Mechanical pressure gauges require no external power and provide a reliable means of accurate pressure measurement. To properly select a pressure gauge you need to consider the process medium (liquid or gas it will be used on), range, environment, accuracy, dial size, connection and mounting requirements.

1. Process Medium

The wetted parts of the pressure gauge, the bourdon tube and socket must be compatible with the process medium. Bourdon tubes are most commonly made from phosphor bronze, 316 stainless steel, or monel. If it is not compatible or the media is extremely viscous or dirty, a diaphragm seal / gauge isolator can be used but it will add an additional minimum error of around 0.5% to the gauge / seal assembly.

2. Pulsation and Vibration

Pulsation and vibration will decrease the life of the gauge. In these situations a liquid filled gauge or an internally dampened gauge should be used. Ashcroft offer liquid filled gauges as well as a unique patented method of gauge dampening, that provides the performance of a liquid filled gauge without requiring a liquid fill. Other accessories that minimise the stresses on the bourdon tube / movement include pulsation dampers, pressure snubbers, gauge savers and diaphragm seals but these will all add an additional error to the reading.

3. Pressure Gauge Range

ASME B40.1-1998 recommends that the normal operating pressure should be confined to 25% to 75% of the scale. If pulsation is present then the maximum operating gauge pressure should not exceed 50% of the full-scale range.

4. The Environment

Environment considerations include temperature, airborne particulates (particles in the air), condensation, humidity, water and exposure to other chemicals. Temperature can affect the accuracy and integrity of the gauge. Gauges are usually available either temperature compensated or non-temperature compensated. When ambient conditions are corrosive, contain a large number of particulates or maybe exposed to a wet or humid environment including wash-downs or rain, a weatherproof/hermetically sealed or liquid filled gauge should be used.

5. Accuracy

Accuracy is usually defined as a percentage of the full scale range. As a guide the following is often used:

• Test gauges: 0.1% to 0.25%
• Critical Processes: 0.5%
• General Industrial Process: 1.0% to 2%

Refer to ASME B40.1-1998 or the DIN specifications for more information on accuracy.

6. Dial Size

Dial sizes typically range from 1.1/2" to 16" but the most commonly used sizes are 100mm (4") and 150mm (6"). Generally readability requirements, space limitations and required gauge accuracy determine the dial size.

7. Gauge Connection

Gauges are available with a variety of connection threads including BSP, NPT, DIN, JIS & SAE. 3/8" and 1/2" are the most commonly used.

8. Mounting Requirements

Usually available as:

• Direct stem mount - Lower connection
• Wall/Surface Mount - Lower connection
• Panel surface mount - Back connection
  
• Panel hole U-Clamp flush mount - Back connection
• Panel hole front flange flush mount - Back connection

Glossary of Gauge Related Terms:

• Gauge Pressure: Indicates pressure from a single source and uses ambient pressure as zero.
• Absolute Pressure: The pressure measured above a perfect vacuum. i.e. Absolute Pressure = Gauge Pressure + Atmospheric Pressure
• Differential Pressure: The difference between two independent, but related pressures.
• Compound Pressure Gauges: Indicate pressures above and below ambient pressure - positive and negative pressure.
• Duplex Gauges: Utilises two independent sensing elements each connected to different but related pressure sources.