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Topic: thermocouple questions (9 msgs / 142 lines)
1) From: Peter Zulkowski
Merry Christmas to everyone :)
For the last dozen or so roasts first crack has been occurring at higher 
and higher temperatures.
At this point I do not hear first until nearly 435F!
It used to happen about 410F but has slowly crept up.
Yup, the inside of my PGR is covered with brown, including the 
thermocouple (gave up cleaning it after I did it one time).
So I figured that perhaps all the coffee residue is skewing the reading.
However, at end of roast, about 461F, second crack is happening, and 
that has always been the case.
Any ideas as to why first crack is happening at increasingly higher 
It does not seem to matter how large a roast I do, or what type of bean. 
Same for 75 Gr as for 750 Gr.
I just did that test today, with different beans also.

2) From: Rob Stewart
After 325 roasts in my Rosto both it and the tc probe are a nice brown, 
having never been cleaned and still get 1st at about 414f.  Have you 
checked at what temp water boils. Maybe the lower end is acting up or 
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Peter Zulkowski wrote:

3) From: Rob Stewart
Also if your using just the bead, could the bare wires just behind it 
have gotten mashed together?

4) From: Peter Zulkowski
The TC is pushed through a small diameter SS tube (salvaged from a 
turkey fryer thermometer). The bead just pokes out the end, but it does 
not seem to matter if it is withdrawn or extended out further.
There is no more insulation on the wires close to the bead.
For my new project I acquired a probe from a microwave oven.
The kind you push into a roast.
It had a thermistor in it :( , so I removed it and just tested the shield.
First I put a bare TC in hot water and got a reading.
Next I pushed the same  TC into the SS shield and bottomed it out 
against the pointy end, and put it into the hot water.
There was only one degree difference.
Close enough for roasting I think.
Still roasting  by time, smoke, temperature, sound, smell and intuition, 
here in LHC.
Rob Stewart wrote:

5) From: Rob Stewart
So .... You don't think there could be a problem with the wires now 
contacting each other or the ss tube?
Peter Zulkowski wrote:

6) From: Peter Zulkowski
Good idea, I will attempt to insulate the wires from each other and from 
the tube. At a glance the wires do not appear to be touching anything 
other than where they are welded into a bead, but I will inspect more 
Rob Stewart wrote:

7) From: Peter Zulkowski
Thanks again Rob, I hesitate to pull the TC out of the tube as it is a 
bit complicated to get it back in... oh my!
I could just push it further through and put the end in boiling water 
huh... duhhh.
Trying to watch out for old age, but it keeps creeping in, here in LHC
Rob Stewart wrote:

8) From: Erik Gilling
Rob Stewart wrote on Sat, 24 December 2005 15:15
This shouldn't matter as a thermocouple measures the temperature at the point of the junction between the two wires.  The only thing special about the bead in a thermocouple is that the two wires have been welded together.  If the wires were mashed together just behind the bead the junction would be moved a few mm closer but should not affect the temperature reading greatly.

9) From: Rob Stewart
Thanks Erik,  I'll add that info to my meager understanding of 
thermocouples. I had thought the bead had some special properties and 
significance other than a connection.

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