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Weird readings from TC-08/USB

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Weird readings from TC-08/USB

Postby dutch_dk » Tue Jan 15, 2013 3:00 pm

Hi all

I am running a TC-08 on a small laptop, measuring temperatures on a Mitsubishi heat pump.
From time to time the TC-08 goes crazy, it looks as the polarity on the K probes changes from + to - !?
It can switch back again riht by itself.
It happened again today, and I reinstalled the software to R5.22.8.
The problem persists, the system started with showing the wrong values.
What to do?

This image is a screendump from 13/01-2013.
Picolog mysterius readings.gif
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Re: Weird readings from TC-08/USB

Postby Hitesh » Tue Jan 15, 2013 5:36 pm

Hi dutch_dk,

Are your thermocouples touching any metal surfaces?

Please also check in case you have any grounding issues, is your PC on the same grounding as the equipment being measured?

Have you got a USB or serial TC-08?

Regards,
Hitesh

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Re: Weird readings from TC-08/USB

Postby dutch_dk » Sat Jan 19, 2013 10:38 am

Hi again

Yes 1 of 6 thermocouples are tuching the tubings on the heating unit, and grounding?
On a laptop, grounding is not possible as the powersupply connect to main, but no ground, and a ground connetion is hard to do permanently, but I have tried, simply a wire from the metal body of the heating unit to the outer shield on USB-connector, (Loosely held by hand).
The result is taht the readings changes differently, the one with metal connection rises 7-8 C, the outdoor reading drops 2-3, others unaffected.

What is the propper way to set up the rig?


/Jan Holland
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Re: Weird readings from TC-08/USB

Postby dutch_dk » Sat Jan 19, 2013 2:10 pm

Ooooops, I forgot the model/type, it is an USB-model, the blue one ,

/Jan Holland
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Re: Weird readings from TC-08/USB

Postby Hitesh » Mon Jan 21, 2013 4:42 pm

Hi Jan,

You can connect a wire from the heater to a screw terminal on the laptop if it has something like a VGA port, otherwise you have to expose some of the braiding in the USB cable and wrap the wire around it.

You can insulate the tip of the thermocouple with some clear varnish.

Hope this helps.
Hitesh

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Re: Weird readings from TC-08/USB

Postby dutch_dk » Mon Jan 21, 2013 8:55 pm

The behavior has been normal since I made the ufinished grounding.
I was just wondering, at my job we have been using these systems for years, and never experienced a problem like this, but anyway, it seems to be solved by now.
I just have to calibrate / check values from the two sensors that changed values due to the grounding.

Thank you for helping.

/Jan Holland
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Re: Weird readings from TC-08/USB

Postby olepagh » Tue Jan 22, 2013 5:10 pm

dutch_dk wrote:The behavior has been normal since I made the ufinished grounding.


Hi Jan.

Your solution with grounding is correct.
I have had the same experience with several USB TC-08´s and they are very sensitive to even small voltage differences between input terminals and computer chassis/ground.

Even isolated thermocouple (2 mm steel tubing type, type T) made occasionally troubles.
Remember that the Seebeck voltage is only 39 µV/K around 0 deg.C

We have taken the consequences and provided all of the Labtops used for TC-08 with approx. 4 m soft cable from the chassis and a crocodile clip and instructed the user to always connect the measuring object with the computer.
/ Best regards
Ole
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