Which statement about gauge, absolute, and differential pressure is correct?

Which statement about gauge, absolute, and differential pressure is correct?

• A. Gauge pressure is always greater than absolute pressure.
• B. Absolute pressure is the pressure relative to atmospheric pressure.
• C. Differential pressure is the difference between two pressures.
• D. Gauge pressure is the sum of absolute and atmospheric pressures

Answer: (C)

Understanding these pressure types helps see why the statement about differential pressure is correct. Absolute pressure is the total pressure relative to a perfect vacuum (zero). Gauge pressure is what a typical pressure gauge shows and is the pressure relative to atmospheric pressure. Because gauge pressure equals absolute pressure minus atmospheric pressure, gauge pressure isn’t inherently always greater than absolute pressure; it depends on the actual numbers. The idea that gauge pressure is the sum of absolute and atmospheric is not correct, since the proper relationship is gauge = absolute − atmospheric. Also, absolute pressure is not defined relative to atmospheric pressure; it’s defined relative to vacuum. The key idea here is that differential pressure is simply the difference between two pressures, which is exactly what this option states. For example, if one side is 16 psi absolute and atmospheric is 14.7 psi, the gauge reading would be 1.3 psi, and the difference between two pressures is the differential pressure, which is what DP measurements use.

Understanding these pressure types helps see why the statement about differential pressure is correct. Absolute pressure is the total pressure relative to a perfect vacuum (zero). Gauge pressure is what a typical pressure gauge shows and is the pressure relative to atmospheric pressure. Because gauge pressure equals absolute pressure minus atmospheric pressure, gauge pressure isn’t inherently always greater than absolute pressure; it depends on the actual numbers. The idea that gauge pressure is the sum of absolute and atmospheric is not correct, since the proper relationship is gauge = absolute − atmospheric. Also, absolute pressure is not defined relative to atmospheric pressure; it’s defined relative to vacuum. The key idea here is that differential pressure is simply the difference between two pressures, which is exactly what this option states. For example, if one side is 16 psi absolute and atmospheric is 14.7 psi, the gauge reading would be 1.3 psi, and the difference between two pressures is the differential pressure, which is what DP measurements use.