What is Gauge Pressure ?

Gauges often measure pressure in pounds per square inch (psi). You may also see gauge pressure represented in psig, where the “g” indicates that the measurement is from a gauge rather than another source, like atmospheric pressure. Many gauges display pressure in two units: psi and bar. Bar is the metric unit for measuring pressure.

Absolute pressure is the total pressure measured from zero or in relation to a perfect vacuum. Gauge pressure doesn’t account for the total pressure, because it doesn’t measure atmospheric pressure. To determine the absolute pressure, you must add the gauge and atmospheric pressure. Here’s the formula:

P_abs = P_g + P_atm

• P_abs: Absolute pressure
• P_g: Gauge Pressure
• P_atm: Atmospheric Pressure

For example, your air compressor gauge might read 110 psi. The average atmospheric pressure at sea level is 14.7 psi. If you operate your air compressor at sea level, the absolute pressure would be 124.7 psi. Absolute pressure is the most accurate measurement of the total pressure within your system and is essential when analyzing performance or making precise engineering calculations.

Air compressor gauges work very similarly to a vehicle speedometer. The gauge is circular, with markings around the outside edge. Lines divide the markings into smaller increments. As the pressure in the system rises, a needle on the gauge moves. You can determine the pressure within the system by looking at the tip of the needle.

In everyday operations, the gauge pressure is usually sufficient to determine whether your air compressor is running at the proper pressure for your application. However, some scenarios may require you to add the pressure from the gauge to the atmospheric pressure to determine absolute pressure.

Like many instruments, a pressure gauge may drift over time, becoming less accurate than initially. Gauges need calibration before installation and at least annually during other maintenance checks. The International Organization for Standardization (ISO) calls for yearly audits. You can test your gauge using the following steps:

1. Gather a pressure hand pump, the digital gauge, the pressure gauge you want to test and adaptors to change the threading.
2. Attach the digital indicator to the top of the pressure pump and the gauge you want to test to the adaptor on the pump’s end.
3. Close the pump’s air escape valve, turn on the digital gauge and slowly add pressure. If your pressure gauge is accurate, it should display the same pressure as the digital gauge.
4. Test the pressure at four or more set points to determine your gauge’s accuracy at different pressures.

This testing process tells you whether your gauge is accurate. If the indicator fails the test, you may need to replace it or send it to a certified pressure gauge calibration expert.