JP3273298B2 - 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 gas concentration measuring device provided with such a conventional drift correction device. 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. Furthermore, in the conventional apparatus, correction of drift was taken into consideration, but correction of a change in sensitivity of a detection signal with respect to a change in absolute light amount in the detection unit was not performed. Accordingly, the present invention has been made in view of the above problems, and provides a carbon dioxide concentration measuring apparatus capable of performing drift correction and sensitivity correction of a detection signal without using a mechanism for continuously interrupting light required for a photodetector. The purpose is to do. It is still another object of the present invention to provide an apparatus capable of measuring a carbon dioxide gas concentration more accurately by correcting sensitivity in addition to drift correction.

[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 maximum value during the current intake is detected and stored.From the detection signal of the heat detector, the maximum value during the current intake is detected and stored, and before the maximum value is detected during the next intake. When the maximum value of the detection signal exceeds the stored maximum value, the maximum value is updated and stored, the stored maximum value is read out in chronological order, processed by a Kalman filter, and a correction value is output. Drift detection means for calculating a difference between the detection signal and the detection signal, and a light source is momentarily turned off to detect a signal minimum value of the detection when the light source is off, and to store the maximum value stored therein. Calculate the difference from the Sensitivity correction means for storing a reference value at the maximum received light amount at that time as "0", calculating a ratio between the reference value and the density signal, correcting the sensitivity of the density signal and calculating a density component, It comprises a control means for obtaining the carbon dioxide concentration based on the concentration component, and a storage means for storing the maximum value and the reference value.

According to a second aspect of the present invention, in the carbon dioxide concentration measuring apparatus 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 device according to the first aspect, the light source is turned off when the correction value changes beyond a predetermined range.

[0011]

According to the first aspect of the present invention, the maximum value at the time of current inspiration is detected from the detection signal detected by the heat detector after receiving the infrared light transmitted through the breathing gas and stored in the storage means. Before detecting the maximum value at the next intake, heat detector 2
If the maximum value of the detection signal exceeds the stored maximum value,
The stored maximum value is updated and the larger maximum value is stored. The maximum value stored in this way is read out in time series, and a Kalman filter process is performed to obtain a correction value. Drift correction is performed to calculate a difference between the correction value and the detection signal to obtain a density signal that changes in a time series. 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, and the carbon dioxide concentration “0” is used to obtain a reference value at the maximum received light amount at that time. Remember. The ratio between the reference value and the concentration component is determined, and the sensitivity of the concentration component is corrected based on the ratio to determine the carbon dioxide concentration.

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

In the invention according to claim 3, 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 correction value has changed beyond a predetermined range.

[0015]

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 value. 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. 9 is an explanatory diagram for turning off the light source when the correction value in the embodiment of FIG. 7 exceeds a predetermined range from the sensitivity correction reference value.
FIG. 8 is a waveform diagram of the carbon dioxide gas concentration obtained by 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 for detecting the amount of heat that changes according to the concentration of carbon dioxide in exhaled gas. Although the thermopile has less drift and is inexpensive than PbSe conventionally used, 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 from the light source, fogging or dirt on 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.

Further, since the amount of transmitted light is reduced and the output sensitivity of the thermopile changes due to fogging or dirt on the window of the exhaled gas detector, 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 gas 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, this drift correction is effective when the drift fluctuates gently. However, when the drift fluctuates relatively quickly, a discontinuous point occurs and sufficient correction cannot be performed.

Therefore, in the present invention, by introducing a correction process using a Kalman filter, drift correction can be performed using a Kalman filter that can sufficiently follow the fluctuation even if the drift varies rapidly.
It is known that the Kalman filter can be processed in real time and has good followability of renewed data.

The drift correction of the thermopile output based on the correction value output by the Kalman filter shown in FIG. 3 will be described. In FIG. 3, the maximum values detected at the time of intake are P1, P2, P3 and P4, for example, P2
The maximum value at the time of inhalation of the detection signal of the thermopile detected at the time of is taken as Vm (point on the solid line in the figure). The correction value VI output as the optimal estimator of the Kalman filter indicated by the broken line in FIG.
Can be calculated by the following equation using the maximum value Vm as input data.

VI (n + 1) = VI (n) + (Vm−VI (n)) / B (n + 1) (1)

Here, B (n + 1) is expressed by the following equation.
B (n + 1) = (1 + α · B (n)) Here, the Kalman coefficient is determined in advance, and the correction characteristic of the filter changes depending on the value of α. VI (n + 1) represents the filter output at the present time, and VI (n) represents the filter output at the previous time. Vm is the maximum value of the detection signal at the present time.

That is, when the current maximum value Vm of the thermopile is inputted, the correction value VI (n + 1) of the Kalman filter of the above equation (1) is obtained.

This correction value is expressed by a broken line in FIG.
Asymptotically following the maximum value input in time series detected during inspiration. Therefore, a concentration component corresponding to the concentration of carbon dioxide can be obtained by calculating the difference between the correction value and the detection signal.

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".
Is obtained and stored as a reference value at the maximum light quantity at that time. This reference value is obtained and updated each time the light source is turned off / on.

Further, the ratio of the previously obtained and stored reference value to the concentration signal obtained for each exhalation is determined, the sensitivity correction of the concentration signal is performed, the concentration component is calculated, and the carbon dioxide concentration is calculated by the concentration component. Calculate. The ratio between the reference value and the concentration signal is constant if the concentration of carbon dioxide is the same, 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. 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, description will be given of a case where the sensitivity of the detection signal of the thermopile is corrected based on the above principle. 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 at the time of 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. Maximum value Vc
And the output value Vd, the density signal Vx is calculated as (Vc−
Vd).

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 of the density signal is calculated based on the ratio to obtain a density component. The carbon dioxide concentration is calculated from the concentration component.

The density component can also 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, the 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, the concentration component is obtained, and the 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 subjected to sensitivity correction based on the ratio to obtain a density component, and the carbon dioxide 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 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 composed 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-to-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 determination of an output 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, and measured carbon dioxide concentration data. 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.

Numeral 10 denotes a display unit comprising a plurality of light emitting elements such as LEDs (light emitting diodes) or an acoustic element such as a buzzer. The display unit 10 displays the measured carbon dioxide gas concentration in a bar graph according to the change in the concentration. 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. Is recognized as intake air, the maximum value of the heat detector 2 at the time of current intake is detected from the detection signal, and 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.

If the maximum value of the detection signal of the heat detector 2 exceeds the maximum value stored in step 2 before detecting the maximum value at the next intake, the stored maximum value is updated. The maximum value exceeding this maximum value is stored in the RAM 8 (Step S3).

The control unit 6 sequentially reads out the stored maximum values in a time-series manner, performs the above-described processing by the Kalman filter, and outputs a correction value. (Step S4).

The difference between the detection signal of the heat detector 2 following the current maximum value detection point and the correction value obtained in step S4 is calculated to obtain a time-varying density signal (step S5).

In the ordinary processing, the process proceeds from step S5 to step S6 described below. Steps S20 and S21 surrounded by a dashed line will be described later.

Next, the light source 1 is turned off instantly (step S
6) 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 at the carbon dioxide concentration of “0” and the maximum amount of light received at that time (Step S7). 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 S8). Based on this concentration component, the carbon dioxide concentration is calculated and displayed. The density data is sent to the device 10 and displayed (step S9). When displaying, when the display device 10 is configured by a conventional bar graph display device, the carbon dioxide concentration is displayed as a change in the length of the bar graph according to the carbon dioxide concentration waveform shown in FIG.

As described above, in the embodiment described above, the Kalman filter is introduced to perform drift correction for correcting the detection signal by following the drift of the heat detector 2 caused by a rapid change in the ambient temperature, and to change the light source. It is turned off instantaneously, the difference between the minimum value at that time and the maximum value at the time of the immediately preceding intake is set as a reference value, and a sensitivity component is corrected by a ratio between the reference value and the density signal to obtain a density component. Since the gas concentration is calculated, an accurate and stable carbon dioxide gas concentration can be obtained.

Although the light source is arbitrarily turned off / on in the flowchart of FIG. 2 described above, the light source may be turned off at a constant cycle T longer than the respiratory cycle as shown in FIG. The cycle 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 period shown in FIG. 5, the light may be turned off at the time of expiration, and the detection data at this time cannot be used. However, by synchronizing with the inspiration, the respiration data is surely corrected, and the measurement becomes more difficult. Become stable.

In the case shown in FIG. 6, after the maximum value is detected,
It is turned off after a predetermined time. This is because when detecting the maximum value from the detection signal at the time of intake, the maximum value is obtained from the magnitude of the detection signal before and after the detection, so a predetermined time is set to reliably detect the maximum value I do.

As shown in FIG. 7, when the variation of the correction value, which is the output of the Kalman filter, is greater than a predetermined range, the correction value at the time when the variation exceeds the predetermined range is stored as a sensitivity correction reference value. Then, the light source 1 can be turned off when it exceeds a predetermined range from the sensitivity correction reference value. The processing is performed by adding steps S20 and S21 in the flowchart of FIG. 2 described above. That is, when the density signal is obtained based on the correction value drift-corrected in step S5 of FIG. 2 described above, it is determined whether the correction value has exceeded a predetermined range, for example, 4 mmHg, from the sensitivity correction reference value (step 20). If it exceeds, the next step S6 (FIG. 2)
Then, the light source 1 is turned off. In this case, the correction value at the time of turning off the light source is stored in the RAM 8 as a new sensitivity correction reference value (step S21), and the process returns to step S20. Also,
If it is determined in step S20 that the correction value does not exceed the sensitivity correction reference value, the process returns to step S20 to monitor the sensitivity correction reference value.

[0052]

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

According to the second aspect of the present invention, by automatically turning off / on the light source at predetermined intervals, the output sensitivity due to the rapid fluctuation of the ambient temperature and the output sensitivity due to the fogging of the window and the like can be 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, respiration 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 clouding of the window of the ventilation pipe.

According to the present invention, the drift is corrected by using the Kalman filter.
Correction can be made to follow a rapidly changing drift, and accurate measurement of carbon dioxide concentration can be performed even in an environment where the ambient temperature changes drastically.

[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 illustrating an output of a Kalman filter that performs 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 of turning off the light source in the embodiment of FIG. 1 when the sensitivity correction reference value changes by a predetermined value or more.

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 maximum value at the time of the current intake is detected and stored, and the maximum value of the detection signal is stored before the detection of the maximum value at the next intake. If the maximum value is exceeded,
The maximum value is updated and stored, the stored maximum value is read out in chronological order, processed by a Kalman filter, a correction value is output, and the difference between the correction value and the detection signal is calculated to change in chronological order. Drift correction means for obtaining a concentration signal to be turned off, a light source is momentarily turned off, a minimum value of the detection signal at the time of off is detected, a difference from the stored maximum value is calculated, and a carbon dioxide gas concentration of "0" is calculated. Sensitivity correction means for storing a reference value at the maximum received light amount at that time, calculating a ratio between the reference value and the density signal and correcting the sensitivity of the density signal to calculate a density component; A carbon dioxide concentration measuring device comprising: control means for obtaining a carbon dioxide concentration based on the above; and 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 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 the correction value changes beyond a predetermined range.