What is Instrument Index?

Instrument index is a document containing list of instrument devices within a plant.

Instrument index shall include tag number of all physical instruments (e.g. field instrument, physical alarm and indicator) and pseudo instruments which commonly named “soft tag” (e.g DCS indication, alarm, controller).

Instrument index shall be created at the beginning of project and considered as a live document which should be kept updated even though the plant has been operated.

Instrument index shall be revised if there is any plant or system modification which impact to additional, removal, or resetting of instrument

In instrument index document, the following information should be stated but not limited to:

  • Tag number
  • Loop Number
  • Type of Instrument
  • Location
  • Service description
  • P&ID Number
  • Line number or equipment number
  • I/O Type
  • Control System
  • Range or set point along with engineering unit used
  • Applicable reference Document (Instrument Data Sheet Number, Hook-up Drawing Number, Instrument Layout Number, Loop Drawing Number)
  • Package Number
  • Manufacturer
  • Model Number

The following references are required in preparing instrument index to make it complete of information:

Reference drawing: P&ID, HMB

  1. P&ID (Piping and Instrumentation Diagram)

From P&ID all instrument tag number are gathered along with its associated information such as service description, instrument type, line/equipment number, set point. A good quality P&ID may have distinct symbol that distinguish control system to which each instrument connected.

  1. HMB (Heat and Material Balance)

We can determine measurement range of instrument based on process data on each stream from HMB (Heat and Material Balance).

Reference document: Cause & Effect

Instrument index should include fire and gas related tag numbers. Fire and gas devices usually do not appear in P&ID, they are stated in cause and effect instead.

The purpose of instrument index

From its self explanatory name, instrument index will be referred as an index for many purposes as follow:

  • As a basis to prepare the I/O list by extracting only the tag number which has I/O point

  • Searching, Listing, Filtering a tag number

The following link is typical instrument index (for example only):


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