JPH08233810A - Carbon dioxide concentration measuring device - Google Patents

Abstract

(57) [Abstract] [Purpose] The purpose is to measure the carbon dioxide concentration by correcting the drift of the detector without using a rotating mechanism for interrupting light. [Structure] In a carbon dioxide concentration measuring device for irradiating a breathing gas with infrared rays and detecting a signal corresponding to the amount of permeation to measure the carbon dioxide concentration, a heat detector 2 for detecting the amount of infrared permeation
And a switch SW for turning on / off the light source 1, a maximum value at the time of current inspiration is detected and stored from the detection signal of the thermopile 2, and a detection signal following the maximum value detection time and the maximum value stored. Difference between the drift correction means for calculating the concentration signal that changes in time series and the minimum value of the thermopile 2 when the light source is turned off momentarily and the stored maximum value at the time of inspiration. A sensitivity correction means for calculating a difference, calculating a difference between the reference value and the concentration signal by storing the reference value of the maximum received light amount at that time when the carbon dioxide concentration is “0”, and obtaining a concentration component for sensitivity correction It comprises a control unit 6 for calculating the carbon dioxide concentration based on this concentration component, and a RAM 8 for storing the maximum value and the reference value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carbon dioxide concentration measuring device for measuring the concentration of carbon dioxide contained in exhaled gas.

[0002]

2. Description of the Related Art Generally, when measuring the concentration of carbon dioxide in exhaled gas using infrared rays, a photodetector is used to measure the amount of light corresponding to the absorption of light by carbon dioxide during exhalation. There is known a device which corrects a drift of an output voltage of a photodetector such as a change in irradiation intensity of a light source and a change in light amount due to a stain on a window of a detection unit (Japanese Patent Publication No. 60-4).
4614).

FIG. 9 shows the configuration of a carbon dioxide concentration measuring device equipped with such a conventional drift correction device.
In FIG. 9, reference numeral 40 denotes a connecting pipe through which breathing gas passes, and one is a connecting end for the subject to add to the mouth, the other is branched into two and one is an open end, and one is an examinee. It is connected to a servo-ventilator 41 that sends in air when a person inhales. Connection tube 4
A pair of windows 41a and 41b made of glass or the like for transmitting light are formed in the middle portion of 0. The light source 4 is provided below the window 41b.
2 is arranged, and an optical interrupter 43 having a light transmission hole which is rotationally driven by the motor M is arranged above the window 41a. A filter 44 that absorbs only light having a wavelength absorbed by carbon dioxide is disposed above the light interrupter 43, and a photodetector 45 is disposed above the filter 44. Four
6 is an amplifier for amplifying the output voltage of the photodetector 45, and 47 is a rectifier. 48 is a divider, 49 is a logarithmic amplifier, 50
Is a recording device. Further, 51 is a field effect transistor (FET), which is turned on by the output of the servo fan 41 during the intake period. Further, 52 is a memory, which holds the voltage corresponding to the carbon dioxide concentration "0" in the intake period and outputs it to the divider 48.

In such a structure, the light emitted from the light source 42 passes through the breathing gas in the window 41b and the connecting pipe 40, and is emitted as the light interrupted by the optical interrupter 43 from the window 41a through the filter 44 and the carbon dioxide concentration. The amount of light corresponding to is detected by the photodetector 45. The output signal of the photodetector 45 is given as an exponential function, is amplified by the amplifier 46, and is rectified by the rectifier 47.
Is rectified by.

The output of the photodetector 45 has a filter 44,
A drift such as a change in the amount of light due to dirt on the windows 41a and 41b or a change in the light intensity of the light source 42 is included. Therefore, in order to remove the drift component from the output voltage output from the rectifier 47, the
A positive signal is output to the FET 51 to make it conductive, and the memory 52
The voltage corresponding to the carbon dioxide concentration "0" is held and output to the divider 48. On the other hand, since the positive signal from the servo fan 41 disappears at the end of the intake period, the FET 51 is turned off, the output of the rectifier 47 (the signal corresponding to the carbon dioxide gas at the time of exhalation) is output to the divider 48, and the memory 52. The drift component is removed by division by the voltage corresponding to the carbon dioxide concentration "0" held at, and the zero point is calibrated. The output of the divider 48 is output to the logarithmic amplifier 49, and an output signal proportional to the carbon dioxide concentration is obtained.

[0006]

However, the carbon dioxide concentration measuring device equipped with the drift correction device for the conventional photodetector described above is an expensive PbSe as a photodetector of this type.
Are using. PbSe has a fast response speed, but when infrared rays are continuously irradiated, the temperature of the element itself rises, the resistance value decreases, and the drift becomes large. Therefore, PbSe continuously discontinues at a cycle shorter than the respiratory cycle, for example, 200 Hz. However, it is necessary to detect the amount of light, and a drive unit such as an optical interrupter and a motor that rotationally drives the optical interrupter is arranged to detect the amount of light passing through the breathing gas. Therefore, there is a problem that the device is downsized, the power consumption is reduced, the robustness is limited, and the device is expensive. Therefore, in view of the above problems, the present invention can correct the drift of the output voltage without using a mechanism that continuously interrupts the light required for the photodetector.
It is an object of the present invention to provide a carbon dioxide concentration measuring device capable of correcting a change in sensitivity and more accurately measuring the concentration of carbon dioxide in addition to drift correction.

[0007]

A carbon dioxide concentration measuring device of the present invention according to claim 1 irradiates respiratory gas with infrared rays,
In a carbon dioxide concentration measuring device for measuring a carbon dioxide concentration by detecting a signal corresponding to the amount of transmitted light, a heat detector 2 for detecting the amount of transmitted infrared radiation, a switch means SW for turning on / off the light source 1, and a heat detector. The maximum value at the time of current inspiration is detected and stored from the detection signal of the device 2, and the difference between the detection signal following the maximum value detection time and the stored maximum value is calculated to change the concentration in time series. The carbon dioxide concentration "0" is calculated by calculating the difference between the drift correction means for obtaining the signal and the minimum value of the heat detector 2 when the light source 1 is momentarily turned off and when the light source 1 is off, and the stored maximum value during intake. ) Is stored as a reference value for the maximum amount of received light at that time, and the sensitivity correction means for calculating the ratio between the reference value and the concentration signal to obtain the concentration component whose sensitivity is corrected is provided. Control means 6 for calculating concentration and maximum value Those comprising a storage means 8 for storing the fine reference values.

According to a second aspect of the present invention, in the carbon dioxide concentration measuring device according to the first aspect, the light source is turned off at a predetermined cycle longer than the breathing cycle.

According to a third aspect of the present invention, in the carbon dioxide concentration measuring device according to the first aspect, the light source is turned off in synchronization with the intake air. According to a fourth aspect of the present invention, in the carbon dioxide concentration measuring device according to any of the first to third aspects, the light source is turned off when the difference from the maximum value detected during adjacent inspiration is a predetermined value or more.

[0010]

According to the first aspect of the present invention, the amount of infrared rays transmitted through the breathing gas from the light source is detected by the heat detector, and the maximum value at each inspiration is detected from the detection signal and stored in the storage means. The difference between the detection signal following the value detection and the stored maximum value is calculated to obtain the concentration signal and drift correction is performed. Next, the light source is momentarily turned off to detect the minimum value of the heat detector when the light source is turned off, and the difference from the maximum value stored is calculated, and the carbon dioxide concentration "0" is used as the reference for the maximum amount of received light at that time. The value is obtained and stored in the storage means. Further, the ratio between the reference value and the concentration signal is calculated, the sensitivity of the concentration signal is corrected by this ratio to obtain the concentration component, and the carbon dioxide concentration is obtained based on this concentration component.

According to the second aspect of the invention, the light source is turned off in a cycle longer than a predetermined respiratory cycle.

According to the third aspect of the invention, the light source is turned off in synchronization with the intake air.

In the invention according to claim 4, the light source is turned off when the difference from the maximum value detected at the time of adjacent inspiration exceeds a predetermined value.

[0014]

Embodiments of the carbon dioxide concentration measuring apparatus of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention. FIG. 2 is a flow chart showing the process of calculating the carbon dioxide concentration in the embodiment of FIG. FIG. 3 is a diagram showing detection points when drift correction is performed on the detection signal of the heat detector. FIG. 4 is an explanatory diagram in the case where the detection signal of the heat detector is subjected to sensitivity correction. FIG. 5 is a diagram showing a case where the light source is turned off at a predetermined cycle in the embodiment of FIG. FIG. 6 is a diagram showing a case where the light source is turned off in synchronization with intake air in the embodiment of FIG. Figure 7
FIG. 2 is an explanatory diagram for turning off the light source when the difference between the maximum values during inhalation before and after exceeds a predetermined value in the embodiment of FIG. 1. FIG. 8 is a waveform diagram of the carbon dioxide concentration obtained in the embodiment of FIG.

Prior to the description of the embodiments, the principle of the present invention will be described. In the present invention, a thermopile is used as a heat detector that detects the amount of heat that changes according to the concentration of carbon dioxide in the exhaled gas. The thermopile has less drift and is less expensive than the conventionally used PbSe, but has a unique property, and it is required to be used corresponding to this property. That is, the response speed required for the carbon dioxide concentration measuring device is 200 ms or less, but since the response speed of the thermopile is as slow as 50 ms to 200 ms, the response speed of 200 ms or less is achieved by the conventional method of chopping the light of the light source. Is difficult to do.

However, a drift occurs in the detection signal due to, for example, a change in the amount of infrared rays from the light source, clouding or dirt on the window of the exhaled gas detecting section, and the structure of the thermopile itself. Among these, the drift of the detection signal due to the structure of the thermopile itself occurs with the change of the operating environment temperature, and therefore needs to be corrected. That is, the thermopile has a hot junction and a cold junction, and a drift occurs in the detection signal due to the difference in thermal time constant between these two contacts. The hot junction, which has a small heat capacity, responds quickly to a rapid change in ambient temperature, but the cold junction, which is in thermal contact with the container, has a large heat capacity, and therefore the response is delayed compared to the hot junction. Therefore, when detecting a signal output according to the temperature difference between the hot junction and the cold junction, a drift occurs until the cold junction thermally reaches equilibrium with the ambient temperature.

Further, since the amount of transmitted light is reduced and the output sensitivity of the thermopile is changed due to clouding or dirt on the window of the exhaled gas detecting portion, the measured carbon dioxide concentration also changes and stable measurement cannot be performed.

Therefore, in order to use the thermopile, it is necessary to correct the drift of the detection signal and the fluctuation due to the output sensitivity and then measure the carbon dioxide concentration.

According to the present invention, the output sensitivity of the thermopile changes when a drift occurs in the detection signal due to the structure of the thermopile due to a rapid temperature change, or when the window of the detection unit becomes cloudy or the light amount of the light source changes. In that case, the drift correction and the sensitivity correction are performed respectively.

Normally, the maximum value for each inspiration is detected and stored from the detection signal of the thermopile, and the output value at each subsequent exhalation is detected, and the stored maximum value and the output value at expiration are stored. Drift correction is performed to calculate the difference and obtain the concentration signal.

FIG. 3 is an explanatory view for drift correction, where A, C and E are the maximum detection points detected from the detection signal of the thermopile during inspiration, and B and D are points A and C. Is a detection signal at the time of exhalation. For example, the maximum value Va at the time of inspiration at point A is detected and stored, the output value Vb at point B at the time of subsequent expiration is detected, and Vb at expiration is subtracted from the maximum value Va. This is repeated for each inspiration and expiration.
Therefore, the maximum value is stored and held for each inspiration and expiration,
Since the difference from the output value at the time of exhalation is obtained, it is effective when the drift is gentle or small.

Further, in the present invention, when the sensitivity is corrected, the light source is instantly turned off and turned on. That is, the maximum value is detected from the detection signal of the thermopile at the time of intake immediately before the light source is turned off and stored, and the minimum value is detected from the detection signal of the thermopile when the light source is turned off. The difference between the maximum value and the minimum output value is calculated and stored as the reference value at the carbon dioxide concentration "0" and the maximum light amount at that time.

Further, the ratio between the stored reference value and the concentration signal obtained at the time of the above-mentioned drift correction is calculated, the sensitivity of the concentration signal is corrected to obtain the concentration component, and the concentration of carbon dioxide gas is calculated based on this concentration component. Calculate. The ratio between the reference value and the concentration signal is constant as long as the carbon dioxide concentration is the same even when the detection signal is lowered due to a rapid change in the light amount of the thermopile due to the dirt on the window. Therefore, even if the output sensitivity of the thermopile changes by instantaneously turning off the light source during the measurement of carbon dioxide concentration, the reference value is calculated as described above and the ratio to the concentration signal is calculated to correct the sensitivity of the concentration signal. Since the concentration component is obtained by performing the above, and the concentration of carbon dioxide gas can be corrected based on this concentration component, it is possible to always perform stable measurement of the concentration of carbon dioxide gas.

With reference to FIG. 4, a description will be given of detection values when drift and sensitivity correction of the thermopile output based on the above principle are performed.

Normally, the maximum value Vc at the point C during the current inspiration is detected, and the output value Vd at the point D reduced by the carbon dioxide gas during the subsequent exhalation is detected. The difference between the maximum value Vc at the time of inspiration and the output value Vd at the time of expiration is calculated as the concentration signal Vx from (Vc-Vd). In addition to detecting the maximum value for each inspiration, the output value at the time of expiration following the maximum value detection is detected, and the concentration signal is calculated from the difference between the maximum value and the output value at the time of expiration to determine the ambient temperature. Is drastically changed and the detection signal of the thermopile is changed, the drift correction is performed.

Next, the light source is turned on / off, but the maximum value (peak value) of point A at the time of intake just before turning off is detected and stored, and the offset voltage of the thermopile when the light source is turned off is used. The minimum output value Vb at a certain point B is detected. next,
The difference between the maximum value Va at the time of inspiration and the output value Vb at the time of expiration is determined by (Va-Vb) as the reference value Vo at the maximum amount of received light at the time when the carbon dioxide concentration is "0", and is stored. Further, the maximum value Vc at the point C during the current intake
And the output value Vd at point D decreased by the carbon dioxide gas during the subsequent exhalation. The difference between the maximum value Vc and the output value Vd is calculated and the concentration signal Vx is calculated as (Vc-V
Calculated from d).

Then, the ratio (Vx / Vo) between the stored reference value Vo and the concentration signal Vx obtained by calculating the difference between the output value detected at the time of expiration and the output value detected at the time of maximum value detection at the time of inspiration.
Then, the sensitivity of the concentration signal is corrected based on this ratio to obtain the concentration component, and the carbon dioxide concentration is calculated based on the concentration component.

Next, the density component can be obtained in the same manner even if the output sensitivity of the thermopile is reduced due to a decrease in the amount of light due to dirt on the window. As shown in FIG. 4, when the amount of received light changes and the output of the thermopile decreases, the maximum value Ve at point E is detected and stored from the detection signal of the thermopile during inspiration. The minimum value Vf which is the offset voltage of the thermopile at the point F when the light source is turned off is detected, and the reference value V ₀₁ at the maximum light amount at the time of the carbon dioxide concentration "0" is obtained from (Ve-Vf). Remember. The maximum value Vg at the point G at the time of the next inspiration and the output value Vh at the point H decreased by the carbon dioxide gas at the time of the subsequent exhalation are detected, and the concentration signal V _X1 is obtained from (Vg-Vh). Then, the ratio V _X1 / V ₀₁ of the reference value V ₀₁ and the concentration signal V _X1 at the time of exhalation is calculated, and the carbon dioxide concentration is calculated based on this ratio.

[0029] As described above, even when the amount of received light is decreased, the ratio of the carbon dioxide concentration is the reference value and the density signals of the same time constant, i.e., because it is _{Vx / V0 = V X1 / V} 01, the light source Is turned on / off to obtain the ratio between the reference value and the concentration signal, the concentration signal is subjected to sensitivity correction by this ratio to obtain the concentration component, and the carbon dioxide concentration can be calculated based on this concentration component. That is, by obtaining this ratio, it is possible to obtain an accurate carbon dioxide gas concentration even if the output sensitivity of the thermopile output changes due to changes in the amount of light.

In FIG. 1, T is a ventilation pipe through which the exhaled gas and the inhaled gas flow, and windows W1 and W2 made of a transparent material such as sapphire are formed at opposing portions at predetermined positions. One end (left in the drawing) of the ventilation pipe T is an insertion end that is inserted into the mouth of the subject, and the other end (right of the drawing) is an open end to the atmosphere. The windows W1 and W2 are antifogging processed to prevent fogging due to water vapor in the exhaled gas. A light source 1 such as a lamp is arranged near the window W1 and irradiates the window W1 with light. Further, below the window W2, the heat detector 2 made of the thermopile described above is arranged to detect the infrared rays transmitted from the light source 1 and transmitted through the windows W1 and W2. Further, on the light receiving surface of the heat detector 2,
A filter F having a wavelength (about 4.3 μm) that is absorbed by carbon dioxide in the exhaled gas is arranged.

Reference numeral 3 is a light source drive section which is composed of, for example, a constant current circuit and is turned on / off by a switch SW. The switch SW is composed of, for example, a semiconductor switch such as a transistor, and is turned on / off by a control signal output from the control unit 6 described later.

Reference numeral 4 is an amplifier (for example, logarithmic amplifier) that amplifies the detection voltage of the heat detector 2, and 5 is an analog-digital converter that converts the output of the amplifier 4 into a digital signal. The control unit 6 described above is composed of, for example, a CPU
The device is controlled based on a control program for measuring the carbon dioxide concentration stored in the OM 9.

Reference numeral 7 denotes an operation unit including, for example, a plurality of push buttons, which sets parameters such as an ON / OFF cycle of the light source 1 and an upper limit value of a detection signal of the heat detector 2 and sets required data.

Reference numeral 8 denotes a RAM which temporarily stores and holds set parameters, maximum values detected from detection signals of the heat detector 2, calculated reference values, measured carbon dioxide concentration data, and the like. Reference numeral 9 denotes a ROM, in which a control program for automatically measuring the carbon dioxide concentration by performing drift correction and sensitivity correction on the detection signal of the heat detector 2 according to the above-described principle of the present invention is stored in advance.

Reference numeral 10 designates a display unit comprising a light emitting element such as a plurality of LEDs (light emitting diodes) or an acoustic element such as a buzzer, and displays the measured carbon dioxide concentration in a bar graph according to the concentration change, or the buzzer. The modulated sound according to the density change is notified by. Alternatively, both an LED and a buzzer can be provided. By equipping both of them, the breathing state of the subject can be monitored both visually and auditorily.

Next, the operation of the above configuration will be described with reference to the flowchart of FIG. At the start of measurement, the light source 1 is turned on at the same time when the power switch (not shown) is turned on (step S1), and carbon dioxide gas accompanying breathing that comes and goes through the insertion end of the ventilation tube T inserted into the mouth of the subject. The transmitted light due to the concentration change is received by the heat detector 2, the time when the detection signal of the heat detector 2 becomes large is recognized as intake air, and the maximum value is detected from the detection signal of the heat detector 2 at the time of intake air and the RAM 8 (Step S2). The maximum value can be detected by calculating the difference value between the data before and after the detection signal of the heat detector 2 on the time axis.

The decreased detection signal following the maximum value detected in step S2 is recognized as exhalation, and the output value decreased by the carbon dioxide contained in the exhalation is detected from the detection signal of the heat detector 2 to detect the exhalation. The difference between the maximum value of and the output value at the time of exhalation is calculated to obtain the concentration signal (step S3).

Next, the light source 1 is momentarily turned off (step S4), and the difference between the minimum value and the stored maximum value of the heat detector 2 when the light source 1 is off is calculated, and the carbon dioxide concentration is "0" and The reference value for the maximum amount of received light at that time is obtained and stored (step S5).

Carbon dioxide concentration "0" obtained in step S5
Based on the ratio between the reference value corresponding to the density signal calculated in step S3 and the density signal calculated in step S3, the density component whose sensitivity is corrected is calculated (step S6). The carbon dioxide concentration is calculated based on the sensitivity-corrected concentration component, and the concentration data is displayed on the display unit 1.
0, and the length is changed according to the carbon dioxide concentration shown in FIG. 8 and displayed as, for example, a bar graph (step S
7).

As described above, the above-described embodiment uses the heat detector 2 composed of a thermopile, and the drift correction and the difference between the maximum value detected for each inspiration and the subsequent output value at the time of exhalation are obtained. Since the carbon dioxide concentration is calculated while the light source 1 is instantly turned off and the sensitivity is corrected, the stable concentration of carbon dioxide can be measured.

In the flow chart of FIG. 2, the light source 1 is arbitrarily turned off / on, but as shown in FIG. 5, the off cycle T of the light source 1 can be set in advance. The cycle T may be set in advance in the ROM 9 and stored, or may be set via the operation unit 7. By predetermining the cycle, the detection signal of the heat detector 2 can be automatically corrected. The predetermined cycle can be set, for example, every 30 seconds or every 1 minute according to the situation of the ambient temperature.

Further, as shown in FIG. 6, the light source 1 may be turned off every a plurality of inspirations or every one breath (inspiration and expiration). The number of intakes may be set in advance in the ROM 9 and stored, or may be set via the operation unit 7. If the light source is turned off at the above-mentioned constant cycle, it may be turned off during exhalation, and the detection data at this time cannot be used, but by synchronizing with inspiration, respiratory data is reliably corrected and measurement becomes more stable. . Alternatively, the light source can be turned off for each intake.

Further, as shown in FIG. 7, a predetermined value may be set for the drift of the detection signal of the heat detector 2, and when the predetermined value is exceeded, the light source may be momentarily turned off.
The predetermined value is set to, for example, 4 mmHg as the difference between the maximum values immediately before and at the time of current intake, and when the difference exceeds this value, the light source 1
Turn off. In FIG. 7, the light source 1 is turned off on the assumption that the maximum values P1 and P2 and P3 and P4 at the time of intake exceed predetermined values. Similarly to the above, the predetermined value is set, for example, via the operation unit 7 or stored in the ROM 9 in advance, and the control unit 6 monitors the predetermined value to control the off operation of the light source 1. By providing the predetermined value, the measurement can be performed reliably and stably in an environment where the ambient temperature fluctuates drastically.

[0044]

As described above, according to the carbon dioxide concentration measuring apparatus of the present invention as set forth in claim 1, by using the heat detector composed of the thermopile, the chopper (intermittent light interruption) necessary for the conventional photodetector is used. Since there is no need for a mechanical component such as a container) or a motor for rotating the same, there is an advantage that the device can be easily downsized, the robustness can be improved, and the cost can be reduced. Further, the sensitivity can be corrected by turning on / off the light source at any time.

According to the second aspect of the present invention, a change in the output sensitivity due to a drift of the output due to a sudden change in the ambient temperature or a cloud of the window is automatically generated by turning the light source off / on every predetermined period. Since it can be corrected to, there is an advantage that a stable carbon dioxide concentration can be accurately measured.

According to the third aspect of the present invention, the respiratory data can be reliably corrected even if there is a change in the sensitivity of the heat detector due to a change in the ambient temperature or a fogging of the window of the ventilation pipe.

According to the fourth aspect of the present invention, the light source is turned off when the output value of the heat detector exceeds a predetermined value set in advance, so that the carbon dioxide is stable even in an environment where the ambient temperature fluctuates drastically. Can measure gas concentration.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a carbon dioxide concentration measuring device of the present invention.

FIG. 2 is a flowchart illustrating a processing operation of the embodiment of FIG.

FIG. 3 is a diagram illustrating drift correction in the embodiment of FIG.

FIG. 4 is a diagram illustrating sensitivity correction in the embodiment of FIG.

5 is an explanatory diagram for turning off the light source in the embodiment of FIG. 1 at a constant cycle.

FIG. 6 is an explanatory diagram for turning off the light source in synchronization with the intake air in the embodiment of FIG.

FIG. 7 is an explanatory diagram of turning off the light source when the detection signal of the heat detector in the embodiment of FIG. 1 exceeds a predetermined value.

FIG. 8 is a waveform diagram of carbon dioxide concentration obtained according to the embodiment of FIG.

FIG. 9 is a configuration diagram of a carbon dioxide concentration measuring device including a conventional drift correction device.

[Explanation of symbols]

 1 light source 2 thermopile 3 light source drive unit 4 amplifier 5 analog / digital converter 6 control unit 7 operation unit 8 RAM 9 ROM 10 display unit

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Masami Ito 1-31-4 Nishinishiai, Shinjuku-ku, Tokyo Inside Nihon Kohden Kogyo Co., Ltd. (72) Masayuki Inoue 1-1-31 Nishiochiai, Shinjuku-ku, Tokyo No. Nihon Kohden Kogyo Co., Ltd. (72) Inventor Sugiura Regular No. 1-314 Nishiochiai, Shinjuku-ku, Tokyo Nihon Kohden Kogyo Co., Ltd.

Claims (4)

[Claims] 1. A carbon dioxide concentration measuring device for irradiating breathing gas with infrared rays to detect a signal according to the amount of transmitted light to measure the concentration of carbon dioxide, wherein a heat detector for detecting the amount of transmitted infrared radiation, and a light source. And a switch means for turning on and off, and detecting and storing the maximum value at the time of current intake from the detection signal of the heat detector, and detecting the signal following the maximum value detection time and the stored maximum value. Drift correction means for calculating the difference to obtain a concentration signal that changes in time series, and the light source is momentarily turned off to detect the minimum value of the heat detector at the time of off, and the stored maximum value at the time of intake is stored. Sensitivity for calculating the difference from the value and storing it as a reference value for the maximum amount of received light at the time of carbon dioxide concentration "0", and calculating the ratio of the reference value and the concentration signal to obtain the sensitivity-corrected concentration component Compensation means, and the concentration An apparatus for measuring carbon dioxide concentration, comprising: a control means for calculating the carbon dioxide concentration based on the minute and a storage means for storing the maximum value and the reference value. 2. The carbon dioxide concentration measuring device according to claim 1, wherein the light source is turned off at a predetermined cycle longer than the breathing cycle. 3. The carbon dioxide concentration measuring device according to claim 1, wherein the light source is turned off in synchronization with the intake air. 4. The carbon dioxide concentration measuring device according to claim 1, wherein the light source is turned off when the difference from the maximum value detected at the time of adjacent intake is a predetermined value or more.