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Process Instrumentation - ISA Codes

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It is common to make letter combinations in PI&D diagrams according ANSI/ISA S5.1-1984 (R 1992) "Instrumentation symbols and identification".

In addition to the letter combination a succeeding number as a unique identification of the actual instrument is commonly used. The numbering practice varies - some use a sequential number, others use a number related to the process line number or similar.  

First Letter

First letter indicates a measured or initiating variable, or a modifier, such as Current (I), Speed (S) or Flow (F).

Measured or Initiating Variable

  • A - Analysis
  • B - Burner, combustion
  • C - User's choice
  • D - User's choice
  • E - Voltage
  • F - Flow rate
  • G- User's choice
  • H - Hand
  • I - Current (electrical)
  • J - Power
  • K - Time, time schedule
  • L - Level
  • M- User's choice
  • N- User's choice
  • O- User's choice
  • P - Pressure, vacuum
  • Q - Quantity
  • R - Radiation
  • S - Speed, frequency
  • T - Temperature
  • U - Multivariable
  • V - Vibration, mechanical analyses
  • W - Weight, force
  • X - Unclassified
  • Y - Event, state or presence
  • Z - Position, dimension


  • D - Differential
  • F - Ration (fraction)
  • J - Scan
  • K - Time rate of change
  • M - Momentary
  • Q - Integrate, totalizer
  • S - Safety
  • X - X-axis
  • Y - Y-axis
  • Z - Z-axis

Second or Succeeding Letters

Second or succeeding letters indicates a readout or passive function, output function or a modifier function.

Readout or Passive Function

  • A - Alarm
  • B - User's choice
  • E - Sensor (primary element)
  • G- Glass, viewing device
  • I - Indication
  • L - Light
  • N- User's choice
  • O- Orifice, restriction
  • P - Point (test connection)
  • R - Record
  • U - Multifunction
  • W - Well
  • X - Unclassified

Output Function

  • B - User's choice
  • C - Control
  • K - Control Station
  • N- User's choice
  • S - Switch
  • T - Transmit
  • U - Multifunction
  • V - Valve, damper, louver
  • X - Unclassified
  • Y - Relay, compute, convert
  • Z - Driver, actuator

Modifier Function

  • B - User's choice
  • H - High
  • L - Low
  • M - Middle, intermediate
  • N- User's choice
  • U - Multifunction
  • X - Unclassified

Examples - P&ID codes

 Flowmeter - Indicating

  • FI 001

Temperature - Transmitter

  • TT 001

Control Valve

  • FV 001

Position Switch - High Level

  • ZSH 001
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