Problem with long Thermocouples and DAQ91xx

[SPR123]

Hi

I have a problem measuring the temperature of long thermocouples.

I use a 0.5mm type N thermocouple which is 20 meter long

and a 0.5mm type N thermocouple which is 5 meter long

and two other thermocouples which where about 1 meter long.

I made the test with a DAQ9179 chassis and a 9212-temperature module.

I repeated the test with a DAQ9184 and a 9113-temperature module.

Both Tests give me the same result.

The 20m long Thermocouple shows 24°C and all the other Thermocouples show about 19 °C.

For the Data acquisition I used Signal Express Manager with 50 Hz and 10 Hz (Fast acquisition) and 1Hz (precise acquisition).

I also used a measuring System of iba and a cheap portable measuring instrument and both times ALL the Thermocouples show 19 °C.

Anyone has an idea why this failure appears or has a solution to solve it?

Thank you

[SteveD123]

No real ideas, but some suggestions you could try:

Do all the thermocouples have the same construction? Is the long thermocouple Metal-Shethed Mineral-Insulated (MIMI)? Is the sheath touching the thermocouple wires anywhere (a simple resistance measurment will check)?

Are there any Grounding issues?

YOu could try a ferrite bead on the thermocouple wire next to the instrument in case it is an RF pickup problem.