Extreme Negative Saturating Signals

[Tino_1111]

Hi, I have a thermocouple program to read temperature over a period of time, but the problem is that I am getting negative 'spike' in the ranges of -2000000 degrees Celsius! I have checked through my program and I don't think this is a problem because the neg signals are so sporadic. I am using a PCI-6071 with a SCB-100, but I don't know how to troubleshoot this problem to check if my hardware being used is working properly. If someone could please help me to solve this problem it would be unbelievable!! New UserLabview 8    

[mr_]

Tino_1111--

Are you sure you are using a K type thermocouple?  Also, are you using any CJC (cold-junction compensation)?  The reason I ask is because thermocouples require some form of temperature reference to compensate for unwanted parasitic “cold” junctions. The most common method is to measure the temperature at the reference junction with a direct-reading temperature sensor and subtract the parasitic junction voltage contributions. This process is called cold-junction compensation. You can simplify computing cold-junction compensation by taking advantage of some thermocouple characteristics.

Is your thermocouple floating or grounded? From the setup of your VI, it appears as though your thermocouples are floating sources.  If it is floating, you don't need to do a differential measurement, you can just directly connect the negative terminal to AIGND. If it is ground referenced, your best method is to do a differential measurement. A differential measurement helps eliminate noise by rejecting noise that is common to both lines of the differential pair. If these lines are not close together, they can have independent or out-of-phase noise on each of them which could actually amplify the noise level. We recommend using shielded, twisted pair wires for the two signals. Connect the shielding to AIGND or the ground of your thermocouple.

I have some additional questions about your general setup.  Do you have more than just two thermocouples?  Do all of thermocouples exhibit this "negative spike" behavior?  In your screenshot I see the one numeric indicator with a large negative number, but the graph looks like it is at 0.  Have you tried different pins on your DAQ card? 

Have you looked at the examples included with the DAQmx driver?  There are some great examples for analog input, temperature-thermocouple.  Once in the Example Finder (Help»Find Examples) navigate to Hardware Input and Output»DAQmx»Analog Measurements»Temperature.  This folder will have examples that reference CJC as well.  This is something I think you may want to look into.  

Here is a website that talks about signal conditioning in general.

Cheers!

[Tino_1111]

Hi Tyler, Thanks for the advice but to tell you the truth I am somewhat confused. I'll let you know what I have tried to make things more clear. I have 6 K type thermocouples that need to be read continuously for a given time at a certain defined rate. My usual setup in the VI is to have the CJC at a constant value of 25 and select K type for my thermocouple option. I have connected the thermocouples a few different ways to diagnose the problem. 1. Connected the positive end to ACH# (ACH8) and negative to AOGND. 2. connected the positive and negative ends using for example ACH16(PC7) and ACH24 (GND) then ACH 17 and ACH25 and so on. This does work good for all 6 thermocouples only if the temperature each is reading are above 25 degrees celsius if one goes below 25 degrees celsius then this one will get the 'negative spikes.' The toggles on the SCB-100 are set for 100 generic terminals (temp. sensor and accessory power off). I have tried all 4 toggle setups as referenced by the SCB-100 label, but I still get the same effect or no reading. I have tried the 'Acq CJC Temperature.VI in the examples and it does work reading at ~40 degrees celius or 106 F but I don’t know what I need to do to my VI or hardware to prevent these negative spikes. I don’t think I am using my CJC properly because I don’t know how to. I do agree that it is something to do with my setup but I have no clue what else to do. If you need more information please let me know. Thank you again for the reply. Using Lab View 8 P.S This a link to the SCB-100 referance guide. http://digital.ni.com/public.nsf/ad0f282819902a1986256f79005462b1/f011ad8b0f4bd35086257050006cbb70/$FILE/ATTXMKR3/SCB-100%20E%20Series.pdf

[mr_]

Hey Tino_1111--

If it is working for you with configuration #2, it sounds like your thermocouples are meant to be measured differentially.  

I am a bit confused from your last post.  Did you mention that the example does work, but not your VI?  Does the example show negative spikes? If the Acq CJC Temperature.vi works, you might want to try making the following changes:

This uses the internal CJC sensor, which the Acq CJC Temperature.vi uses the built in sensor as well.

Cheers!

Message Edited by Tyler C on 05-15-2009 05:51 PM