ThermoCouples

[Vidmas30]

Good evening,

Last week we had test drive for our temperature and pressure control. PID works fine, but there's a little problem with thermocouple inputs (we use ATC8 module). As I mentioned earlier, there were temperature error with sensor of outdoor temperature, but when we filled heating system and started testing I saw that all the sensors have the error of 4-5C (they all show 4-5C higher temperature) . There are connected 3x water temperature sensors ant 1x outdoor temperature sensor. We use K type thermocouple sensors (produced by our local manufacturer, as I gave Unitronics TC input specifications) connected with K type compensating cable. What should I do?

I can subtract those 5C from every sensor mathematycally, but but I don't know if there will be the same error when temperature will rise 10, 20 or more degrees..

Please help..

[nagaraj]

Are all the thermocouples showing an error or only the ones immersed in water? Are you connecting the thermocouples directly to the PLC? If you are using more wires as extensions, you will be creating more junctions and you will definitely see an error. Also, make sure only the tip of the thermocouple is immersed in water and not the insulation. We use thermocouples in our units all the time and I have found it to be better to linearize the output from the thermocouple using reference temperature at least 30 deg C apart rather than using the thermocouple output directly.

[Vidmas30]

Hi,

Thanks for answer! All thermocouples show an error..

Your idea about wire extensions is interesting - compensating cable connected to cabinet's terminals. From terminals to PLC IO-ATC8 module signals wired with simple copper wires (~1-2m long). Maybe it's good idea to connect directly to IO module. I'll try..

And I didn't understood about linearization of TC input.. Can you explain it to me?

[Emil]

TC is in fact connection between two types of alloy. According to the temperature in this point, the connection point generates mV. The wires are part of the sensor itself and need to be from the same two types of alloy. In the moment you connect TC cable to ordinary couper one, you perform another two TC in the point of connection.

When you connect T/C cable to the pins of the controller, there is another temperature sensor built in, which measures te temperature of he connector and compensate the error. This is so called "Cold junction".

I hope now you understand where the inacuracy comes and how to prevent it - you need ot use cable from the same type TC and special connectors - from the same type. You need to be sure the polarity of the cables match.

[Vidmas30]

Thank you, Emil, tomorrow I'll try to connect TC directly and see what the results will be. Thank you!

[Phil Salkie]

If you're using compensating cable (that is, extension wire which has the same alloys as the wires of the thermocouple) you shouldn't be creating measurement error with the wires themselves. Are you using thermocouple terminal blocks? If you land the thermocouple on a standard terminal block, then land the wire on the other side of that terminal block, you are essentially adding in any temperature difference across that block. We keep compensating terminal blocks in stock for our customers who are involved in temperature sensing applications.

As for linearization, you can more precisely calibrate your thermocouple input by sending the input signal through a linearization function block. That block gives you a linear transform between two ranges of values - for example, if an analog input gives you a value between 0 and 1000, you can use the linearization block to convert that to a span of 0 to 250 pounds. (I would usually make that 0 to 2500, then show my display with one decimal point to preserve measurement accuracy.) In the case of temperature, you can set a low and high calibration point, place the thermocouple in a low temperature, store the value it is producing and enter the value it should produce. Then do the same for a high temperature - that will correct the end-points of your measurement range, and (all things being equal) you should have a much more accurate temperature reading within your working range.

[andrewpaul]

A standard Type K sensor will only offer you a ±1.1°C accuracy. Make sure that you are using thermocouple grade cable from the sensor to the meter. If you are looking for greater accuracy, I recommend the use of an RTD which has an accuracy of ±0.15°C.

Edited December 7, 2017 by Ausman
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