Thermocouples measurement - cRIO and NI 9213

[PaoloBaldracchi]

Hello,

I'm performing temperature measurements with thermocuples (type T) using cRIO and NI 9213. We are measuring temeratures within the range of -20 to +50 °C and as stated in the data sheet of the of NI 9213, the measurement error shoud be aorund 0.8°C + thermocouple accuracy. I would like to know if there is any way to reduce this error via calibration of the whole measurement chain. Is there any standard or procedure to follow? would it be usefull to have a more precise cold reference unit to be cupled with NI 9213?

Thanks a lot in advance,

kind regards,

Paolo B.

[enpe]

Ciao Paolo,

Non è possibile migliorare la specifica presente sul datasheet in quanto è quella relativa al modulo appena calibrato. Con il passare del tempo l'accuratezza ovviamente peggiorerà e un'eventuale calbiazione servirà a riportare la precisione come da specifica. 

Saluti,

Enzo P.

[JamesBeams]

Hi Paolo,

A majority of the error comes from isothermal error of the CJC, and the fact that it is a single CJC is located inside the module on the other side of the large connector.  To achieve higher accuracy you want to do as much as possible to reduce any thermal gradient across the connector so the CJC is sensing as close as possible the value of the junctions where the thermocouple wires are connected.  To do this you want to make sure you avoid an environment with moving ambient air, typically close to room temp, and orient the chassis so that the self-heating of the backplane has the least effect.  This is difficult because turning it on its side could potentially cause a gradient from top to bottom of the connector, but orienting it with the module faced upwards results in the backplane heat passing through the module.  It could be possible that mounting the chassis upside down (so all heat dissipates away from the connector) could be the best orientation, but I'm not sure that is a tested orientation.  Also the channels closer to the center of the connector will perform better isothermally because the CJC is closest to these channels. 

Other things you can do is be sure to use "high resolution" mode to reduce the effects of noise if your application can handle a slower sample rate.  You could also consider the 9214, which has better isothermal performance given it has multiple CJCs located next to the TC connections in a terminal block instead of behind a large, thick connector.

Even with these hints, it is still up to you to determine how much better the measurement will be through your own testing.  You can connect a known "temperature" from a thermocouple calibrator or stable voltage source and measure the resulting "temperature" seen by the module and use that to calculate the overall error seen by the module.  For this a Fluke 5520A or Ectron 1140A would be recommended for highest accuracy.

-James