[AR] Re: Best Practices for Measuring Engine Temps with a Thermocouple
- From: Norman Yarvin <yarvin@xxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Fri, 18 Oct 2013 14:59:52 -0400
On Fri, Oct 18, 2013 at 08:11:18AM -0400, Ed Kelleher wrote:
>A Swagelok 1/8" tube fitting, with 1/8" diameter
>stainless steel shell thermocouple (TC) will seal
>up nicely, though part of the fitting remains
>permanently attached to the TC. You can remove
>the TC and use it on other thrust chambers, but
>it will be locked into that initial position/extension.
One thing to remember about such setups, though, is the limits of the
theory behind why it's okay to weld a thermocouple to the chamber in
the first place (as opposed to keeping it electrically isolated like a
normal sensor). The theory is that as long as all the hot-end
junctions between dissimilar metals are at the same temperature, it
doesn't matter how many junctions there are: their effect nets out to
zero. So if you have part of the current going from thermocouple lead
A directly to thermocouple lead B, and another part of it going
through the chamber wall C, it doesn't matter how much current goes by
which path, because all the junctions between A, B, and C are all at
about the same temperature. Or at least they are, to a decent first
approximation, when you're measuring the outside of the chamber.
If you're trying to measure the temperature of the inside of the
chamber wall, on the other hand, you need to electrically isolate the
thermocouple from the outside of the chamber wall. Otherwise you'll
get some mix of inside and outside temperatures, the details being
dependent on exactly what currents are flowing where. (Well, the heat
equation being what it is, you'll be getting a mix anyway, not the
temperature of the very innermost micron of the surface. But this
will make it much worse.)
--
Norman Yarvin http://yarchive.net/blog
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