Tube and spring pressure gauges
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Tubular spring pressure gauges are the most commonly used pressure and vacuum gauges in industry due to their simplicity, reliability, compactness and low cost.
The main element of these devices is a single-coil tubular spring (1) bent along the arc of a circle with a radius R of ellipsoidal or planar-oval cross-section (Fig. 1). One end of the tube is rigidly fixed in the holder (2), through the nipple of which pressure is supplied to the inner cavity of the tube, and the other end is free.
When pressure is applied to the tube, its radius increases and it straightens; when it is depressurized, its radius decreases and it twists. Under the action of pressure P, the free end of the tube moves by the amount A, and through the lever (3), the toothed sector (4), the tribocage (5) moves the arrow of the device (6), which is located on the axis of the tube. 
