Temperature probe starts fluctuating at 140

[ntabb]

The issue I seem to be having is my 12'"probes from Brewershardware start fluctuating when they reach 140 degrees and it gets worst the higher the temp gets. One second it will read 152 and then it jumps to 165 and then 141 and then 170 etc.... They work great up till I get above 140. I also have two 4" inch probes that I bought from this site and they work great. No issues at all above 140. I think the thermistors in the 12" probes may be bad or somehow i got water in them, but I'll list out what I've done to troubleshoot and if anyone has any ideas please let me know.

1.) I tried another 12" probe on the same temp input
2.) I tried a different temp input that worked great on the 4" probe
3.) Turned off all electric
4.) Changed the coefficients as listed on this page http://www.brewershardware.com/temperat ... tions.html
5.) Tried the default coefficients
6.) power cycled the bcs460

I use CAT6A shielded cable with the ground wire grounded. The weird thing is they work great until I get above 140 degrees. Any suggestions are appreciated.

[Baron Ken]

Anything else going on when you reach 140F? E.g. do you turn on a pump or anything? (or is that covered by your #3?)

[ntabb]

All power remains off. The only things powered on is the BCS460, PC and monitor.

[ECC]

You have more than one 12" BH probe, and they both behave the same?

Any way that you could get a screenshot (or just paste in the values) of the "Calibrate Sensors" page around 140 when they start fluctuating? That will give us resistance/temp values to help track this down.

[ntabb]

I do have two of them and there both acting the same. Its killing me because the 4" probes on the same temp input works great.

I'll definitley take some screen shots when I get back home and paste them in here.

Thanks for the quick responses guys!

[ECC]

Ok, also a screenshot of the coefficients in the System Settings page might be helpful.

To calculate temperature from resistance, the BCS uses those coefficients to basically curve-fit the R/T response. It almost sounds like you're hitting a point in the curve that is incorrect/unstable, i.e., small changes in resistance are somehow yielding large changes in temperature. We'll know more when you get screenshots, but my first thought would be a typo in the coefficients.

[ntabb]

So I've been troubleshooting for about 2 hours and I think I found the issue. Its definitely something with the thermowell or the thermistor. I took one of the probes apart and found what I assume to be thermal paste on the thermistor. I cleaned it off and cut the thermowell down to about 5" and reinserted the thermistor and its reading perfect now. I took the other 12" probe apart and found no what I assume is thermal paste on it. I noticed that the thermistor was only about 3/4 way down the thermowell when I took it out. I don't know if that's a big deal or not just something I noticed. I reinserted it back into the 12" thermowell and it was doing what it did before, the temp was jumping all over the place. I cut that thermowell down to 5" and put it back into the water and it stopped fluctuating.

Does this make any sense?