Why Zero Out A Manifold Gauge?
In a RSES tech tip I read Quoted directly from Ritchie Engineering about analog gauges "Why Zero Out A Manifold Gauge?"
It said a gauge can get off 0 due to change in altitude or barometric pressure. It is important that you set it back to 0 before charging.
FWIW A bourdon tube analog gauge is not affected by by altitude or barometric pressure. If your open to ATM gauge is reading 0 at sea level and you take it up to 10,000 ft open to ATM it will still read 0. Additionally if you are using the PT chart printed on the gauge face for charging or troubleshooting at altitude with the gauge open to ATM you should adjust the needle off 0 and set it to the equivalent psia of the environment you are in.