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Pressure Cheat Sheet

A one page pressure reference: unit conversion table, gauge and absolute pressure, hydrostatic head, force and the practical rules for selecting gauges and transmitters.

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Unit conversion (1 unit equals)

Unit	bar	psi	kPa
1 bar	1	14.504	100
1 psi	0.068948	1	6.8948
1 kPa	0.01	0.14504	1
1 kg/cm²	0.98067	14.223	98.067
1 atm	1.01325	14.696	101.325
1 mmHg (torr)	0.0013332	0.019337	0.13332
1 mmH2O (4 °C)	0.000098064	0.0014223	0.0098064
1 inH2O (4 °C)	0.0024909	0.036127	0.24909
1 MPa	10	145.04	1000

Gauge, absolute, differential

Absolute	P_abs = P_gauge + P_atm
Standard atmosphere	1.01325 bar a = 14.696 psi a
Vacuum	below atmospheric, often shown as negative gauge
Differential	ΔP = P_HP − P_LP

Hydrostatic pressure

Head	P = ρ × g × h
Water	1 m ≈ 98.07 mbar ≈ 1.422 psi
10 m water	≈ 0.981 bar
g	9.80665 m/s² (standard)

Force and area

Force	F = P × A
Example	10 bar on 100 cm² = 10,000 N

Gauge selection rules

Normal pressure in the middle third of the scale (about 25 to 75 % of full scale)
Full scale about 1.5 to 2 times the normal operating pressure
Pulsating service: liquid filled gauge or snubber
Steam: siphon (pigtail) to protect the element
Corrosive or viscous media: diaphragm seal

Transmitter selection rules

Calibrated span within the sensor range, within the allowed turndown
Check the over pressure and static pressure limits
Accuracy of the total installed loop = √(sum of squares of errors)
Absolute transmitters for vacuum and vapour pressure

Atmospheric pressure with altitude

Approximation	P ≈ 1013.25 × (1 − 2.25577 × 10⁻⁵ × h)^5.25588 hPa
Rule of thumb	about −12 hPa per 100 m near sea level

Accuracy class (EN 837-1 gauges)

Class	Error
0.1 to 0.6	Test gauges
1.0	± 1 % of span
1.6	± 1.6 % of span
2.5	± 2.5 % of span

Related Calculators and Tools

Pressure Unit ConverterFree on instrumentationblog.in Hydrostatic Pressure CalculatorFree on instrumentationblog.in Pressure Gauge Range SelectorFree on instrumentationblog.in Atmospheric Pressure at AltitudeFree on instrumentationblog.in

Frequently Asked Questions

How many psi is 1 bar?

14.504 psi.

What is the difference between bar g and bar a?

bar g is relative to atmosphere; bar a is relative to vacuum. bar a = bar g + about 1.013 at sea level.

How much pressure does 1 m of water create?

About 98.07 mbar or 9.807 kPa at standard gravity.

Learn more on the blog

Static pressure vs differential pressure

A practical guide on instrumentationblog.in that explains the theory behind this cheat sheet.

Read Full Article

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