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Topic: Thermocouple probes (5 msgs / 143 lines)
1) From: Marchiori, Alan
After seeing the link for the $20.00 thermocouple thermometer over and over
again (one more time:http://store.yahoo.com/webtronics/digtherwkpro.html)I
finally gave in and got one... Now I want more probes and having never
worked with thermocouples before I am looking for some advice on where to
begin.  Some of my general questions are:  What type of insulation is best
for a roaster application i.e. what will last the longest at 500F+?   I see
glass braided (and think hmm glass doesn't sound like it would melt) and
then Teflon and think that sounds good too.  20 gauge 30, 36 gauge?  I would
rather go for the larger gauge so it is less fragile?  And my last question
is if I just buy thermocouple wire (much cheaper than buying pre made
probes) is there an easy way I can join the ends together to make probes
(that doesn't require a welder)?
thanks,
Alan
ps.  I apologize that this message contains no poetry.
pps.  I do think that science is an art, however.
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2) From: Dan Bollinger
Telfon works for me, but I'm not using it every time.  The ends must be
welded, just buy the premade probes. Dan
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3) From: Jason Molinari
You shuold get the glass covered wire. At about 450 def. F teflon emits toxic gasses...not a good thing:) Also the glass covered is more flexible... You COULD just twist the wires together for a thermocouple, but i dont thikn its as good as a weld..you will just have to make sure that the probe is in contact with the heat at any place hte wires are touching each other ..since thats where it makes a short and detects temperature. It will work, just not pretty:)
Gauge is up to you . I prefer small gauge since it reacts much faster. I put mine in the bean mass in my popper and it kind of floats around swirled with the beans..
jason
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4) From: Tom Gramila
The best spec's that I could find said that the teflon insulated probes 
were fine for continued use at 450F, and for intermittant use up to 
500F.  I have used teflon insulated TC's with teflon as a probe in the 
bean mass of my poppers without any trouble.
The fiberglass insulated ones are good to a much higher temperature.  I 
uses this type for monitoring the post-heater, pre-chamber air 
temperature, which gets much hotter than 500F.  The spots that get close 
to the heater wirres lose the color in the glass insulation, but the 
glass stays put.  Touches between the TC and the heater destroys the 
glass.
As far as making the TC joints go, any solid electrical joint the can 
survive the temperature that it is exposed to will work fine.  The thing 
that I worried about in setting mine up was the fact that by using a 
disimilar metal (like solder) to make the joint that you would be 
introducing additional thermocouples that might make your measurement 
wrong.  This turns out to actually be relatively unimportant, as long as 
the TC wire 1 - solder - TC wire 2 junction is isothermal.
The reason this is so, I believe, is that the dominant voltage that you
measure is actually developed along the wire, and not specifically at the
junction.  The is a very small voltage that exists between disimiliar
metals, but the workhorse of the TC is the fact that any material with a
hot and a cold end will develop a voltage difference.  ( Think of the heat
as influincing the velocity of the electrons at each end -- at the hot end
they run away faster, so at equilibrium, there must be fewer around, --
the voltage you get then comes from charge accumulation).  TC work because
the hot end - cold end voltage difference is different for the two metal
types that make up the thermocouple.
My setup confirms the unimportance of the specific junction material.  I 
use both spot-welded junctions, and tin-lead soldered junctions.  Since 
I measure the TC voltages with a straight voltmeter (about 22 microvolts 
per degree F), I use a reference junction setup: two back to back TC's.  
The voltage measured in that case is the temperature difference between 
the two junctions.  (If you want to use NIST calibration tables, for 
example, this is the voltage they list, with one of the junctions cooled 
with an ice bath.)  But anyway, the voltage that I measure with the two 
sensors at the same temperare (one is welded, the other soldered with 
ordinary tin/lead)   is really small  less than the 
equivalent of a couple of tenths of a degree.  So the most important thing 
is that the joint does not melt!!  not so much what its made of.
Tom G.
On Fri, 24 Jan 2003, Jason Molinari wrote:
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5) From: Mike McGinness
Temp probes for NU-701 digital wireless Q thermometers are available rated
to 1200f to withstand flareups. These puppies are way cool BTW. Can monitor
3 different wireless probes all set for different high or low temp alarms
etc. I've just recently discovered them. Probes take heat, the units
themselves only monitor to 392f  however, more than high enough for low and
slow Q but not high enough for coffee roasting. I'll be investigating
finding base units that read higher...
MM;-) aka Kona Konnaisseur miKe mcKoffee
Dual Variable Transformer Rosto Roasting
Rocky grindin' - Miss Silvia brewin'


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