Temperature readings with USB-6009 and K type thermocouple

[Allistar]

Firstly I'm using the "ContAcqThermocoupleSamples_IntClk.2010" C# example in Visual Studio 2010 that came with the installation CD for the USB-6009. It was install to here...

"C:\Users\Public\Documents\National Instruments\NI-DAQ\Examples\DotNET4.0\Analog In\Measure Temperature\ContAcqThermocoupleSamples_IntClk\CS"

I have my K Type thermocouple wire to the AI3 + and - terminals (which as I understand is differential?) I'm a total newbie...

I've run the application and I seem to get random garbage from the output... I've attached a screen grab of the application running.

As you can see I've set the CJC to a constant of 25 deg C which is read from a good old mercury thermometer sitting on top of my 6009.

I have no idea what I'm doing wrong, can some please point me in the right direction?

[Jacob_R]

Hello Allistar,

If you are running off of an example, the code should be fine. What we need to do is troubleshoot your signal with a simple tool such as Measurement and Automation explorer.

Once open, you can configure MAX. From the screenshot, it looks like the connection has not been set up properly.

Once you have that connected and working well in MAX, you can go on to program your application in Visual Studio.

One thing to note, is that the 6009 will not have very accurate temperature readings as explained here.

[Allistar]

Sorry about the delay in getting back to you... I'm slowly learning...

I've followed the steps to get this setup in Max and it looks like I still have the same problem (or similar at least) as you can see from my screen shot my samples vary wildly between about -25 and -75.

I have the thermocouple sitting here on my desk and the air temp is about 25 degrees at the moment.

Anymore ideas?

[Jacob_R]

Hello Allistar,

It really seems like it is a connection issue or the actual thermocouple.

Make sure you are connecting everything correctly according to the the following color codes.

Also try a different thermocouple and a different AI channel.

Hope that helps!

[Allistar]

The wiring is correct I'm certain of it...

I doubt that it's the thermocouple as it's one that came with my multimeter and when use with it with the multimeter it shows me the correct temp. Also it's clearly got "Thermocouple Type K" marked on it. I'm pretty sure I haven't screwed that up.

Another interesting peice of information is that when I run the same MAX thermocouple test but don't have the thermocouple wired up to the USB-6009 I get very similar varying readings (as screenshot).

Am I wrong to expect that this should read a constant ? All be it at 0 ??? Why is it varying so much ? Could I possibly have been shipped a faulty USB-6009 ?

Also I setup another test to read voltage in MAX and I get the same mV varying wildly in that also... although very unlikely but this could be the device I'm using to generate the voltage here but my multimeter doesn't show the same flucuations. Any help would be appreciated as I'm about to throw this thing out the window...

[SnowMule]

The thermocouple is a fancy form of a resistor.  It is a passive component.

The 6009 reads voltage on the analog inputs.  The AO's do not source voltage, or provide signal conditioning for external passive devices.

What you need is a circuit between the TC and DAQ to convert the resistance of the TC to a voltage the DAQ can measure. 

If you want a plug-in-simple thermocouple to USB adapter, the USB-TC01 is the module you want.  For more channels/higher accuracy, USB-9211 or USB-9213 and a CDAQ Carrier.

[Jacob_R]

Hello Allistar,

The USB-6009 is definitely not designed for thermocouple meaurements. As SnowMule recommended there are other devices that are much better suited. You will most likely need to amplify the signal before routing it to your device.

You can also get some OK measurements from the USB6009 by using a VI called Convert Thermocouple Reading; which converts a voltage to a temperature reading.

There is an enitre seperate forum on this issue here. The post by Otis is very insightful.