A gauge pressure instrument should always read zero when exposed to atmospheric pressure. Standard pressure is a constant value used for calculation purposes. Atmospheric pressure can change with many influences, such as elevation and weather. Absolute pressure is referenced against absolute zero pressure, or a complete vacuum. If you hold an absolute pressure instrument in the open air, the reading should be well above zero, in the range of 14.7 to 12 psia.
Absolute pressure is used when calculating flow and fluid properties.
This is important to understand because gauge pressure is normally converted to an absolute pressure value for calculating purposes. To ensure accuracy, the gauge pressure must be accompanied by the atmospheric pressure at the time of the reading. Lets say 1 psig is being measured with a gauge pressure instrument. This needs to be converted to absolute units. The actual atmospheric pressure though is 12.5 psia.