IR-gas analyzer for humidity measurement in incubators

Selective humidity measurements in IR with a measuring wavelength of 1.85/spl mu/m, a chamber length of 20cm and an accuracy of 2%rH in the range between 70%rH and 100%rH at temperatures between 30/spl deg/C and 50/spl deg/C require the measurement of radiation absorption with a reproducibility of about 0.5% The drift in intermittent operated devices can reach 5%/min in the radiation signal and 0.3%/min in a ratio signal using measuring and reference wavelengths. Because it is to be found that the drift is nearly complete a zero drift a method for online correction of the zero point has been developed. The measuring process is divided into three stages: - a "zero" measurement with ambient air, whereas the humidity content of the air, which is determined with a capacitive sensor, is used to re-calculate the IR zero point; the actual measurement step and a flushing step. The complete measuring and calibration scheme allows mutual parameter corrections between the IR-optic and the capacitive humidity sensor.