Selecting a Pressure Gauge
Pressure gauges can be found in virtually every industry. Depending on your application, these instruments play an important role in measuring the air, gas or liquid pressure in your system to keep it running safely and smoothly. However, for optimum efficiency, you will need to make sure the pressure range you choose for your gauge aligns with your application requirements and a few other factors.
To properly select a pressure gauge, consider the gauge process, range, environment, accuracy, dial size, connection and mounting requirements.
Accuracy
For a mechanical pressure gauge, accuracy is defined as a percentage of the full-scale range. While requirements differ from one industry to another, the following are general guidelines:
| ASME B40.1 | Typical Use | Accuracy |
| Grade 3A | Test Gauges | +/-0.25% of span |
| Grade 2A | Process Gauges | +/-0.5% of span |
| Grade 1A | Industrial Gauges | +/-1.0% of span |
| Grade A | Industrial Gauges | +/-2-1-2% of span* |
| Grade B | Commercial Gauges | +/-3-2-3% of span* |
*Accuracy expressed as +/-2-1-2% equals +/- 1% of the middle half of the range with +/-2% over the remaining half of the scale. +/-3-2-3% equals +/-2% over the middle half over the range and 3% over the remaining half.
Gauge accuracy standards as determined by ASME B40.100
Dial Size
Pressure gauge dial sizes range from less than 1˝ to 16˝ diameters. Generally, readability requirements, space limitations and required gauge accuracy determine dial size. Accuracies of 0.25% or 0.5% generally have dial sizes of 41⁄2˝ or larger since more dial graduations are required.
Case Style/Material
Environmental considerations include ambient temperature, air-borne particulate, condensation, humidity, water and chemicals, all of which can affect gauge performance. Ambient temperature may affect the accuracy and integrity of the gauge. Gauges are available either temperature compensated, or non-temperature compensated. Ambient conditions may require that the gauge be isolated from temperature extremes. When required, the gauge should be isolated from temperature extremes with a flexible line assembly. When ambient conditions are corrosive, contain a large number of particulate or if the gauge will be exposed to a wet or humid environment like humidity, wash-downs or rain, specify a gauge that is weatherproof/hermetically sealed or liquid filled.
Media/Wetted Parts
The wetted parts of the pressure gauge, the Bourdon tube and socket must be compatible with the process media. If not compatible with the wetted parts of the gauge, corrosion will occur. Corrosion of gauge wetted parts will eventually cause gauge failure and possibly safety issues. When the gauge wetted parts are not compatible with the process media, a diaphragm should be considered.
Connection Size
Gauges are available with a variety of connections including NPT, DIN, JIS, BSP & SAE. Process pressure gauges with 4 1⁄2˝ dial sizes or larger are most often supplied with a 1⁄2˝ NPT connection to best support the gauge. Factors to consider when selecting a pressure gauge connection include process pressures, gauge size and weight, space limitations, leak integrity, and past experience.
Connection Location
Consider the following mounting options when selecting a pressure gauge:
- Stem mount lower connect
- Wall/surface mount lower connect
- Panel mount back connect
- U-clamp flush mount back connect, for panel mounting
- Front flange flush mount back connect, for panel mounting
Pressure Ranges
ASME B40.100 recommends that normal operating pressure be confined to 25%-75% of the scale. If pulsation is present in the process, maximum operating gauge pressure should not exceed 50% of the full-scale range.
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