Instrument symbols
On a P&ID, every instrument is a bubble — a circle carrying a short tag. Two things tell the whole story: the outline of the bubble says where the function lives, and the letters inside say what it measures and what it does. Here are the four bubble outlines drawn in clean ISA style, the XX-NNN tag format decoded, and how to get each symbol as PNG/SVG.
The instrument bubble and its outline
All instrument symbols start from the same circle. Small additions to the outline tell you the mounting / location class — which is what determines whether an operator can actually see and act on the loop:
A horizontal line drawn solid means the function is normally operator-accessible (a primary display); some drawings use a dashed line for a function that is accessible but not normally displayed, and a square for logic that lives in a PLC. Modern HMIs blur DCS and PLC, but the outline persists on P&IDs because it reflects where the logic runs, not where the value happens to be shown — a PLC-controlled loop behind a SCADA faceplate is still drawn with the square.
Reading the tag: the XX-NNN format
The text inside the bubble is an ISA-5.1 tag in two parts joined by a hyphen — for example FCV-104 or LIT-201:
- Function letters (1–4 letters). The first letter is the measured variable; the succeeding letters describe the function.
- Loop number
NNN(usually 3 digits) identifies the specific loop. Conventionally grouped by area —1xxfor unit 1,2xxfor unit 2, and so on.
Optional suffixes (A/B/C, S, X) handle redundancy, switches and accessories: PT-101A, HS-204, FCV-104S.
First letter — the measured variable
| Letter | Variable |
|---|---|
F | Flow |
T | Temperature |
P | Pressure |
L | Level |
A | Analysis (pH, O₂, conductivity…) |
W | Weight / force |
S | Speed / frequency |
V | Vibration |
Z | Position |
H | Hand (manual) |
Succeeding letters — the function
| Letter | Function |
|---|---|
I | Indicator (local readout) |
C | Controller |
T | Transmitter |
R | Recorder / historian |
S | Switch |
V | Valve / final control element |
Y | Computing / converting (I/P, math) |
E | Sensing element (raw) |
H / HH | High / high-high alarm or limit |
L / LL | Low / low-low alarm or limit |
Putting the two halves together:
FCV-104— flow control valve on loop 104.LIT-201— level indicating transmitter on loop 201.PIT-101— pressure indicating transmitter (a transmitter with a local display).FIC-301— flow indicating controller (a controller with a built-in display).LSHH-401— level switch high-high (a discrete trip device for safety logic).
This is just the working subset. The full letter table — every first letter, modifier and succeeding letter — is in our ISA-5.1 instrumentation symbols reference.
Discrete vs shared / computer function
The same letters describe two physically different kinds of device, and the difference matters for safety reviews:
- Discrete instruments are dedicated hardware doing one job — a standalone transmitter, a vibrating-fork level switch, a limit switch.
LSHH-401wired into the safety system is typically a separate discrete device, independent of the DCS, as functional-safety practice (ISA-84) usually mandates. - Shared display / shared control functions live inside a DCS or PLC and share processing and screens with many other loops. ISA-5.1 marks these with the line-through-circle (shared display) or the square (PLC) outlines shown above, sometimes using a hexagon for a dedicated computer function.
So LIT-401 on a line-through bubble is an analog value shown on a shared DCS screen, while LSHH-401 on a plain field bubble is a hardwired trip — same loop, different worlds.
Where to get instrument symbols
Instruments are the easiest family to produce yourself: the geometry is four outlines and the content is a tag. The four bare mounting bubbles and the signal-line styles are in the generic icon library; tagged instruments (PT-101, LIT-201, LSHH-401…) are published by engineers in the community library, or you compose them in the builder — pick the bubble, type the tag, export:
Get instrument symbols as PNG / SVG
Browse the tagged instruments engineers have published (CC BY 4.0) and download with a free account — 5 a month on Free — or compose your own bubble and tag in the builder, which is free to use without an account.
Open the builder → Browse community instrument symbols →Frequently asked questions
What does an instrument bubble symbol mean?
An instrument is a circle (the bubble) carrying a tag. The outline says where the function lives: a plain circle is field-mounted, a circle with a horizontal line is a DCS / shared display, a circle inside a square is a PLC, and a circle with a double line is a shared / auxiliary location.
How do you read an ISA-5.1 instrument tag?
Use the XX-NNN format. The first letter is the measured variable (F flow, T temperature, P pressure, L level, A analysis); the following letters are the function (I indicator, C controller, T transmitter, R recorder); NNN is the loop number. So FCV-104 is a flow control valve on loop 104 and LIT-201 a level indicating transmitter on loop 201.
Field vs DCS instrument symbol — what's the difference?
A field instrument is a plain circle, visible only at the equipment. A DCS / shared-display instrument adds a single horizontal line and is accessible from the control room. A PLC function adds a square around the circle.
Where can I download instrument symbols?
The four bare mounting bubbles and signal-line styles are in the icon library. Tagged instruments are in the community library (Instrumentation), or take a minute in the Symbol Builder: pick the bubble, type the tag, export PNG/SVG.
Keep reading
- ISA-5.1 instrumentation symbols reference — the full letter tables & mounting bubbles.
- P&ID symbols explained — the full guide to every symbol family.
- P&ID valve symbols — bodies, actuators, fail-safe positions and tags.
- Build your own symbols — free online editor.
- Community instrument symbols — tagged bubbles published under CC BY 4.0.
- Ask the community — a tag combination or a mounting case the standard leaves open.