Common Instrument Tags List (ISA 5.1 Examples)
A searchable list of the instrument tags you meet on every P&ID and instrument index, with the meaning of each, grouped by measured variable.
How to Read a Tag
The first letter is the measured variable (F flow, P pressure, L level, T temperature, A analysis). A modifier may follow (D differential, Q totalise). The next letters give the function (I indicate, T transmit, C control, S switch, A alarm, V valve) and H or L give the limit. The number is the loop number, shared by every instrument in the same loop.
Frequently Asked Questions
What does PSV stand for?
Pressure safety valve, a relief valve that protects equipment from overpressure.
What is the difference between LSH and LAH?
LSH is a hardware level switch; LAH is an alarm, often generated in the control system from a transmitter.
What does XV mean?
An on off or shutdown valve; X is the unclassified first letter commonly used for such valves.
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