Temperature measurement and CJC using thermocouples on a USB-6229 BNC

[Mikea01]

Hi,

I am trying to get accurate temperature measurements using K type thermocouples on the usb-6229 BNC. The SignalExpress software wont let me select 'Built In' for the CJC Source. I therefore have to select 'Constant', which means I then have to monitor the ambient temperature change during the day with a separate meter and adjust the CJC Value accordingly to maintain an accurate reading. Is there any way round this to avoid having to monitor the ambient temperature at the junction?

Thanks in advance

Mike

[AEI_JR]

Mike,

The USB-6229 does not appear to have a built-in compensator, which is why you can't select 'Built-In'.  There are a few ways around this, here are two: 

1)  Replace the 6229 with a device designed to measure TCs, like the USB-9211A.  This does have internal compensation. 

2)  Include a RTD or thermistor channel, measuring temperature at the USB-6229.  Set the compensation to 'Channel', and select the channel accordingly.  You should make sure to insulate the entire device, with the RTD or thermistor inside, to prevent gradients between the CJC sensor and your connection points.

Hope this helps.

JR

[Mikea01]

Hi JR,

thanks for your reply. I will use your second option but just have a few questions for you as follows:-

1. When I select Channel for CJC Source, then the CJC Channel also needs entering. I thought that a drop down list of all the channels would appear if I clicked within this box. This did not happen though. It just requires a text entry. What exactly should I enter here? Assuming the RTD is on CH0, should I simply enter Dev2_ai0 (which is how the RTD channel is displayed in the step input)?

2. Should the RTD be connected single ended or differential?

3. Should the the thermocouples be connected single ended or differential? I ask this because in the manual for the 6229 it shows the input configuration for a thermocouple as differential, but I'm sure I've seen on this support site a reply recommending single ended as well. I'm using 8 thermocouples, which is half the channels taken up in differential mode, so single ended would be nice.

4. Where exactly should I put the RTD? My configuration is as follows:-

I am currently in differential mode. From the BNC connectors, I have 19cm leads (stranded copper wire) to a terminal block. From the other side of the block, I have 5cm leads (stranded copper wire) to the 8off K type socket housings all in close proximity to each other to which I plug my thermocouples into. The whole terminal block just hangs over the edge of the logger. Eventually the logger will be put into a rack, with the K type sockets on the front panel. Should the RTD be next to a BNC or attached to one of the K type socket housings?

Thanks again

Mike

[AEI_JR]

Mike,

Thermocouples are always measured differential, since they have two wires which generate a signal.  Follow this link for a tutorial:

http://zone.ni.com/devzone/cda/tut/p/id/7108

I must warn you that though it is possible to measure TCs this way, I really don't recommend it as a method of general practice.

In order to select the RTD as a channel, you must configure it in Measurement and Automation Explorer as a Virtual Channel.  If you need help with that, I can walk you through it, but the details will depend on how you condition the RTD. 

RTDs come in a variety of flavors, depending on the measurement application.  They can be made of different materials, and have different nominal resistances, but they all work essentially the same way - calibrated to be at nominal resistance at 0 deg C, their resistance will change predictably with temperature.  Passing a small, constant current source through them will generate a voltage proportial to resistance via Ohm's Law. Since your USB device does not have the capability to source current, you will need to provide an external source.  You may select a source which will condition the RTD output so that voltage is directly proportional to temperature.  The output of these devices is generally single-ended, referenced to ground, so your input would be NSRE.

You'll want your RTD located as close to all your TC connection points as possible, to avoid temperature gradients, and be sure to insulate the entire block.

[Mikea01]

Hi JR,

thanks for all the useful info you have given me. I may use the RTD configured as a Virtual Channel in the future on smaller projects. Being as thermocouples should only be connected differentially, this would mean that too many channels would be needed for my temperature measurements when I know I would also need quite a few channels for other voltage measurements too. Now that I know the 6229 does not have cjc built in, I have decided to use the AD595C (thermocouple amp with cjc) for each thermocouple. Because the output is with respect to 0V, then I only need to use one channel per thermocouple. I wouldn't have come to this conclusion without your help though, so thanks again. If you see anything wrong with this, then please let me know.

Regards

Mike

[RER · Accepted solution]

Hiya Mike,

It seems that you have managed to find a suitable solution to your the problem, which is great news.

However, for future reference, if you had purchased the 6229 with mass termination (rather than the BNC interface) then you could have interfaced the device with a SCB-68 terminal block (see following link)

http://sine.ni.com/nips/cds/view/p/lang/en/nid/1180

The SCB-68 does have a built-in CJC source, so you could have used this in conjunction with 6229 (mass termination).

A bit of an aside, but I thought you may be interested.

Best wishes to you all

[Mikea01]

Hi Rich,

thanks for the info. I actually inherited this project, so the hardware had already been selected. We will be using more of these units in future so that would be another good solution.

Thanks for all your help.

Mike