Selecting a Pressure Gauge per EN 837: Range, Accuracy Class, Nominal Size and Accessories
The Bourdon tube gauge is the most common pressure instrument in plant engineering and maintenance. Selection mistakes rarely show immediately – they appear as trembling pointers, permanent zero shifts or a burst measuring element. This post summarises the selection rules of the EN 837 series and works through an example.
A German version of this article is available — switch the language to Deutsch.
:::info Tool With the Bourdon Gauge Configurator you combine nominal size, range, connection, filling and accessories and export a PDF datasheet. :::
The EN 837 series
| Part | Scope |
|---|---|
| EN 837-1 | Bourdon tube pressure gauges – dimensions, metrology, requirements and testing |
| EN 837-2 | Selection and installation recommendations for pressure gauges |
| EN 837-3 | Diaphragm and capsule pressure gauges – dimensions, metrology, requirements and testing |
EN 837-2 is the working document for selection; EN 837-1 provides the characteristics of the instruments you choose between.
1. Scale range
The single most important step. A gauge working permanently at full scale fatigues; one that only moves through the first tenth cannot be read accurately. The common recommendation:
| Loading | Operating pressure at most |
|---|---|
| steady | 3/4 of full scale |
| fluctuating | 2/3 of full scale |
Full-scale values follow a standard series, for example 0–1 / 1.6 / 2.5 / 4 / 6 / 10 / 16 / 25 / 40 / 60 / 100 bar.
Worked example
Steady operating pressure 6 bar
required full scale ≥ 6 bar / 0.75 = 8 bar → range 0–10 bar
Fluctuating operating pressure 6 bar
required full scale ≥ 6 bar / (2/3) = 9 bar → range 0–10 bar
Fluctuating operating pressure 7 bar
required full scale ≥ 7 bar / (2/3) = 10.5 bar → range 0–16 bar
Half a bar more operating pressure – and the next range up is needed.
2. Accuracy class
The class states the permissible error as a percentage of the span. EN 837-1 defines classes 0.1 – 0.25 – 0.6 – 1 – 1.6 – 2.5 – 4.
Class 1.0 on 0–10 bar → ± 0.1 bar
Class 1.6 on 0–16 bar → ± 0.256 bar
Because the error refers to the span, an oversized range worsens accuracy at the operating point – another reason not to double the range "to be safe".
The fine classes 0.1 to 0.6 are for test and reference gauges; in plant operation 1.0 and 1.6 are typical.
3. Nominal size and connection
The nominal size is the case diameter in millimetres. Standard sizes include 40, 50, 63, 80, 100, 150, 160 and 250. Rule of thumb: the longer the reading distance and the finer the class, the larger the size.
The process connection is typically G ¼ B for small sizes and G ½ B from size 100 upwards, lower mount or back mount. Check the thread type at the installation point – an NPT thread seals differently from a parallel G thread with a sealing ring.
4. Operating conditions and accessories
| Situation | Measure |
|---|---|
| Vibration, pressure surges, pulsation | Liquid filling (e.g. glycerine) damps pointer and movement; with strong pulsation add a throttle screw |
| Steam, hot media | Siphon: condensate protects the Bourdon tube from the medium temperature |
| Aggressive, viscous or crystallising media | Diaphragm seal instead of direct media contact |
| Replacement during operation | Fit a gauge isolating valve |
| Gases and vapours at high pressure | Choose a safety pattern per EN 837-1 |
| Oxygen | Instrument free of oil and grease, marked accordingly |
The highest safety pattern, S3, has a solid baffle wall between measuring element and dial and a blow-out back: if the Bourdon tube fails, the medium escapes to the rear instead of through the window towards the operator.
Checklist for datasheet and order
- Medium, operating pressure, loading (steady / fluctuating)
- Range per the 3/4 or 2/3 rule
- Accuracy class
- Nominal size and reading distance
- Connection thread and position
- Wetted materials of measuring element and connection
- Filling, safety pattern
- Accessories: isolating valve, siphon, throttle screw, diaphragm seal
- Measuring point tag
The Bourdon Gauge Configurator captures exactly these items and summarises them in a PDF datasheet.
:::caution Note This post summarises common selection rules and does not replace the current editions of EN 837-1 to -3 or manufacturer data. Safety-related measuring points are additionally subject to the plant design requirements. :::
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