Pressure Measurement · Glossary

What is Vacuum?

mbar a, Pa, TorrPressure Measurement

Vacuum is any pressure below atmospheric pressure, from slight suction to near perfect vacuum.

Vacuum Explained

Vacuum can be stated as negative gauge pressure or, more clearly, as absolute pressure. A perfect vacuum is 0 bar a.

Near full vacuum, gauge readings become inaccurate because atmospheric pressure varies, so absolute sensors are used. Very low vacuum needs special gauges such as Pirani or ionisation types.

Vacuum Formula

Vacuum (gauge) = p_atm − p_abs

p_atm	atmospheric pressure
p_abs	absolute pressure in the vessel

Example: 200 mbar a with 1013 mbar atmosphere is 813 mbar of vacuum, or −813 mbar g.

Vacuum: Typical Values

1 Torr	133.3 Pa = 1.333 mbar
Rough vacuum	down to about 1 mbar a
Perfect vacuum	0 bar a

Calculators for Vacuum

CalculatorAbsolute Pressure CalculatorFree online calculator with formula and worked example.CalculatorVacuum Pump Down Time CalculatorFree online calculator with formula and worked example.CalculatorBoiling Point CalculatorFree online calculator with formula and worked example.

Frequently Asked Questions

What is Vacuum?

Vacuum is any pressure below atmospheric pressure, from slight suction to near perfect vacuum.

Can a gauge pressure transmitter measure vacuum?

Yes, as negative gauge pressure, if its range goes below zero. For deep vacuum an absolute transmitter is better.

Why does water boil at low temperature in vacuum?

Boiling happens when vapour pressure equals the surrounding pressure; lower pressure means a lower boiling point.

Learn more on the blog

Gauge pressure transmitter: working principle and applications

A practical guide on instrumentationblog.in with more detail on vacuum and how it is applied on real plants.

Read Full Article

More Pressure Measurement Terms

Pressure termHydrostatic PressureHydrostatic pressure is the pressure created by the weight of a liquid column; it depends only on height…Pressure termPressure HeadPressure head is pressure expressed as the height of a liquid column that would produce it.Pressure termStatic PressureStatic pressure is the pressure of a fluid measured at right angles to the flow, without the effect of…Pressure termUpper Range LimitUpper range limit is the highest value a transmitter sensor can be calibrated to measure.Pressure termTransmitter TurndownTransmitter turndown is how far a transmitter span is reduced from its sensor upper range limit.Pressure termOverpressure LimitOverpressure limit is the highest pressure a sensor can take without damage or loss of its calibration.

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