DIY is not everyone’s cup of tea; nor is it necessarily ideal for industrial usage, where large numbers are needed, and installation, commissioning, maintenance and replacement must be considered. So, more permanent and operationally convenient ready-made thermocouple sensors are, of course, available. Thermocouple sensors can be bought as separate units from a very wide range of types and styles. At the most basic level there are simple bare wire thermocouples with junctions welded as described previously. These may be insulated according to the applicational needs with anything from PVC to ceramics. Frequently, the thermocouple conductors will be fitted into a closed end probe of some kind, outer sheath (protection tube), or thermowell made from a suitable heat-resistant alloy or refractory material.
And, here again there are almost as many sizes and styles as there are applications. Sizes range from sub-miniature (around 0.25mm diameter - sheathed), through miniature and on up to standard (around 6mm) and beyond (20mm and much more) - to cover for all requirements. As for styles, there are general purpose welded sheath probes, bolt style probes, hand-held probes (of several designs), surface probes equipped for direct attachment to pipes of most sizes, others designed to monitor the temperature of point or moving surfaces and so on - all protected by insulation and sheath materials (if required) chosen for the thermocouple and the application with or without grounded sheaths, quick disconnect systems and so on.
Additionally, there are sheathed probes for autoclave temperature measurement (incorporating flexible stainless steel conduit and pressure entry glands), bayonet and compression fitting style thermocouples for the plastics and other industries) and heavy duty and high temperature industrial sheathed thermocouples with options like head-mounting terminal assemblies and thermowell extension pieces.
In all cases, great care is taken by the suppliers to ensure that the conductors are correctly manufactured, and installed into the sensor housing under closely controlled conditions. Thus, the amount of change the heated region of the conductors may experience during service (which affects uniformity) is minimised. This is important, since it is this unit that will almost certainly be sitting in the area of greatest temperature gradient, and therefore contributing to most of the output voltage.
An alternative form of construction uses mineral insulated (MI) cable where the thermocouple conductors are embedded in a closely compacted, inert mineral powder, and surrounded by a metal sheath (like stainless steel or nickel alloy) to form a hermetically sealed assembly. The sheath functions as a useful protective cover. This kind of device is available with outer diameter dimensions ranging now from 0.25mm up to 10.8mm, while lengths can be from a few millimetres up to tens of metres and beyond.
For rather special applications, where high speed response is needed, it can be advantageous for an MI thermocouple to be manufactured with the junction itself exposed. However, this kind of departure needs the expertise and skill of a regular practitioner.
Thermocouple sensors are often provided with a connection or terminal box which allows convenient linking to the rest of the circuit. Alternatively, a special plug can be fitted, in which the connecting pins are made from the thermoelectric material concerned - as are those in the mating socket.
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This information is an extract from TC's outstanding FREE guide to Themocouple and Resistance Thermometry.