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Problems with inaccurate TC-08 readings

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Problems with inaccurate TC-08 readings

Postby FuseHead » Tue Mar 07, 2017 9:47 pm

Hi,

I'm having problems with the TC-08 USB. I am using these mineral insulated thermocouples: http://www.farnell.com/datasheets/1918884.pdf

with these suitable K-type 5 meter extension cords:

http://www.farnell.com/datasheets/19188 ... 1480947831

I am measuring seven channels and when these seven sensors are placed in ice/water mix in a plastic bucket they all show around 0 celcius degrees. Also when warm water from a tap is put on a bucket they all show 45 degrees. Readings were also checked to be correct with a Mercury temp gauge.

So all good untill these sensors are put in real enviroinment which are microbiologial reactors (with different gas atmoshperes and very moist conditions) readings between sensors differ from 10 to 20 celcius degree even though all are in same temp (temperatures checked with a Mercury temp gauge).

Same thing happens when I put these sensors in a steel pan which is heated with a steel hot plate, readings differ from 10 to 40 celcsius degree (some show 60 and some show 100 even though correct temp was 80).

To me this sounds like a potential difference/voltage problem because readings are stable in a plastic bucket and differ when placed in enviroinment where there could be potential differencies (inside a reactor, steel pan on a hot plate with 1kW heater element, especially if the shells of sensors are touching each other etc...) but how could this be as the MB-ISK-S10-500-MP-ANSI thermocouples should be mineral insulated so the outer shell shouldn't have any connection to the thermocouple (nominal resistance between the shell and plug tips was something in a range of tens of mega ohms).

Also the reactors in question are with a plastic shell so there shouldn't be any grounding issues with those. TC-08 is connected to a non grounded laptop though.

Or could this be an extension cord problems as the sensor-extension cord plug is at ~35 celcius degrees while the extension cord - TC-08 plug is at 15 celcius degrees. But I doubt as those cables should be quite good quality so no cold junction should be formed between plugs.

So should I begin to shield all the wiring, grounding the laptop/TC-08 usb or where to start solving the problem? There is quite a lot of electrical interference also because there are lot's of electrical equipment near the reactors.

Best Regards,

-Marko
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Re: Problems with inaccurate TC-08 readings

Postby FuseHead » Wed Mar 08, 2017 11:47 am

Answering to my own problem, error was caused by non earthed laptop power supply. Earthing the laptop's motherboard with a separate earth wire solved the problem! (Also disconnecting the PSU would have solved the problem but this wouldn't be a solution in a continuous measurement system).


-Marko
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Re: Problems with inaccurate TC-08 readings

Postby Martyn » Thu Mar 09, 2017 2:08 pm

Thanks for the update, and good to hear it is working :)
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