HomeRoast Digest


Topic: Problem with temperature probe (6 msgs / 164 lines)
1) From: Rudy Ramsey
I recently bought the thermocouple-on-a-wire digital thermometer from SM,
and tried it last weekend for the first time. It never got above 340F during
my roast, even though I successfully roasted that batch to FC+ (well, a bit
into second crack, anyway).
I'm using a FreshRoast-8 fluid-bed roaster. I bent the wire (crimped it
lightly over the edge of the lip of the glass roasting chamber) so as to
place the thermocouple squarely in the center of the bean mass, and was able
to control its position by moving the wire from the outside, so that I'm
confident it was always inside the bean mass.
I tested the thermometer in boiling water, and it reaches 204F (6000 ft
elevation here in Parker, CO, southeast of Denver). That seems right on.
Does anyone have an idea what I'm doing wrong?
Rudy

2) From: Stephen Niezgoda
First I would call SM. As long as you didn't abuse it I am sure they would
happily send you out a new one.
If you can verify that the problem is with the probe and not the meter, I
can send you about 10ft of K type wire to make yourself a new probe.  Just
contact me off list.
Steve Niezgoda
On 4/21/06, Rudy Ramsey  wrote:
<Snip>

3) From: Chris Peltz
This is a multi-part message in MIME format.
Rudy -
I have a similar problem. I purchasd the thermocouple digital thermo from
SM, and I'm using it with my StirCrazy/Convex Oven. I actually drilled a
hole in the bottom of the base so that I could place the probe directly in
the middle of the bean mass. 
I've tried 2 roasts, and it looks like both are getting to 2nd crack at
around 350. I have been able to go higher than that, but the beans get
pretty dark at that point.
I'm only based in Colorado (Windsor - 1hr north of you), so I don't know if
altitude has any impact.
Chris.  
From: homeroast-admin
[mailto:homeroast-admin] On Behalf Of Stephen Niezgoda
Sent: Friday, April 21, 2006 12:50 PM
To: homeroast
Subject: Re: +Problem with temperature probe
First I would call SM. As long as you didn't abuse it I am sure they would
happily send you out a new one.
If you can verify that the problem is with the probe and not the meter, I
can send you about 10ft of K type wire to make yourself a new probe.  Just
contact me off list.
Steve Niezgoda
On 4/21/06, Rudy Ramsey  wrote:
I recently bought the thermocouple-on-a-wire digital thermometer from SM,
and tried it last weekend for the first time. It never got above 340F during
my roast, even though I successfully roasted that batch to FC+ (well, a bit 
into second crack, anyway).
I'm using a FreshRoast-8 fluid-bed roaster. I bent the wire (crimped it
lightly over the edge of the lip of the glass roasting chamber) so as to
place the thermocouple squarely in the center of the bean mass, and was able
to control its position by moving the wire from the outside, so that I'm
confident it was always inside the bean mass.
I tested the thermometer in boiling water, and it reaches 204F (6000 ft
elevation here in Parker, CO, southeast of Denver). That seems right on. 
Does anyone have an idea what I'm doing wrong?
Rudy

4) From: raymanowen
THERE IS NOTHING WRONG WITH THE SM DIGITAL METERS!
Any thermocouple generates a potential dependent only on the temperature at
the junction of the wires. H2O has a large thermal mass, so it easily
supplied the heat subtracted by the thermocouple lead wires when the
junction was immersed in the boiling H2O. 204 degrees at this altitude
proved it out.
Hot air has very low thermal capacity, so the thermocouple wires themselves
pull heat away and lower the temperature of the junction faster than the ho=
t
air can supply it. So, it reads low and pretty unstable. Heat flow-
Thermodynamics- will have an effect whether you agree or not.
Cheers -RayO, aka Opa!
--
"When the theme hits the bass, I dance the Jig!" - -Virgil Fox at the Might=
y
Wichita WurliTzer- 1976

5) From: Steve Hay
On 4/24/06, raymanowen  wrote:
<Snip>
r
<Snip>
t
<Snip>
Interesting theory... I've never had this problem with an iRoast2.  I alway=
s
figured that the mass flow of the air through the roaster and the relativel=
y
thin wires caused the reading to be okay.
--
Steven Hay
hay.steve -AT- gmail.com
Barry Paradox: Consider k to be the greatest element of the set of natural
numbers whose description require maximum of 50 words: "(k+1) is a natural
number which requires more than 50 words to describe it."

6) From: raymanowen
Mount the probe with a substantial segment of it in the hot air stream. If
the probe points upstream towards the heat source, the natural thermodynami=
c
flow of heat, from the high temperature sink to the low temperature sink
won't sap the heat before the junction heat develops the Peltier- Seebeck
potential for measurement.
Phew! Don't even try to diagram that last sentence. My reply proves the
Barry Paradox. Just realize that a thermocouple does not have any inherent
accuracy or precision. That comes from the physical installation.
All you are seeing on the digital meter is the temperature of the
thermocouple junction, and that's not necessarily the temperature of the
process of interest. One joke I've seen quoted is to measure internal bean
temperature by drilling a hole in it and installing a measuring probe.
That bean won't roast anything like a normal bean. If you keep using the
same instrumented bean out of convenience, that's really SNAFU.
Cheers -RayO, aka Opa!
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