Incorrect TC readings

[Lerxst]

I'm having trouble getting reliable readings from a group of thermocouples.  I'm using SCXI 1102B, TC-2095, and a E-series daq board.  The object that I'm trying to get a temp profile on is a metal (conductive) cylinder.  When I connect the k-type tc's, I read anywhere from 2 deg c below the actual temperature to 6 deg C above with a few reading the correct temperature.  Is it possible that the conductive surface is causing a problem between the tc's?  BTW, I'm using open tc detection and the channels are switched to ground-reference off on the back of the tc-2095.  I've tried every combination of open tc detection/ground ref on/off and this gives me the closest results, although still incorrect.  What is the best way to achieve this measurement?  And as I have it configured, would it be valid to simply subtract the delta t to make all channels agree initially?  Any help would be greatly appreciated.

[AnalogKid2DigitalMan]

Alex, is that you??? (a Rush/Alex Lifeson reference)

A couple things to check:

Have you tried mounting them all together in very close proximity on other materials (noncondcutive and then conductive) to see if they read the same?

Are they mounted securely to the cylinder?

What method or instrument are you comparing their readings to?

[Lerxst]

Some of the thermocouples are welded to the interior wall of the cylinder and some are suspended from the top of the cylinder, not touching the metal at all.  Now it seems to me that the suspended tc's should be floating, so the tc-2095 should be switched to "ground reference on" for those channels.  However, the readings get more erratic with them configured that way.  Also, changing the ground ref setting on one channel seems to effect other channels.  Anyone have any ideas.

[Sean_C.]

Hello,

How many thermocouples are you reading from?  Do you have any open thermocouple channels that you are scanning?  How fast are you scanning these thermocouples?  Try reading from one thermocouple at a time and see if that makes any difference.

Also, try setting all thermocouples to GND REF = OFF.  It may be that you are forming a ground loop with the different grounds (the cylinder and the SCXI GND).

Let me know your results.

Regards,
Sean C.

[Lerxst]

I'm reading from a total of 9 thermocouples (1kHz sample rate).  Initially all of the Tc's were on consecutive channels, starting at 0.  I've found that spacing the tc's out (having a few open channels between them) helped the problem.

[Henrik_Volkers]

1kHz is quite fast , do you have such fast temperature changes on your cylinder ? 

 (How big is your cylinder? A big oven pipe of 20m or a small combustion element? )

What is your temperature range of interest?

You wouldn't need 9 TCs if you don't have temp gradients on your cylinder.

If you have a conducting (metal) cylinder with a temperature gradient and multiple point not isolated TCs you might get error EMFs (e.g.. error voltages) by your cylinder material !

A good source for TC knowledge:
Manual on the use of thermocouples in temperature measurement,
ASTM PCN: 28-012093-40,
ISBN 0-8031-1466-4
(Page1)'Regardless of how many facts are presented herein and regardless of the percentage retained, all will be for naught unless one simple important fact is kept firmly in mind. The thermocouple reports only what it "feels." This may or may not the temperature of interest"