Weird (phanton) Sonofresco Temp Spike

[slim]

Sonofresco and/or electrical engineering friends:

I'm getting an occasional (maybe once every 4 or 5 roasts) spike in temp that's causing the Sonfresco to shut down (it senses a fault.)  The whole thing is exceptionally clean, from the chimney, to the thermocouple, to the chaff screen etc.

I haven't been able to see any burning chaff in the chamber, nor have I seen any evidence of it.

I'm leaning toward believing this is an erratic thermocouple, but it also happened on occasion with the previous thermocouple. (Both came directly from sonofresco.) 

My question is, do you think it's a (another) sketchy thermocouple or could there be something else going on? I have a third thermocouple that I can swap out (also from sonofresco) but at this point, I'm thinking of getting an aftermarket. I remember some one posting in the sonofresco thread the equivalent that could be bought elsewhere.

Any and all thoughts and opinions will be appreciated.

Jim

[Ascholten]

It does seem like a loose connection possibly BUT the 'spike' lasts more than just an instant looking at your graph.  Check all your connections, especially if you plug this thing into a 'extension cord' for lack of a better word to bring it to the board, in case you have a multi junction issue.

K TC wire is cheap to get, and you don't have to have it as a 'probe' really if you are a bit savvy with it.

Aaron

[jspain]

Don't have a Sono anymore... No idea what the issue is. ever happened to me on either of my units. I'm with you however and can't believe it's the same issue three time in a row.... Did you call Sono? I always found them helpful.

[CrabApples6]

Slim, short story I second the "contact Sonofresco" comment. 

Longer story:

Normally a thermocouple that experiences a failure will create an open circuit yielding a zero voltage and low temperature.  About 95% (or more) of the thermocouple failures will create a low voltage.  On devices that  could be negatively affected by a low temperature failure, a secondary protective circuit is installed that outputs a high voltage when an open circuit is suspected.  This causes a high temperature spike and thus trips the unit which is safer than a false low indication allowing increased heat input and possible catastrophic failure.  I am not sure if the Sonofresco has this type of protection or not.

It is unlikely that multiple thermocouples are experiencing failures.  Not impossible, but improbable. 

It is also possible that the cold junction compensation is causing a problem.  Thermocouples are relative temperature devices.  A galvanic voltage proportional to temperature is  produced at the hot junction and again at the cold junction.  The resulting differential is converted to the temperature. As temperature increases, the voltage increases between the high voltage and low voltage side.  If the ground was loose on the high voltage side a high voltage would result and a high temperature would be the resulting indication.  So Aaron's advice to check all your connections should address this.  The cold junction compensation may also be an integrated circuit that is failing or has a bad connection.  If it is an integrated circuit it may have it's own temperature sensing device for cold junction temperature built in that could be failing.

Does it seem to happen at the same time each roast or randomly?  Start noting differences and similarities on when this occurs.  This is all valuable information and can help you and Sonofresco troubleshoot the problem.
 
You can check the thermocouple in a boiling water bath which puts you close to the temperature of the spike as well. 

 

[sea330]

All I have is J, T, TC wire, I'm going to order some 22 gauge K TC wire.  As soon as it gets in I would be happy to make one up for you, wire runs about .80 cents a foot for high quality Omega TC wire and a couple bucks for shipping. I agree with Aaron ether bad connection or the junction is going bad. Lucky me, I have a TC welder so when one starts getting flaky I just cut off the end and weld a new junction. Let me know if I can help. I often wondered if a chemical reaction to the metallurgy of the TC is why I repair TCs 2 or 3 times a year. 

Rich 

[Ascholten]

If it's bare wire probe then yes that can cause issues.  If it's embedded in say a stainless sheath then it is protected from elements and other stuff causing problems to it.  If you have solder on it, or corrosion or run it to another piece of wire, it can have issues at cross over points in temperatures.  Pretty much any two different types of wires can be made into a TC, (or peltier in reverse), it's just that we use certain ones for our J, K, etc because they are more reliable and predictable. Given this, any junction in the circuit can add / subtract to the overall 'equation' for your temperature reading.  You could be at some threshold where at that range, some hokus pokus is happening with wire joinings in places, causing stray voltages.  Even a slightly loose or corroded connection, temperature can cause it to expand just enough to cause problems in a range, or vibration and temp.  Ive seen TC's do some pretty stupid stuff over time, but it's job security for me    Loose junctions in a nasty environment can also cause some galvanic events to happen as well which will also throw your readings off.  For something so simple it can be a real booger sometimes to keep working.

Aaron