Every reading in a plant has a name. 31TT0231.PV. P1_PIT01. RoomA:Sensor__CO2. To the engineer who chose it, the name says everything. To everyone else, including every piece of software that has to use it, it is a code. This is how to read one, and why reading it is never quite enough.
The letters: what is measured, and what the instrument does
Most process plants follow the letter code of the standard ISA-5.1. The first letter says what is measured. The letters after it say what the instrument does with it.
| First letter | Measures | Following letters | Does |
|---|
| T | temperature | I | indicates (shows the value) |
| P | pressure | T | transmits (sends it on) |
| F | flow | C | controls |
| L | level | V | is a valve |
| A | analysis (oxygen, pH, CO2…) | A | raises an alarm |
| S | speed | H / L | high / low |
| Z | position | S | is a switch |
So PIT is a pressure indicating transmitter, FCV a flow control valve, LIT a level transmitter that also shows its value, and TIC a temperature controller. P1_PIT01, from a public test plant in Korea, is pressure transmitter 01 in process 1.
The numbers, and the ending
Around the letters sit numbers. A common pattern puts the area first and the loop number after: in 31TT0231, area 31, temperature transmitter, loop 0231. Instruments that work together in one control loop share the loop number, so 31TT0231, 31TIC0231 and 31TV0231 are the sensor, the controller and the valve of the same loop. Common, not universal: every site bends the pattern.
The ending says which value of the instrument you are looking at. .PV is the process value, what is actually measured. .SP is the setpoint, what the controller is trying to reach. .OP or .OUT is what the controller tells the valve to do. Confusing the measurement with the setpoint is one of the most common mistakes in industrial data, because the two often sit close together and look alike.
Other industries, other codes
Power plants often use KKS, a German system that codes the plant, the system and the component in one string, or its successor RDS-PP (IEC 81346). Norway's offshore industry codes tags after NORSOK Z-DP-002. Building management systems mostly use free text, like Heating:DH__water_temperature__supply. Water utilities use all of these, and a fair share of names that only make sense to whoever typed them in 1998.
Why the standard is never enough
A standard tells you what a name should mean. It cannot tell you that the transmitter named TT was moved to another pipe in 2011, that LT03 was replaced by LT03B and nobody removed the old one, or that a reading labelled in % is really in millimetres. That knowledge sits in people. When they retire, it goes with them.
This is the problem we work on. Our software reads a plant's own exports and works out what each reading is from three things at once: the name, the way the numbers behave, and the physics they must obey. When the evidence is not strong enough, it says so and asks a person, instead of guessing. And when someone asks in their own words, “the pressure at PIT01 in process 3” or “CO2 in room D”, it finds the reading by the plant's own names, and says plainly when the plant has no such thing.