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Decontamination Process Validation

Thermocouples
How Do They Work & Why Do Things Go Wrong?

Thermocouples are made and used all the time and there’s not much more thought needed than that, until they stop working, something that every validation engineer will have experienced! It is therefore worth being aware of what a thermocouple is actually doing to prevent issues arising in the first place.

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How a Thermocouple Works

A thermocouple is made from two dissimilar wires, joined at one end to form the ‘hot junction’ where measurements are taken. The wires are then insulated from each other along their length before rejoining at the ‘cold junction’ – usually inside a recorder where the temperature can be controlled.

When the hot junction is exposed to a temperature different to that at the cold junction, a voltage is generated between the cold junction wires, which can be read by the attached instrument. The relationship between voltage and temperature is a standard depending on the various thermocouple types, and this can be used to translate the voltage received into a temperature measurement at the hot junction. In theory this relationship should be absolute, but in practice a variety of outside elements, such as purity and condition of the thermocouple wire, ambient temperature and cold junction temperature can affect the voltage received, so calibration of thermocouples is generally required to ensure accuracy of measurement.

Thermocouple Diagram

Like any other metal, thermocouples are prone to damage, oxidation and general wear and tear – this results in inaccurate temperature readings and, eventually, an open circuit and complete loss of data. Of particular note is oxidation and corrosion due to chemical attack – clean steam and peracetic acid are particularly damaging to thermocouples – and using an exposed thermocouple can lead to water and chemicals being pulled up the thermocouple sheath causing corrosion beyond the tip. For this reason when remaking thermocouples, the wire should be cut back until clean metal can be seen before the junction is created and care taken to avoid any kinks along the length of the cable.

It is also worth noting that the hot junction point is the first point of contact closest to the cold junction rather than the tip of the thermocouple. A very long twisted junction may therefore create inaccuracies of measurement, as the point of measurement will not be where you may expect it to be. Furthermore a short across the two wires (either due to damaged sheathing or overstripped wires touching in the input board) will lead to the reading being taken at the short rather than the thermocouple tip.

Finally, a loose or badly made hot junction that allows the wires at point of contact to move against each other will cause spiking, incorrect readings or repeated brief open circuit readings.

Bad Thermocouple
Bad Thermocouple
Terrible Thermocouple (1)
Really Thermocouple

Good Thermocouple - long enough to hold together without being too long, both wires twisted evenly around each other

Poor Thermocouple - too long, poor initial connection due to uneven stripping of sheath

Bad Thermocouple - poor stripping, initial connection not tight, too long, wire one twisted around wire two

This is unlikely to work at all!

It is also worth noting that the hot junction point is the first point of contact closest to the cold junction rather than the tip of the thermocouple. A very long twisted junction may therefore create inaccuracies of measurement, as the point of measurement will not be where you may expect it to be. Furthermore a short across the two wires (either due to damaged sheathing or overstripped wires touching in the input board) will lead to the reading being taken at the short rather than the thermocouple tip.

Finally, a loose or badly made hot junction that allows the wires at point of contact to move against each other will cause spiking, incorrect readings or repeated brief open circuit readings.

thermo group

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).

Calibrating Thermocouples

  • Calibration of a thermocouple requires three temperature points. The lowest and highest temperatures likely to be relevant to a test, and a checkpoint between these to ensure that all parameters are met across the whole temperature band.
  • This calibration data will be used for all tests performed using this calibration file until another calibration is performed. A Calibration Check works retrospectively on tests that have already been completed.
  • You should check the guidelines to which you are working to see how frequently you are required to recalibrate your equipment.
  • You will require a suitable temperature calibrator or thermal bath, and depending on the guidelines to which you are working, you may also require an independent reference probe.
  • It is important to insert the reference probe into the correct hole in the well to minimise air flow around the probe. The probe should reach all the way to the bottom of the well to ensure accurate calibration
  • Gather enough thermocouples to fill one of the holes in the well. This will minimise air flow an improve calibration accuracy. Ensure the thermocouples ends are level, and insert the bunch into the well
  • Ensure that all the thermocouples are positioned at the bottom of the well. If any are positioned higher than the rest calibration of this probe will be inaccurate

Hermetically Sealed Thermocouple

Once the tip has been 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
• Water tracking up the lead
• Interference caused by the wire touching the machine

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