“Absolute” Pressure (psia) vs “Gauge” Pressure (psig)

Here is the scenario:

1. You have just received your brand new Ashcroft Test Gauge.
2. Why??? Because you know that the gauge on your canister is not accurate enough for making reliable quantitative dilutions.
3. You notice that your brand-spanking-new Ashcroft Test Gauge is not reading zero, like so:

4. Now you are starting to FREEAAK OUT!!! Because let’s face it… these gauges are not cheap.
5. But before jumping off a cliff and/or throwing your gauge off one... let us review the following:

Like the gauge on your canister, the Ashcroft Test Gauge will read pressure/vacuum in what is called “gauge” pressure. This is not to be confused with “absolute” pressure, which is related. What am I talking about here?

Absolute pressure is zeroed against a perfect vacuum. Gauge pressure is zeroed against the local atmospheric pressure. This means when you are standing at sea level your body is experiencing ~14.7 (14.696) pounds/square inch of pressure, due to the atmosphere above you. We call this atmospheric pressure [i.e., 1 atmosphere (atm) of pressure]. You have no clue you are experiencing this every day, because you are used to it. However, you do realize things are different when you dive into a pool and experience an increase in the pressure or fly in a plane and experience a decrease in pressure. I know you know what I am talking about. In any event, this atmospheric pressure may be measured as 14.7 psia (“a” for absolute), at sea level.

However, your fancy new Ashcroft Test Gauge has been zeroed against this local atmospheric pressure. So when you are standing on the beach with your gauge it would read 0 psig. To further elaborate, when you are at sea level and you pressurize your canister to 30 psig (“g” for gauge), you have actually put 44.7 psia in the canister.

So when you see that your zero is off on the gauge, this is probably due to the fact that it was zeroed at a different elevation. This does not mean it is un-calibrated. In fact, we ship all of these calibrated, but you do have to adjust your zero. In the case of the customer example above, it just so happens they work at a higher elevation, which had a lower atmospheric pressure (e.g., 14 psia) than the elevation the gauge was originally zeroed at. This lower atmospheric pressure expresses itself as a lower gauge pressure, which looks like the zero is off.

Quick recap:

Gauge pressure = absolute pressure – atmospheric pressure

Using the pressurization examples above:

30 psig = 44.7 psia – 14.7 psi (atmospheric pressure at sea level)

0 psig = 14.7 psia – 14.7 psi (again, at sea level)

Now how about re-zeroing that test gauge? It as simple as the following: The smaller outer screw at six o'clock (red arrow) is a set screw. Loosen the set screw and then adjust the zero with the larger, inner screw (green arrow). Then re-tighten the set screw.

Mixing up gauge pressure and absolute pressure seems to be a fairly common mistake. So always be sure to double check what exactly are you dealing with.