Explanation:

When a pressure gauge is used to measure the pressure of a fluid, the reading obtained is called the gauge pressure. The gauge pressure is the difference between the pressure being measured and the atmospheric pressure. The atmospheric pressure is the pressure exerted by the weight of the atmosphere on the earth's surface. The gauge pressure is expressed in units of psi, kPa, or bar.

Atmospheric Pressure:

Atmospheric pressure is the pressure exerted by the weight of the atmosphere on the earth's surface. It is measured using a barometer. The atmospheric pressure is dependent on altitude and weather conditions. The standard atmospheric pressure at sea level is 101.325 kPa or 14.7 psi.

Gauge Pressure:

Gauge pressure is the pressure measured with the help of a pressure gauge. It is the difference between the pressure being measured and atmospheric pressure. Gauge pressure can be positive when the pressure being measured is greater than atmospheric pressure or negative when the pressure being measured is less than atmospheric pressure.

Absolute Pressure:

Absolute pressure is the pressure measured relative to a perfect vacuum. It is the sum of gauge pressure and atmospheric pressure. Absolute pressure is used in calculations where the pressure in a system is required to be measured relative to a perfect vacuum. It is expressed in units of psi, kPa, or bar.

Mean Pressure:

Mean pressure is the average pressure over a specific period of time. It is calculated by dividing the total pressure by the time period. Mean pressure is used in calculations where the pressure in a system is required to be measured over a specific period of time. It is expressed in units of psi, kPa, or bar.

Therefore, the pressure measured with the help of a pressure gauge is called gauge pressure because it is the difference between the pressure being measured and atmospheric pressure.