NI9212, NI9213: Temperature Offset on long thermocouples

[freshward]

Hi everybody,

we're having some issues with our NI data acquisition systems. In recent tests we discovered temperature offsets of about 5 °C when using longer thermocouples.

DAQ1:

- Chassis: cDaQ - 9179

- Module: NI9213

DAQ2:

- Chassis: cDaQ - 9184

- Module: NI9212

Compared Thermocouples (all Type N):

- 0.5 mm sheath TC, 20 m length

- 0.5 mm sheath TC, 5 m length

- 1 mm sheath  TC, 20 m length

- 2 mm sheath TC, 20 m length

- 0.5 mm non-insulated TC, 20 m length

Software: Signal Express 2015

Tested sampling rates: 1 / 10 / 50 Hz

Tested modes: High Accuracy / High Speed

All thermocouples measuring air temperature at the same position (purple is the non insulated one).

When changing the datalogger (other suppliers), the temperature difference lies within 1 °C. 

What could be the problem here?

Thanks in advance!