Setting the CJC source constant for a SCC-TC02 module

[_Bryan]

Hello,

I have a SCC-TC02 module with a SC-2345 box.  I am using the uncompensated panelette for a type T thermocouple and I am trying to find out how to set the CJC value so I can get an accurate reading.  None of my searches have turned up anything useful.

Thanks

Bryan

[Conseils]

The compensation is in the TC02 module not on the connector on the panelette

" The National Instruments SCC-TC01 and SCC-TC02 are 1-channel modules for conditioning a variety of thermocouple types, including J, K, T, B, E, N, R, and S, and millivolt inputs with a range of ±100 mV. The NI SCC-TC0x modules include a 2 Hz lowpass noise filter, instrumentation amplifier with a gain of 100, and buffered outputs for maximum scanning rates by an E Series DAQ device. The input circuitry of the SCC-TC0x modules also includes high-impedance bias resistors for open-thermocouple detection, as well as handling both floating and ground-referenced thermocouples. The SCC-TCox modules include an onboard thermistor for cold-junction compensation."

You can find the information here

[_Bryan]

When I use the built in CJC source my temperature is +10F (+6c) above actual.  NI sells type J, K and Uncompensated panelettes.  Since I am using a type T I purchased the uncompensated panelette.  I though that was how it worked.  So why would I be +10F above actual?

Bryan 

Message Edited by CCI_Bryan on 03-28-2006 12:57 PM

[Conseils]

Could I perhaps ask a question.....

What makes you suspect that you have an error of 10F......

More often than not problems are associated with ground loops or earth potentials when working with thermocouples or possbily induced noise.
Do you have a calibrated temperature simulator or are you trying to compare the temperature to something that you believe is correct ?

[_Bryan]

I was using a cup of ice water to compare temperatures.  If I use the built in CJC option I was reading 42F, but if I use a CJC constant of 25 I get 32F.  The constant of 25 was just a guess, so I was wondering about the correct way to get the CJC constant value.

[Conseils]

Well now that takes me back a bit....
Firstly is your ice water truly slushy??????, a long, long time ago my first job in a morning as a trainee was to make the slushy ice flasks of reference ice for the cold junctions. No, only joking... the boss used to make me re-do it if it wasn't just the right amount of slushy...

It's most likely that by using the uncompensated panelette an error exists at that point due to temperature differences at the various junctions and disimilar metals. In fact I think it says as much in the manual somewhere. But it does sound like quite an error.

To get the cold junction temperature check out the manual given at the previous link, I would have pasted bits in for you but if I remember correctly they use lots of subscript fonts and it doesn't copy at all well. Basically there is a conversion from volts to temperature and some factors for the thermistor curve.

So is that interconnect cable copper or some other material, rather than the same materials used in the thermocouple. One other thing.... The thermocouple, it is connected the right way round isn't it? I know it's a daft question but one has to ask..

Message Edité par Conseils le 03-28-2006 10:48 PM

[_Bryan]

I have read the manual on getting the CJ temperature reading, which I get 1.19 volts, but I get lost in the equation.  I think I might just use a calibrated hand held t/c reader and adjust the CJC value and do an ambient test and a ice bath test and hope for the best.  NI has a nice KnowledgeBase article about the SCB-68 and this issue, too bad they don't have one for SCC-TC02.

  And yes the thermocouple is connected correctly   You could be correct about the interconnecting wire material from the panelett and the TC02.  I would hope that that would have been accounted for when it was designed, but who knows......

[Conseils]

What version of Labview are you using?


I know there is already a 'convert thermistor reading' VI available in earlier versions, that's what I would start with. As I am on 6i at the moment I won't post it as the newer version (you may be on 😎 may bet better for your needs.

[Conseils]

I will be on all main versions (except 5.x) later today.

[_Bryan]

I am running version 8.  I did find Acq CJC Temperature.vi in the example finder and if I did it properly the CJC temp is 84.34F, ambient in the room is 72F.  I was looking at the SCC-TC Input Module users guide and there is a section on calibrating the module.  It looks like if I can find a themocouple calibrator I can adjust the potentiometer on the top of the module.  According to the manual I need to scale the voltage measurements and adjust the pot by voltage and not actual temperature units.  Do you think I could get around that by reading a thermocouple with a calibrated device then adjust the pot on the module until the temperature matches?  I know it would not be the perfect soloution, but it might work.