ISA-5.1 Tag Letter Codes: A Reference Table
When you already know how to read a tag but need to confirm exactly what a given letter means, you want a table, not an explanation. This page is that reference: the ISA-5.1 first letters that name the measured variable, and the succeeding letters that name the function, laid out so you can look up any code you meet on a P&ID. It is written for the engineer or technician who hits an unfamiliar combination and needs to resolve it correctly rather than guess.
ISA-5.1 tag letter codes in one line: In ISA-5.1 tag letter codes, the first letter names the measured variable and the following letters name the functions. First letters include A analysis, F flow, L level, P pressure, T temperature, and S speed. Succeeding letters include I indicator, R recorder, T transmitter, C controller, V valve, S switch, and A alarm, with H and L as high and low modifiers. Read the first letter as the variable and every letter after it as a function in order.
First Letters: The Measured Variable
The first letter answers one question: what is being measured or initiated. The table below lists the common first letters. Treat any letter in this position as the variable, and remember that a few carry an implicit differential or ratio modifier.
| Letter | Measured or initiating variable |
|---|---|
| A | Analysis (composition, quality) |
| F | Flow |
| L | Level |
| P | Pressure or vacuum |
| T | Temperature |
| S | Speed or frequency |
| H | Hand (manual) |
| I | Current (electrical) |
| J | Power |
| W | Weight or force |
The letter D used as a modifier after the first letter means differential, so PD is pressure differential and TD is temperature differential. A leading modifier changes the variable, not the function, which is why you resolve the whole variable before moving to the function letters.
Not every project uses every letter, and some sites define user letters for variables the standard does not name, documenting them in the drawing legend. When you meet a first letter that is not in the standard set, check the P&ID legend before assuming, because a custom letter carries the site's own meaning. The ISA-5.1 symbology overview covers how those legends fit alongside the standard letters.
Succeeding Letters: The Function
After the variable, each letter names a function the device performs. The table below lists the common function letters and the modifiers that refine them.
| Letter | Function or modifier |
|---|---|
| I | Indicate |
| R | Record |
| T | Transmit |
| C | Control |
| V | Valve or final element |
| S | Switch or safety |
| A | Alarm |
| G | Gauge or glass (viewing) |
| Y | Relay, compute, or convert |
| H / L | High / Low modifier |
So FIT reads flow indicating transmitter, PIC reads pressure indicating controller, and LAH reads level alarm high. A doubled modifier such as HH or LL marks a more extreme trip, which usually drives a shutdown rather than a warning.
The safety letter S deserves care, because a leading or standalone S can mean switch, and in a safety context an added S can flag a device that is part of a protective function. When a tag combines S with a doubled modifier, for example PSHH, it is almost always an initiator of a trip, and you should treat it as safety-related and confirm its role against the cause-and-effect matrix. Devices tied to a safety instrumented function carry extra rigor around their tags for exactly this reason.
Using the Table to Resolve an Unfamiliar Tag
To resolve any tag, split it into the leading variable and the trailing functions, then read each part off the table in order. AIT decodes to analysis indicating transmitter, TDT to temperature differential transmitter, and FQI to flow quantity indicator, a totalizer. Because the codes are compositional, the table lets you decode combinations you have never seen without memorizing every possible tag.
When a code still does not resolve cleanly, the drawing legend and the instrument index are the authorities that override any general table. Sites tailor ISA-5.1 to their needs and record the tailoring in the legend, so a letter that looks wrong against the standard may be correct against the project. Reading a tag well means using the table for the standard letters and the legend for the local ones, and reconciling the two when they seem to disagree.
Frequently Asked Questions
What does the letter Y mean in an ISA-5.1 tag?
Y in the function position means relay, compute, or convert. It marks a device that transforms a signal rather than measuring or displaying it, such as a signal converter, a square-root extractor, or a computing relay. So FY is a flow computing or converting element, often the block that turns a differential pressure into a flow value.
Is there a single ISA-5.1 letter for every variable?
No. ISA-5.1 defines a standard set of first letters, but it also allows user-defined letters for variables it does not name, and it lets sites tailor usage. Those custom letters are recorded in the P&ID legend. When you meet a first letter outside the standard set, check the drawing legend before assuming its meaning.
What is the difference between H and HH in a tag?
A single H is a high modifier marking one trip or alarm level, and a doubled HH marks a second, more extreme high-high level. The high-high point usually drives a shutdown or interlock while the plain high point drives a warning. The same applies to L and LL on the low side. Read the doubled modifier as the higher-priority trip.
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