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Topic: Digital thermometer with thermocouple: the blues (13 msgs / 299 lines)
1) From: Scott Marquardt
http://www.sweetmarias.com/prod.roastkits.shtml"The thermocouple lead can eventually wear out - or if you contact a metal
surface during roasting, it can melt a bit. It is best to use this
thermometer to measure air temperature in a roaster, or the temperature in
the beans only."
I installed this in a way that embeds just the tip in just the bean mass.
There's some blue sheathing just up from the tip, apparently some heat
shrink which turns out to be pretty vulnerable to heat. It just melts! Based
on how I mounted the TC, I suspect I'm now risking a short across the leads.
It would be good to have the vulnerability of the blue heat shrink noted in
the product description. It's not shown in the picture -- so this is
probably a mod since Tom posted his original product description.
The meter seems to work very well indeed, however.
-- 
Scott

2) From: David Liguori
No need to worry about a short across the leads.  A thermocouple _is_ a 
short!  Just put more heat shrink on it if you're worried.  Radio Shack 
sells it.
  --
David Liguori
Scott Marquardt wrote:
<Snip>

3) From: Scott Marquardt
True. But does a non-welded short in any way compromise the character of the
coupling?
As for adding more things to melt in my bean mass, I'll pass.  ;-)
Having a coupling itself in the mass sure makes for responsive, fast
readings. I don't think I'll be doing that for long (will have an industrial
K in there soon), but for now it's pretty fun to use this thing.
On 6/23/06, David Liguori  wrote:
<Snip>
-- 
Scott

4) From: Peter Schmidt
This is a multi-part message in MIME format.
Scott,  you may want to consider a digital meat thermometer, i.e. a BBQ =
thingy.
After fraying two wired K types, I went with a meat probe design.  It =
reads fast enough and only at the tip, and it can sit in the beans =
forever- no wear.
An added plus on some models is a preset alarm that can be set to go off =
at say, 300.  That's where I plateau for a drying phase.
peter

5) From: Terry Stockdale
At 10:26 AM 6/23/2006, Scott wrote:
<Snip>
Yes.  If you fray the insulation and the wires touch anywhere except 
via the weld, you'll get a false reading.
I've got a thermocouple for a kitchen digital thermometer that I 
managed to destroy when BBQ'ing -- I managed to expose the wires 
where they came out of the probe.  The resulting short gives a temp 
value of HI.  If I untwist the wires carefully, I can get a real reading.
Terry
--
Terry Stockdale -- Baton Rouge, LA
My Coffee Pages:http://www.terrystockdale.com/hottopMy RK Drum pages: http://www.terrystockdale.com/rkdrumTerry's Computer Tips: http://www.terryscomputertips.com<Snip>">http://www.terrystockdale.com/coffeeMy Hottop pages:http://www.terrystockdale.com/hottopMy RK Drum pages: http://www.terrystockdale.com/rkdrumTerry's Computer Tips: http://www.terryscomputertips.com<Snip>

6) From: Scott Marquardt
I have that same one, with the alarm, I'll wager. Works well enough. I'll
probably put it into my bean mass along with the TC as a backup!
On 6/23/06, Peter Schmidt  wrote:
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-- 
Scott

7) From: James House
online link for purchase?
On 6/23/06, Scott Marquardt  wrote:
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8) From: Dean
you can just peel the heat shrink tubing back off--it looks like its 
there mostly to keep the jacket from fraying too fast (and of course to 
look nice).  I stripped two of these and stuck them into the bowls on my 
Rostos, and they're holding up pretty good. (its the little blue TC 
thermometer from Tom & some wire from 'somebody else' (I forget who))
even if the wires touch a bit, i don't know if they'll change the 
reading much--there's >probably< an oxide layer that gives a little 
insulation to the wires, plus you could draw the series/parallel circuit 
that gets created and study it to see if it's likely to be a problem
just my two cents
Dean
Scott Marquardt wrote:
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9) From: David B. Westebbe
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Likely the probe is a thermistor and not a thermocouple.

10) From: Peter Schmidt
<Snip>
David,
Just out of curiousity, what's the diff?
I get a free edumacation every day on this list?
peter

11) From: Michael Wascher
Peter,
A thermocouple is two dissimilar metal wires twisted together. The junction
of two dissimilar metals generates a small voltage. The voltage changes as a
function of temperature.
A thermistor is a semiconductor device whose resistance varies as a function
of temperature.
Thermistors are relatively inexpensive, primarily because the associated
circuitry required to use them is simpler. But thermocouples are a probably
a better choice for coffee roasting applications. They are usable at higher
temperatures and are more rugged.
Both have nonlinearities that must be accounted for.
--MikeW
On 6/24/06, Peter Schmidt  wrote:
<Snip>
-- 
"There is nothing new under the sun but there are lots of old things we
don't know." --  Ambrose Bierce

12) From: Scott Marquardt
No.
It's easy to tell. A TC is just a little welded bead.
On 6/24/06, David B. Westebbe  wrote:
<Snip>
-- 
Scott

13) From: raymanowen
A thermocouple is, "...a short across the leads," Scott. If the bead breaks
or breaks off, you can quickly fix the thermocouple by cutting the wires at
the bead, stripping the wires  1/4 inch, burnishing the wires with steel
wool, and just twist or crimp the wires together.
You can duplicate the junction more permanently if you can MIG weld or gas
weld the junction with a slightly reducing flame. Either way works well.
If you embed just the tip in the bean mass, the cooler lead wires will sink
heat away from the measuring junction [tip]. Then the accuracy will depend
on the rates at which heat is applied to and removed from the junction.
It may seem like magic if the heat paths are ignored.
Cheers -RayO, aka Opa!
On 6/22/06, Scott Marquardt  wrote:
<Snip>
-- 
"When the theme hits the bass, I dance the Jig!" - -Virgil Fox at the Mighty
Wichita WurliTzer- 1976


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