The phenomenon of decrease of resonant frequency of an AT-cut quartz crystal vibrating in thickness shear mode, due to the deposit of foreign materials on its surface, has been utilized for instantaneous thickness determination of vacuum evaporated thin films. The resonance frequency shift has a nearly linear relationship with the thickness deposited. The factors governing the possible extent of deviation from linearity are discussed. The instrument needs calibration only at a few points. Actual calibration has been made for several commonly used metals like Ag, Cu and Au by means of multiple beam interferometer. The instrument being mass-sensitive can measure masses from a minimum of less than one microgram to the maximum of a few miligrams. Several factors regarding the choice of the crystal, depending on the range of the thickness to be monitored, have been discussed. The frequency shift due to the absorption of moisture by the deposited film can be made use of in metering humidity. Possibility of such a use has been observed during experiment.
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