₁ . It is easy to show that the duration of the pulse emitted by the And circuit will be determined by formula (7).

In FIG. 5 shows an embodiment of the threshold device 9. The trailing edge of the polling signal of the logarithmic device starts the waiting multivibrator, which generates a pulse with a duration of τ _n . The trailing edge of the pulse of the waiting multivibrator latches the trigger, the input of which receives the pulse obtained after the conversion in converter 8. Thus, if the duration of the input pulse is longer than the pulse duration from the logarithmic converter, then the output of the trigger is set to 0, if less, then 1.

The proposed device allows to exclude when determining the concentration of CO ₂ in exhaled air the influence of other components without the disadvantages inherent in the prototype, which, in turn, leads to an increase in the accuracy of determining the concentration of CO ₂ .

Literature
1. Patent EPO N 430859, cl. G 01 N 29/02, 1991

 2. Gershgal D.A., Fridman V.M. Ultrasonic equipment for industrial use. M .: Energy, 1967, p. 253.

3. Patent GDR N 216329, CL G 01 N 29/02, 1984 (prototype)
4. Ultrasound. Little Encyclopedia. Ed. Golyamina I.P. M .: Soviet Encyclopedia, 1979, S. 258.

 5. Physical acoustics. Ed. W. Mason, trans. from English, vol. 1, part A, Moscow: Atomizdat, 1968, p. 174-178, 181-183, 192.

Claims (1)

 A device for measuring carbon dioxide concentration in exhaled air, including a measuring chamber connected to the patient’s respiratory tract and containing an ultrasonic emitter and a sound reflector, a pulse generator, the first output of which is connected to the emitter connected to the first input of the amplifier, and an output signal processing channel, comprising a computing circuit, characterized in that it is equipped with a temperature control unit, the input and the first output of which are connected, respectively, to the temperature sensor To the heater and the heater located in the measuring chamber, the output signal processing channel contains a first amplitude detector, the input of which is connected to the first output of the generator, a second amplitude detector, the input of which is connected to the amplifier output, waiting for a multivibrator, the input of which is connected to the second generator output, and the output connected to the second input of the amplifier, a logarithmic converter, the first and second signal inputs of which are connected, respectively, with the outputs of the first and second amplitude detectors, a new device, the signal input of which is connected to the output of the logarithmic converter, while the first input of the computational circuit is connected to the output of the threshold device, the second input is connected to the output of the logarithmic converter, the third input is connected to the second output of the temperature control unit, the fourth and fifth inputs respectively setting calibration points and controlling the operating mode, the first output is connected to the control inputs of the logarithmic transducer and threshold device, and the second output connected to the input of the indicator block.

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