High amount of noise in thermocouple reading

[camerond]

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@jab10015 wrote:
That's where I am confused. There are tons of attenuators out there it's just a matter of finding one that eliminates high frequencies. From what I found in a short time, a VSWR resistance network was mentioned and could reduce these high frequencies (>50kHz). As for where to put it, I am stumped on that as well as I am not very familiar with these circuit elements. Would it connect to the NI 9213 unit we have or the RFE probe?

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That's where your advisor (or as Rocky and Bullwinkle would say, "Mister Big.") comes in. Hey, that's what we called ours, anyway .

[johnsold]

Well, I have been using thermocouples for 40+ years and this is the first time I ever heard anyone suggest an attenuator as a solution to a noise problem!  (Read what I said about faculty members above).

For an attenautor to be of some assistance it would need to attenuate the high frequencies without affecting the low freqeuncies and DC - AND WITHOUT introducing any new thermal junctions (meaning the attenuator needs to be made of the same materials as the thermocouple). By the way, such a device (not including the thermal effects) is called a "filter."

The proper engineering approach to a problem like this is to (1) Identify the coupling mechanism(s) between the interference source and the thermocouples. (2) Find ways to reduce the coupling. (3) Apply hardware filters to keep the interference out of any electronic circuits which might be driven into non-linear operation. (4) Apply signal processing in hardware or software to reduce the effects of any remaining interference.

Without doing (1) anything else you try has a significant probablility of being a solution to the wrong problem. If (1) and (2) are done well, the remaining steps might not be required.  Sometimes (2) requires redesigning the system to correct grounding or common impedance problems.

If you do not mind reporting this publically, what is your geographical location?

Lynn

[jab10015]

Hey all,

We have constructed an external low-pass filter on a breadboard that seems to be improving the noise present in our signal. The next step is fine tuning the capacitor and resistor values to more effectively reduce the noisy signal. I am also creating an external radio frequency filter that I came across the other day. Again, thank you for the help.

[camerond]

Since you (reportedly) didn't have any problem when the TC was suspended in air, did you ever try coating it with something nonconductive (even fingernail polish, if there are any ladies in your lab, or plastic model cement, or Vaseline for a short-term test) to see if isolating it electrically from your heater works?

Cameron

[Henrik_Volkers]

jab10015,

you can build inductive schoke filters with the TC wires. That wouldn't introduce additional junctions. (Never tried, but even Cs soldered/clamped to the TC wire and held at equal temperature shouldn't introduce (big) EMF errors)

But again: The EMC that add noise to your TC EMF is most probably due to inductive coupling. (inductive heater!  :D)

Tightly twisting the TC wire to minimize the TC loop area. 

Identify the magnetic field ( by theory , or  use a paper with iron shavings and apply some strong DC current pulses) and let the TC wire follow these field lines.

Finally: Hook up the TC wire to a scope: You will see the RF

Now you can untwist a small area of the TC wire near the coil, building a small antenna loop. By changing the orientation of that loop you should be able to to minimize (if not cancel) the RF on the scope.

A picture of the heater and your TC would really help.