Method and apparatus for spectroscopic measurement of the concentration of a gas

A spectroscopic method to measure the concentration of a gas in a gas sample, the method comprising the steps of:

• intensity modulating a light of a laser diode and frequency modulating the light by performing stepwise modulation with a modulation signal;
• passing the modulated light through the gas sample along a measurement path and through a reference cell of a predetermined pressure and concentration;
• detecting intensity of the light passed through the gas sample and the reference cell and generating a measurement signal and a reference signal, respectively, representative thereof;
• automatically compensating the measurement signal for nongasrelated transmission variations in the measurement path by a first and second demodulating step, said first demodulating step comprising the steps of:
• forming a first difference signal between a monitoring signal representative of the modulated light and the measurement signal,
• forming a second difference signal between the monitoring signal and said reference signal, and
• automatically adjusting a first and second gain level of the measurement signal and the reference signal such that the first and second difference signals are equal to zero with respect to an amplitude of a fundamental frequency of the modulation signal,
• said second demodulation step comprising the step of:
• sampling the measurement signal and reference signal at corresponding points on an absorption spectrum after compensation of the measurement and reference signal,
• receiving the sampling of the reference and measurement signal and generating a concentration signal which is indicative of the concentration of the gas in the gas sample.

An apparatus for spectroscopic measurement of a concentration of a gas in a gas sample, comprising:

• means for intensity and frequency modulating a light of a laser diode by a modulation signal to lock the light to an absorption line of a gas within a reference cell of a predetermined pressure and concentration;
• means for distributing the modulated light through the gas sample along a measurement path and through the reference cell;
• means for detecting the intensity of the light passed through the gas sample and the reference cell and for generating a measurement signal and a reference signal, respectively, representative thereof;
• means for automatically compensating the measurement signal for nongasrelated transmission variations in the measurement path, the compensation means comprising:
• means for forming a first difference signal between a monitoring signal from a monitor diode representative of the modulated light from said laser and the measurement signal,
• means for forming a second difference signal between the monitoring signal and said reference signal, and
• means for automatically adjusting a first gain level of the measurement signal and a second gain level of the reference signal such that the first and second difference signals are equal to zero with respect to an amplitude of a fundamental frequency of the modulation signal;
• means for detecting the harmonics of the measurement signal and reference signal with respect to the fundamental frequency of the modulation signal;
• and
• means for receiving the harmonics of the compensated measurement signal and reference signal and for generating a signal which is indicative of the concentration in the gas sample.