A thermocouple junction is formed by simply welding the two wires together, the weld then either being left exposed or fitted into a housing, a tube or similar, for protection. Exposed junctions have a rapid response to temperature changes whereas those fitted in a housing not only respond much slower but also the housing can influence the temperature being measured. Housings often enclose the junction itself in thermal silicone so improving thermal contact between junction and the outside world. Mechanical restraints such as alumina powder and resins are also used all of which, combined with the material and thickness of the tube as well as its physical position, can affect the final reading. It is important to remember the thermocouple is measuring the temperature at its junction.
Letter designates junction types, the more common ones are:
(1) Not to be used above 700°C due to magnetic changes in the material
(2) Non magnetic materials, often used where there are magnetic fields present
(3) Gives same output at 0C and 42C making the minimum useable temperature 50C
(4) Usually used at high temperature due to cost and low sensitivity
(5) High cost but very stable
To minimise electrical pickup, which could mask the small thermocouple output, wires should be twisted, routed away from any electrical noise, especially wires carrying mains frequency, and kept as short as possible.
The above is offered in good faith but Watch Hill Ltd cannot be held responsible for errors or values shown.
