The instrument index is the master register of every instrument tag on a project. One row per tag, one workbook or database that every discipline joins to. Its columns are not decorative: each field holds a value that a downstream deliverable will read, and each value has a home document where it is authoritative. The tag number, service, and loop number come off the P&ID. The measured range and signal type come off the instrument datasheet. The loop diagram number comes from the loop drawing register, and the manufacturer and model arrive once the device is purchased. This note walks every field in a working instrument index, states what it holds, names the source document the value comes from, and shows how the register stays consistent when a drawing is revised.
This is the field reference for the index specifically. For how the index sits alongside its neighbours, see instrument index vs I/O list vs line list. For the columns of the wired subset, see I/O list columns explained.
The full field table
The columns below are ordered the way a working register reads them: identity first, then drawing references, then process data, then device and procurement, then classification and status. On any given project some columns are combined and a few extra ones are added, but this is the set that a complete index carries.
| Field | What it holds | Source document | Example |
|---|---|---|---|
| Tag number | The unique identifier for the instrument | P&ID | PT-101 |
| Instrument type | Function letters without the loop number, used for grouping and counts | Derived from the tag | PT |
| Service description | Plain-language statement of what the instrument measures or controls | P&ID, process | Separator inlet pressure |
| Loop number | The control or measurement loop the tag belongs to | P&ID, tag | 101 |
| Area / unit | Process area or plant unit the tag sits in | P&ID title block, plot plan | Unit 34 |
| Location plan reference | The layout drawing that shows the physical position | Location, plot plan | Unit 34 layout |
| P&ID number | The drawing the instrument appears on | P&ID title block | PID-014 |
| P&ID revision | The version of that drawing the row was built from | P&ID | Rev C |
| Loop diagram number | The loop drawing that details the tag's wiring | Loop drawing register | LD-101 |
| Installation detail | The hook-up or mounting standard detail | Installation standard | HK-04 |
| Process line | The line number the instrument sits on | P&ID, line list | 4"-CS-101-A1A |
| Process fluid | The medium being measured | P&ID, line list | Sour gas |
| Design conditions | The pressure and temperature basis | Process data sheet, line list | 5500 kPa, 60 degC |
| Measured range | The calibrated span of the device | Instrument datasheet | 0 to 1000 kPa |
| Engineering units | The unit that qualifies the range | Instrument datasheet | kPa |
| Manufacturer | The selected vendor | Datasheet, requisition | Meridian Process Co. |
| Model number | The selected model | Datasheet | MX-500 |
| Datasheet number | The specification sheet reference and its revision | Datasheet register | DS-0101 Rev B |
| Requisition / PO | The purchasing reference | Procurement | RQ-4400 |
| Process connection | The line size and connection type | Datasheet | 2 in flanged |
| Signal / supply type | The output signal or supply the device provides | Datasheet | 4-20 mA, 2-wire |
| Wired signal | Whether the tag carries a wired signal to a controller | Derived from type, P&ID | Y |
| System | Whether the tag belongs to the control or the safety system | Cause-and-effect, SIS register | SIS |
| Hazardous area | The area classification and the device Ex rating | Area classification drawing, datasheet | Zone 1 IIB T4 |
| Status | New, existing, or removed on the current revision | Revision markup | New |
Identity columns
The identity columns name the row. Every other field qualifies it.
The tag number is the primary key and must be unique across the whole register. It is read off the P&ID bubble and it is the string every other document joins on. The instrument type is the function letters carried separately from the loop number, so a project engineer can filter the index to every transmitter or every control valve without parsing text. The service description states what the instrument does in words a reader who does not know the process can follow, and it should match the description used on the loop diagram and the datasheet so three documents do not disagree. The loop number groups the tags that participate in one loop, which is what lets the index be sorted to show a complete loop at a glance.
Tag structure follows the project's identification standard. On most projects that is ISA, covered in ISA 5.1 instrument tags, but the index carries whatever dialect the drawings use, and the type column is derived from the tag under that dialect's rules.
Drawing reference columns
These columns tie the tag back to the documents that define it.
The P&ID number is the drawing the tag appears on, and the P&ID revision is the field that makes the reference useful. The number alone tells you which drawing. The revision tells you which version of that drawing the row was built from. The loop diagram number points to the loop drawing that shows the wiring and terminations for the tag, and the installation detail points to the standard hook-up drawing for the mounting and process connection. These are references, not data: the index holds the document number, and the detail lives on the document.
Process and device columns
The process columns describe what the instrument sees. The device columns describe the instrument that was bought to see it.
The process line, process fluid, and design conditions come from the P&ID and the line list, and they set the basis the instrument is specified against. The measured range and engineering units come from the instrument datasheet, because the calibrated span is a property of the selected device, not of the drawing. The same applies to the manufacturer, model number, process connection, and signal type: these fields are blank in the preliminary index and are filled as datasheets are issued and devices are purchased. The datasheet number and requisition columns are how the index links to the procurement package, so a reviewer can move from a tag to its specification and its purchase order without a separate lookup.
What the index does not carry
The boundary that most often gets blurred is the one with the I/O list. The index records whether a tag is wired, in a simple yes or no column. It does not record the signal class. The AI, AO, DI, DO classification, the PLC rack, slot, and channel, and the wire numbers all belong to the I/O list and the cable schedule, not the index. Keeping that line clean is what stops the index from turning into a second, unmaintained copy of the I/O list. The full set of wired-signal columns is covered in I/O list columns explained.
Which deliverable reads each field
Every column earns its place because a downstream document reads it. The map below shows where the load falls.
| Index field | Deliverable that consumes it | Why it is needed there |
|---|---|---|
| Tag number | Loop diagram, I/O list, cable schedule, datasheet | The key every downstream document joins on |
| Loop number | Loop diagram | Groups the loop's instruments onto one drawing |
| P&ID number and revision | Every deliverable | Traces each row to the drawing version it came from |
| Wired signal flag | I/O list | Selects which tags become I/O rows |
| Service description | Datasheet, loop diagram | Carries a consistent plain-language identity |
| Process line and fluid | Datasheet, calibration | Sets the wetted-material and range basis |
| Manufacturer and model | Maintenance system, spares | Identifies the physical device installed |
| Datasheet number | Procurement package | The requisition attaches the datasheet by this reference |
| Area and location | Cable schedule, junction box allocation | Drives field routing and marshalling |
| Hazardous area and certification | Installation, inspection | Confirms the device suits the area classification |
Format: one row per tag, locked columns
Most instrument indexes are Excel workbooks, because the register is tabular and every discipline that touches it has Excel. Larger projects hold the index in a database-backed engineering register and export to Excel for each issue. The format matters less than the discipline: one row per tag, the tag number as the unique key, a column order that is locked at the first issued revision, and new columns inserted at the right edge rather than between existing ones so that any tool reading the sheet by position does not break. An instrument index template gives a starting column set that already follows these rules.
How the fields stay consistent across revisions
The index is a living document, and the P&ID revision is the parent that drives it. When a drawing is revised, the sequence is fixed: the index is updated first, then the I/O list and the other downstream registers follow, because they depend on it.
At the field level, three rules keep the register honest. First, every row carries the P&ID revision it was built from, so a reviewer can tell a current row from a stale one without checking every drawing. Second, a status column marks each row as new, modified, or removed on the current revision, which is what turns a re-issued index into a readable change record rather than a fresh list. Third, the tag number is the join key: when a tag is added or retagged, the change propagates through every document that references it by that key. Under management of change, a row whose P&ID revision trails the current issued drawing is a finding that has to be closed. For the wider publication rhythm across FEED, issued for construction, and as-built, and for a revision comparison workflow that produces an added, removed, modified diff, see the instrument index vs I/O list vs line list reference.
Who owns the index
The lead instrument engineer owns the instrument index. Document control holds the controlled master and issues revisions. Procurement builds the purchase package from it, commissioning builds the loop-check register from it, and maintenance inherits it as the as-built tag register, but none of them edit it. Fixing that ownership at kickoff is what prevents the parallel, diverging copies that surface during commissioning.
A structured extraction from a P&ID set produces the index directly, every bubble becoming a row, delivered as an instrument index in Excel with the source columns populated and the datasheet and range fields left open for detailed design to complete. See P&ID to I/O list for the wired subset that follows.