# Question about vacuum and vacuum meter

## Main Question or Discussion Point

I wonder why vacuum is sometimes measured in range from 0 to -1 bar. Is it because of it's measured relatively to normal atmospheric pressure?
I guess that normally, vacuum shoud be in range of 0 to 1 bar or something?
Can you help me understaning what terms "underpressure" and "overpressure" really means?
Thanks

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Ivan Seeking
Staff Emeritus
Gold Member
I wonder why vacuum is sometimes measured in range from 0 to -1 bar. Is it because of it's measured relatively to normal atmospheric pressure?
Yes, there is PSIG, or gauge pressure [or vacuum], and PSIA, or absolute pressure. Gauge pressure uses normal atmospheric pressure as zero. Absolute pressure is gauge pressure minus 1 bar. A measure of 0 PSIA [absolute] would be a complete vacuum.

I guess that normally, vacuum shoud be in range of 0 to 1 bar or something?
Can you help me understaning what terms "underpressure" and "overpressure" really means?
Thanks
Since you are starting with a gauge pressure of zero, which means that we are starting with atmospheric pressure [ 1 bar] as zero, anything lower than this would be negative, which is a vacuum.

Underpressure and Overpressure mean just what they say. If you have a system that operates between 1 and 2 bars, and you measure it operating at 3 bars, it would be 1 bar overpressure. If you measured it operating at 0.5 bars, it would be 0.5 bars underpressure.