Pressure Gauge Dial Symbols and Markings Explained

Why pressure gauge dials include more than the reading

A pressure gauge dial is not only a place to read the current process pressure. On many industrial gauges, the dial also carries information about the instrument’s construction, measuring range, accuracy, calibration conditions, intended service, and safety design. These details help users identify whether the gauge is suitable for a particular application and whether the indicated pressure should be interpreted under any special conditions.

This is especially important for mechanical pressure gauges covered by the EN 837 series. In this context, the main gauge types are Bourdon tube gauges, diaphragm gauges, and capsule gauges. EN 837-1 applies to Bourdon tube pressure gauges, while EN 837-3 applies to diaphragm and capsule pressure gauges. The standards define a combination of required and optional dial markings. Some markings are basic, such as the pressure unit and manufacturer identification. Others are conditional, appearing only for certain accuracy classes, installation orientations, process media, or safety-pattern constructions.

Understanding pressure gauge dial symbols is therefore useful for selection, installation, calibration review, inspection, and safe use. A gauge that appears similar in range and size may have different limitations if its accuracy class, medium suitability, wetted materials, mounting angle, or safety construction differs.

Pressure unit shown on the scale

The pressure unit is one of the fundamental dial markings and is required under EN 837. Without a clearly marked unit, a pressure value cannot be interpreted correctly.

For Bourdon tube pressure gauges, bar is the preferred unit. Where SI indications are used, Bourdon tube gauges may show pressure in kPa or MPa. The appropriate unit depends on the pressure range and the intended technical context. For example, kPa may be more convenient for lower industrial ranges, while MPa is often used for higher pressures.

For diaphragm and capsule pressure gauges, bar is also preferred. However, mbar is used for measuring ranges of 600 mbar or below, which reflects the common use of diaphragm and capsule elements in lower pressure and differential pressure applications. Where SI units are used for diaphragm and capsule gauges, the dial may show Pa, kPa, or MPa.

The unit marking should always be read together with the numerical scale. A scale marked 0–10 bar is not equivalent to a scale marked 0–10 MPa, even though the numbers appear identical. Unit marking is therefore a basic but critical part of pressure gauge identification.

Accuracy class marking

The accuracy class tells the user the permitted error band of the gauge, expressed as a percentage of span. For example, an accuracy class of 1.6 means the allowed indication error is based on 1.6% of the measuring span under the standard’s specified conditions. It is not a statement that the gauge is accurate to 1.6% of the current reading.

Accuracy class marking is required on the dial. This marking is important because the same pressure range may be available in different accuracy classes, depending on the gauge type and nominal size. A small utility gauge and a larger test or reference gauge may both have a 0–10 bar scale, but their permitted errors can be very different.

For Bourdon tube pressure gauges, the defined accuracy classes are:

For diaphragm and capsule pressure gauges, the defined accuracy classes are:

Not every accuracy class is available for every nominal size. EN 837 uses nominal size limits to determine which accuracy classes are permitted for each gauge type. In practice, higher accuracy classes generally require suitable dial size, mechanical design, and scale readability. When comparing gauges, the accuracy class should therefore be checked together with the gauge type and nominal size, not in isolation.

Pressure element type symbol

Some pressure gauge dials identify the internal sensing element by a symbol. This marking helps distinguish how the pressure is mechanically converted into pointer movement.

For Bourdon tube gauges, the pressure element may be a C-type Bourdon tube, a spiral Bourdon tube, or a coiled Bourdon tube, depending on the range, sensitivity, and construction. A C-type Bourdon tube is common in many industrial gauges. Spiral and coiled forms provide a different geometry and may be used where the design requires greater movement or a particular measurement behavior.

For diaphragm and capsule gauges, the dial may similarly use a symbol to identify the pressure element. A diaphragm element uses a flexible membrane that deflects under pressure. A capsule element generally consists of two diaphragms joined together to create a chamber, often suited to low-pressure measurement.

Under EN 837, marking the pressure element type on the dial is optional rather than mandatory. Its presence can still be helpful, especially when the external case style does not clearly reveal the internal measuring principle.

Maximum steady working pressure indication

Some pressure gauges are designed to operate continuously at the maximum scale value without long-term loss of accuracy. This is not true for every gauge. Many gauges should normally be selected so that the working pressure lies below the top of the scale, allowing margin for pressure fluctuations and reducing mechanical stress.

Where a gauge is intended for continuous operation at the maximum scale value, it must carry a specific identifying mark at the maximum scale value. This marking indicates the gauge’s maximum steady working pressure capability. For full-scale loadable Bourdon tube gauges, the mark is commonly positioned at the end of the scale, making it possible to distinguish this capability from the outside.

This marking should not be confused with a general overpressure rating or a permission to ignore process pressure limits. It specifically identifies that the gauge is suitable for continuous operation up to the maximum scale value while maintaining its accuracy over time according to the applicable standard context. Users should still consider pulsation, vibration, temperature, pressure spikes, and the compatibility of the gauge with the process.

Dial inclination angle marking

Mechanical pressure gauges are affected by mounting orientation because gravity can influence the movement, pointer, and elastic measuring element. For many gauges, the standard reference condition is a vertical dial. If a gauge is intended to operate at a non-vertical angle, that inclination angle must be shown on the dial.

The dial inclination marking indicates the calibrated mounting position. In other words, the gauge should be calibrated in the same orientation in which it will be used. If a gauge is calibrated vertically but installed at a significant angle, the indication may shift, especially for more sensitive or higher-accuracy instruments.

This marking is particularly relevant when gauges are mounted on sloped panels, angled equipment faces, or installations where visibility requires the dial to be tilted. The symbol or angle indication informs the technician that the orientation is not incidental; it is part of the gauge’s calibration context.

Calibration reference temperature

The EN 837 reference calibration temperature is 20°C. Temperature matters because mechanical pressure gauges rely on elastic elements, linkages, and materials whose behavior can change with temperature. Even when the process pressure is stable, changes in ambient or instrument temperature may influence the reading.

For accuracy classes 0.1, 0.25, and 0.6, the calibration temperature must be indicated if it differs from the 20°C reference temperature. This requirement is conditional. It is mandatory only for those specified accuracy classes and only when the calibration temperature is not the reference value.

The marking helps prevent misinterpretation of a high-accuracy gauge. If a gauge was calibrated at a temperature other than 20°C, the user needs to know that condition when evaluating measurement uncertainty or comparing it with calibration certificates and reference instruments. For ordinary process monitoring, this may be less visible in day-to-day operation, but for inspection and metrological work it can be significant.

Gas or liquid service indication

For certain high-accuracy Bourdon tube gauges, the accuracy may be achieved only when the gauge is used with a gas or only when it is used with a liquid. In that case, the dial must identify the applicable state of aggregation.

This requirement applies to Bourdon tube gauges in accuracy classes 0.1, 0.25, and 0.6 when the stated accuracy is achieved only for one type of service. Gas service may be identified by G, and liquid service may be identified by F. Equivalent wording may also be used, or the process medium may be named directly where that is clearer.

The reason is that gas and liquid service can impose different mechanical and dynamic conditions on the measuring system. Effects such as damping, compressibility, and movement behavior may influence the indicated pressure, especially in higher-accuracy gauges.

This marking should not be read as a general chemical compatibility statement unless the medium is specifically named for that purpose. It indicates the service condition under which the stated accuracy applies. EN 837-3 does not address this state-of-aggregation marking for diaphragm or capsule pressure gauges.

EN 837 standard number on the dial

A pressure gauge dial may identify the EN 837 standard that the gauge follows. This can be useful because the standard number helps identify the measuring principle and applicable construction requirements.

An EN 837-1 marking indicates a Bourdon tube pressure gauge. An EN 837-3 marking indicates a diaphragm or capsule pressure gauge. These markings can support inspection and documentation by showing which part of the EN 837 series is relevant to the instrument.

However, marking the followed standard number on the dial is optional. A gauge may still be designed according to the standard even if the standard number is not printed on the dial. Conversely, when the marking is present, it should be interpreted together with the other dial markings and the gauge documentation.

The standard number is particularly helpful when evaluating safety-pattern markings, pressure element type, and requirements that apply only to Bourdon tube gauges or only to diaphragm and capsule gauges.

Manufacturer or supplier identification

The pressure gauge dial should identify the manufacturer and/or supplier. This is a required marking and may appear as a printed name, trademark, or logo.

Manufacturer or supplier identification supports traceability. It allows users to connect the physical gauge to documentation, specifications, calibration records, certificates, and replacement information. In maintenance environments, this can be important when several similar gauges are installed across a plant or machine.

A name or logo is not, by itself, proof of suitability for an application. It should be read with the range, unit, accuracy class, materials, process-medium markings, safety markings, and any documentation supplied with the gauge. However, without identification, it becomes much more difficult to verify the instrument’s origin, design standard, or service history.

Device serial number marking

Serial number requirements depend on gauge type, accuracy class, and whether the gauge is used for metrological purposes. A serial number gives an individual identity to a specific instrument, rather than only identifying a model or manufacturer.

For Bourdon tube gauges, serial number marking is mandatory for accuracy classes 0.1 and 0.25. These are high-accuracy classes, where individual traceability is especially important.

For Bourdon tube gauges in accuracy classes 0.6 through 4, serial number marking is generally optional unless the gauge is used for metrological purposes. In such cases, the need for traceability can make individual identification necessary even for gauges with less demanding accuracy classes.

For diaphragm and capsule pressure gauges, serial number marking is mandatory when the gauge is used for metrological purposes. Where the gauge is not used for metrological purposes, the serial number requirement may not apply in the same way.

Serial numbers support calibration traceability, maintenance history, quality control, and regulated measurement records. They help ensure that a calibration certificate, inspection report, or repair record belongs to the exact gauge installed in the system, not merely to another gauge of the same type.

Wetted-parts material identification

The wetted parts are the parts of the gauge that come into contact with the process medium. Depending on the gauge design, these may include the pressure connection, Bourdon tube, diaphragm, capsule, seals, or other internal pressure-containing components.

The dial may identify the material used for wetted parts. This marking is especially relevant when the wetted parts are not made from common listed materials such as brass, bronze, tin, or hard solder. If a different material is used for corrosion resistance or process compatibility, it may be printed on the dial. A common example would be a stainless steel grade used for improved resistance to corrosive media.

Under EN 837, wetted-parts material marking is optional. Its absence does not necessarily mean the materials are unsuitable, but it does mean the user should verify material compatibility through documentation, datasheets, or purchase specifications.

This marking should be treated as part of the selection process. Even if the pressure range and accuracy class are correct, unsuitable wetted materials can lead to corrosion, leakage, contamination, or premature failure.

Safety pattern gauge marking

Certain Bourdon tube safety pattern gauges may carry an S marking or S symbol together with the EN number. This marking identifies a safety construction type covered in the Bourdon tube gauge context.

The marking applies to specified gauge sizes and safety construction types. In general terms, safety-pattern gauges are designed to reduce risk to personnel if the pressure element fails. Relevant safety features may include blow-out protection, a baffle wall, or safety glass, depending on the construction type.

Type S2 applies to smaller gauges without a baffle wall. Type S3 applies to a wider size range with a baffle wall. The baffle wall is an internal barrier intended to help separate the pressure element from the front of the gauge, improving protection if a failure occurs.

Safety pattern marking is associated with Bourdon tube gauges under EN 837-1. EN 837-3 does not cover safety pattern dial marking for diaphragm or capsule pressure gauges.

The presence of an S marking should be considered when selecting gauges for hazardous services, high pressures, or locations where personnel may be exposed to the front of the instrument. It does not remove the need for correct installation, compatible materials, suitable pressure range, and appropriate process protection.

Oxygen and acetylene process-medium markings

For Bourdon tube gauges used with oxygen or acetylene, the process medium must be marked on the dial in English as oxygen or acetylene. These markings are safety-related, not merely descriptive.

For oxygen service, the required no-lubrication symbol must also be marked. This is because oil and grease can be hazardous in contact with oxygen and may ignite spontaneously. A gauge intended for oxygen service must therefore be handled and maintained so that the pressure system remains free from unsuitable lubricants and contamination.

Acetylene service also requires clear identification because acetylene has specific safety considerations. The dial marking helps prevent accidental use of a gauge in an incompatible or unsafe service.

These process-medium markings apply in the Bourdon tube gauge context. EN 837-3 does not cover oxygen or acetylene process-medium indication on diaphragm or capsule pressure gauge dials.

When reading these markings, the user should treat them as application limits. A gauge marked for oxygen or acetylene is not automatically suitable for every other gas, and an unmarked gauge should not be assumed safe for oxygen service. The dial marking is one part of a broader safety requirement that includes cleaning, materials, pressure range, installation practice, and contamination control.