Counting instruments on a P&ID without missing scope.
What counts as an instrument on a bid I/O count, how to handle redundant pairs, what to ignore, and the tag-suffix patterns that flag scope you would otherwise miss.
What counts as an instrument
For the bid I/O count, an instrument is anything that.
- Takes a wire to a PLC analog input card, AI,
- Takes a wire from a PLC analog output card, AO,
- Takes a discrete input wire, DI, or
- Takes a discrete output wire, DO.
The I/O list lives in those four buckets. Anything else gets recorded but does not consume PLC channels. Mechanical gauges, sight glasses, thermowells without an inserted transmitter, manual block valves with no position feedback. They appear on the instrument index. They do not appear on the I/O list.
The first error a junior engineer makes is counting every circle on the P&ID. Half the circles are not wired devices.
How to read second-letter codes for signal class
ISA 5.1 second letters tell you the signal class without measuring anything.
- Transmitter, T. Usually 4-20 mA analog input. FT, LT, PT, TT all become AI on the PLC.
- Indicator-controller, IC. The controller logic lives in the PLC, not as a separate device. It does not consume an I/O channel. The controlled-variable transmitter and the manipulated-variable output do.
- Computing element, Y. Mostly internal to the PLC. The field signal it computes from has already been counted.
- Switch, S. Digital input. PSH, FSL, LSL, ZSC all become DI.
- Solenoid, Y on a discrete actuator. Digital output, DO.
- Element, E without a transmitter. Mechanical only, no signal.
- Glass, sight gauge, G. Mechanical only, no signal.
Tag the second-letter pattern on every circle, sum the AI, AO, DI, DO buckets per page, sum across pages. That is the bid I/O count.
SIS prefixes. Filter them out into a separate count
Tags with double-letter alarm or switch suffixes, PSHH, PSLL, FAHH, TALL, LSLL flag SIS scope. These cannot share a PLC card with BPCS scope per IEC 61511, ISA 84. They go on a separate I/O list against a SIS-certified logic solver. For the bid.
- Count them separately so the SIS cabinet sizing is its own line item.
- Apply SIL-allocation hardware multipliers, 1oo2 voting doubles the transmitter count, 2oo3 triples it.
- Quote the SIS scope at a higher per-channel cost than BPCS, because the certified hardware is more expensive.
A typical petrochemical bid might run 20 percent SIS on the total instrument count. Missing that split at bid time produces a margin disaster at the build phase.
Future, vendor-supplied, and by-others tags
Most P&ID notes blocks call out three categories of tag that are not in the integrator's scope.
- Future. Reserved tag, no instrument installed yet. Not in bid scope.
- By vendor, package controller. A packaged skid, compressor, refrigeration unit brings its own controller. Integrator does not wire individual instruments inside the package. The package interconnect is in scope. Everything inside the dotted-line skid boundary is not.
- By others. Some other contractor wires this instrument. Common with mechanical-trade-supplied valves, electrical-trade-supplied motor controls, or tank gauging on existing tanks.
On bid drawings these are flagged in the legend, the notes block, or as a tag suffix. They look like real instruments and quietly inflate the count if you are not looking. A 60-page bid drawing set might carry 10-15 percent of nominal tag count in these excluded categories. Scrub them out before you lock the count.
Redundant pairs
A redundant transmitter pair, FT-101A, FT-101B is two instruments. Each takes its own AI channel. Voting hardware, 1oo2, 2oo3 is annotated on the drawing near the redundant block. The count must reflect every transmitter, not just the loop.
The trap. Voting annotations are usually on the SIS-prefixed tags. Engineers who skim past the annotation count one instrument per loop and miss the redundant pair. On a hydrocracker bid this is a 30 percent under-count on the SIS scope.
Cross-page connectors and off-page tags
Multi-page drawing sets carry off-page connectors, circles with arrows pointing to "from page X to page Y". The instrument tag itself appears on one page. The connector appears on others. Counting connectors is a double-count. Counting only one tag per page misses tags drawn close to the page boundary that show up on both pages with off-page connector arrows.
The discipline. Count tags by their unique tag number, not by their drawing position. If FT-101 appears on pages 4 and 12, it is one instrument.
Counting under a tight bid clock
Manual approach. Split the drawing set across two engineers, count independently, reconcile the deltas. Two independent counts catch each other's misses. One solo count by a tired engineer the night before submission does not.
Either way. Keep the SIS subset as a separate column. Strip the future, by-vendor, by-others tags before the count locks. Cross-reference your final count to the equipment list. A vessel with no level transmitter is suspicious. A column with no temperature instruments at all is wrong.
Locking the bid number
Once the I/O count is clean, the rest of the math is mechanical. Card sizing rule, multiply each signal class by the channels-per-card divisor, spare-capacity uplift, your house standard, usually 20-25 percent, cabinet count, terminal density per cabinet, usually 200-400 terminals depending on your panel builder. Multiply by the per-cabinet build cost, add labor for engineering and commissioning, apply your contingency, lock the number. The I/O list creation guide walks through turning that raw count into a properly structured I/O list with all required columns.
The whole exercise lives or dies on the I/O count. Get the count right and the bid stands up to senior review. Get it wrong by 10 percent and the margin disappears either way you missed it.
If you want a structured template that goes from extracted instrument list to bid card-sizing rollup, the I/O list template is a starting point. If you want to skip the count entirely on the next bid, Tagsight extracts the I/O list directly from the drawing set and produces the bid-ready signal class breakdown.
Before you lock the count, review what does not belong on the I/O list. Legend entries, line designations, and vendor model codes look like real tags and silently inflate the number. Once the count is classified and clean, building the I/O list from P&IDs covers the column-build step that turns the count into an I/O list.