Labeling of instruments

The receiving gauge in the control room works on the transmitted pneumatic pressure, 15 psi giving full scale, but has its dial calibrated in terms of the plant pressure that it is indicating. The Bourdon tube of such a gauge is capable of withstanding only a limited amount of overpressure above 15 psi before it will burst. Furthermore, the material of the Bourdon tube is chosen for air and may be unsuitable for direct measurement of the process fluid pressure. 

A pressure gauge of this sort with a scale reading up to 1,200 psi was installed directly in the plant. The plant gauge pressure was 800 psi, and the gauge was damaged. 

Gauges of this type should have the maximum safe working pressure clearly marked in red letters on the face. 

If more than one sort of unit is used in your plant for measuring pressure or any other property, then the units used should be marked on instruments in large, clear letters. You may use different colors for different units. Everyone should be aware of the differences between the units. However, it is better to avoid the use of different units. 

A damper on the equipment was operated by a lever, whose position was indicated by a scale, calibrated in degrees of arc. These were converted to °F  

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