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Topic: Digital Temperature probe OK for air popper? (7 msgs / 174 lines)
1) From: Darren Conrad
Howdy,
Been reading about folks adding a digital temperature probe to their roasters
and suddenly remembered that I happened to have one of these.  It was a freebie
that the vendor threw in when I purchased my DVM.  But, I am concerned whether
it can be used with my air popper.  Here's the specs:
Type K thermocouple sensor
Fast responding handles temp range of -40F to 482F
Designed for gas and surface measurements
The tip of the cable has a beaded sensor for air measurements
What do y'all think?  Can I poke this through the cover on my popper and seal
the opening with some high temp silicon glue?  Does this have enough range for
coffee roasting?
Thanks for your sage advice!
Darren
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2) From: Rick Farris
Darren wrote:
<Snip>
Sure.  Note that if you permanently attach it to your popper, you won't be
able to throw it in the dishwasher anymore...
-- Rick
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3) From: EskWIRED
<Snip>
It is very unlikely that you wold ever exceed 482 in your popper.  I'd go
for it.
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4) From: dewardh
Peculiar . . . I find a reply to this question, but can't find the original 
post.
<Snip>
If you're talking about the $19.95 device mentioned previously . . . it's hard 
to figure how they arrived at the "482F" upper limit.  The bead probe that 
comes with it certainly works at temperatures well above that (as does the box 
itself).  Most probably the "limit" comes from a little piece of shrink wrap at 
the end of the probe lead, there to prevent fraying of the fiber insulation, or 
the insulation itself (which should be fiberglass, but isn't . . . I just flame 
tested it and it burns).  I'd at least cut off the shrink wrap . . .
But, more generally, be careful with bead or wire thermocouples.  Thermocouple 
wire is quite brittle, and mechanical failure of the junction is almost a 
certainty if it's long exposed to banging around in a mosh pit of roasting 
beans. I posted a "hint" a few days ago about how to make a simple (and cheap) 
thermowell (1/8" thinwall tubing . . . SS is about $3.00 a foot at the local 
hardware, high temp heat sink compound from Radio Shack or anywhere (you only 
put it in the last 1/4" of the tube, just enough to connect the bead to the 
wall, so you don't need much), crimpers or pliers you've already got, threaded 
fittings or swagelocs you don't need) . . . you probably saw it already. 
 You'll get more consistent and more reliable readings with a well, and, if you 
put a dab of sealant in the "other end" of the tube, a much higher temperature 
rating for the insulation (no air, can't burn ).
The "art" of temperature measurement lies in knowing what you want to measure,  
 and getting a probe in place to measure it.  Rick Farris posted an 
interesting note last week comparing thermocouple with an "analog" (mechanical 
dial) thermometer.  The thermocouple disclosed that the dial thermometer was 
*not* reading bean temperature, but an undefined combination of bean 
temperature and roast chamber temperature (and thus always too high).  While 
the dial got closer to correct at the end of the roast (where the bean/chamber 
differential is smallest), and clearly has at least some utility for 
determining roast endpoint, it would be useless for "profile roasting" since at 
no point during the roast did it accurately measure bean temperature.  That 
same dial thermometer, were it mounted so more of the tip/shaft was "in the 
beans" and the rest of the shaft was either at ambient or insulated from the 
roast air, would probably work just fine.
I can (obviously ) go on about this stuff far beyond most peoples interest 
(or patience) . . . if you've got questions probably better to ask direct (off 
the list) so as not to bore everybody . . .
Deward
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5) From: Darren Conrad
Rick,
I'm not planning on putting this one through the diskwasher.  The probe is very
small and attached to a long braided nylon cord that the wires run through. I
don't think it will hold up to washing and the box states specifically that it
is not designed for use in liquids.
I know some build up will occur over use from coffee smoke and oils, but I
don't think that I should have to remove it to clean it occasionally. Is the
temp range on this probe OK?
Thanks,
Darren
Darren wrote:
<Snip>
Rick replied:
<Snip>
<Snip>
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6) From: Spencer W. Thomas
My problem with my dial thermometer is that the setting "drifts" (for 
lack of a better term).  Last week it seemed consistently 25deg high 
(based on its "room temperature" reading, and the reading during first 
crack). This week it seems about 25 deg low.  Significantly annoying.
=Spencer
dewardh wrote:
<Snip>
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7) From: dewardh
Spencer:
<Snip>
lack of a better term).
There are lots of "problems" with "stick and dial" thermometers . . . but they 
still manage (usually) to work reliably and well.  I've seen one actually "skip 
a tooth" on its drive gear (when being used near the top of its range) . . . 
"permanent" re-calibration, that .  And they, like all thermometers, make 
certain "assumptions" about use and environment.  It used to be (with 
glass/mercury in particular) that thermometers had on them an "immersion line" 
.. . . the presumption being that below the line was in whatever was being 
measured, above the line was at some (human compatible, unless otherwise 
specified) "ambient".  I've seen "digital" (thermocouple/RTD) probes specified 
thus, but it's no longer common, and I've never seen it on an "inexpensive" 
instrument (although what's doing the "measuring" is pretty obvious with an 
exposed bead thermocouple).  In fact the temperature of the upper part of the 
stem (and the dial, in "mechanicals") can, as in Rick's case, make a 
significant difference in the reading.  There can be a problem with thermowells 
also, especially if the sample is small, the sample/environment differential is 
high, and the thermal conductivity of the well itself is not well attended to. 
 I've seen probes with one TC at the tip, and another "compensating" junction 
further up the probe.  Not cheap, though . . . usually the only "compensating 
junction" is in the box.
Roasting, like politics and, heh, dare I say life itself? can seem simple on 
the surface . . . but it can get quite complex as one digs into the details. 
 That's why we (usually) settle for "good enough" . . . (or, in politics, "not 
good enough, but the best we can get at the moment").
Deward
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