The units in which we measure pressure are often misunderstood or misapplied. An instrument must call out what type of pressure is being measured in order to be accurate. The difference between the types of pressure units is the reference point. Gauge pressure is referenced against the atmospheric pressure at the measurement point. 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. 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.