K thermocouples

[CactusPots]

How important is it to use K thermocouple wire?  What is the difference? 

Just to eyeball it, it looks like single pair 16 gauge stranded or thereabouts.

[Bill Kielb]

4 hours ago, CactusPots said:

How important is it to use K thermocouple wire?  What is the difference? 

Just to eyeball it, it looks like single pair 16 gauge stranded or thereabouts.

It’s important as any dissimilar wire sets up another thermocouple junction. Tcouple wire is made of the same stuff the thermocouple is. One side will be slightly magnetic and the other not, just like your tcouple. If you take some tcouple wire and simply twist the ends together you have a thermocouple all on its own. So using any other wire sets up it’s own junction which will produce a voltage on its own. In practice these are equal and offset, but on occasion when the physical temperature is different it will introduce significant error. The compatible Wire  and holder for that matter are always recommended.

[Dick White]

The operation of a thermocouple is the galvanic effect when two dissimilar metals in contact with each other create a very small, but measurable, electric current. The amount of the current is proportional to the temperature at the point of contact. A pyrometer (or the programming in a kiln controller) measures the current and mathematically converts the millivolts to temperature. Going to your specific question, the thermocouple wire is made of the same two metals as the thermocouple, so that there is a continuous circuit back to the pyrometer of the proper metals. For a K-type, the specific metals are nickel-chromium and nickel-alumel.

[neilestrick]

The only time you can use non-thermocouple specific metals is in junctions or switches where there's not a temperature difference across the junction. For example, a switch that toggles a readout between two different thermocouples, or the brass connectors on the ceramic block that holds the thermocouple. Because the two ends are so close to each other, there's no temperature difference across the connection. Along a wire, however, would be a problem.