Thermocouple Cable; the Hero of Process Control.

Remember that little bit of cable that spans the physical gap between the process and the temperature control instrumentation?  It probably got almost no thought at the time of installation yet it could be the most critical part of your process control.

Thermocouple cables do all the heavy lifting when it comes to temperature control, what type of probe you have on the end or the expensive instrument it connects too are only ever as good as the thermocouple cable that spans them, and whilst it looks like a simple bit of cable, there’s a lot going on under the skin.

David Hamilton, Managing Director at SK Wiring Products explains: “A common misconception is that the probe and the instrument are the important parts of the system, in reality the probe is nothing more than a method of protecting the exposed end of the thermocouple cable and the instrument is doing nothing more than converting a mV signal into a temperature understandable to humans”

Thomas Johann Seebeck discovered that when any conductor is subjected to a thermal gradient, it will generate a voltage. This is known as the thermoelectric effect or Seebeck effect. Measurement of this voltage involves connecting another conductor to the “hot” end. This additional conductor will then also experience the temperature gradient, and develop a voltage of its own which will oppose the original. Using a dissimilar metal to complete the circuit creates a circuit in which the two legs generate different voltages, leaving a small difference in voltage available for measurement. That difference increases with temperature.

It is of critical importance then that the thermocouple cable is of the highest quality, made to the most exacting standards in order to have accurate process control.  Anyone who has bought thermocouple cable will have noticed there can be a big difference in price between suppliers for the same thing.  That difference is almost certainly down to the quality of the wire used in its manufacture, and the level of testing it has undergone before reaching the market place.  Test equipment for thermocouple cables is expensive and requires highly trained meteorologists to operate effectively; testing must also be done in a very controlled environment, further adding to the cost.  A well-controlled test environment allows the manufacturer to state the uncertainty of the thermocouple cable measurement and it is this factor that is most overlooked when buying thermocouple cables.

David Hamilton explains “Thermocouple cables are all manufactured to operate to a standard, this standard allows for a small amount of deviation from the measured temperature typically stated as a percentage of the measured temperature i.e. +/- 0.004t or 0.0075t but if the test system is of poor design it could have large amounts of uncertainty built into the answer, without knowing these uncertainties you could never be sure that the calibration certificate you have is in anyway accurate.  If the uncertainty of the system is greater than the allowable deviations then it’s very possible that the cable will not be within spec, but may appear to be when looking at the calibration certificate.”

If the uncertainty exceeds the allowable deviations then your process control temperatures will be wrong, this typically leads to either a loss of efficiency in the process or at worst the scrapping of high levels of material.  It wouldn’t take long for that cheap bit of cable you bought to start costing a lot of money in lost production or scrappage costs.  It is important then that you know the source of the cables that you are buying and that the calibration certificate that you should insist comes with it has the uncertainties stated.  With a certificate that has both uncertainty and deviation stated you will be able to tell if the cable you plan to install is up the standard you require.  The smaller the uncertainty statement the more accurate the calibration certificate will be.

David Hamilton adds “We have recognised over the last few years that buyers are becoming increasingly savvy when it comes to thermocouple cable purchases and that engineers are starting to more fully understand the importance of high quality cables in their process control and are becoming increasingly demanding of accurate information.  That is why we have invested heavily into our testing facility.  Our industrial lab allows us to test to high accuracy in a very tightly controlled environment and is the most advanced lab operated by a thermocouple cable manufacturer in the UK and quite probably the E.U.”

Next time you are changing a probe or replacing a thermocouple cable take a minute to give some thought to how important accurate temperature control is to your process, if the answer is that it matters then don’t chase down the cheapest cable you can, instead chase down the best quality that you can.  Taking this approach will pay dividends in the long run.




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