ISA 5.1 identification letters

Tag letters inside any bubble, e.g. "FIC" over "101": first letter = measured variable, following letters = functions.

FIRST LETTER (measured or initiating variable, with optional modifier): A analysis, B burner/combustion, C user's choice (often conductivity), D user's choice (often density), E voltage, F flow (modifier F ratio), G user's choice (often gauging/position), H hand, I current, J power, K time/schedule, L level, M user's choice (often moisture), N user's choice, O user's choice, P pressure/vacuum, Q quantity (modifier Q integrate/totalise), R radiation, S speed/frequency (modifier: S safety), T temperature, U multivariable, V vibration/machinery analysis, W weight/force, X unclassified, Y event/state/presence, Z position/dimension. First-letter modifiers: D differential, F ratio, J scan, K time rate of change, Q integrate/totalise, S safety, X/Y/Z axis. SUCCEEDING LETTERS: A alarm, B user's choice, C control, D differential (modifier), E sensor/primary element, G glass/gauge/viewing device, H high, I indicate, K control station, L light or low, M middle/momentary, N user's choice, O orifice/restriction, P point (test connection), Q integrate, R record, S switch, T transmit, U multifunction, V valve/damper/louvre, W well (thermowell), X unclassified, Y relay/compute/convert, Z driver/actuator/final control element. Examples: PT pressure transmitter, PDT differential-pressure transmitter, FE flow element, FIC flow indicating controller, LSHH level switch high-high, TW thermowell, PSV pressure safety valve, XV on/off valve, HS hand switch, ZSO position switch open, AT analyser transmitter, FQ totaliser, LG level gauge.