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TC-08 USB temperature readings fluctuate

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TC-08 USB temperature readings fluctuate

Postby GiladO » Mon Jul 06, 2009 2:05 pm

When my laptop (DELL) is connected to the mains via it's adapter the temperature readings fluctuate. This does not happen when the battery powers the laptop. It is necessary that this computer be connected to the TC-08 and due to long term readings it must be connected to the mains.
Reading some earlier posts it does seem that it has already occurred to other people at least once before.
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Postby Pico Stuart » Mon Jul 06, 2009 3:04 pm

This problem is almost certainly caused by the thermocouple being used touching something which is at a different potential to the TC-08/PC earth. If this voltage is greater than about +/-7V the channel is pulled out of he common mode range of the TC-08. This causes all channels to read very incorrect values. You must ensure that the metal object you are touching with the exposed end of the thermocouple is earthed correctly and at the same potential as the PC earth. If that cannot be done, the thermocouple must be electrically isolated from the metal surface.

regards
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It is insulated ...

Postby GiladO » Mon Jul 06, 2009 9:13 pm

Actually, the thermocouple is insulated in an alumina sleeve. It is placed within a high temperature resistance furnace. Connecting the thermocouple to a Eurotherm controller provides the correct temperature.
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Solved

Postby GiladO » Tue Jul 07, 2009 8:35 am

The computer power supply was connected to an insulating transformer thus eliminating the problem. I recommend that you improve the TC-08 design in order to eliminate such problems. It may result in a cost increase but I'm sure that some customers are willing to pay the additional cost.
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Postby Pico Stuart » Tue Jul 07, 2009 9:22 am

Good Morning,

Thankyou for your response. The TC08 cannot be a totally isolated design as there would be no way to power the front end. The power supples are produced via the USB.

Kindest regards
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Re: Solved

Postby olepagh » Sat Aug 22, 2009 1:57 pm

GiladO wrote:The computer power supply was connected to an insulating transformer thus eliminating the problem. I recommend that you improve the TC-08 design in order to eliminate such problems. It may result in a cost increase but I'm sure that some customers are willing to pay the additional cost.


We had a similar problem with the TC-08USB and 8 type T sensors in a calibrating bath.
It was solved by connecting the labtop's ground to the bath's ground.
(ground = chassis)

It seemed to be a frequently occurring problem, so we permanently connected a small piece of wire to a chassis screw on the PC and supplied the portable test set with 2-3 meters of test leads and some banana plugs and clips and an instruction to the user to always establish a compensation connection between the PC and the object being measured.
/ Best regards
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Re: TC-08 USB temperature readings fluctuate

Postby Michael Hayes » Mon Nov 02, 2009 5:09 am

This question although old is timeless and as I just saw it for the first time, I thought I might share my experience. I sometimes use the TC-08 to measure temperatures within a Forehearth being heated by Gas burners. The readings are fine until we introduce Glass and Electric Heating and then the readings go wild. The problem is that although the Thermocouples are "Isolated" there is enough capacitive coupling or leakage between the channels and the through the laptop power supply to cause the interferrence. Remember also that flames are conductive. I also tried soldering an earth wire onto the shell of one of the D connectors into the PC and taking it to a 3 pin plug but this was not a great solution when multiple channels were in use.
The past few times I have made these measurements, I have individually earthed one of the legs from each thermocouple and from memory, I don't think I also needed to earth the laptop chassis but this could also be done.
Obviously this scheme will not work if each of the thermocouples has a low impedance connection to a common mode voltage.
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Re: TC-08 USB temperature readings fluctuate

Postby spinner » Mon Nov 02, 2009 1:37 pm

Has anyone had similar experiences with the ADC-24?

Also, please check the following?

Hi there

I am trying to find out the difference between the two loggers above.
I want to record as many channels as possible (hence the ADC 24), but would like to know of any reasons why the TC-08 is different from the ADC 24. Besides the thermocouples, is there any other hardware I would need with the ADC 24 that is not needed with the tc-08? ARe there major technical differences/advantages?
I am measuring a casting process, hence the time resolution does not have to be very fast, probably 0.2 s?

Thanks
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Re: TC-08 USB temperature readings fluctuate

Postby ziko » Thu Nov 05, 2009 9:10 am

Hi this is normal and will be common on the ADC20/24 or any product that measures a differential input, if the CMRR of the chip is exceeded your readings will not read true.

Now in terms of comparing the ADC24 with the TC-08 it all depends on what you are looking for. The ADC24 is not ideal for temperature as you need CJC. If however you are not looking to measure temperature then you have to compare specs.

The ADC24 has 20 bits noise free resolution, it has 8 differential inputs (16 single ended), the conversion time of the ADC varies from 60ms to 660ms, it has voltage ranges from +/-39mV to +/-2.5V. Different voltage ranges will have different resolutions, you can also choose a quicker conversion time for a lower resolution. For more info please refer to the user manual under section 4.4:

http://www.picotech.com/document/pdf/adc20044.pdf

The ADC24 has a Common Mode range of +/-1.9V (from +/-39mV to +/-1.25V) and +/-3.0V (+/-2.5V).


In terms of the TC-08 it has a noise free resolution of 16.25 bits, a conversion time of 100ms for the ADC. The voltage range is +/-70mV, it has a Common Mode Range of +/-7.5V.

With both devices they have one ADC and the multiple inputs are multiplexed.

I hope this helps.

Kind regards
Ziko

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Re: TC-08 USB temperature readings fluctuate

Postby Roachtech » Fri Jan 08, 2010 5:32 am

I am also having this problem of incorrect readings with the TC-08 while testing electric kilns near 2000F. The readings return to normal when the electric elements cycle off, but return when the kiln controller activates the element relays. Using 7 inputs, 6 are isolated by ceramic insulators and 1 is contacting metal. Removing the thermocouple from contacting the metal does not effect the other 6 readings, nor does unplugging the laptop charger. Only temporarily shutting off the heating element power supply corrects the fluctuation problem. I'll try adding additional earth grounds to the kiln and laptop.
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Re: TC-08 USB temperature readings fluctuate

Postby GiladO » Sun Jan 10, 2010 4:18 pm

This problem cannot be resolved properly by grounding the computer. Unfortunately this product was not designed with such applications in mind. I undertook my measurements connecting the instrument to a netbook which was powered by a car battery. I did not find a simpler way around it. I hope they design the product better so it can be incorporated in experimental and production environments. Currently I think that a warning concerning its limitations should appear in the products description.
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Re: TC-08 USB temperature readings fluctuate

Postby olepagh » Sun Jan 10, 2010 7:43 pm

GiladO wrote: I hope they design the product better so it can be incorporated in experimental and production environments. Currently I think that a warning concerning its limitations should appear in the products description.


You are so right!

Another serious issue is that the TC-08USB's Cold Junction compensation, which is located in air and not near the input terminals, therefore can (will) add measuring deviations up to 1 -2 °C if the temperature around the converter varies.
The old serial TC-08 was much better in that way, as all the input terminals were placed next to each other and the CJ sensor was placed very near the input terminals.

The answer, I got in another thread some time ago, was that the TC-08USB is a 'value for money' item, and that is of course correct in a way, but it should not prevent the development department to think about.

I solved the CJ issue partly by placing the converter in an isolated cardboard box.
/ Best regards
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Re: TC-08 USB temperature readings fluctuate

Postby Guest » Sun Jan 10, 2010 10:12 pm

I see you like to disassemble your instruments ... :wink: . A good design is a good design and the instrument is not that cheap. I use many temperature controllers that overcome those problems at a fraction of the cost. It is a matter of taking care of details the knowledge is available. In an adjacent lab they have a similar instrument that does not suffer from those problems. It costs 100$ less.
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Re: TC-08 USB temperature readings fluctuate

Postby GiladO » Sun Jan 10, 2010 10:16 pm

The above is not a guest it's me 8)
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Re: TC-08 USB temperature readings fluctuate

Postby olepagh » Mon Jan 11, 2010 4:17 am

I see you like to disassemble your instruments ... :wink: .
.

Usually no..., but when when the 4 inputs to the left drifted about 1-1,5 °C within 20 minutes, compaired to the 4 right inputs just because the converter was placed 50 cm from a labtop PC, I had to find the reason.
As responsible for an accredited thermometry laboratory, I have to know what is going on...

In an adjacent lab they have a similar instrument that does not suffer from those problems. It costs 100$ less.


That is interesting. Could you reveal which brand and type?
/ Best regards
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