Pressure · 2.5

Pressure gauges to EN 837: accuracy class, scale range, safety case and accessories

26 October 2026 · 9 min read · EN 837-1, EN 837-2, EN 837-3

In short

A gauge's accuracy class is its maximum error as a percentage of full scale: class 1.0 on 0–16 bar is ±0.16 bar anywhere on the dial.
Choose the scale so that the working pressure is at most ¾ of full scale for steady pressure and ⅔ for fluctuating pressure.
For gas and high-pressure service, use the S3 safety pattern: solid front, blow-out back.
Add a siphon for steam, a snubber or liquid filling for pulsation, and a diaphragm seal for corrosive or clogging fluids.

The humble pressure gauge is the instrument operators trust most, because it needs no power and no screen. It is also often specified by copying a line from the last project. EN 837 gives clear rules, and following them avoids gauges that are unreadable, inaccurate or dangerous when they fail.

Where this matters

Local indication during start-up, maintenance and when the DCS disagrees with the field.
Pump and compressor discharge, where pulsation destroys gauges quickly.
Steam lines, where hot condensate can damage the Bourdon tube.
Gas at high pressure, where a ruptured tube can blow out the dial window towards the person reading it.

How it works

Most gauges use a Bourdon tube: a curved, flattened tube that straightens slightly under pressure and drives the pointer through a gear movement (EN 837-1). For low pressures and corrosive fluids, diaphragm (plate) gauges are used (EN 837-3), and for very low gas pressures, capsule gauges.

Accuracy class

maximum error = class % × full-scale value

Class	Error on 0–16 bar	Typical use
0.6	±0.096 bar	Test gauges
1.0	±0.16 bar	Process gauges, 100 mm and larger
1.6	±0.26 bar	General service
2.5	±0.40 bar	Small dials, utilities

The error is fixed in bar, so it is a larger share of a small reading. This is one reason not to oversize the scale.

What the standards and practice say

EN 837-1 covers Bourdon tube gauges: classes, sizes, cases and tests. EN 837-3 covers diaphragm and capsule gauges.
EN 837-2 gives selection and installation recommendations. The working pressure should not exceed ¾ of full scale for steady pressure, or ⅔ for fluctuating pressure.
Safety pattern (S3): a solid baffle between the tube and the dial, and a blow-out back, so that a rupture vents backwards, away from the reader. This is usually required for gas and for high pressure.
Standard dial sizes are 63, 100 and 160 mm. 100 mm is the usual process size.
Liquid filling (glycerine or silicone) damps pointer vibration and lubricates the movement in pulsating service.

Accessories

Accessory	When
Siphon (pigtail or U-tube)	Steam and hot vapours: holds a condensate seal so hot steam doesn't reach the tube
Snubber or pulsation damper	Pump discharge, reciprocating compressors, hydraulic systems
Overpressure protector	Where pressure can exceed the scale, e.g. a low-range gauge on a higher-pressure line
Diaphragm seal	Corrosive, viscous or clogging fluids (article 2.4)
Gauge valve or 2-valve manifold	Isolation, venting and testing without a shutdown

Worked example: a pump discharge gauge

Input	Value
Normal / maximum pressure	6 / 8 barg, pulsating
Fluid	Water at 40 °C
Accuracy needed	To about ±0.2 bar

Step 1, scale. The pressure is pulsating, so the working value should be at most ⅔ of full scale. For 8 barg maximum, full scale must be at least 8 / (2/3) = 12 bar. The next standard scale is 0–16 bar. On a 0–10 bar dial, 8 barg would be at 80 %, too high for pulsating service.

Step 2, class. Class 1.0 on 0–16 bar gives ±0.16 bar. That meets ±0.2 bar. Class 1.6 (±0.26 bar) would not.

Step 3, case and accessories. Water at 8 barg does not need an S3 case on a strict reading, but many specifications require it for all process gauges. Pulsation calls for a liquid-filled case and a snubber.

Option	Max at	Error	Result
0–10 bar, class 1.6	80 %	±0.16 bar	Too close to full scale for pulsation
0–16 bar, class 1.6	50 %	±0.26 bar	Accuracy not met
0–16 bar, class 1.0, filled, snubber	50 %	±0.16 bar	Meets both

What this example shows

Pulsation decides the scale (⅔ rule), and the scale then decides the class needed.
Accessories are part of the specification, not an afterthought.
Standard scales are 1, 1.6, 2.5, 4, 6, 10, 16, 25… bar, so round up to the next one.

When it fails: the fixes in order

A shaking pointer or worn movement: fit a snubber and a liquid-filled case. Check the mounting is not on a vibrating pipe.
Reading stuck after a steam event: fit a siphon and fill it with water before commissioning.
Zero shifted: the gauge was over-ranged. Use a higher scale or fit an overpressure protector.
Blocked or corroded: fit a diaphragm seal or change the wetted material.

Common mistakes

Scales chosen so that normal operation is at 90 % of the dial.
No siphon on steam service.
Standard cases, not safety pattern, on gas and high-pressure service.
Small 63 mm dials where operators need to read from a distance.
Copper-alloy wetted parts in ammonia or sour service.
No isolation valve, so the gauge can't be changed without a shutdown.

Checklist

The working pressure is ≤ ¾ of full scale (steady) or ≤ ⅔ (fluctuating).
The accuracy class gives the error needed at full scale.
The dial size suits the reading distance (100 mm or larger for process).
The case is S3 safety pattern for gas or high pressure, or per company rule.
The wetted parts suit the fluid. Use stainless for most process service.
A siphon is fitted for steam, and a snubber or filling for pulsation.
A diaphragm seal is fitted for corrosive or clogging fluids.
An isolation valve or 2-valve manifold is fitted.
Oxygen-service gauges are cleaned and marked accordingly.

Quick reference

Error = class % × full scale.
Scale: working ≤ ¾ FS (steady), ≤ ⅔ FS (fluctuating).
Standard scales: 1, 1.6, 2.5, 4, 6, 10, 16, 25, 40, 60, 100 bar…
S3: solid front, blow-out back.
Accessories: siphon (steam), snubber or filling (pulsation), seal (corrosive), protector (over-range).