Why do we need atmospheric pressure in different pressure gauges?

Question

I want to know, how a pressure gauge give reading , gauge pressure and atmospheric pressure together

Answer 1

THere are 3 different types of pressure measurements.

1. Absolute pressure: pressure compared with a vacuum

2. Gauge pressure: pressure compared with the atmosphere

3. Differential pressure: one pressure compared with another pressure on the other side of the measuring diaphram.

Sometimes the gauge pressures are measured against an opening to the atmosphere, so if there is a low-pressure weather front in the area, the gauge pressures will read higher than if it is a nice day outside (high atmospheric pressure).  Also, gauge pressure is dependant on altitude.  The higher you are in altitude, the higher the gauge pressure will read (assuming all other process variables are equal for both altitudes low, and high).

Sometimes a standard atmosphere of air is trapped inside the gauge and it is closed off from the outside.  So the gauge pressure will always be compared with a standard atmosphere (approx. 14.5 psi).  This will not vary with the weather, or with altitude.

For very high pressure measurements (>10,000 psi) it really doesn't matter if you are measuring the pressure against 0 psi, 13.8 psi, or 14.7 psi.  The instrument won't be accurate enough to distinguish the difference of the reference pressure.

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