Parallel thermocouples connected in NI 9213

[herclau]

Good morning,

How to connect to one of the channels, in module Ni 9213, five thermocouples connected in parallel to mesure the average temperatura of room? The others channls is occupied for other measurements.

I nedd to conncet compesation resistors ("swamping resistors") for each of the thermocouple wich are in parallel? How to calculate its values?

Thank

[David22JRA]

Hi herclau

Actually you can use NI MAX and only create a task and select the option that says “Connection Diagram” and should see the way for connecting your thermocouple. Use this document that show those steps: http://www.ni.com/getting-started/set-up-hardware/data-acquisition/thermocouples

For accurate wire circuits should be identical. Since the resistance of the actual thermocouple will vary, the effect of these variations can be minimized by using swamping resistors. The values of the swamping resistors should be high in comparison with the change or difference in resistances encountered. A resistor value of 1500 ohms generally works well.

Regards

[AnalogKid2DigitalMan]

herclau:

I did not know of that technique. Thanks to you, now I do.

Kudo for you!

Thanks

-AK2DM

[herclau]

Hi David,

Thanks for your response and attention. I changed the thermocouples in parallel, of different lengths, for thermocouples of equal length. The thermocouples have the following resistances:

new(Ohms) old(Ohms)

10.74           14.09

10.36           5.23

10.75          12.87

10.44          5.11 

11.20          6.96

Do I have to connect the resistance compensation?

Searching, I found the following formula:

"Inaccuracy due to wire length is Caused by the voltage divider effect produced by a combination of the input impedance of the amplifier and the source resistance of the TC.

A first order approximation for inaccuracy use this equation:

% Inaccuracy = 1- [Ra / (Ra + Rt)] x 100

Where:

Ra=Amplifier input resistance.

Rt = Thermocouple wire resistance (total of Both wires)"  

Could you use it to calculate the compensation resistance?

Thank

[Daniel_Eduardo]

Hello herclau,

Could you be more specific and explain more in what you mean when you say that if you have to connect the resistance compensation?

Thanks! 

[herclau]

Hello Daniel_Eduardo,
By placing thermocouples of the same length the resistance of the same are identical (left column). The wire circuits is identical. And my question is: Is necessary including resistance compensation , even with these small variations of resistance for more accurate measurement of average temperature?
thanks