Low Temperature Reading

We met a problem with our Eurotherm 3508. The reading of temperature below 0C is different from the reading obtained from thermocouple. I heard that Eurotherm's reading might be not accurate at low temprature range (100K-270K). So, how can I solve this problem? Do you have any software update package or something like that?
Thanks!
Peng
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  • There's no difference in input measurement accuracy and precision in low ranges, but the choice of sensor may have an effect.

    Can you let me know what type of sensor or thermocouple you have connected to the 3508, and also how your reference "thermocouple" is being measured? Are the two sensors (3508 and reference) positioned in the same part of the apparatus?
    • We used K Type thermocouple to connect the sample in UHV chamber with two pins outside chamber. For Fluke multimeter, the voltage is measured between the two pins, which is -5.8mV (-194C according to reference table). Room temp is 297K, then sample temperature is 103K. For Eurothem, two silver wires connect V+, V- ports with the two pins. The reading is 120K. 3508 and multimeter are all positioned on the two pins, which are connected with the sample by K Type thermocouple.
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  • Jon (Official Rep) February 27, 2013 17:04
    Hello Peng,
    Could you provide some more information on the setup:

    Is this just a basic multimeter? so you are just reading the mV input and deducting the ambient temperature away from the reading or is this a Fluke thermocouple meter?

    Are you using type K compensating cable between the chamber and the 3508 controller?

    You seem to be parallelling up the reading of the same thermocouple, does the reading alter in the 3508 if you remove the Fluke meter?

    Do you have a copy of the controller configuration? If so, please send to support@eurotherm.co.uk or you can contact me on 01903 268500 x 7026 to discuss this further.
    Regards, Jon
    • Hi, Jon.

      Yeah, this is a basic multimeter. We were just reading the mV input and deducting the ambient temperature.

      We are using common cable, not type K compensating cable. What's compensating cable for?

      It doesn't change if we remove the Fluke meter.

      For the controller configuration, is that like:
      3508/CC/VH/1/X/1/4/XXX/G/D4/ET/XX/XX/ENG/XXX/XXXXX/XXX/STD

      Thank you so much!

      Peng
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  • Jon (Official Rep) March 05, 2013 08:25
    Hi Peng,
    The problem with using copper wire to the thermocouple is that you are creating thermoelement at the transition point because of the dissimilar metals so could experience temperature errors. Using Compensating cable uses a low cost alloys, which has a similar thermoelectric property to the actual thermocouple. On this basis there is no thermal voltage at the transition point between the thermocouple and the compensating cable.

    Are you able to confirm that the 3504 is actually configured for type K thermocouple and no PV offset or High/Low offsets have been applied? To verify - Go to level 3 access, then page round to PVInput and use the scroll button(repeatedly) to enter/view Lin type offset etc..

    Also, where are you located?

    Regards, Jon
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