Optopneumatic Detector with Platinum Flow Sensor

In optopneumatic detectors, operating on principle of the gaseous infrared detectors, the radiation absorbed in the gas is changed into pressure fluctuation, followed by periodic flow of the gas between detector chambers. This flow is usually measured by means of hot-wire or hot-film anemometers. Their parameters in a great measure determine the quality of the whole detector. The flow sensor used in optopneumatic detectors should have short time constant, high sensitivity, possibility to distinguish the flow direction, and high chemical resistance to the gas which the detector is filled with. To meet these requirements, micro-electro-mechanical system flowmeter consisting of two platinum hot films placed in series in the flow direction was made. Platinum film of 0.3 μm thickness was deposited by magnetron spattering on thin membrane, which was composed of polysilicon substrate (0.5 μm) and two $Si_{3}N_{4}$ layers with thickness of 0.1 μm. The obtained platinum elements have resistance of 32 Ω and temperature coefficient of resistance of 2.31× $10^{-3}$/°C. Hot film does not change its parameters up to 350°C, whereas typical working temperature of such films ranged from 150 to 200°C. Anemometer characteristics and voltage-current dependences were determined in this work. The linear range of operation from 0 to 10 ml/min and sensitivity suitable for optopneumatic detectors were found. The time constant of the flowmeter is equal to 11 ms (95% of final signal), that enables the detector to operate with frequency up to 100 Hz. The flowmeters were applied in detectors filled with sulphur dioxide and were tested for six months at temperature of hot-film about 150°C. During that time no changes of their parameters were observed.