In order to achieve the accurate measurement of gas stable isotope, according to the structure of gas cavity and the characteristics of optical elements, a constant temperature and pressure gas isotope measurement experimental system based on tunable diode laser absorption spectroscopy (TDLAS) technology is designed, which can achieve high-precision and stable control of temperature and pressure in the box. The hardware circuit system including high-precision constant-current source, two-channel temperature sampling module, conditioning circuit and pressure control circuit is designed in the system. The corresponding system closed-loop control software is developed to ensure the stability of the temperature and pressure of the optical cavity. Through the experimental verification, the temperature is controlled at 35ºC and the pressure is controlled at 150 mbar in the present work. The measurement results verified the stability of the temperature and pressure system, as follows: the optimal temperature fluctuation control is ±0.005ºC and the pressure stability is ±0.1 mbar. Then, in our experiment, the CH4 standard gas is introduced to measure carbon isotope (12CH4 and 13CH4). The results clearly show that the control system is sufficiently stable to be used and the δ13C value can reach 0.52‰ according to the Allan variance.

At present, gas monitoring technology based on optical principle has been widely used in atmospheric environment, industrial emissions, smart city and other monitoring fields. For the requirement of high precision and high stability control of temperature and pressure of laser absorption spectroscopy measuring instrument, the temperature and pressure control scheme of the system is studied. The temperature and pressure control system is identified and the simulation model is established. The consistency of temperature and pressure models is 99.35% and 95.55% by using Process Models module to debug continuously. The simulation compares the output response curves of different control methods with the same step signal input, and the Fuzzy Adaptive PID parameter control algorithm achieves the design requirements in response time, overshoot and other aspects.