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Choosing the Correct Thermocouple

There are several different types of thermocouple and multiple variations within these types. The most common are:

Type K

– a general purpose thermocouple with a wide temperature band (-200 C to 1350 C where high degrees of accuracy are not required. Usually has a green sheath and connectors, and is composed of chromel and alumel, both of which contain high levels of nickel – this is magnetic which can cause issues in some applications. Commonly used for high temperature applications

Type T

– suitable for use between -270 and 300 C, with a relatively high accuracy. Usually has a brown sheath and connectors (originally blue as British Standard). Composed of Copper and constantan, both of which are non-magnetic. Commonly used for autoclaves, instrument washers etc.

Flat pair

– two strands of thermocouple individually insulated, with an additional sheath holding both together. Offers good mechanical strength and flexibility, the outer sheath protecting the internal strands from damage. Increased overall diameter.

Twin twisted

– two strands of thermocouple individually insulated, twisted together with no outer sheath. Offers increased flexibility and reduced overall diameter for narrow entry fittings, more vulnerable to damage and more prone to tangling

Single strand

– each thermocouple consists of 2 single strands of dissimilar conductive metals. These provide greatest consistency and homogeneity to give more accurate and stable readings, but any damage to either wire will significantly affect the reading.

Multi strand

– multiple fine wires making up each strand of cable. Provides greater flexibility and reduces cold working errors. However although the wire as a whole will provide a temperature reading, accuracy will be affected if any wires do not make proper contact at the hot or cold junction or break during testing.

Welded

– the tips of the thermocouple are welded rather than twisted together. This guarantees the point of contact is stable and gases cannot form between the elements. However, if the tip is exposed the thermocouple is still subject to corrosion and wear and tear, and re-welding may not be possible on site if a probe is damaged.

Sealed/capped

– once the tip has been made (usually welded) the outer sheath is extruded over the junction and hermetically sealed or covered with a cap. This completely prevents corrosion and damage to the tip at the expense of slight loss of reaction time, as well as preventing water being pulled up the sheath by force of vacuum or capillary action. This type can be very useful in very corrosive environments to maintain thermocouple integrity

Making and maintaining Thermocouples

For many applications a twisted tip is sufficient, as long as it is properly made. An exposed junction has a faster reaction time, and in a corrosive environment a welded tip does not offer any real benefit, as the tip needs remaking on a regular basis anyway.
The intent is to create a clean, tight bond between the two wires of the thermocouple - the twist is simply a means of holding the join in place, as the measurement will be taken from the first point of contact.
Cut and strip both strands to about 10mm of bare wire, taking care to ensure both strands are stripped back the same amount.
Bend both wires out to form a ‘V’ shape
Twist the two strands together around the midpoint. Both wires should be twisted around each other in a double helix – a single strand twisting around a straight one will cause an electrical inductive effect and lead to incorrect measurements.
The first twist is the most vital, and should be tight enough to prevent any movement of the connection. Continue until the whole tip is twisted together
Once the twist has been created, the tip can then be trimmed to the required length (about 5mm is recommended).