Instrument Codes

In general, every conventional measuring or controlling instrument Installed in a process unit is identified by three separate codes as follows.

􀂾 A location number code indicates the specific process unit in which the instrument is installed.

􀂾 A function letter code indicates the property or process variable being measured or controlled.

􀂾 A serial number code identifies the specific instrument and therefore prevents confusion when there are several Instruments In a single process unit, each having the same function letter code.

The combination of the three codes is known as the Instrument tag number, which has the basic format -

xx a - yyy


“xx” is a two-digit number used to identify the process unit.

'a’ is a letter code containing two or more capital letters and is used to identify the instrument function.

‘yyy’ is a three-digit number used to identify the particular instrument.

When the instrument code or tag number is written on a drawing or document, a dash is inserted between the ‘a’ and the ‘yyy’ sections of the format. For example, a pressure indicating controller installed in a process unit coded 10 and identified by serial number 101, is described in written form as 10 - PIC - 101.

In the case of the same tag numbers, the process pressure correcting element, usually a control valve, often has the same tag number as the control instrument.

However, when the controller operates two valves in a split range mode, the valves are tagged and numbered consecutively, for example,

10 - PIC - 101 >>>>>>>>>>>>>>>>>>>> 10 – PCV – 101-1 and 10 – PCV – 101-2

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