How to do Cold Junction Compensation using USB-6218

[MrDomino]

I'm pretty certain that the DAQ itself doesn't have a built in thermistor for CJC so I wanted to see if I could potentially use an extra thermocouple to account for it. The whole purpose of this section of the DAQ program I'm building is to measure engine temperatures. Can I use an extra thermocouple that isn't measuring anything for cold junction compensation?

[blawson]

Yes, you can use one thermocouple reading to do CJC for other thermocouple readings.

The best way to understand it is to do it in MAX.  Open up max and create a task for temperature, choose one channel thermocouple input to be your cold junction measurement.  Set the CJC source to constant.

Now add your measurement channels and set their CJC source to "channel" and select the first channel.

[MrDomino]

Ok so I tried messing around in MAX and I see where you can set up a channel for CJC. There’s only one problem. I’m not sure how to set up the CJC thermocouple. For the other thermocouples I see where I can set the CJC Source to CJC Channel and then select a MAX task.

I’m not sure how to set up the CJC task. I go to create new task – Add Channel – Thermocouple – Select Channel – and then choose the channel that the thermocouple is wired in to. Now I have the follow options that I set.

Max – 100 deg C

Min – 0 deg C

Thermocouple type – K

CJC Source (I have three options)

-          Built-in

-          Constant – specify constant CJC value

-          CJC Channel – specify channel

I’m not sure which of these three I should pick when setting up the CJC thermocouple.

[blawson]

well you can't use built-in because your card has no built in CJC.  use constant and go with that.  You'll have some error because you aren't compensating that channel, but it's better'n nothing.  This is why boards use a thermistor.  Alternatively, I think you could use a thermistor, but you'd need external excitation.

There is an internal channel "_boardTempSensor_vs_AiGnd" listed in the channels available in the test panel section of MAX, but im not sure if you can really read that channel on that card. (sometimes MAX shows you options that are more general than what your specific HW supports, like how it shows you "built in" CJC option when your card can't do that.)

[MrDomino]

I'm not exactly familiar with thermistorrs but I'm guessing that they don't use any kindof CJC? I know they're just resistors whose resistance is dependent on temp. Could I use a thermistor instead of a thermocouple and not have to deal with CJC of the compensating thermocouple?

[blawson]

come to think of it, using a TC for CJC is probably a bad idea, since if you assume the thermocouple junction and the measuring point are at the same temperature, you would get no voltage across the sensor.  

can you read from the onboard temp sensor channel?

[MrDomino]

I'm almost certain that the USB-6218 doesn't have an onboard temp sensor which is why I'm going to need some kind of external compensation.

[blawson]

I believe that M-series devices all have the internal channel I mentioned above.  Getting there is not a beginner topic though.

Why don't you just run it with constant compensation with an input for room temperature at the time of the experiment? Step one is to get it running and find out if imprecise CJC is even a problem.  If you apply a constant, your readings will be off by whatever error is in the constant.  You can't count on the board temp sensors to be better than +/- 0.5C anyway.  You're likely to have bigger problems with noise, sampling errors, resolution, etc.

If you have an extra AI channel, you could prototype a temp sensor from one of several temperature ICs.  Look for LM35, LM94022, etc.

[blawson]

I believe that M-series devices all have the internal channel I mentioned above.  Getting there is not a beginner topic though.

Why don't you just run it with constant compensation with an input for room temperature at the time of the experiment? Step one is to get it running and find out if imprecise CJC is even a problem.  If you apply a constant, your readings will be off by whatever error is in the constant.  You can't count on the board temp sensors to be better than +/- 0.5C anyway.  You're likely to have bigger problems with noise, sampling errors, resolution, etc.

If you have an extra AI channel, you could prototype a temp sensor from one of several temperature ICs.  Look for LM35, LM94022, etc.

[MrDomino]

Wait, I'm using a USB DAQ, not an M-series device...