[Homeroast] Behmor Temp Help

raymanowen at gmail.com raymanowen at gmail.com
Thu Nov 11 23:26:07 CST 2010


Hank, a thermocouple develops a "Seebeck" potential at the junction of the
two wires. The potential depends on the metallurgy or exact composition of
the two wires and the temperature at the junction.

The junction temperature is exactly what you are reading on your meter.
Except Except Except- if the wires pass through a sharp hole in a metallic
piece, the insulation could be abraded, and the wires short to each other or
the metal plate. The short is your new measuring junction, probably not too
hot.

Assume a nice clean electrical installation with no spurious junctions or
corroded connections. (A corroded wire junction is totally different from a
clean junction- a completely different thermocouple junction than the
meter's calibration, so the readings are false.) A good thermocouple from an
electrical standpoint is part of the story.

A type J thermocouple (red, white wire insulation; black connector bodies)
uses iron and constantan wires for the couple. It develops a higher
potential for the same temperatures- it's useless with a K-calibrated meter.
Coat hangers are iron wires- strip off the paint and you instantly have
rusty iron wire. Bad for coat hangers and thermocouples.

Metal-sheathed thermocouples are quite rugged mechanically, but you have to
account for heat flow to- and away from- the junction. The measurand
supplies heat to the junction; everything else takes heat away including the
sheath and the thermocouple wires themselves.

Anything that moves the thermocouple, even slightly, during the roast or
from roast-to-roast with a poor installation will change its sensitivity and
give different numbers. Apparent advantage: IR

Cheers, Mabuhay -RayO, aka Opa!

Absent the Grinder, Roasting is a caseless hope.


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