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Reduce noise on thermocouple readings on NI PCI-6225

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Hello All

I have a problem with what appears to be static electricity induced noise on thermocouples.

 

We are testing with a sand like medium flowing across a plexiglass window and measuring temperature in the flow. The interaction of the air/sand mixture flowing across the plexiglass creates a significant amount of static and our thermocouple readings are severly impacted. Is there a way to filter the signal so my data and signal analysis tools ignore obvious off-scale readings ?

The filtering i see seems to only apply to high freqency noise. I would like to trim the signals from the TCs to ignore any measurements above 700DegF and below 32Degf.

 

Thank you

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Accepted by topic author AeroEng

Hi AeroEng,

 

It might be helpful to see what the current state of your acquisition VI is - just to get an idea of your overall architecture. I can think of a couple of alternatives.

 

  1. You could do some external signal filtering to get rid of the extra charge on your thermocouple wires - refer to this very helpful White Paper on the topic of Wiring: http://www.ni.com/white-paper/3344/en/
  2. As far as software-defined filtering, you could use the In Range and Coerce function (http://zone.ni.com/reference/en-XX/help/371361J-01/glang/in_range_and_coerce/) on your data and use the boolean output to hide your actual data or simply coerce it to your MAX/MIN values

Hopefully this helps!

Xavier
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Thanks Xavier 

 

I was headed down the path of the coerce function but i wanted to make sure that there was not a simpler way to alleviate the issue.

I thought that in NI-MAX one could limit the range of the measured signal. It seems to work with other analog measurements that the range that is set in the task, sets the limits of the measured signal. 

 

thanks 

 

Peter

 

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AeroEng,

 

You are completely correct - that is another alternative way. A higher voltage seen on the 6225's input will be clipped at its maximum value. However, you will need to know which voltage on your thermocouple leads is associated with your desired limits and select the appropriate input range accordingly. You also will only have a couple of options (from it's specifications page, the 6225 has ±10V, ±5V, ±1V and ±0.2V as possible ranges - any other selection for Max and Min will coerce to one of these values) so you might clip too much or not enough of your undesired signal.

 

As I said, there are various methods to achieve what you are looking for. Another one (which would give you more custom control over the behavior of your data) could be to use a custom scale that deals explicitly with values that are over a certain threshold (http://digital.ni.com/public.nsf/allkb/0464919A11C092E386256D800079C3BB).

 

Hope it helps!

Xavier
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Myself, I would try to dissipate the static charge.  It can't be good for the equipment...

Bill
CLD
(Mid-Level minion.)
My support system ensures that I don't look totally incompetent.
Proud to say that I've progressed beyond knowing just enough to be dangerous. I now know enough to know that I have no clue about anything at all.
Humble author of the CLAD Nugget.
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