A quartz reference gauge is a device used to measure vacuum. The working principle is pretty much same to that of a bourdon tube. Here, 2 bourdon tubes are used and a formed into a helix. When a pressure difference between the two occurs, the setup begins to rotate. This rotational deflection is picked up using an optical circuit as show in the figure below.
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The reason for using quartz is that it has good spring characteristics and if the unit is kept at a constant temperature environment, the angular deflection per unit pressure is repeatable. The main disadvantage with the device is that it cannot be used as a vacuum gauge in gases with fluorine content as this erodes quartz.
The rotational deflection is connected into an electronic signal, after it passes through the optical circuit. This electronic circuit is further amplified and then the output is annulled using a servo-control system. The corresponding output is displayed by analogue techniques or counted digitally, which can be directly in pressure units. With a tachogenerator on the servomotor shaft a damping adjustment facility can be provided, if necessary. The device is known to have a resolution of 1 milliTorr for 100 milliTorr full scale reading.