Research Of System About SAW Formaldehyde Gas Sensor

Surface acoustic wave (SAW) is a kind of sound wave spreading along the surface of solid. At present, the generally used SAW devices are based on Rayleigh wave. The spread of the SAW is very sensitive to ambient conditions, therefore, by covering some gas sensitive material on the way of the wave spreading surface, various gas sensors are studied for toxic gas detection purpose. When the gas sensitive material selectively absorbs some specific gases, the transmission characteristic of the SAW will be changed and so does the phase between the transmitter and receiver of the wave. This thesis develops such a kind of SAW gas sensor for detecting the specific gas in tiny concentrations in air.This thesis expatiates on the system of SAW gas sensor and its application for formaldehyde sensor applications. The hardware and software of the system are described in detail in this thesis. Based on the experimental results of the sensors to low concentration formaldehyde and some other interference gases, analysis on precision and dissolution of the sensors is discussed and some advice for further study is also given.In the SAW gas sensor system, the SAW gas sensor is covered with gas sensing film. The oscillating signal generated by the SAW oscillator is amphfied with a RF amplifier, and then transmitted to a frequency mixer whose local oscillating end is connected to the signal from another SAW oscillator without sensing film. The mixer sends compares of these two signals to a lowpass filter. After reshaping and amplifying, the signal is processed by MSP430 microprocessor into digital signal and then sent out by a RF transmit module. On the other end, a receiver can collect the data from the RF module and send it to computer through RS-232 port. The computer transforms and displays the data on the screen.The sensitivity and the selectivity of the SAW formaldehyde gas sensor are deeply correlated to the sensitivity and the selectivity of the gas sensing film. Using the film which has better performance can greatly increase the characteristic of the sensors, so how to select and design more sensitive films is studied and some suggestions for further study are also pointed out by this thesis.