Absolute vs Gauge Pressure

As a quick reminder, absolute pressure refers to the pressure relative to an absolute vacuum - i.e. a volume with zero molecules occupying the space. Gauge pressure, on the other hand, is the pressure relative to atmospheric pressure on Earth, which is defined as 1 atm or 1.01325 bara. Therefore, you can have negative gauge pressure, but never negative absolute pressure.

They are related by: $P_a = P_{atm} + P_g$

If you're reading this, I'm sure you already knew that, so instead, this article is just a reminder to make it clear which pressure you are referring to. If I see: "operating pressure of 10 bar" - I have no idea if that is 10 bara or 10 barg. And that can make a pretty big difference!

One last reminder: when you are designing pressure vessels where the external pressure is not atmospheric, you need to use the pressure difference. E.g. if you want to operate a pressure vessel at 10 barg, but that vessel has a vacuum jacket (at ~0 bara), then the pressure differential is actually 11 bar.

To reduce confusion, if you are working under atmospheric pressure, use barg. If you are working under vacuum, use bara.