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Join Date: Aug 2017
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Difference in Absolute and Relative Pressure Gauge

08/04/2017 6:09 AM

Good day everyone,

What is the reading in my absolute pressure gauge and my relative pressure gauge if I measure let's say a 2 bar pressure? It's just that I want to know their difference in reading when measuring same pressure.

Thank you

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#1

Re: Difference in Absolute and Relative Pressure Gauge

08/04/2017 6:32 AM

The difference between them will be [1 atmosphere, corrected for the local barometric ambient pressure allowing for weather conditions and altitude above sea level], by definition of the terms.

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Join Date: Aug 2017
Posts: 6
#2

Re: Difference in Absolute and Relative Pressure Gauge

08/04/2017 10:51 PM

If it is a relative pressure gauge such as a tire pressure gauge, then "one atmosphere" means that the pressure inside the tire is one atmosphere greater that the pressure outside the tire. The absolute pressure inside the tire is accordingly two atmospheres since the pressure outside the tire is normally one atmosphere. At higher altitudes, say at 3,000 meters elevation, the outside pressure is less by a third, so the same gauge will read 1.33 atmospheres. On the other hand, an Absolute pressure gauge will read one atmosphere laying all by itself at sea level, two atmospheres when connected to a tire there, and still two atmospheres when at an altitude of 3,000 meters. A weather barometer is an Absolute pressure gauge.

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Join Date: Apr 2017
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#3

Re: Difference in Absolute and Relative Pressure Gauge

08/04/2017 11:31 PM

In absolute terms, Absolute Pressure is the pressure above absolutely nothing (not to be confused with the Kelvin temperature of Absolute Zero). Relative Pressure is always relative to some other pressure (usually background pressure). Now here is where it gets weird: If you believe in Quantum Physics, then there is no such thing as Absolutely Nothing. They say that even in a perfect vacuum, that there is still something there. I don't know how they treat absolute values, but I don't really believe in quantum physics anyway (not completely).

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#4

Re: Difference in Absolute and Relative Pressure Gauge

08/05/2017 10:02 AM

We have two ways of measuring pressures, depending on what we use as reference. They are:

• Absolute pressure
• Gauge pressure

Absolute Pressure

One way to measure pressure is to use absolute zero pressure (like in space) as the base value against which other pressures are measured. The pressure measured relative to this absolute zero pressure in a vacuum is what scientists call absolute pressure. A pressure measurement below atmospheric pressure is called negative pressure, or vacuum pressure.

So you might be thinking ''How is this possible? On Earth, how do we make measurements based on a vacuum?'' We use an absolute pressure sensor. Scientists have found a way to create a vacuum behind a diaphragm in the sensor. They do this by removing the air and then sealing the instrument so that it does not have direct contact with the outside. So, absolute pressure is also called sealed-pressure and is typically given in the unit of psia.

This is really helpful because even if the outside ambient pressure changes, we have an accurate method to measure the pressure.

DHAYANANDHAN.S

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Join Date: Dec 2017
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#5

Re: Difference in Absolute and Relative Pressure Gauge

12/21/2017 1:42 PM

You may be wondering why you need both types.

Some processes such as evaporators are effected by weather conditions so it's important to have a pressure transmitter measuring the absolute pressure.

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