Development of handheld SAW vapor sensors for explosives and CW agents

This article reports about the development of various discrete (single sensor) surface acoustic wave (SAW) handheld sensor systems, for detection and quantification of explosives and chemical warfare agents (CWA). The sensing element is a SAW device of delay line, resonator or dispersive delay line type with frequency of operation ranging from 36 MHz to 434 MHz. The SAW devices are coated with various polymers having good selectivity to explosives and CWA. Oscillator with the SAW device in the feedback loop as a frequency determining element is used. Dual oscillator configuration with one coated and one uncoated SAW device is used. The outputs of the oscillators are mixed and signal conditioned before frequency measurement. The frequency measurement is carried out by a high-speed high-resolution reciprocal counting method using microcontroller-based readout circuitry. Data acquisition software written in Visual Basic performs online display and storage. The performance of various parts and the sensor systems is analyzed for their stability, resolution, accuracy and sensitivity.