Instrument Tag or Number

An Instrument number or tag has two parts which is combination of letter and number.

• Identification letter for indicating the function and purpose of use.
• An individual tag number/Loop No./serial number.

Letter is used to classify the instrument by it's function and number is used to identify the loop/interlock. Identification letters on the ISA symbols indicate:

• The variable being measured (e.g. flow, pressure, temperature).
• The device’s function (e.g., transmitter, switch, valve, sensor, indicator).
• Some modifiers (e.g., high, low, multifunction).

In the instrument tag number in above figure the initial letter indicates the measured variable. The second letter indicates a modifier, readout, or device function. The third letter usually indicates either a device function or a modifier.

In that cases where the instrument is considered to be identified by it's location/area then the instrument tag number format will be as bellow: