We buy instrumentation cable.

Pairs, triads and the shields between them

The unit of construction is the pair: two insulated conductors twisted together, one positive and one negative. A triad does the same job with three conductors, used where a signal needs a third leg, as with a three-wire RTD. A single jacket can hold one pair, four pairs, eight pairs, or a mix of pairs and triads. Multi-pair reels are named by the pair count.

Shielding comes in two layers and both matter. Individual shields wrap each pair and stop one signal from talking to its neighbour. An overall shield wraps the whole bundle and drains interference to ground. Between them sits a drain wire, usually stranded tinned copper, that carries the noise away. Cable with both layers is the usual specification for analogue work in a plant, and it is the version most often left on a shelf.

Reading a cut end tells you which construction you have. Foil shields appear as a thin metallised film with a drain wire laid against it. Braid shields show as a woven copper tube. Cable with individual shields only will show foil at every pair, and cable with both will show a shield around the bundle as well.

• Single pair, multi-pair and triad constructions
• 16 to 24 AWG, stranded for pulls and solid for short runs
• Individual shields, overall shield, or both
• Drain wire present and continuous at the ends
• PVC, PE or low-smoke jackets depending on the area classification

Thermocouple extension wire by type and colour

Thermocouple extension cable is not ordinary instrument wire. The conductors are made from alloys chosen to match the thermocouple they extend, so the junction stays at the measuring point and the wire does not introduce a second junction of its own along the run. Use the wrong alloy and the reading drifts with the temperature at the cabinet door.

Type is matched to the sensor: J, K, T and E are the common ones in Houston plants, and each has its own colour code and its own polarity convention. Getting the polarity backwards at one end produces a reading that is wrong by a consistent offset, which is harder to spot than a dead signal.

That is why extension cable is usually printed or colour-coded along its length and why the outer jacket is often marked with the type. A reel with the marking worn off and no tag is still reviewed, but the type has to be established before it can be used again, and that is a slower answer.

Never join two extension wires of different types to make a length. Send them as separate pieces and say what each one is.

Why small cable carries the most per reel

A reel of 24 AWG pairs weighs almost nothing and holds a surprising amount of copper relative to its jacket. It is also the cable a plant is least likely to have spare when a transmitter fails, which is why it holds its usefulness long after heavier cable has been written off.

The logic is about scarcity of the right construction rather than size. A 500 MCM feeder can be substituted by pulling a new run from a drum that already exists on site. A specific 8-pair individually shielded cable with a particular jacket cannot, and the plant that needs it needs exactly that.

The same rule applies to what we buy. A small reel of the right instrumentation construction is reviewed seriously. A large reel of the wrong one is a metal lot. Construction, not weight, is what carries the review. Send the pair count and the shield arrangement, and the answer follows quickly.

Reading an instrumentation lot from a photograph

Send the pair count, the gauge, the shield arrangement and the jacket material if it is marked. Then send three photographs: the reel tag if it survives, the jacket print, and the cut end fanned out so the pairs and the shields are visible. Those three answer almost everything a first pass needs.

If the lot includes thermocouple wire, list the type and the colour code you can see, and keep the reels of different types apart. Mixed extension wire is the one thing in this category that cannot be sorted by eye later, so the description has to carry it. A reel of eight individually shielded pairs answers a specific problem that a reel of single pairs cannot.

The rest is logistics. Instrument reels are light and load by hand, and most arrive from an instrument department, a warehouse shelf or a project closeout with the pallet still intact. Say where the material is and whether it is boxed, since boxed reels are easier to count than loose ones.

• Pair and triad count, and total conductor count
• Gauge, and whether the conductors are solid or stranded
• Shield arrangement and drain wire condition
• Thermocouple type and colour code, kept separate
• Length on each reel, and whether it has been outdoors