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ADC 24 versus TC 08

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ADC 24 versus TC 08

Postby spinner » Mon Nov 02, 2009 1:38 pm

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
Spinner
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Re: ADC 24 versus TC 08

Postby gruntman » Tue Nov 03, 2009 6:19 pm

Spinner,

There are several differences between the USB TC-08 and the ADC-24. The main difference however is that the TC-08 was designed with thermocouple data logger in mind for it's primary purpose. While you can log thermocouple readings with the ADC-24, it was designed to fit multiple applications rather than just one. The TC-08 has one key thing that the ADC-24 does not - Cold Junction Compensation. Refer to the thermocouple article available online at http://www.picotech.com/applications/thermocouple.html to see the why this compensation is important in making accurate measurements.

You stated the need to measure a large amount of channels. With the USB TC-08 up to 20 units can be connected to the same computer for a large amount of temperature logging simultaneously.

Overall, considering the two units and your application, I would recommend the TC-08 over the ADC-24.

Regards,

Richard Boyd
Crag Technologies, Inc
http://www.pc-oscilloscopes.com
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Re: ADC 24 versus TC 08

Postby spinner » Wed Nov 04, 2009 9:59 am

Hi Richard

Thank you for your reply.
Given the number of channels (double) and the price, it seems that ADC has more value. How far out typically would the ADC be?
We would only need 0.5 degree accuracy.
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Re: ADC 24 versus TC 08

Postby gruntman » Wed Nov 04, 2009 7:35 pm

Spinner,

If you ignore the error in the thermocouples themselves, a thermocouple logging system will be accurate based on the following:

1) Cold Junction Error
2) Amplification Error
3) Linearity Error

Thermocouples measure the difference between the hot (the casting process) and the cold (data logger) junctions. In order to determine the actual temperature you must know what temperature the data logger is at precisely. You then compensate all the temperature readings against this known temperature. This is called cold junction compensation. The ADC-24 has no on board temperature sensor to determine the amount to compensate. In order to use the ADC-24 you will have to design a circuit to measure the temperature of the unit, and then apply a equation to all the channels to offset their readings. Linearity will also be based off this compensation. You may find that you can get the ADC-24 accurate at certain temperature - yet off by several degrees on other ranges. It will vary based on how accurate your CJC circuit and equations are. The USB TC-08 has the CJC built in already with accuracy specifications and calibration certificates available to back them up. There is a reason why many calibration houses around the world use the TC-08 for testing other devices.

For $130 more for 2) TC-08s compared to 1) ADC-24, me personally, would save my time and purchase a unit built for my project rather than spending time on building a circuit.

Overall, the ADC-24 is a very good unit with features that you can't find in hardly any other unit at its price range. That being said, for your application I still believe you would be happier in the long run with the TC-08's.

Regards,

Richard Boyd
Crag Technologies, Inc
http://www.pc-oscilloscopes.com
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Re: ADC 24 versus TC 08

Postby spinner » Mon Nov 09, 2009 10:33 am

Hi Richard

Thank you for that very clear answer!
One last question: When ordering the unit, is there any additional hardware that I need, besides the thermocouples themselves? How exactly do the leads connect to the unit? There are no photographs of that part of the unit...
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Re: ADC 24 versus TC 08

Postby spinner » Tue Nov 10, 2009 1:26 pm

Hi Richard
Once again, thanks for your help.
I have to establish the typical accuracy from each unit to convince my higher powers. Please could you help me with some values.
Thanks
J
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