nuckollsr(at)cox.net Guest
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Posted: Sun Feb 12, 2006 8:46 pm Post subject: High temperature epoxy for ungrounded thermocouple probe |
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At 04:48 PM 2/11/2006 -0800, you wrote:
Quote: |
<jmfpublic(at)comcast.net>
Listers:
I have a Jabiru 3300 6 cylinder engine that uses spark plugs with a thread
diameter of 0.645 inches. I wish to use ring terminals of 3/8 inch
diameter drilled out to 15/32=3D0.4688 inches. (Using a Unibit will allow
a clean cut.) This will center the probe more accurately than some
commercial probes, and it allows me to make my thermocouples using type J
wire that I bought from Omega. I don't want any splices, and will custom
fit each wire, with an appropriate service loop.
My question is this. I wish to insulate the 1/4 inch or less soldered end
of the thermocouple. Should I use a bit of tefzel insulation, or plumbing
Teflon tape? Is there a thin ceramic that is used that I never heard
of? The main question is what sort of potting compound to use. I haven't
researched high temperature epoxy yet, and would appreciate any
suggestions. I am using a thermocouple amplifier, and a grounded
thermocouple will not work well at all.
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Last time I needed to instrument an engine, I used the AD594
style chips and grounded thermocouples. I used a separate
wire to crankcase to take the -VS, +Vs midpoint ground to the
crankcase. Common mode on the chips is more than capable
of working this way as long as you strap the signal ground
to the engine.
When you use grounded thermocouples, there are no concerns about
any common-mode noises (or differential for that matter,
t/c's are VERY low impedance sources) . . . so concerns
about proximity to other wires including spark plug wires
is nil. The hard part is arranging for the crankcase ground
reference. For a permanently installed instrumentation
system, you might have to figure out a way to "float" the
T/C signal conditioners . . . or simply use crankcase for
ALL instrument system grounds.
If you must insulate the T/C, I think I'd dip them in a
high-temp epoxy like JB Weld. Put a couple of thin coats
on each T/C before using JB weld to cement it into the
barrel of the terminal. See:
http://jbweld.net/products/jbweld.php
This is pretty amazing stuff . . . in spite of it's
metal bearing properties, it's still and insulator
and rated for up to 500F.
Now, if you float the T/C's at the engine, I recommend
you tie a pair of 100 ohm resistors from each leg of
the amplifier chip to signal ground. This gives you a very low
common mode impedance to signal ground and has no effect on
accuracy of measurements over the short distances encountered
in small aircraft.
Bob . . .
< What is so wonderful about scientific truth...is that >
< the authority which determines whether there can be >
< debate or not does not reside in some fraternity of >
< scientists; nor is it divine. The authority rests >
< with experiment. >
< --Lawrence M. Krauss >
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