NI 9211

[anthonycrowther]

Hello,

I am currently developing a transient heat conduction lab experiment for use by heat transfer students. The rig is fairly basic but the main aim is to generate temp time curves and use this data to determine values for the thermal diffusivity in various materials. I am currently using the NI 9211 with J-Type thermocouples. I have encountered several problems these are;

The temperatures I am recording are between room and 100 deg C. The nature of the measurement is transient and constantly changing. I have found that the thermocouples take between 3-15 seconds to read actual temps felt at the thermocouple end depending on the temperature difference. Is this correct. Can I improve this? I am aware that there are other temp measurement devices available that are more accurate, but I need something reasonably robust for the intended environment.

Another problem is the callibration of each thermocouple. Do each J-types require individual callibration as it seems to me that they can differ significantly from each other.

Is it best to use the CJC value of 25 or should some other value be used? (Or internal setting)

I would greatly appreciate some assistance with these issues.

[Aaron_W.]

Anthony,

When I typically see thermocouples used they are used on a much larger temperature range, so it makes me think that you will not get the accuracy that you might desire from your measurements. You might look into some of the documentation that we have on RTDs.  I will link some documents below. When you say that it takes 3-15 seconds do you mean, for instance, if you place the thermocouple in a bath of hot water that it will take that amount of time before the thermocouple gives a voltage that would correctly identify the temperature? 

As far calibration goes for each thermocouple, I would recommend calibrating a couple of them and see if the temperature-voltage curve for those are similar. If they are similar it might not be necessary to calibrate every one, but if you notice a substantial difference then you would more than likely need to calibrate all of them. 

For your CJC value, this just removes the effect of the voltages generated by the cold junctions for a more accurate temperature measurement. The NI-9211 has a built-in CJC, so I would recommend using that as it should be much more accurate then using a constant value.  Some thermocouple data sheets will give a CJC value that you could use as a constant. 

Taking Thermocouple Temperature Measurements

Temperature Measurements with RTDs

Temperature Measuremetns with a Thermocouple or RTD