Measuring High Temperature: Thermocouples, Pyrometers, and What Each Lies About

Two dissimilar metals joined at a point generate a small voltage that depends on the temperature of that junction. It is cheap, robust and reads continuously, which is why it runs almost every kiln and furnace controller. But it reads the JUNCTION. A thermocouple in the roof of a kiln tells you about the roof; one in the gas stream tells you about the gas. Neither is necessarily the temperature of the pot on the shelf, and the difference can be a hundred degrees. Put the tip where the work is, give it time to equalise, and remember it has its own thermal mass — a heavy sheathed probe takes minutes to follow a change. The type matters too: a K-type covers most workshop use and a cheap one drifts with age, so check it in boiling water occasionally.

Look through it at the hot object and a small electric filament appears superimposed on it. Adjust the current until the filament disappears against the background, and the current is a measure of the temperature. It is elegant because it needs no contact and no assumption about distance — brightness matching is independent of how far away you are, since both the object and the filament are seen through the same optics. Its weakness is the same emissivity problem plus a human judgement: 'disappeared' is a decision your eye makes, and different people settle on slightly different points. It is also useless below visible glow. The published optical pyrometer blueprint builds one and is worth reading alongside this.

Put a thermocouple in the fire beside the work, heat a bar, and write down what each colour reads. An hour spent doing this converts a vague colour scale into your own calibrated instrument. Do it in the light you normally work in, with the fuel you normally use, because both of those affect the judgement — which is the whole point of the colour rung's two conditions. After that the eye is faster than any instrument and always with you, and the instrument becomes a check rather than a necessity. That is the same progression as the go/no-go gauge in the measurement batch: use the precise tool to establish the reference, then use the fast one.