The companion identification system to KKS: where KKS identifies plant systems and equipment (area-first), ANSI/ISA-5.1 identifies the individual field instrument itself, by the variable it measures and the function it performs — the FT/PIC/LAH-style tags you see in a P&ID bubble. Complete official letter table, plus the everyday tag combinations built from it.
How a tag is built — and how it relates to KKS
An ISA-5.1 tag reads left to right as: first letter (the measured/initiating variable, or a modifier) + one or more succeeding letters (the function the instrument performs) + a loop number. PT-101 is a Pressure Transmitter on loop 101; TIC-205 is a Temperature Indicating Controller on loop 205. Redundant instruments on one loop add a trailing letter — FT-101A and FT-101B are two transmitters on the same loop.
KKS and ISA-5.1 solve genuinely different halves of the same identification problem, and a real plant typically carries both: KKS’s multi-segment code (e.g. 10LAC01CT001) identifies where a device sits in the plant — unit, system, sub-system, item number — largely independent of what the device actually does. An ISA-5.1 tag (e.g. FT-101) identifies what the device does — measures flow, transmits it — largely independent of where it physically sits. KKS is the dominant convention in German-influenced and much of European/Indian power-station engineering; ISA-5.1 dominates North America and most international process-industry P&IDs. On a real project using both, the same physical transmitter can carry a KKS location tag on the plant equipment list and an ISA-5.1 loop tag on the P&ID and instrument index — two labels for one device, answering two different questions.
Readout/passive functionZ-axis, Safety Instrumented System
Output/active functionDriver, Actuator, Unclassified final control element
Function modifier—
Common Tag Combinations
PT — Pressure Transmitter
PI — Pressure Indicator
PIC — Pressure Indicating Controller
PIT — Pressure Indicating Transmitter
PDT — Pressure Differential Transmitter
The D is a variable modifier applied right after the first letter, not a separate function — same pattern as TDT (Temperature Differential Transmitter) or FDT.
PSH — Pressure Switch High
PSL — Pressure Switch Low
PSHH — Pressure Switch High-High
A doubled modifier (HH, LL) marks a second, more severe alarm/trip threshold above the first — e.g. PSH warns, PSHH trips.
PSLL — Pressure Switch Low-Low
PSV — Pressure Safety Valve
S here is the "Safety" modifier, not "Switch" — context (a valve, not an electrical contact) disambiguates it; this is the mechanical relief valve, not an instrument loop.
FT — Flow Transmitter
FIC — Flow Indicating Controller
FFRC — Flow Ratio Recording Controller
F as a succeeding letter (ratio) plus F as the first letter (flow) — the two F's mean different things depending on position.
FCV — Flow Control Valve
FE — Flow Element
The primary sensing element itself (e.g. an orifice plate) — distinct from FT, the transmitter reading it.
LT — Level Transmitter
LIC — Level Indicating Controller
LG — Level Gauge
G here is "Glass, Gauge, Viewing Device" — a local sight glass, not a transmitted signal at all.
LAH — Level Alarm High
LAL — Level Alarm Low
TE — Temperature Element
The sensor itself (e.g. an RTD or thermocouple) — distinct from TT, the transmitter.
TT — Temperature Transmitter
TIC — Temperature Indicating Controller
TRC — Temperature Recording Controller
The first succeeding letter (R, passive/readout) records for a human operator; the second (C, active/output) is the automated control action — this passive-then-active letter order is a general ISA-5.1 convention, not specific to temperature.
AIT — Analytical Indicating Transmitter
A covers any analysis not given its own letter — the specific analysis (O2, pH, conductivity...) is defined outside the tag bubble, in the instrument index or P&ID note.
AE — Analyzer Element
SIC — Speed Indicating Controller
ST — Speed Transmitter
ZI — Position Indicator
Commonly a valve limit-switch or damper position indication.
ZSO — Position Switch, valve confirmed Open
A common project-level extension: O/C suffixes for open/closed confirmation aren't in the base ISA table itself, but are near-universal practice.
HS — Hand Switch
An operator pushbutton or selector switch — H as first letter means "Hand" (manually initiated), not a measured process variable at all.
HIC — Hand Indicating Controller
A manual loader/setpoint station — the operator directly sets the output, there's no automatic control action.
YS — Relay/Compute, Switch function
Y is reserved for a function that doesn't fit the standard measured-variable categories — commonly a relay, solenoid, or logic/compute block.
About This Calculation
ANSI/ISA-5.1 is the standard used across most process, power, and manufacturing industries worldwide — dominant in North America, and widely used internationally alongside KKS and its own IEC 60617 counterpart — to identify what an individual field instrument actually measures and does. Where KKS answers "where in the plant does this sit", ISA-5.1 answers "what does this device do": a tag like FT-101 or TIC-205 is built entirely from a first letter (the measured variable), one or more succeeding letters (the function performed), and a loop number. This reference presents the complete official identification letter table from ANSI/ISA-5.1-2009 — not an abbreviated vendor cheat sheet — alongside the everyday tag combinations (PT, FIC, PSHH, and dozens more) actually built from it, each with a short note on why the combination reads the way it does.
Frequently Asked Questions
How do I read a multi-letter tag like PDIT or TRC?
Left to right: the first letter is the measured variable (or a modifier applied to it, like D for differential), and every letter after that adds a function. PDIT is Pressure Differential Indicating Transmitter — P (pressure) + D (differential modifier) + I (indicate) + T (transmit). TRC is Temperature Recording Controller: the passive/readout function (R, record — for a human) comes before the active/output function (C, control — automated), which is the standard ISA-5.1 letter ordering, not specific to temperature.
Why does the same letter mean different things in different positions?
ISA-5.1’s table has separate columns for exactly this reason: a letter’s meaning depends on whether it’s the first letter or a succeeding letter. R as a first letter means Radiation; as a succeeding letter it means Record. S as a first letter means Speed; as a succeeding letter it can mean Safety, Switch, or Stop depending on which succeeding-letter column it appears in. Always check the column, not just the letter.
What’s the difference between FE, FT, and FIC on the same flow loop?
They’re three different devices on one measurement, not three names for the same thing: FE is the Flow Element, the physical primary sensor (e.g. an orifice plate) that actually contacts the process; FT is the Flow Transmitter that converts the element’s signal into a transmittable value; FIC is the Flow Indicating Controller that displays it to an operator and can act on it. A real loop diagram often shows all three tied to the same loop number.
How does a KKS tag and an ISA-5.1 tag relate to the same physical device?
They can both exist on the same instrument, answering different questions: the KKS tag (e.g. 10LAC01CT001) fixes its location in the plant hierarchy — which unit, which system, which item number — while the ISA-5.1 tag (e.g. FT-101) fixes its function — what it measures and what it does. A project using both typically cross-references the two in its instrument index, rather than picking one over the other.
Are "N" and "O" really left undefined by the standard?
Yes, deliberately — ISA-5.1 reserves certain letters (most notably N and, in some editions, parts of the table) as "User’s Choice", handed back to each project to define in its own instrumentation legend sheet. This reference marks those slots as User’s Choice rather than inventing a meaning for them; always check the specific project’s legend for what they’ve been assigned to mean there.