Vacuum Leak Detection Using Piezoelectric Film

A technique for detecting a small leak of gas into a vacuum involves the use of a diaphragm made of a thin film of poly (vinylidene fluoride) [PVDF]. To exploit the piezoelectricity of PVDF for this purpose, both sides of the film are coated with thin, electrically conductive layers that serve as electrodes. Wires connect the electrodes to the input terminals of a buffer amplifier and associated circuitry that measures the voltage induced between the electrodes by the piezoelectric effect in the film. In operation, the diaphragm is moved around in the vacuum in the vicinity of, and facing toward, a suspected leak. When the diaphragm crosses the stream of leaking gas, the pressure of the gas impinging on the diaphragm bends the diaphragm, thereby inducing a voltage. In an experiment, a prototype sensor based on this concept generated a signal of about 60 mV from air leaking into a vacuum through an orifice 10 μm wide at a rate of 0.017 standard cm3/s. The noise floor of the sensor was found to be about 5 mV. It was concluded that even this initial unoptimized sensor should be able to detect leaks somewhat smaller than 0.01 standard cm3/s.