Measuring in the Gas Phase – Absolute vs. Gauge Pressure

 

Last week I wrote a piece about gas phase measurement units, but before I uploaded it to the blogosphere I realized there was a part that needed to come first, so here goes.

Different units — whether they are pressure, volume, length, or something as obscure as kinematic viscosity — can be daunting. This is especially true if you are trying to communicate with someone who’s used to using certain kinds of units and needs to switch gears and use a different unit. I want to focus on pressure units, but before we do that let’s define a simple but confusing concept, which is the difference between gauge pressure and absolute pressure.

Absolute pressure is defined as force per unit area that a fluid or gas exerts on the walls of its container. If you take a bottle filled with air from sea level to 10,00 feet high, the pressure in the container will be the same, but when you open the lid, gas will escape until the internal pressure is the same as the atmospheric pressure outside the bottle. Absolute pressure is the pressure exerted upon us by the pressure of the atmosphere on earth.

In a perfect vacuum, absolute pressure would indicate an absence of gas molecules and a pressure of 0.000, regardless of the units used to express the vacuum. For instance, 0.000 pounds/per/sq/inch absolute (psia) is equal to 0.000 in all absolute pressure units.

Since we don’t live in a vacuum, we mainly use gauge pressure, in which atmospheric pressure has already been taken into account, so the units used are 0.000 gauge pressure. In other words, gauge pressure is “an absolute pressure,” minus atmospheric pressure. Unless you are measuring in a vacuum, gauge pressure always starts at zero and is not concerned with the pressure of the earth’s atmosphere.

It’s a lot easier to be precise about the amount of pressure we’re using if we don’t have to account for atmospheric pressure. Here’s an example: Say you need 32.0 pounds/per/sq/inch gauge (psig) in your forklift tire. In absolute pressure at sea level that would be 47.5 psia and at 10,000 feet elevation it would be 42.7 psia. Since we want to specify the same amount of pressure in the tire regardless of the atmospheric pressure, it’s easier to work in gauge pressure.

When does gauge pressure not work? When you are calibrating an instrument, measuring a gas at an elevated pressure or need to know your altitude or weather conditions, you need to distinguish the absolute pressure to get precise results. Most gas analyzers either measure the atmospheric pressure or assume the pressure is at sea level and give you tables to compensate for differences at higher altitudes.

If we want to measure the gas concentration in tanks or vessels that are above atmosphere pressure we have two ways of doing so. The first is to have a remote pressure sensor to take in account the extra molecules due the increase in pressure of the compressed sample. She second is to bring the sample back to atmospheric pressure and measure at ambient pressure.

The next blog post will talk about measuring in “volume barometric” and other obscure units. VBar is used to measure a gas phase concentration while taking into account absolute pressure and then properly compensating for altitude or weather conditions. The VBar barometric units assume that the measurement sensor is at ambient pressure.

My final thought is to understand absolute vs. gauge pressure. There is a time when it matters, so knowing so will help avoid confusion.

 

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