Instrument range shall be selected based on operating range of process to be measured. In common practice, it is recommended to select instrument with full scale range that can give operating process reading between 30% and 70% especially for pressure gauge. The following is an example of selecting pressure gauge range
Given Process data:
Minimum operating pressure: 5 barg
Maximum operating pressure: 9.5 barg
Maximum Design pressure: 25 barg
Available standard Pressure Gauge range: 0 – 10 barg, 0 – 16 barg, 0 – 25 barg
Selecting 0-10 barg will give 95% reading during maximum operating condition. Instrument has only narrow margin above maximum operating pressure.
Selecting 0-16 barg range will give best reading during operating condition. However, the maximum overpressure of pressure gauge is commonly 1.3x of the maximum scale which is only 16×1.3 = 20.8 barg. In such case, pressure gauge shall be equipped with overrange protector to withstand design pressure.
Selecting 0-25 barg can withstand to 32.5 barg (25 x 1.3) without using overrange protector, however it would give poor measurement indication during normal operating condition.