Manufacturer of Thermocouple Sensors

How does a Temperature Thermocouple work?

Thermocouples have been developed as a result of the discovery of the Seebeck effect, by Thomas J. Seebeck in 1821.

Two dissimilar metals are joined together at both ends in an electrical circuit. One “junction” is the measuring junction or “hot end”. The other is the reference junction or “cold end”. A sensitive voltmeter is connected into one of the conductors.

Under laboratory conditions the reference junction would be held at a known temperature, usually 0°C but in normal industrial practise the junction is left at ambient temperature and an external sensor is used to compensate for this variation (known as cold junction compensation, usually a thermistor bead is used to measure the ambient temperature).

Quite simply as the temperature rises or falls at the measuring junction a voltage is generated within the circuit which correlates directly to temperature and can easily be converted by reference to the appropriate tables.

What is meant by Exposed, Ground and Insulated Hot Junction?

Exposed Hot Junction – An exposed junction is where the two wires that make the junction are left without an external protection sheath. In the simplest form this could mean just a bare wire design or the junction can be constructed so that it extends out the end of a protection sheath. Exposed junctions give the fastest response time of any thermocouple construction but are only suitable for unpressurised, dry and non-corrosive applications.

Grounded Hot Junction – A grounded thermocouple has the thermocouple electrically connected, or grounded, to the protection sheath, normally by welding or silver soldering so that it becomes an integral part of the sensor tip. This type of construction can also give fast response times but with the added advantage of being suitable for pressurised applications. This junction type is popular in the Plastics Industry.

Insulated Hot Junction – As the name suggests the thermocouple junction in this design is electrically insulated from the protection sheath. Response times are slower with this construction but this is now the most common type due to its ability to avoid interference from earth loops in machinery.

What does Mineral Insulated mean?

A Mineral Insulated Thermocouple is a reference to the type of construction used. The thermocouple conductors are extruded along with the protection sheath and a densely packed oxide powder that provides the electrical insulation. Various protection sheaths are available as well as many of the standard thermocouple types giving massive versatility.

A mineral insulated thermocouple has advantages over a traditional fabricated thermocouple as follows:

Semi-flexible – The probe part of the thermocouple can be bent to almost any shape on site without special equipment. The oxide powder inside the probe helps to support the tubular sheath during bending, avoiding kinking. Minimum bend radii apply.

Fast Response – Mineral insulated thermocouples can be made in remarkably small diameters, down to 0.25mm diameter. More common sizes range from 1mm to 6mm. The smaller the diameter, the faster the response time and they can be used in pressurised applications.

Robust – The mineral insulated cable used to make this type of thermocouple is extremely resilient and can be used in high pressure applications. It can even be used in applications where there is a risk of crushing.

Long lengths – Due to the way the mineral insulated cable is made, it is supplied in long lengths. It is therefore quite possible to manufacture a thermocouple probe that is many metres long. This makes it ideal for temperature measurement in arduous or extreme applications where traditional cables are not appropriate.