Instrument symbols
Instruments are drawn as a bubble with a tag inside. The shape of the bubble tells you what kind of device it is, the line through it tells you where it is located, and the letters tell you what it measures and does.
- Field-mounted instrumentA field-mounted instrument is installed in the plant at the process, read and maintained there.
- Panel-mounted instrumentA panel-mounted instrument is on the front of a panel in the main control room, where the operator can see and use it.
- Behind-panel instrumentA behind-panel instrument is in the control room but not on the panel face, so the operator does not normally use it.
- Local panel instrumentA local panel instrument is on a panel in the plant, away from the main control room, such as a compressor or package control panel.
- DCS shared displayA shared display function lives in the distributed control system (DCS) and is shown to the operator on a screen shared with many other loops.
- PLC functionA PLC function is carried out in a programmable logic controller, often for sequences, interlocks and packages.
- Computer functionA computer function is carried out in a separate computer rather than the DCS or PLC, for example an advanced control or optimisation calculation.
- InterlockAn interlock symbol marks a logic function that trips or permits something when conditions are met, such as stopping a pump on low level.
- Pressure gaugeA pressure gauge shows the pressure locally on a dial, with no signal sent anywhere.
- Pressure transmitterA pressure transmitter measures pressure and sends it as a signal to the control system.
- Level transmitterA level transmitter measures the liquid level in a vessel or tank and sends it to the control system.
- Flow transmitterA flow transmitter turns the reading from a flow element into a signal for the control system.
- Temperature transmitterA temperature transmitter turns the reading from a temperature element, a thermocouple or RTD, into a signal for the control system.
- Level gaugeA level gauge shows the liquid level directly, through glass or with a magnetic float indicator, with no signal sent.
- Orifice plateAn orifice plate is a flat plate with a hole in it, clamped in the line.
- VenturiA venturi tube narrows the flow smoothly and then widens it again.
- RotameterA rotameter shows flow by the height of a float in a tapered tube.
- Magnetic flow meterA magnetic flow meter measures the flow of conductive liquids from the voltage induced as they pass through a magnetic field.
- ThermowellA thermowell is a closed tube inserted into the line or vessel that protects a temperature element and lets it be removed without opening the process.
Instruments in ISO 10628-2
About ISO 10628-2Instruments in IEC 62424
About IEC 62424- Field-mounted instrumentLocal PCE request
- Panel-mounted instrumentPCE request in the central control room
- DCS shared displayPCE request in the central control room
- Pressure gaugeLocal pressure indication
- Pressure transmitterPressure indication in the control room
- Level transmitterLevel indication in the control room
- Flow transmitterFlow indication in the control room
- Temperature transmitterTemperature indication in the control room
- Level gaugeLocal level indication
A circle is a discrete instrument, a circle in a square a shared display such as a DCS, a diamond in a square a PLC, and a hexagon a computer function. No line means field-mounted, a solid line means in the main control room, a dashed line means behind the panel.
The letters follow ISA 5.1: the first letter is the measured variable (P pressure, T temperature, F flow, L level), the following letters are the function (I indicator, T transmitter, C controller, E element, G gauge).
The cards at the top use the forms of US legend sheets (ISA 5.1). The grids below them show the same symbols as ISO 10628-2 or IEC 62424 draw them. See ISA vs ISO P&ID symbols for the differences.
Frequently asked questions
What are instrument symbols on a P&ID?
Instruments are drawn as a bubble with a tag inside. The shape of the bubble tells you what kind of device it is, the line through it tells you where it is located, and the letters tell you what it measures and does.
How many instrument symbols are there?
This legend covers the 19 most common ones: field-mounted instrument, panel-mounted instrument, behind-panel instrument, local panel instrument, DCS shared display, PLC function, computer function, interlock, pressure gauge, pressure transmitter, level transmitter, flow transmitter, temperature transmitter, level gauge, orifice plate, venturi, rotameter, magnetic flow meter, thermowell. Project legends may add company-specific variants.
Can I download these symbols?
Yes. Each symbol page has a free SVG and PNG, and the whole legend is available as a printable PDF chart on the P&ID symbols page.