Date: Winter 2012-2013
Is a "PSIG" gauge always calibrated to 14.7 PSI, or does it very by the atmospheric pressure of your area? Would a PSIG reading in a vacuum read -14.7 PSI?
PSIG is simply a measure of pressure as compared to whatever the
atmospheric pressure is that surrounds the gauge. Simple pressure
gauges are always measuring PSIG. This is best illustrated by
example as follows:
Suppose you are measuring tire pressure using a simple tire pressure
gauge. Any tire gauge measures in PSIG. Let us say that your tire
gauge reads 40 PSI. Normal atmospheric pressure is about 14.7 psi
with respect to a vacuum. So your tire pressure in absolute terms (that
is, tire pressure with respect to a vacuum) is 54.7 PSIA. In other
words, the pressure in the tire is 40 PSI higher than the normal
atmospheric pressure of 14.7 PSI that surrounds the gauge.
Normal atmospheric pressure is all around us, and PSIG is a useful
way to measure how much higher a pressure in a tire, or any other
pressure vessel, is as compared to atmospheric pressure. Normally
you do not care what your tire pressure is as compared to a vacuum,
because you are never going to drive the car in outer space. You only
care how much higher it is than atmospheric pressure..... in other
words, you are only interested in how many PSIG are in the tire.
If the pressure vessel instead contained a vacuum, then the gauge
would read -14.7 PSIG (as you suspected).
Not only is measuring pressure in PSIG generally more useful in
everyday life, it is a lot easier to make a gauge that measures in PSIG.
PSIG gauges are really simple (google "Bourdon Tube"). Gauges that
measure absolute pressure (PSIA), that is, pressure with respect to a
vacuum, are much more expensive and bulky.
So just remember..... simple pressure gauges always measure in
PSIG, which means that they measure how much higher or lower a
pressure in a pressure vessel is, AS COMPARED TO whatever the
atmospheric pressure is around the gauge.
Thanks for the question. Yes, a PSIG is calibrated to atmospheric pressure (or 14.7 PSI). This means that the gauge is measuring the net pressure that a vessel experiences and not the absolute pressure. You are correct in that the atmospheric pressure does vary in different areas. This variation is measurable (and important since it affects weather). Yes, a PSIG reading of an evacuated vessel (vacuum) would be -14.7 PSIG. However, in practice the reading may be -14.5 since some gauges do not response linearly.
I hope this helps. Please let me know if you have any more questions.
The abbreviation PSIG is “pounds per square inch gage” which is the gas pressure above atmospheric pressure (~14.7 PSI). The abbreviation (PSI) is the “pounds per square inch” compared to vacuum – which is “pounds per square inch” less than the gage pressure. The difference depends upon the type of pressure apparatus being used.
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Update: November 2011