Incorrect temperature readings (TC02 + AI03)

[mdelsol]

Hi,

The equipment is a PXI-8175 (PXI-1031DC), with the following cards: PXI-4472, PXI-6221 and the carrier SC-2345 with four TC02 and two AI03 modules.

When I read a single channel the measurement is good, problem starts when I read more than one channel at the same time, then the temperature readings have different offsets. And if I open one of the temperature channels the others vary (one 4 degrees down, other 2 up and another 10 up). If the analogue input from the AI03 is open also the temperatures vary up and down… when I connect it everything changes again. As I can connect the equipment to a DC battery, I tried this with a 12 volts car battery and happens the same. I’m using a K type thermocouples and “Built in” CJC.

Can any one help me?

Thanks in advance.

Marc.

[mdelsol]

Hello,

I discovered that this phenomenon only happens when the acquisition is performed at rates higher than 500 Hz. Does any one know why does it happens and how to solve it.

Where can I find information about this phenomenon?

Thanks,

Marc.

[Cristobal]

Marc,

Which are the positions of the SCC modules? Are you trying from LabVIEW or MAX? Could you include your code to try to test it?

I have a SCC with two SCC-TC02 ( J1 and J2 ) and a SCC-AI-03 ( J3 ). I have connected just two thermocouples to TC02 modules and from MAX, I have created a task with two daqmx global channels. When I disconnect the thermocouple from J1 module, the temperature of J2 doesn't change. But when I disconnect the thermocouple from J2, the temperature of J1 goes up nearly 1 ºC.

I am not using SCC-AI-03 in this test.

crisR

[Cristobal]

Hello,

I have also tried connecting 5 volts to a SCC-AI03, and when I disconnect this voltage, the measurement of the first thermocouple goes down a lot. My scan list is thermocouple1, 2, 3, and voltage.

When I disconnect the voltage, this input goes a -10 volts, so when the board scans the thermocouple1 and to pass from -10 volt to a positive voltage, this is a big step and there is no time for the PGIA and ADC to set the gain a digitize on time. It seems that there is a crosstalk between these two channels and the voltage couples on the thermocouple1 measurement.

This is the behaviour your are seeing. I think there is no way to solve this unless you don't disconnect the signals that you are acquiring.

http://digital.ni.com/public.nsf/websearch/B9BCDFD960C06B9186256A37007490CD?OpenDocument

Regards

crisR

[mdelsol]

Hello crisR,

You are right, this is probably the problem… but as you probably know in testing can happen any thing and some times happens that you have sensors failure, and in this case one sensor failure causes bar readings on the rest…

I think that this is an important limitation of the system and that should be reported to the clients before to by it… or at least to be explained better on the user manual. Any way it’s good to know what’s happening with the system and to know that I’m not doing any thing wrong…

Thanks you so much for interest regarding this item.

Regards,

Marc.