03-25-2008 05:54 AM
03-26-2008 04:23 PM - edited 03-26-2008 04:24 PM
Usually you will use either the build in CJC compensation or a channel to bring in the CJC value. Since the 9205 is for general analog input it does not have built in CJC compensation.
When you use a the channel method you will want to use a thermistor or RTD for the CJC channel. However, this is not an option since both Thermistors and RTD’s take a current/voltage excitation that the 9205 cannot supply.
The best bet for the 9205 is to just use the constant option and leave it at the default value (room temperature). You can set this in Measurement and Automation Explorer (MAX) as a DAQmx task or in a DAQ assistant.
03-26-2008 05:13 PM
03-27-2008 05:44 PM
Imd2,
Just to make sure I understand your application; you want to use the 9205 (b/c of the higher channel count) to acquire across several thermocouples while using a 9211 to bring in a reference reading. This will allow you to make corrections to the 9205 readings.
Am I correct in the way I described your question?
If my assumptions are correct I do not see why you couldn't use the 9211 as a reference for the 9205. This would have to be done in software, by first comparing the input of the 9205 thermocouple readings to the 9211 reference reading and then adding an offset or a gain to the 9205 values.
I would still recommend just using the constant value for the cjc compensation for the 9205.
What kind of temperatures are you reading in? Is there a lot of variation in the readings?
Are your thermocouple connections close to a hot or cold source that would cause a lot variation?
More importantly are you seeing a lot of variation between your acquired signal and your actual value?
03-27-2008 06:29 PM
03-28-2008 12:15 PM
Imd2,
Using the NI 9211 you can definitely adjust the data that comes from your 9205. However, you would have to do the scaling or conversion in LabVIEW using lower level math/comparison VI’s.