SCXI 1120 thermocouple

[sgregor249]

Hello all. I currently have a system setup that is reading several thermocouples which are setup to read between -20 and 121 deg C. I need to increase one of the thermocouples from a maximum of 121 to 450 deg C.

The system has sevreal K type thermocouples wired into a TBX-1328 which is connected to a SCXI-1120 card then to a PCI DAQ card in a PC. The hardware gain on the SCXI card is 1000.

I've attached my LabVIEW code that allows me to read the TCs just fine when the min and max are set to -20 and 121 deg  C. However when I increase the maximum to 450 deg C, the DAQmx read.vi throws the following error:

(DAQmx Read (Analog 2D DBL NChan NSamp).vi
Property: AI.Max
You Have Requested:  450.000000
Valid Values Begin with: 81.739252
Valid Values End with:  121.951756

Channel Name: TE400A

Task Name: TCs

Does anyone know what is driving the 121.95 upper bound? Does it have something to do with the gain jumpers on the SCXI 1120D? How can I get this TC to read up to 450 deg C?

[Broken_Arrow]

From the look of the error, the channel that you want to read up to 450 is still set like the other channels which read a max of 121. You have to set the min / max independently per channel.

Another thing to look out for, if you have open thermocouple detection enabled (by using resistors), then you can't read quite that high.

[sgregor249]

Broken Arrow, I don't really understand what you mean. The code that I posted is a stripped down version of something that sets up multiple TC channels. The attached version only does one channel. I realized there is a mistake in the code, [450] should be wired into the max scaled input of Init DAQ.vi instead of [-121]. Can you elaborate on what you mean by "still set like" in your first sentence? How do I set the min/max independently on each channel? I thought that the DAQmx Create Channel (AI-Temperature-Thermocouple).vi is doing exactly that.

[sgregor249]

I just ran the stripped down code on the test rig and I realized that the error -200077 that I posted earlier is generated by the DAQmx Start Task.vi.

[Broken_Arrow]

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sgregor249 wrote:

Can you elaborate on what you mean by "still set like" in your first sentence?

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I meant that you may be doing something down stream that conflicts with how you set up the Min/Max in your VI.

BTW, I ran your VI and it works fine. I configured it to have 4 T/C's, with the 3rd T/C being 121 to 450, the others being -20 to 121. I have to put in > 2500 in the MAX to get your error.

Message Edited by Broken Arrow on 06-16-2010 08:12 AM

[sgregor249]

Were you using an SCXI 1120? Was the product of the two hardawre gains 1000? Did you do anything in MAX?

If the code worked on your setup that makes me believe that its a hardware issue.

Message Edited by sgregor249 on 06-16-2010 09:42 AM

[Broken_Arrow]

I have a 1102 which has 32 inputs and an input range on 10V. Yours has an input 5V and 8 inputs.

Why are you using a gain at all? When you set the input type to TC and set an input range, the card will set gain for you.

[sgregor249]

The gain settings on the card for the channel of interest was set to 1000 which specifies a +-5mV range, which in turn means a maximum of ~121degC. I changed the gain setting to 250 which specifies a +-20mV range, allowing my thermocouple to read up to ~485degC. I ran the LabVIEW code and was able to read ambient temperature when the new gain settings without any errors. I am going to use a thermocouple simulator to see if I successfully read the required 300degC, but I am confident that it will work now.