Pressure Measurement · Glossary

What is Absolute Pressure?

Symbol: bar a, psiarelative to perfect vacuumPressure Measurement

Absolute pressure is pressure measured relative to a perfect vacuum, so it is never negative.

Absolute Pressure Explained

Absolute pressure sensors have a sealed vacuum reference. They are used for vacuum processes, barometric measurement and wherever the gas laws are applied.

Gas density, standard volume flow, boiling point and vapour pressure calculations all need absolute pressure.

Absolute Pressure Formula

p_abs = p_gauge + p_atm

p_gauge	gauge pressure
p_atm	local atmospheric pressure

Example: 5 bar g at sea level (1.013 bar) is 6.013 bar a.

Calculators for Absolute Pressure

CalculatorAbsolute Pressure CalculatorFree online calculator with formula and worked example.CalculatorGas Law CalculatorFree online calculator with formula and worked example.CalculatorBoiling Point CalculatorFree online calculator with formula and worked example.

Frequently Asked Questions

What is Absolute Pressure?

Absolute pressure is pressure measured relative to a perfect vacuum, so it is never negative.

When must absolute pressure be used?

In gas law, density and flow compensation calculations, and for vacuum service where atmospheric changes would cause large errors.

Can I convert gauge to absolute with 1.013 bar?

Only near sea level. At altitude use the local atmospheric pressure, which is lower.

Learn more on the blog

Gauge pressure transmitter: working principle and applications

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

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More Pressure Measurement Terms

Pressure termDifferential PressureDifferential pressure is the difference between two pressures measured at two points.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.

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