JP3273297B2 - Carbon dioxide concentration measurement device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus 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 detect and measure the amount of light corresponding to the absorption of light by the carbon dioxide during exhalation. A device is known 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. Sho 60-4).
4614).

FIG. 9 shows a configuration of a carbon dioxide concentration measuring apparatus provided with such a conventional drift correction apparatus. In FIG. 9, reference numeral 40 denotes a connection pipe through which the respiratory gas passes, and the connection pipe is used by the subject to add one to the mouth, the other is branched into two, one is an open end, and one is the subject. Is connected to a servo ventilator 41 that feeds air at the time of air intake. Connection pipe 4
A pair of windows 41a and 41b made of glass or the like that transmit light are formed in the middle part of 0. The light source 4 is provided below the window 41b.
2 is disposed above the window 41a, and an optical interrupter 43 having a light transmitting hole that is driven to rotate by the motor M is disposed above the window 41a. Above the light interrupter 43, a filter 44 that absorbs only light having a wavelength that is absorbed by carbon dioxide is disposed, and above the filter 44, a photodetector 45 is disposed. 4
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. Reference numeral 51 denotes an FET (field effect transistor), which conducts during an intake period by the output of the servo ventilator 41. Further, a memory 52 holds a voltage corresponding to the carbon dioxide concentration “0” during the inspiration period and outputs the voltage to the divider 48.

In such a configuration, the light emitted from the light source 42 passes through the window 41b and the respiratory gas in the connecting tube 40, and passes through the filter 44 as light intermittently transmitted from the window 41a by the light interrupter 43. Is detected by the photodetector 45. The output signal of the photodetector 45 is given as an exponential function, amplified by the amplifier 46, and output from the rectifier 47.
Is rectified.

The output of the photodetector 45 includes a filter 44,
Drift such as a change in light amount due to contamination of the windows 41a and 41b or a change in 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 servo ventilator 41 outputs
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 ventilator 41 disappears at the end of the intake period, the FET 51 is turned off, and the output of the rectifier 47 (the signal corresponding to the carbon dioxide gas at the time of expiration) is output to the divider 48 and the memory 52 The drift component is removed by dividing by the voltage corresponding to the carbon dioxide concentration “0” held in the above, and the zero point is calibrated. The output of the divider 48 is output to a logarithmic amplifier 49 to obtain an output signal proportional to the carbon dioxide concentration.

[0006]

However, the above-mentioned conventional carbon dioxide gas concentration measuring apparatus provided with the drift correction device of the photodetector is expensive as this type of photodetector.
You are using Although PbSe has a fast response speed, the temperature of the element itself rises when infrared rays are continuously radiated, the resistance value decreases, and the drift increases. Therefore, it is intermittently shorter than the respiratory cycle, for example, 200 Hz. It is necessary to detect the amount of light transmitted through the respiratory gas by arranging a drive unit such as an optical interrupter and a motor for rotating the optical interrupter. For this reason, there is a problem that the size of the device is reduced, the power consumption is reduced, the robustness is limited, and the device is expensive. Further, in the conventional device, the drift is corrected based on the voltage corresponding to the carbon dioxide concentration “0” at a certain point in time during inhalation. Therefore, if there is a large drift before the next reference value is obtained, At the time when the next reference value is used, discontinuity such as jump occurs in the corrected signal. Accordingly, the present invention has been made in view of the above-described problems, and therefore, it is possible to perform drift correction and sensitivity correction with less discontinuity of a detection signal without using a mechanism for continuously interrupting light required for a photodetector. It is an object to provide a measuring device.

[0007]

According to a first aspect of the present invention, there is provided a carbon dioxide concentration measuring apparatus which irradiates a respiratory gas with infrared rays.
In a carbon dioxide concentration measuring device for measuring a carbon dioxide concentration by detecting a signal corresponding to a transmission amount, a heat detector for detecting an infrared transmission amount, switch means SW for turning on / off a light source, and a heat detector From the detection signal, the current maximum value at the time of intake is detected and stored, a straight line connecting the current value and the maximum value at the time of the immediately preceding intake is calculated, and a correction straight line extended until the maximum value at the next intake is detected. Drift correction means for calculating the difference between the detection signal following the current maximum value detection time and the correction straight line, and calculating the concentration signal that changes in time series, and the light source is momentarily turned off to detect the off-state detection signal. Detect the minimum value, calculate the difference from the stored maximum value, and store it as the reference value at the maximum received light amount at that time with the carbon dioxide concentration “0”,
A sensitivity correcting means for calculating a ratio between the reference value and the density signal to obtain a density component corrected for sensitivity; a control means for obtaining a carbon dioxide gas concentration based on the density component; and storing a maximum value and the reference value. Storage means.

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 respiratory cycle.

According to a third aspect of the present invention, in the carbon dioxide concentration measuring apparatus according to the first aspect, the light source is turned off in synchronization with the intake.

According to a fourth aspect of the present invention, in the carbon dioxide concentration measuring apparatus according to any one of the first to third aspects, when the difference between the maximum values detected at the time of adjacent intake exceeds a predetermined value, the light source is switched off. Turn off.

[0011]

According to the first aspect of the present invention, the amount of infrared light transmitted through the respiratory gas is detected by the heat detector, and the maximum value at the time of current inspiration is detected from the detection signal and stored in the storage means.
A straight line connecting the current value and the maximum value at the time of the immediately preceding intake is calculated to obtain a correction straight line, which is extended until the maximum value at the next intake is detected. The difference between the detection signal following the maximum value detection time at the time of the current intake and the correction straight line is calculated, and the concentration correction signal that changes in time series is obtained to perform drift correction. Next, the light source is turned off instantaneously, the minimum value of the detection signal at the time of off is detected, the difference from the stored maximum value is calculated, a reference value is obtained and stored in the storage means, and the stored reference value is stored. Then, the ratio of the concentration signal to the concentration signal is calculated to determine the concentration component for which the sensitivity has been corrected, and the carbon dioxide gas concentration is determined from the corrected concentration component and displayed.

In the invention according to claim 2, the light source is turned off at a constant cycle longer than a predetermined respiratory cycle.

In the invention according to claim 3, the light source is turned off in synchronization with the intake air.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a carbon dioxide concentration measuring apparatus according to the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing the configuration of the embodiment of the present invention. FIG. 2 is a flowchart showing a process of calculating the concentration of carbon dioxide in the embodiment of FIG. FIG. 3 is an explanatory diagram for correcting a drifting detection signal with a correction straight line. FIG. 4 is an explanatory diagram of sensitivity correction for a detection signal having changed sensitivity. FIG. 5 is an explanatory diagram in which the light source is turned off at regular intervals in the embodiment of FIG. FIG. 6 is an explanatory diagram of turning off the light source in synchronization with the detection signal at the time of intake in the embodiment of FIG. FIG. 7 shows FIG.
FIG. 13 is an explanatory diagram for turning off the light source when the maximum value at the time of adjacent intake exceeds a predetermined value in the embodiment of FIG. FIG.
FIG. 2 is a waveform diagram of a carbon dioxide concentration obtained by the embodiment of FIG. 1.

Prior to the description of the embodiments, the principle of the present invention will be described. The present invention provides a thermopile (such as S60 manufactured by Dexter Research Center, USA) as a heat detector that detects the amount of heat that changes according to the concentration of carbon dioxide in exhaled gas.
It was used. The thermopile is a conventional Pb
Less drift compared to Se and inexpensive,
It has a unique property and is required to be used in accordance with this property. That is, the response speed required for the carbon dioxide concentration measurement device is 200 ms or less, but the response speed of the thermopile is as slow as 50 ms to 200 ms, so that the response speed of 200 ms or less is achieved by the conventional method of chopping the light from the light source. It is difficult to do.

However, a drift occurs in the detection signal due to, for example, a change in the amount of infrared light of the light source, fogging or contamination of the window of the exhaled gas detector, and the structure of the thermopile itself. Of these, the drift of the detection signal due to the structure of the thermopile itself occurs with a change in the use environment temperature, and thus needs to be corrected. That is, the thermopile has a hot junction and a cold junction, and the detection signal drifts due to a difference in thermal time constant between the two junctions. A hot junction having a small heat capacity responds quickly to a rapid change in the ambient temperature, but a cold junction in thermal contact with the container has a large heat capacity, and thus has a slower response than the hot junction. Therefore, when detecting a signal output according to the temperature difference between the hot junction and the cold junction, drift occurs until the cold junction thermally reaches equilibrium with the ambient temperature.

[0017] Further, since the amount of transmitted light is reduced due to fogging or dirt on the window of the exhaled gas detection unit and the output sensitivity of the thermopile changes, the measured carbon dioxide gas 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 before measuring the carbon dioxide concentration.

In the present invention, when the detection signal drifts due to the structure of the thermopile due to a rapid temperature change, or when the output sensitivity of the thermopile changes due to fogging of the window of the detection unit or fluctuation of the light amount of the light source. In this case, drift correction and sensitivity correction are performed.

Normally, in the output of the thermopile, the maximum value for each inspiration is detected and stored, and the output value in the expiration period reduced by the carbon dioxide gas for each expiration is detected, and the maximum value and the output value at the time of expiration are detected. Is calculated, a density signal is obtained, and drift correction is performed. However, in this drift correction, if the drift is large at the time of recognizing the maximum value at the time of intake, the difference between the maximum values at the time of intake before and after the intake becomes large, which may be discontinuous. Therefore, in the present invention, in order to reduce this drawback, the variation of the maximum value at the time of intake is approximated by a straight line to reduce the discontinuous variation.

FIG. 3 shows a correction straight line for the drift of the detection signal. That is, the maximum value detection points A, B,
C and D are each connected by a straight line, and are extended until the maximum value at the time of the next inhalation is detected as shown by the broken line, and the difference between this correction straight line and the output value at the time of expiration following the maximum value detection time is calculated, This is for obtaining a density signal that changes in a time series.

The correction straight line is obtained as follows.
In FIG. 3, a straight line connecting two points is represented by v = a, where v is the maximum value difference between the two points and v is the time difference between the two points.
It can be calculated by storing t in a memory. For example, as for the straight line connecting the points B and C, two maximum values during inhalation are detected, and those values are stored, and the distance in the horizontal direction is determined by the time difference t between the points B and C, the vertical direction. Distance of B
Since the difference v between points and the maximum value of points can be calculated, the gradient a is (2
The difference v) of the maximum value between the points / (the time difference t between the two points),
A correction straight line can be obtained.

The sensitivity correction is performed as follows. The light source is turned off / on instantly, and the maximum value of the thermopile at the time of intake just before the off is detected and stored.
The minimum value of the thermopile when the light source is turned off is detected. The difference between these maximum and minimum values is defined as the carbon dioxide concentration "0".
And a reference value as the maximum light quantity at that time is obtained and stored. This reference value is obtained and updated each time the light source is turned off / on.

Further, the ratio between the previously obtained and stored reference value and the concentration signal obtained for each exhalation is calculated, the sensitivity of the concentration signal is corrected, the concentration component is determined, and the carbon dioxide gas concentration is determined based on the concentration component. Is calculated. The ratio between the reference value and the concentration signal is constant, for example, even when the detection signal decreases due to a sudden change in the amount of light due to the dirt on the window of the thermopile, provided that the carbon dioxide gas concentration is the same. Therefore, even if the output sensitivity of the thermopile changes by turning off / on the light source during the measurement of the carbon dioxide gas concentration, the reference value is obtained and the ratio with the concentration signal is calculated. Since the sensitivity correction is performed to determine the concentration component and the carbon dioxide concentration can be determined from the concentration component, stable measurement can be performed.

Referring to FIG. 4, a case in which the sensitivity of the detection signal of the thermopile is corrected based on the above principle will be described. The maximum value Va at point A at the time of intake immediately before the light source is turned off is detected and stored in the memory, and the minimum value Vb, which is the offset voltage of the thermopile when the light source is turned off, is detected. Next, the difference between the maximum value Va and the minimum value Vb is determined as (Va-Vb) as the reference value Vo at the carbon dioxide gas concentration "0" and the maximum amount of received light at that time, and is stored.

Next, 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 expiration is detected. The difference between the maximum value Vc and the output value Vd is calculated, and the density signal Vx is calculated as (Vc−V
Determined from d).

Then, a ratio (Vx / Vo) of the stored reference value Vo and the density signal Vx calculated for each exhalation is obtained, and the density component is obtained by performing sensitivity correction of the density signal based on this ratio. The carbon dioxide concentration is calculated from the concentration component.

The density component can be obtained in the same manner when the output sensitivity of the thermopile decreases due to a decrease in the amount of light due to a stain on the window or the like. That is, when the amount of received light changes and the output of the thermopile decreases, the maximum value Ve of the point E of the thermopile during intake is detected and stored, and the offset voltage of the thermopile at the point F when the light source is turned off. Are detected, and the reference value V01 at the maximum light quantity at the time when the carbon dioxide concentration is "0" is detected.
(Ve−Vf) and stored. G during inspiration
The maximum value Vg of the point and the output value Vh of the decrease point H due to the carbon dioxide gas at the time of subsequent exhalation are detected, and the concentration signal Vx1 is calculated as (Vg-V
h). Then, a ratio Vx1 / V01 between the reference value V01 and the concentration signal Vx1 is calculated, the sensitivity of the concentration signal is corrected based on the ratio, a concentration component is obtained, and a carbon dioxide gas concentration is calculated based on the concentration component.

As described above, even when the amount of received light decreases, the ratio between the reference value and the concentration signal when the carbon dioxide gas concentration is the same is constant, that is, Vx / V0 = Vx1 / V01 (FIG. 4). The light source is turned on / off to obtain a ratio between the reference value and the density signal, and the density signal is sensitivity-corrected based on the ratio to obtain a density component, and the carbon dioxide gas concentration can be calculated based on the density component. That is, by obtaining this ratio, an accurate carbon dioxide gas concentration can be obtained even if the output of the thermopile changes in output sensitivity due to a change in light amount or the like.

In FIG. 1, T is a ventilation pipe through which exhaled gas and inspired gas flow, and windows W1 and W2 made of a transparent member such as sapphire are formed at opposing portions at predetermined positions. One end (left side in the figure) of the ventilation tube T is an insertion end to be inserted into the mouth of the subject, and the other end (right side in the figure) is an open end to the atmosphere. The windows W1 and W2 are subjected to anti-fogging processing for preventing fogging due to water vapor or the like in the exhaled gas. A light source 1 such as a lamp is arranged near the upper portion of the window W1 and irradiates the window W1 with light. A heat detector 2 made of the above-mentioned thermopile is arranged near the lower portion of the window W2, and detects infrared rays transmitted from the light source 1 through the windows W1 and W2. Also, 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 provided.

Reference numeral 3 denotes a light source driving unit formed of, for example, a constant current circuit, which 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 off / on by a control signal output from a control unit 6 described later.

Reference numeral 4 denotes an amplifier (for example, a logarithmic amplifier) for amplifying the detection signal of the heat detector 2, and reference numeral 5 denotes an analog-digital converter for converting the output of the amplifier 4 into a digital signal. The control unit 6 includes, for example, a CPU.
The apparatus is controlled based on a control program for measuring the concentration of carbon dioxide stored in the OM 9.

Reference numeral 7 denotes an operation unit including, for example, a plurality of push buttons. The operation unit 7 sets parameters such as a set period for ON / OFF cycle period of the light source 1 and an output judgment of the heat detector 2, and sets required data. Do.

Reference numeral 8 denotes a RAM which temporarily stores and holds set parameters, a maximum value detected when the heat detector 2 is inhaled, a reference value calculated when the light source is turned off, measured carbon dioxide concentration data, and the like. I do. Reference numeral 9 denotes a ROM in which a control program for automatically performing the drift correction and the sensitivity correction on the detection signal of the heat detector 2 according to the principle of the present invention and automatically measuring the carbon dioxide gas concentration is stored in advance.

Reference numeral 10 denotes a display unit comprising, for example, a plurality of light emitting elements such as LEDs (light emitting diodes) or an acoustic element such as a buzzer, for performing a bar graph display of the measured carbon dioxide gas concentration in accordance with a change in the concentration, or a buzzer. To notify a modulated sound corresponding to the density change. Alternatively, both an LED and a buzzer can be provided. By equipping both, it is possible to monitor the respiratory state of the subject both visually and audibly.

Next, the operation of the above configuration will be described with reference to the flowchart of FIG. At the start of the measurement, the light source 1 is turned on simultaneously with turning on a power switch (not shown) (step S1). The transmitted light due to the change in the concentration of carbon dioxide gas accompanying the respiration entering and exiting through the insertion end of the ventilation tube T inserted into the subject's mouth is received by the heat detector 2, and the detection signal of the heat detector 2 increases. The maximum value of the heat detector 2 at the time of intake is detected from the detection signal and stored in the RAM 8 (step S2). The maximum value can be detected by calculating a difference value between data before and after the detection signal of the heat detector 2 on the time axis, for example.

A straight line connecting the current and previous maximum values during the intake is calculated, and this straight line is extended until a maximum value during the next intake is detected to be a correction straight line (step S3).

The output value at the time of expiration is detected from the detection signal subsequent to the current maximum value detection time at the time of inspiration, and the difference from the correction straight line obtained at step S3 is calculated to obtain a time-varying density signal. (Step S4).

Next, the light source 1 is momentarily turned off (step S).
5) Find the difference between the minimum value of the heat detector 2 when the light source is off and the maximum value at the time of inhalation immediately before the light source is off, and store it as a reference value for the carbon dioxide concentration of “0” and the maximum amount of light received at that time. (Step S6). The ratio between the reference value and the concentration signal at the time of expiration is calculated, and the sensitivity of the concentration signal is corrected to obtain a corrected concentration component (step S7). Based on this concentration component, the carbon dioxide gas concentration is calculated and displayed. The density data is sent to the device 10 and displayed (step S8). When displaying, when the display device 10 is configured by a conventional bar graph display device,
It is displayed as a change in the length of the bar graph according to the carbon dioxide gas concentration waveform shown in FIG.

As described above, in the embodiment described above, the correction straight line connecting the maximum value during intake using the heat detector 2 is extended until the maximum value during the next intake is detected. Calculate the difference of the output value at the time of the following expiration and perform drift correction to obtain the concentration signal, and turn off the light source instantly, and use the difference between the minimum value at that time and the maximum value at the time of the last inspiration as a reference value, Since the carbon dioxide concentration is calculated while performing the sensitivity correction based on the ratio between the reference value and the concentration component, an accurate and stable carbon dioxide concentration can be obtained.

Although the light source is arbitrarily turned off / on in the flowchart of FIG. 2, the light source may be turned off at a constant cycle T as shown in FIG. Period T
May be set in advance in the ROM 9 and stored, or may be set via the operation unit 7. By setting the cycle T in advance, the detection signal of the heat detector 2 can be automatically corrected. The predetermined period can be set according to the ambient temperature, for example, every 30 seconds or every minute.

As shown in FIG. 6, the cycle of turning off the light source 1 may be, for example, for each of a plurality of inspirations or for each respiration (inspiration and expiration). The respiration rate can be set in advance in the ROM 9 and stored, or can be set via the operation unit 7. When the light source is turned off at a constant cycle shown in FIG. 5, the light source may be turned off at the time of expiration, and the reference value V0 is accurately measured by synchronizing with the inspiration at this time,
Measurement becomes more accurate.

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 the light source may be turned off instantaneously when the predetermined value is exceeded.
The light source is turned off when the difference between the predetermined value and the difference between the maximum value at the time of the last intake and the current intake value, for example, exceeds 4 mmHg. In FIG. 7, the maximum values P1, P2 and P
The light source 1 is turned off on the assumption that the distance between 3 and P4 exceeds a predetermined value. 5 and 6, the predetermined value is set, for example, via the operation unit 7, or a value stored in the ROM 9 is read in advance, and the predetermined value is monitored by the control unit 6 to turn off the light source 1. Control behavior. By setting the predetermined value, measurement in an environment where the ambient temperature fluctuates greatly can be performed reliably and stably.

[0044]

As described above, according to the carbon dioxide concentration measuring apparatus of the first aspect of the present invention, the use of a heat detector composed of a thermopile makes it possible to use a chopper (light intermittent) required for a conventional photodetector. This eliminates the need for a mechanical component such as a device and a motor for rotating the device, so that the size of the device can be easily reduced, the robustness can be improved, and the device can be configured at low cost. Further, the sensitivity can be corrected by turning on / off the light source at an arbitrary time.

Further, since both drift correction and sensitivity correction can be performed, the output signal can be reliably and stably protected against a drift in the detection signal due to a rapid change in the ambient temperature and a change in output sensitivity due to clouding of the window. Carbon dioxide concentration can be measured.

According to the second aspect of the present invention, the light source is automatically turned off / on at predetermined intervals, so that output drift due to a rapid change in the ambient temperature and output sensitivity due to fogging of a window or the like are reduced. The change can be quickly corrected, and stable and accurate measurement of the carbon dioxide concentration can be performed.

According to the third aspect of the present invention, drift correction can be performed with high accuracy even if there is a change in the sensitivity of the heat detector due to a change in the ambient temperature or 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 detection signal of the heat detector exceeds a predetermined set value. The gas concentration can be measured.

[Brief description of the 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 in FIG. 1;

FIG. 3 is a diagram showing a correction straight line for performing drift correction according to the embodiment of FIG. 1;

FIG. 4 is a diagram showing an off state of a light source for performing sensitivity correction according to the embodiment of FIG. 1;

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

FIG. 6 is an explanatory diagram for turning off a light source for each intake in the embodiment of FIG. 1;

FIG. 7 is an explanatory diagram for turning off the light source in the embodiment of FIG. 1 when the difference between the maximum values exceeds a predetermined value.

FIG. 8 is a waveform diagram of a carbon dioxide gas concentration obtained by the embodiment of FIG. 1;

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

[Explanation of symbols]

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

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masami Ito 1-31-4 Nishi-Ochiai, Shinjuku-ku, Tokyo Inside Nihon Kohden Kogyo Co., Ltd. (72) Inventor Masayuki Inoue 1-31, Nishi-Ochiai, Shinjuku-ku, Tokyo No. 4 Inside Nihon Kohden Kogyo Co., Ltd. (72) Inventor Tadashi Sugiura 1-31-4 Nishi-Ochiai Shinjuku-ku, Tokyo Nihon Kohden Kogyo Co., Ltd. (58) Field surveyed (Int. Cl. ⁷ , DB name ) G01N 33/497 A61B 5/08 G01N 21/61

Claims (4)

(57) [Claims] An apparatus for measuring carbon dioxide concentration by irradiating a respiratory gas with infrared light and detecting a signal corresponding to the amount of transmitted light, wherein a heat detector for detecting the amount of transmitted infrared light; From the switch means SW for turning on / off, and the detection signal of the heat detector, the current maximum value at the time of intake is detected and stored, and a straight line connecting the current value and the maximum value at the time of previous intake is calculated. Drift correction to obtain a correction line extended until the maximum value at the time of intake is detected, calculate the difference between the detection signal following the current maximum value detection time and the correction line, and obtain a concentration signal that changes in time series Means, instantaneously turn off the light source, detect the minimum value of the detection signal at the time of off, calculate the difference between the stored maximum value, and calculate the difference in carbon dioxide gas concentration “0” at the maximum received light amount at that time. Stored as a reference value, and Control means for calculating a ratio of the concentration signal to obtain a concentration component corrected for sensitivity by calculating a ratio of the concentration signal, control means for obtaining a carbon dioxide concentration based on the concentration component, and storage means for storing the maximum value and the reference value. A carbon dioxide gas concentration measuring device comprising: 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 respiratory 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 a difference between respective maximum values detected at the time of adjacent intake exceeds a predetermined value.