HomeRoast Digest


Topic: [Homeroast} thermocouple oddity (4 msgs / 110 lines)
1) From: Paul Helbert
Yesterday's roast of 100g SM Sumatra Org. Gayo Mtn Special in the P1 went
beautifully to FC target. 45F ambient, 10 min to FC at 401F and 14:30 EOR at
427F. Cooled in 1 min. Result: FC. Now for the irregularities: The resultant
roast had much more expansion than expected. Noticed a couple of 2nd crack
divots flying out the top of the chimney at 422F around 12:30. After
roasting I found that the wires to the thermocouple bead were not intact.
The bead was hanging by one wire and the other was gone, eroded or beaten
back to the pot metal wall of the popper. So where were these temperature
readings coming from? Conduction through the oiliness of the rotating
coffee? No, because I get a correct ambient temperature reading even after
the roast. Strange...
Anyhow, next thermocouple will not be a bare wire model. I'm looking at
mounting one of these:http://cgi.ebay.com/THERMOCOUPLE-1-4-X1-4-LONG-PROBE-TYPE-K-NEW_W0QQitemZ330213556677QQihZ014QQcategoryZ50971QQssPageNameZWDVWQQrdZ1QQcmdZViewItemA quarter inch projection into the spinning bean mass is about what I had
with the previous setup.
-- 
"There is nothing- absolutely nothing-
half so much worth doing as simply
messing about in boats."
---Ratty to Mole in Kenneth Grahame's
  The Wind in the Willows
Homeroast mailing list
Homeroasthttp://lists.sweetmariascoffee.com/listinfo.cgi/homeroast-sweetmariascoffee.com

2) From: Ken Mary
I am not sure, but you can just twist the wires together over a short 
distance to make the connection. You will get a temperature reading if the
wires touch anywhere or have any metal between them. You may have had
contact at the metal roast chamber wall.
I have used a very small wire exposed thermocouple for 8 years with near
zero wear. The teflon coating is still intact but has turned brown. The same
wire was used in various poppers and now has been used to measure bean
temperature in a drum roaster for the past 3 years.
--
----------
<Snip>
<Snip>http://cgi.ebay.com/THERMOCOUPLE-1-4-X1-4-LONG-PROBE-TYPE-K-NEW_W0QQitemZ330213556677QQihZ014QQcategoryZ50971
<Snip>
Homeroast mailing list
Homeroasthttp://lists.sweetmariascoffee.com/listinfo.cgi/homeroast-sweetmariascoffee.com

3) From: raymanowen
Thermocouple junctions and wires can break, and they can be unplugged while
in use.
All the meters that I've seen give a full scale or "1- - - -" reading when
the thermocouple input becomes open-circuit.
The chart recorders and temperature controllers all went to full scale with
an open thermocouple, otherwise the process heater would be run at the full
power rating, trying to heat the measurand to oblivion.
Imagine a 1000hp steam boiler or a 6000F glass furnace. Thermocouples are
not necessarily the best temperature measuring sensor in every situation,
but there are many ways to misuse or poorly install them.
It doesn't take much ingenuity to botch a signal that is in the millivolt or
microvolt range. No thermocouple is immune to abrasion, so the industrial
thermocouples are installed in metal or ceramic wells, and the lead wires
are often routed in metal conduit like electric wiring.
If the wires pass through a hole in a sheet metal box or wall, you will read
the temperature of the sheet metal if it cuts through the insulation and
touches both wires. If the wires short together, that's your new measuring
junction.
Wherever the tc wires or extension wires touch each other, is your new point
of measurement. The new Surprise junction usually holds very little
technical interest.
Cheers, Mabuhay -RayO, aka Opa!
Call Omega, they'll help you.
On Tue, Feb 26, 2008 at 8:09 AM, Paul Helbert 
wrote:
<Snip>
330213556677QQihZ014QQcategoryZ50971QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
<Snip>
ee.com
<Snip>
Homeroast mailing list
Homeroasthttp://lists.sweetmariascoffee.com/listinfo.cgi/homeroast-sweetmariascoffee=.com

4) From: Paul Helbert
Opa,
That must be it. The new point is at the wall of the popper chamber where I
had routed it in through a small drill hole.
Thanks!
Paul
Homeroast mailing list
Homeroasthttp://lists.sweetmariascoffee.com/listinfo.cgi/homeroast-sweetmariascoffee.com


HomeRoast Digest