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Atmospheric pressure variations are of little importance with gauging pressures typically encountered in refrigeration and A/C service work. With a change of 2 inHg, the gauge changes by only 1 psi. It is unlikely that system pressures would cause concerns if the reading were correct within several psi.
Repeated pressurization (especially over-pressurization) of a gauge does, however, tend to change the response of the tube or diaphragm, and re-calibration may be required.
The easiest way for a service technician to re-calibrate a gauge is to connect it to a source of known, pure refrigerant, and then adjust the re-calibration screw on the gauge to the appropriate pressure reading based on refrigerant temperature.
The most accurate way to re-calibrate is to connect the gauge to a deadweight tester at a re-calibration facility that has equipment traceable to NIST. This is usually cost-prohibitive for most situations, but may be warranted when total documentation is required.
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