cold junction compensation

[Dr Jon Atkinson]

Hello

We have just bought a new NI card (PCI-6052E) connected to a terminal block
(CB-68LP), and so we now have to set-up the cold junction compensation
manually (previously we were using a SCXI-1303 which had it built in).

I would just like to check that the method which we have devised will work.
We are proposing to acquire a thermistor (with known temperature versus
resistance behaviour) and put this in series with a resistor. We would then
put a voltage across these two components and have the voltage drop across
the thermistor as one of the inputs on the terminal block. In the
Measurement and Automation explorer we will set up this channel(called
"cjc") as being a voltage input and tick the box which says it is a
temperat
ure input. We then apply a scaling equation which converts this
voltage into temperature.

We then set up our thermocouple channels in the Measurement and Automation
explorer, and in the CJC source box we say it is user-supplied and make the
user cjc channel the one we made earlier ("cjc").

Is this the correct method of doing this?

Thanks!

Jon Atkinson

[Alex]

[posted and mailed]

j.r.atkinson@bham.ac.uk (Dr Jon Atkinson) wrote in
<917mop$1ra$1@usenet.bham.ac.uk>:

>Hello
>
>We have just bought a new NI card (PCI-6052E) connected to a terminal
>block (CB-68LP), and so we now have to set-up the cold junction
>compensation manually (previously we were using a SCXI-1303 which had it
>built in).
>
>I would just like to check that the method which we have devised will
>work. We are proposing to acquire a thermistor (with known temperature
>versus resistance behaviour) and put this in series with a resistor. We
>would then put a voltage across these two components and have the
>voltage drop across the thermistor as one of the inputs on the terminal
>block. In the Measurement and Automation explorer we will set up this
>channel(called
"cjc") as being a voltage input and tick the box which
>says it is a temperature input. We then apply a scaling equation which
>converts this voltage into temperature.
>
>We then set up our thermocouple channels in the Measurement and
>Automation explorer, and in the CJC source box we say it is
>user-supplied and make the user cjc channel the one we made earlier
>("cjc").
>
>Is this the correct method of doing this?
>

I don't know. However NI have what they call "Application Notes" accessible
from the KnowledgeBase or from the "Developer Zone" and I recall that there are
notes that describe exactly how to do this.

From the Knowledgebase the documents "OU99S5L7" and "1FDPOIN" seem to be
applicable.

cheers, Alex.

--

Alexander C. Le Dain, PhD
ICON Technologies Pty Ltd
http://www.icon-tech.com.au

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* The LabVIEW FAQ http://www.icon-tech.com.au/thelabviewfaq.html *
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[John M. Jarvis]

Hi Jon -

I'll leave the details of the software to those more familiar with
them. I will offer a bit of comment re. the thermister.

Be a little careful with the thermistor approach. Thermistors suffer
from self-heating problems so the value of resistor chosen is important
to limit these effect.

For a few cents more, it might be worth investigating any of the IC
devices that supply a current or voltage proportional to temperature.
There are also dedicated three terminal cold junction ICs that can be
wired-in fairly simply.

Sincerely,

John
"Dr Jon Atkinson" wrote:
> Hello
>
> We have just bought a new NI card (PCI-6052E) connected to a terminal
block
> (CB-68LP), and so we now have to set-up the cold junction compensation
> manually
(previously we were using a SCXI-1303 which had it built in).
>
> I would just like to check that the method which we have devised will
work.
> We are proposing to acquire a thermistor (with known temperature
versus
> resistance behaviour) and put this in series with a resistor. We
would then
> put a voltage across these two components and have the voltage drop
across
> the thermistor as one of the inputs on the terminal block. In the
> Measurement and Automation explorer we will set up this channel(called
> "cjc") as being a voltage input and tick the box which says it is a
> temperature input. We then apply a scaling equation which converts
this
> voltage into temperature.
>
> We then set up our thermocouple channels in the Measurement and
Automation
> explorer, and in the CJC source box we say it is user-supplied and
make the
> user cjc channel the one we made earlier ("cjc").
>
> Is this the correct method of doing this?
>
> Thanks!
>
> Jon Atkinson
>
>


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