TCs with NI 9205

[lmd2]

I have a client who has a NI 9205 for reading pressure sensors and a tach (RPM). He would like to add some TC readings (J type thermocouples) and I was wondering how to handle the cold-junction problem. Is it possible to use one channel to read the ambient of the module and apply some algorithm to correct the temperature readings?
thanx
lmd2

[ryannorthington]

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.

Message Edited by Ryan N. on 03-26-2008 04:24 PM

[lmd2]

Thanx Ryan
can I ask you something; we are thinking another route may be to add an 9211 for TC readings, but here is the thing -would it be possible if we needed more than 4 TCs to use an ambient value from the 9211 to then to use lookup tables containing the characterization data of thermocouple voltages vs. temperatures for a J-type TC? I don't suppose the CJC value of the 9211 could be exposed and used directly?

[ryannorthington]

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?

[lmd2]

Yes, you have the correct understanding. I also prefer using the constant, but I wanted to know my options if the client wasn't comfortable with the constant. The application is monitoring a plastics infector (molten plastic) the DAQ will be in a rack adjacent to the apparatus. They use J type TCs, I haven't seen any data from the legacy system (Dasy Labs).
Thanx for your help,
lmd2

[ryannorthington]

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.