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Topic: thermocouple abrasion (4 msgs / 181 lines)
1) From: Tom Bellerue
What do people do to prevent undo unravelling of the glass braid sheath on a 
thermocouple? I had a couple of tcs give funky readings when more than 1/2" 
of the stuff came loose, or if another spot on the sheath was abraded. I 
have silicon tape on the tc where it is pinned between the edge of the 
popper chamber and can chimney, but it is only rated to 400F so don't want 
to put it near the end of the thermocouple.
thanks!
tom:)

2) From: Deward Hastings
Tom:
<Snip>
sheath on
a thermocouple? I had a couple of tcs give funky readings when more than
1/2" of the stuff came loose, or if another spot on the sheath was =
abraded. 
It's a problem . . . thermocouples are fragile, brittle and subject to
destructive oxidation.  In applications where longevity matters they are =
put
in some sort of thermowell for mechanical and chemical protection.  =
Unless
very fast response is desired this often actually improves reading =
accuracy
by presenting more heat transfer surface to whatever is being measured, =
and
a little thermal inertia to "smooth" the reading.  A thermowell also
generally also reduces electrical noise on the leads.
I've made some simple ones out of thinwall stainless steel tubing from a
local hardware store.  Cut and bend to whatever length and shape you =
want,
put a little dollup of heat sink compound in the "business end", crimp =
it
shut, and thread the TC into it until the bead is in the heatsink =
compound
(as far as it will go ).  The heatsink goop maximizes transfer from =
the
wall to the bead, but you don't want too much or the extra mass will =
slow
down response.  Less is better . . . remember that it will squish up the
tube when you crimp it.  You want just enough to "connect" the bead to =
the
tube wall, and no more.  You get longer sensor life, and consistent
placement from roast to roast with such a thermowell.
If you want faster response (as when measuring air temperatures) but =
still
want some degree of mechanical protection leave the tube end uncrimped =
and
let the bead hang out just beyond the end of the tube.  You get =
consistent
placement, and the leads don't break from flexing.  Doesn't give any
protection from oxidation, though.
Deward

3) From: Nelson, Frank
Tom -
  I am new to the list and don't have much experience with roasting.
  I do however work in a biomechanics lab where we use transducers I =
believe to be similar to your thermocouple.  These gauges generally come =
with a protective sheet already in place on top of two wire leads but =
becuase of the nature of our work we remove this sheet and wires.  We =
then  solder new wires on and do a two coat resurfacing process.  This =
is not for temperature protection but rather abrasion protection.  I =
know the coating material is rated to a fairly high temperature though.  =
I think in the 500s.  All of this work is very tiny and delicate.  If =
you are up for it.  The company selling the thermocouples probably has a =
specific coating material for you to buy.  If they do not you can email =
me directly.  I will try and see if we can work something out.

4) From: Tom Bellerue
<Snip>
<Snip>
Thanks guys, I'll check these things out and post again after the holidays. 
Good info.
tom:)


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