Gauge Pressure vs. Absolute Pressure

This thread should be shut down you have given not just a good answer but the "best answer" thanks for the clarity.

Mike

When we talk about gauge (in measurement) it is comparison with a standard - eg micrometer (in fact in my school days we used to call it a micrometer screw gauge) - in case of pressure gauges - the gauge pressure - is usually with respect to a standard pressure - most commonly atmospheric pressure.

Thus 1 atmospheric pressure will show a 0 value on these gauges.

In case of the absolute pressure - it is with respect to the absolute vacuum pressure and 1 atmospheric pressure will show a value of 1.01 kg/cm2 (1 bar) on these gauges.

Pl note in these both cases it is a pressure Gauge. In case of non gauge (eg a few transmitters etc, the value is not compared and taken as it what should i call face value ?

14.7 PSI

No!

If you go at Mt. Everest this will be substantially less! So this value depends upon the location.

True and if you go to Death Valley "Bad Water area" it would be more so what.

A general answer to the straight forward question was what I gave.

Mike

• Pressure - Wikipedia, the free encyclopedia

• Difference between gauge and absolute pressure - Wikipedia, the ...
• What is the difference between gauge and absolute pressure measurement
• Mercury column

Hi

the difference btwn the two is that absolute press is always almost 1atmosphere above gauge press. Absolute pressure is measured using a vacuum as the reference zero, while gauge pressure uses atmospheric press as its reference zero.

regards

Gage Pressure is pressure specifically measured on a gage within a closed container or a system that uses a gas such as refrigerant. Absolute Gage Pressure is that internal pressure plus atmospheric pressure outside of the system. Atmospheric Pressure is 14.7 PSIA. If your gage pressure is 70 psi then. . .PSIA = 70 +14.7 = 84.7 PSIA.

A lot of good & accurate answers. In my HVAC-R training we specified absolute pressure as a standard of 16.696 at sea-level and 68 degrees Fahrenheit.

Just the atmospheric pressure at the time and position of measurement.

Simple

Absolute Pressure: 0 = total vacuum

Guage Pressure: 0 = atmosphere

All good answers here, but just wanted to point out that absolute vacuum is NOT negative 14.7 psig, -1 bar, 15 Hg, etc.

Absolute vacuum is 30 Hg. That's roughly 2 bars less than 0 psig, roughly 1 bar less than 0 psia.

0 psig is 1 bar / 14.7 psi +/- calibration for altitude and so on. While 0 psi absolute is negative relative to 0 psig, it is not necessarily a vacuum.

Absolute vacuum is 30 Hg. That's roughly 2 bars less than 0 psig, roughly 1 bar less than 0 psia.

Sorry my friend, Just using my fingers I can count 1 minus 1 equals 0.

Could you explain the difference between "absolute vacuum" and 0 psia or 0 bara or... 0 whatever unit-a?

Kind regards

I disagree, my training (RSES & Physics @ UofD) taught Absolute Vacuum : 29.92 In. Hg;

Perfect Vacuum (theoretical) is 30.

I respect your opinion, and I've never questioned anybody training. But... Absolute Vacuum means no one molecule or atom are in the "vacuum chamber", then how can NOTHING exert any PRESSURE?

According to my training, atmospheric normal pressure is 760 mm Hg at Alicante sea level. That's the rough 30 inches Hg you mention. That's the differential pressure between atmosphere and vacuum (760 mm Hg - 0 mm Hg)

I can assure you that Alicante (sea level) is full of air and I don't think somebody could call the beach environment as "Absolute vacuum".

Have you observed a mercury column barometer? Think about the interface between Hg and closed tube end "vacuum" space (really is not absolute vacuum, just the vapour pressure of Hg at ambient temperature)

Kind regards

I respect your statement. As far as absence of motion/ matter , that's the point, it's not exerting pressure- it's a theoretical value used to describe "perfect vacuum". Similar to the use of "empty set" in math- for example the value of any number divided by zero. In HVAC I of course use manometers; how else would you PROPERLY set up a gas heater!

Hello DRFREON,

You've write it. Whats the difference between "absolute vacuum" and "perfect vacuum"? It's the same and of course are theoretical situations. Even in those equipments which need work under very high vacuum, "absolute" is unachievable, even degassing the vessel and using the ultimate design vacuum pumps. The pressure in those equipments is measured too even for very high vacuum normal manometers are not used.

Those types of equipment, which I've used (I've been erecting and making functional tests of a workshop to manufacture "heat pipes" for aerospace use) can reach "absolute pressures" of 10^-6 to 10^-9 bar, but never could reach zero. This would mean no matter remain in the vessel.

P.V = n.R.T or P = (n.R.T)/V and if n (gas mole quantity) is zero whatever the volume be, pressure must be zero.

Kind regards

GAUGE PRESSURE = Pressure which is measured above standard atmospheric pressure

ABSOLUTE PRESSURE = Gauge pressure + standard atmospheric pressure

Thus Example

standard atmospheric pressure = 10kpa

Gauge (measured) pressure = 15 kpa

then

Gauge pressure = 15 kpa

Absolute pressure = 15 kpa + 10 kpa =25kpa