Instrumentation basics · 1.5

Gauge, absolute and differential pressure, and the units that trip people up

25 October 2026 · 9 min read · ISO 80000-4, IEC 60770-1

In short

Gauge pressure is measured against the local atmosphere, absolute pressure against a perfect vacuum, and differential pressure between two process points.
P_abs = P_gauge + P_atm. Standard atmosphere is 1.01325 bar, but at 1,500 m elevation it is only about 0.85 bar.
Gas density, flow compensation, vapour pressure and NPSH all need absolute pressure.
Writing "bar" without g, a or d is the commonest single error on pressure datasheets.

Pressure is the most-measured variable in most plants, and it comes with a hidden trap. The same number means different things depending on what it is referenced to. Before choosing a pressure instrument, you need to know what reference the process data uses, what the instrument measures, and what the control system expects.

Where this matters

Gas flow: density depends on absolute pressure. Using gauge pressure in the density calculation gives a large flow error.
Vacuum service: a gauge transmitter on a vacuum column reads negative values that shift with the weather, while an absolute transmitter reads the true value.
Pumps and vapour pressure: NPSH and flashing checks need absolute pressure.
Plants at altitude: a vendor's standard "atmosphere" may be wrong by 15 % or more.
Relief and design pressures: codes state them as gauge. Mixing references shifts set points.

How it works

The three references

P_abs = P_gauge + P_atm ΔP = P_high − P_low

Type	Reference	Typical suffix	Typical use
Gauge	Local atmosphere (sensor vented)	barg, psig, kPa(g)	Most process pressures, equipment protection
Absolute	Perfect vacuum (sealed reference)	bara, psia, kPa(a)	Vacuum, gas density, vapour pressure, barometric
Differential	A second process connection	bar(d), mbar, kPa	Flow (orifice), level (DP), filter or strainer loss
Sealed gauge	A fixed sealed reference, near 1 atm	bar(sg)	Hostile or submerged locations where venting is impractical

Vacuum is often written as a negative gauge pressure (−0.5 barg) or as "mbar vacuum". Always convert it to absolute before doing calculations.

Atmospheric pressure changes

Atmospheric pressure varies by a few percent with the weather and falls with altitude. A common approximation from the International Standard Atmosphere is:

P_atm ≈ 101.325 × (1 − 2.25577 × 10⁻⁵ × h)^5.25588 [kPa, h in m]

At sea level that gives 1.013 bar, and at 1,500 m it gives 0.846 bar, 16 % lower.

Units

Unit	Equal to	Note
1 bar	100 kPa = 14.504 psi	Common in process work, not SI but accepted
1 kg/cm²	0.980665 bar	Older drawings, often used as if it were 1 bar
1 atm	1.01325 bar	Standard atmosphere
1 mmH₂O	9.80665 Pa	Low DP and draft (at 4 °C water)
1 inH₂O	≈ 249 Pa	The reference temperature (4 °C, 60 °F or 68 °F) changes the fourth digit
1 MPa	10 bar	Common on Asian and European codes

What the standards and practice say

ISO 80000-4 defines pressure and its SI unit, the pascal. Gauge and absolute are not separate units, so the reference must be stated next to the number.
Pressure equipment codes such as ASME and EN 13445 state design and test pressures as gauge.
Gas properties, flow equations (ISO 5167) and valve sizing (IEC 60534) use absolute pressure at inlet.
Good practice on datasheets is to write the reference with every pressure: operating, design, range and set points.
Instrument vendors offer gauge, absolute and DP sensors as different models. The choice is fixed when ordering and cannot be changed in the field.

Worked example: one missing "a" in a gas flow

A gas flow is measured by an orifice plate with pressure compensation. The process data sheet gives "operating pressure 5 bar".

Input	Value
Real line pressure	5.000 barg = 6.013 bara (at sea level)
Compensation configured with	5.000 "bar", taken as absolute
Flow equation	Mass flow ∝ √(ΔP × ρ), with ρ ∝ P_abs for an ideal gas

Step 1, density error. Taking 5.000 instead of 6.013 bara makes the calculated density 5.000 / 6.013 = 0.832 of the true value, which is 16.8 % low.

Step 2, flow error. Flow scales with the square root of density: √0.832 = 0.912, so the flow reads 8.8 % low.

Step 3, the same plant at 1,500 m. The real absolute pressure is 5.000 + 0.846 = 5.846 bara. Even an engineer who adds 1.013 by habit gets 6.013, and is 2.9 % high on density and 1.4 % high on flow.

Pressure used	True value	Flow error
5.000 (gauge taken as absolute)	6.013 bara	−8.8 %
6.013 (sea-level atmosphere added)	5.846 bara at 1,500 m	+1.4 %
5.846 (local atmosphere added)	5.846 bara	0 %

What this example shows

One letter is worth almost 9 % of flow at this pressure, and the error gets larger at lower pressures.
Use the site's atmospheric pressure, not the standard atmosphere, when converting for plants at altitude.
An absolute transmitter for compensation removes the question, because it measures what the equation needs.

When it fails: the fixes in order

Ask the source. Ask the process engineer whether a number is gauge or absolute, and write the answer on the data.
Choose the sensor type to match the use. Use absolute for vacuum and compensation, gauge for protection and general indication, and DP for flow, level and filters.
Fix the units in the DCS. The engineering units text on the tag should carry g, a or d.
Use the site's atmospheric pressure in all conversions.
Re-check set points after any change of reference.

Common mistakes

Writing "bar" with no reference on datasheets and in process data.
Using a gauge transmitter on deep vacuum. Its reading shifts with the weather.
Assuming 1 kg/cm² = 1 bar. It is about 2 % lower.
Adding 1.013 bar at a plant well above sea level.
Mixing inH₂O values with different reference temperatures.
Forgetting that a DP transmitter's static (line) pressure rating is separate from its DP range.

Checklist

Every pressure on the datasheet carries g, a or d.
The sensor type (gauge, absolute or DP) matches the use.
Vacuum services use absolute sensors, or the gauge reading is accepted with its weather error.
The site elevation and local atmospheric pressure are recorded in the design basis.
Calculations (density, flow, NPSH) use absolute pressure.
DP transmitters have a static pressure rating at or above the design pressure.
The DCS engineering units match the reference.
Unit conversions use exact factors, such as 0.980665 for kg/cm².

Quick reference

P_abs = P_gauge + P_atm. Standard atmosphere = 1.01325 bar.
Altitude: P_atm ≈ 101.325 × (1 − 2.25577×10⁻⁵ h)^5.25588 kPa. About 0.85 bar at 1,500 m.
1 bar = 100 kPa = 14.504 psi. 1 kg/cm² = 0.980665 bar. 1 MPa = 10 bar.
Use absolute for: gas density, flow compensation, vapour pressure, NPSH, valve sizing.