Two-wavelength type respiratory gas concentration measuring apparatus

A two-wavelength type respiratory gas concentration measuring apparatus comprising:

• a connector tube equipped with an open tube end to be held in the mouth of an examinee and an opposite open end and airtight windows provided between both ends of said connector tube;
• a light source adapted to apply infrared rays in a path passing transversely through said connector tube by way of said windows;
• a chopper having first and second filters situated thereon and adapted to alternately position in said light path the first filter which allows the transmission of a light component whose wavelength is such that it is absorbed by a particular gas and the second filter which allows the transmission of a light component whose wavelength is such that it is not absorbed by said particular gas;
• a photodetector including a PbSe infrared-ray sensor adapted to convert the infrared rays transmitted through said filters to an electric signal by means of the PbSe infrared-ray sensor;
• a first detector adapted to detect the output of said first filter from said electric signal;
• a second detector adapted to detect the output of said second filter from said electric signal;
• a first power computing circuit adapted to perform the power computation of the output of said second detector with a power exponent m;
• a divider circuit for dividing the output of said first detector by the output of said first power computing circuit;
• a DC level detector for determining the DC level of said photodetector when no infrared rays are applied;
• a second power computing circuit for performing the power computation of the output of said DC-level detector with a power exponent of 2(m-1);
• and
• a multiplier circuit for multiplying the output of said second power computing circuit by the output of said divider circuit;
• said power component m representing the ratio of a temperature-dependent coefficient .alpha..sub.1, with respect to the incident light quantity of the light component of the infrared rays whose wavelength is absorbed by the particular gas, of the variation in resistance, which exponentially fluctuates with respect to temperature changes, of the PbSe infrared-ray sensor to a temperature-dependent coefficient .alpha..sub.2, with respect to the incident light quantity of the light component of the infrared rays whose wavelength is not absorbed by the same particular gas, of the variation in resistance, which exponentially fluctuates with respect to temperature changes, of the PbSe infrared-ray sensor, whereby said power exponent m and said temperature-dependent coefficients .alpha..sub.1 and .alpha..sub.2 are in the relationship:
• ##EQU10## said photodetector circuit supplying a constant voltage to said PbSe infrared-ray sensor, and detecting the current flowing through said infrared-ray sensor as an infrared-ray detection signal.