Odd fluctuations


 
R

RJ Riememsnider

TVWBB Pro
I just noticed something odd. I've had my HM sitting on my office desk today and left it on with 4 new probes I got and this is the graph. What would cause the pit probe fluctuations? All probes are sitting together, practically touching so i would think if this was an actual temperature fluctuation, they would all show it....


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I'm not sure but because it is on the probe's low-resolution range, maybe you're seeing the probe oscillate on one side or the other of a single ADC tick, which is smoothed out by the oversampling and averaging functions. You can check this by figuring out what ADC values correlate to the peak and the valley and seeing how close they are.

If they're separated by more than 2, then I really can't say. You could also try putting a 0.1uF capacitor between the two probe connections (tip and ground) and see if it goes away. That's something I was considering for a future PCB design to reduce the ADC input impedance.
 
As more data about this, here's a grab from my HeaterMeter this morning from around the same temperature:


You can see the straight areas that correspond to one ADC tick indicating the resolution of the pit probe at this range. The little slopey bits are where the oversampling makes a smoother curve out of the transition area, but doesn't do much once the value is firmly into whatever ADC reading corresponds to that temperature. Each straight area is roughly 1 degree F apart. This value would change depending on which probes and fixed resistor you're using, but it looks like you've got a pretty big swing. What's that, like 5F?
 
Yes, I determined it was not the limit of resolution on the ADC and putting a .1uf cap didn't seem to effect it. Moving the probes around to different jacks seemed to change the behavior slightly but it didn't seem to follow the probe. I have relegated this to possible resistance variance from the contacts and the low temperature that the 732s were measuring(I am also using 10k resistors). I was planning on doing some more testing the next cook I do on higher temperature stability.


Thanks for following up Bryan.
 
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