The effect of temperature, mechanical alignment issues, a dysfunctional liquid-fill vent, or mechanical damage will cause zero shift in a mechanical pressure gauge.
The solution to a clogged liquid-fill vent is to fix the vent. Overpressuring a mechanical gauge to the point of permanent deformation ruins the gauge; it does not return to zero after venting the pressure, nor will zero adjustment compensate for mechanical deformation.
Temperature effect and mechanical alignment zero shift are corrected by zero adjustments.
Ametek's Pressure Gauge Handbook (1985)
Effect of Temperature - Zero Shift
Temperature extremes introduce an additional error called "zero shift, " which is caused by the physical change in the dimensions in the various parts of the gauge with temperature. The magnitude of and direction of this error vary with gauges of different designs and are functions of the relative thermal expansion of all of the various parts of the gauge, involving materials, methods of attachment, and dimensions. Zero shift is a constant shift over the entire scale regardless of whether or not pressure is applied to the gauge, and the magnitude of the shift does not vary with applied pressure as it does with range shift.
Zero Shift Error
Zero shift is an error of constant value over the entire span and may be either a plus or minus value. This condition may be caused by a shift of pointer position on the pinion, rotation of the dial, or a change in the tip location of the bourdon (tube).
Correction
Compensation for this condition is accomplished by simply repositioning the pointer or, on gauges with adjustable dials, adjusting the dial to agree with the indication of the standard. Adjustment of the dial may be accomplished by turning a knob, which usually protrudes through the case front. Some gauges are provided with dials adjustable by means of slotted mounting holes. This method necessitates removing the bezel and window, loosening the dial mounting screws, and carefully resetting the dials.
