Thermocouple

A thermocouple is a temperature sensor made from two different metals joined at a junction. Heat the junction and it produces a small, predictable voltage that a controller reads as a temperature.

The junction of two dissimilar metals generates a tiny voltage that varies with temperature. Different metal combinations form different 'types', each with its own range and the matching extension wire it must be connected with.

Thermocouples are polarity-sensitive — reverse the leads and the reading moves the wrong way. The controller must be set to the correct type, or the temperature will read wrong.

Ovens, furnaces, plastics, and process heating where accurate, wide-range temperature measurement is needed. An open thermocouple gives a sensor-break; the wrong type or reversed polarity gives a wrong reading.

Open circuit is the big one: the controller flags a sensor break — or slams to a rail, depending on its burnout setting — and the heating either stops or runs away accordingly. Drift with age reads consistently low or high. Reversed polarity reads backwards: the display falls as the process heats.

Wrong type — sensor or extension cable — is the sneaky one: a stable, believable, wrong reading that can hide for months.

Resistance first: a healthy thermocouple loop reads low ohms end to end; OL means an open sensor or a break in the extension run. Then sanity-check the displayed temperature against something you trust — ambient with the process cold is the easy free check.

The live output is millivolts — tiny, type-dependent, and near zero at room temperature — so a standard meter mostly just tells you 'not open'. Substituting a known-good sensor, or a process calibrator if you have one, settles arguments fast.

Extension runs must be the matching compensating cable, correct polarity, all the way to the controller — a length of ordinary copper flex effectively moves the measuring junction to wherever the copper starts, and the accuracy goes with it.

Colour codes differ between standards — and in some, red is the negative — so never trust colour alone across brands. Prove polarity by warming the tip and watching the reading move the right way. And check the controller is configured for the type actually fitted.