During continuous scanning thermocouples with a SCXI 1120 module is the cjc reference read each scan?

[Randall_T]

Did the developers of the DAQmx drivers improve the temperature scan capability of the SXCI 1120 module over the methods used in the traditional drivers?

When using the SCXI 1120 module for temperature measurement and in the continuous scan mode with traditional drivers the cold junction reference was only checked on the initial start of the scan. In order to maintain accuracy it was necessary to restart the scan.

For the most part I have switched to the 1125 module for thermocouple measurement but I need to use some 1120 modules on a test stand now and I will be using the DAQmx drivers on these. I was in hopes that with the DAQmx drivers this behavior was corrected.

I can conduct testing to determine this but would like to know ahead of time if this is still an issue.

Thanks

[Ben_S]

Randall T,

According to the KnowledgeBase Article, "How Can I Address and Use Cold Junction Compensation (CJC) with SCXI Modules?", the SCXI-1120 module's CJC channel can not be scanned while other analong input channels are being scanned, however, it can can the CJC individually. This is a limitation of the hardware and not the drivers as other modules, such as the SCXI-1125, can scan both input channels and the CJC channel at the same time.

That being said, by default, the CJC will be read only once in the beginning and not checked again. However, if you configure your CJC source to be an actual channel you can include it in the scan.

[Randall_T]

Thank you Ben for the answer.

I checked the article you referenced appears to me that I will be adding the physical address in the channel strings then do the voltage to temperature conversion and then correct the appropriate channels associated with the cjc in question. I will also need to adjust some on board jumpers for the SCXI 1120.  If I am wrong please correct me.

If scanning the physical channel is possible as suggested in this article, would it not be possible to create the channel as a virtual global channel and reference it as the cold junction in Max? Then the temperature adjustment could be done at this level of acquisition rather than having to sort this out in the LabVIEW program. This takes me back to the days with LabVIEW 4.0 before we had the virtual channels and when we had to handle the temperature conversions and other scaling ourselves. Again if I am incorrect please let me know, for I would like to use the method recommended in the article if I don't have to perform all the calculations in the application.

Just wondering if anybody reading this has actually used this method. If so, how did it work for you?

I may end up building a start up routine testing for 1120 modules being used for thermocouple measurement then if found run a stop and restart scan every 5 minutes to prevent large errors from being introduced.

[Ben_S]

Randall T,

I belive this documents address your question about the virtual channels.

How to Create a CJC Virtual Channel for Use with Thermocouple Virtual Channels