HomeRoast Digest


Topic: Comparisons of DTM and ATM [Was RE: +Digital Thermometer] (88 lines)
1) From: Rick Farris
As I promised yesterday when I gave my first report on the $20 Mastech
MS6500 Digital Thermometer (DTM), today I rigged up a way to get the DTM
*and* the analog thermometer (ATM) in the roast chamber at the same time.
I mentioned yesterday that it was hard to hold the thermocouple at any
specific point in the roast chamber.  Today I fished it into the chamber,
wiggled it into position (near the wall of the chamber) and wrapped the
thermocouple wire around the handle.  It worked, but I wouldn't like to
count on it for a long term solution.  I also installed my ATM on the other
side of the roasting chamber.
Yesterday I said that the two thermometers seemed to read about the same.
That was based on the fact that when roasting a well-known bean (Nicaragua
F/T Sabor de Segovia, one I buy in 20# lots!), the DTM read about the same
temperature at 1st crack and at 2nd crack that I'm used to seeing with the
ATM.  As it works out, that was the *only* range where the temperatures
matched.
So, with both thermometers in place and 96g of Brazil Cooxupe Prima Qualita
in the HIP roasting chamber, I started up the roast.  The first thing I
noticed was that although the ATM moved rapidly to about 250, the DTM was
reading only about 150  I gave it some thought and then realized that on
the ATM, the entire thermometer was sitting in the hot air stream, with only
the tip near the edge of the roasting chamber where I like to position it.
That means that from the middle of the thermometer up, the temperature was
probably several hundred degrees.  On an analog stainless steel thermometer,
the large bore shaft conducts a lot of heat down towards the sensing
portion.
On the other hand, the very thin wires connecting to the thermocouple
conduct very little heat, so I concluded that the thermocouple was probably
closer to the actual average bean temperature.
As the roast progressed, I noticed that the thermometers indications were
beginning to get closer and closer, until at about 400 they were almost the
same.  After thinking about it for a while I decided that it made sense.
Early on, the top of the ATM was considerably hotter than the bottom,
leading to a fairly rapid migration of heat down to the tip.  On the other
hand, once the coffee began to approach the temperature of the incoming hot
air, the error introduced by the temperature differential was much smaller.
By the time the beans got up to 450, the readings were close enough
together that I couldn't really tell the difference.
In conclusion, I'd have to say that the $20 DTM didn't offer much
improvement in accuracy over the ATM.  On the other hand, I think I paid
about $20 for my large face, 5" stem ATM.  So, if you're in the market for a
thermometer, I don't really see much reason not to go with the DTM.  I
bought mine not to get better accuracy, but because I was getting a crick in
my neck craning up above the roaster to watch the ATM.  With the DTM I can
sit comfortably and watch the temperature.
About Probes
The probe that comes with the MS6500 is their standard model TP-01 naked
junction probe with a lead-length of about 39".  The manual claims that the
maximum probe operating temperature is 482, or, 572, short term.  That
should be plenty good for coffee roasting, although I can imagine a case
where the probe was in the direct line of heat of a really hot roaster and
melted down.  Of course, it *is* a thermometer, and you'd think you'd notice
if the temp began to shoot up. Note that the problem is not with the
junction itself, but with the lead connecting the junction to the DTM.  As
an optional accessory, probe models TP-02A and TP-03 are available that go
to 1650 and 2200 respectively.  The probe on the TP-02A buries the
junction in a stainless steel tube about 4" long and 1/8" in diameter.  The
TP-03 is about 4" long and 1/4" in diameter.
Note that the connector on the MS6500 is a standard type and you could make
or otherwise obtain any probe of your liking.  I'm thinking about making one
that would be the perfect length and angle to put the thermocouple right
into the center of the beans.
Mastech MS6500
Includes TP-01 K-type probe, manual, and carrying case.
Available from:
Circuit Specialists Inc
220 South Country Club Drive #2
Mesa, AZ 85210
800-528-1417http://www.web-tronics.com/digtherwkpro.html-- Rick
homeroast mailing listhttp://lists.sweetmarias.com/mailman/listinfo/homeroast


HomeRoast Digest