how to measure temperature using digital I/O (pxi 6514) and scb 100

[R1R]

Hi All,
 
I want to measure temperature by using a thermocouple (may upgrade for several measurements with more than 1 thermocouple). For the system, I have PXI-1031 with PXI-8184 Embedded Controller, PXI-6514 Industrial Digital I/O and SCB-100 Connector Block, PXI-2575 Relay Multiplexer, PXI-4060 DMM.
 
I called the NI office in Sydney, and one of the technical support people told me that the PXI 6514 does not accept analog signal from the thermocouple, which is fair enough. Do I have to use other DAQ device? I tried to make a new task in MAX with the DAQmx (I followed the instruction in <a href="http://digital.ni.com/public.nsf/3efedde4322fef19862567740067f3cc/16f71843cc7892d4c1256be3004e1043?OpenDocument" target="_blank">http://digital.ni.com/public.nsf/3efedde4322fef19862567740067f3cc/16f71843cc7892d4c1256be3004e1043?OpenDocument</a>).
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I'm really new in this area, so completely lost and confused now.
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Cheers,
Ryana

[Nathan_R]

Hi Ryana, Thermocouples generate a small voltage based on their temperature. To measure a thermocouple you must be able to read this analog voltage. The PXI-6514 is a digital I/O card that has no analog input channels. That is why it can not be used to measure thermocouples. With your current hardware you can measure a thermocouple using the PXI-4060 DMM. You can use the DMM along with your PXI-2575 mux to expand the channel count. YOu will also need a CJC sensor that the DMM can measure. The 4060 DMM is not programmed with NI-DAQmx. You would need to use the NI-DMM driver. THe switch can be controlled with NI-DAQmx or NI-Switch. Examples for both of these drivers can be found in LabVIEW under Help &gt;&gt; Find Examples...Also here are some Developer ZOne articles that explain how thermocouples work and some considerations when measuring them with a DMM. <a href="http://zone.ni.com/devzone/cda/tut/p/id/4237" target="_blank"> Taking Thermocouple Temperature Measurements</a> <a href="http://zone.ni.com/devzone/cda/tut/p/id/3778" target="_blank">Performing High-Accuracy Temperature Measurements Using a NI Digital Multimeter
and Switch</a> Thanks

[R1R]

Hi Nathan,
&nbsp;
Thank you for your reply.
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I have the CJC sensor in the SCB-100. How should I connect the block to the DMM? Currently it is connected to the PXI 6514.
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For the DMM driver, I've checked it that they are already installed in LabView.
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Thanks a lot.
&nbsp;
Cheers,
Ryana

[Nathan_R]

Hi Ryana, The SCB-100 is not designed to be connected to the PXI-4060 DMM or PXI-2575 switch. To do multiple thermocouple measurements with those two products you will need a terminal block for the PXI-2575 and the wire a CJC sensor and your thermo couples to the switch's channels with the DMM connected to the switches com channel. This will let you scan through your thermocouples and CJC sensor taking a measurement of each using the DMM's single channel. If you do not have a terminal block for the PXI-2575 the NI recommended set up is 1 -&nbsp; <a href="http://http://sine.ni.com/nips/cds/view/p/lang/en/nid/14432" target="_blank">LFH200 to 50-Pin DSub Switch Cable (CH-CH Twisted)</a> cable and 4 - <a href="http://sine.ni.com/nips/cds/view/p/lang/en/nid/202493" target="_blank">NI TBX-50, 50 Pin Dsub Screw Terminal Block</a>. This setup would give you access to all 200 pins on the PXI-2575 and allow you to measure a CJC and 97 thermocouples with the 4060. Let me know if you have any questions and take care. Thanks,<a></a>

[R1R]

Nathan,
&nbsp;
I got the terminal block and the cable as well. The only thing that I'm missing is the CJC sensor. Any recommendation?
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Thanks a lot.
&nbsp;
Cheers,
Ryana

[Nathan_R]

Hi Ryana,Semiconductor sensors, thermistors, or RTDs are commonly used to measure the reference-junction temperature. Basically for a CJC sensor you just need an accurate temperate sensor at the terminal block that can be read by the DMM. Since the DMM can measure resistance I would recommend using an RTD or thermistor for your CJC sensor. Check out these two Developer Zone articles for more info on these transducer types.<a href="http://zone.ni.com/devzone/cda/tut/p/id/7112" target="_blank">Temperature Measurements with Thermistors: How-To Guide</a> <a href="http://zone.ni.com/devzone/cda/tut/p/id/2948" target="_blank">Working with Thermistors and RTDs</a> Thanks,