Re : Multi thermocouples influences themselves

[danydan27]

Hello,

I am building test benches in which i install multiple thermocouples (K-type) lines with femalle connectors at the end. The thermocouple wires are approxiamtly 5 meters long. I use a 6229 Ni-card with the CJC integrated on one of the SCB-68.

I have a strange behaviour on my readings when i move my thermocouple on the several cable terminations.

1) On some lines, i have a good reading on the concerned line and some extreme values on the other, that i consider as a zeros -> Ok for me

2) But on some other lines, i have also a good reading on the concerned line but it influences all or partly of the other lines. My measurement displays showing incoherent values that are always moving. Moreover, some reading that seemed indépendant (case 1), are then influenced by connecting the thermocouple on an other line.

I checked all my wiring and everything seems perfectly screwed and isolated.

Do you have any idea of what happens there?

[Jeremy_M]

Hi danydan27,

I have few questions about the issue you met.

1) What does "extrem values" means? What is the difference between values measured and expected?

2) May you tell me how high are the influences seen on readings?

3) Do you observe the same behavior when replacing thermocouples by batteries?and in MAX?

Is it possible to have screenshots and VI with explanations?

Regards,

[danydan27]

Hi Jérémy, thanks for answering, i was despaired of getting any response.

1) I mean value around -300K/-500K °C that i consider as "none meaningful values" and that i display to the user as 0°C (or -300°C if i work with negative temperatures)

2) When i have no thermocouple connected, the values read are all around -300K/-500K.

When i connected one, some of the not connected channels are begining to show "meaningful values" (0-900 °C), they vary a lot but i display them. The phenomenon is increasing when adding more thermocouples and it depends of the connected thermocouple.

I am not sure that this behaviour may influence my real temperature reading but i am not able to determine which thermocouple is connected, moreover my client is afraid by all this values going up and down...

3) I haven't try with batteries and the behaviour i have just describe have seen with MAX.

Please have a look at the screenshots i linked.

 THK_Channels.jpg are the channels i have configured

 THK_One.jpg is the behaviour with one (the first) thermocouple connected (which is ok for me)

 THK_One_2.jpg is the behaviour with one (the sixth) thermocouple connected (start to see some "wrong" values)

 THK_Reference.jpg shows the configuration of my reference virtual chanel on Ai0

 THK_Several.jpg shows the behaviour with 4 thermoucouples connected ( 0,2,7,8) (it would become difficult to know which one is really connected if the not connected ones was not varying a lot)

I hope i have detailled my problem more precisely.

Regards.