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Letters in an instrument bubble naming its function.
Also known as: ISA tag letters, Instrument identification letters, Loop tag letters
On a piping and instrumentation diagram the letters in an instrument bubble encode what it measures and what it does: the first letter names the measured variable and those following name the function.
Encodes the measured variable in the first letter and the instrument's function in those that follow, so FIC denotes flow indicating controller.
Contexts: Technical drawing
Carries more of the drawing's meaning than the bubble outline does, which is why an instrument symbol cannot be read from its shape alone.
Contexts: Technical drawing
Two or more capital letters written in the upper half of an instrument circle, above a loop number.