[AR] Re: Best Practices for Measuring Engine Temps with a Thermocouple
- From: Ben Brockert <wikkit@xxxxxxxxx>
- To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx>
- Date: Fri, 18 Oct 2013 17:06:46 -0600
You need to differentiate there from the concept of a thermocouple,
i.e. two dissimilar wires joined together in a loop, and a
thermocouple probe, which is what would be installed through the
Swagelok fitting they described. The thermocouple probe is a
thermocouple junction inside a metal sheath, with the actual junction
touching the inner tip of the probe. The wires are otherwise
electrically insulated inside the probe. There's no need to
electrically isolate the thermocouple probe from the jacket when you
want to measure the temperature of the chamber, and there is no need
to isolate the thermocouples from each other with any competent
thermocouple amplifier.
Also, thermocouples as used for temperature sensing are a voltage
source, not a current source. The voltage is, by magical physics,
always and exactly equal to the thermocouple type's Seebeck
coefficient times the temperature difference That's then run through
an amplifier which is also responsible for compensating for the
temperature of the junction inside the amp. While in theory a
thermocouple could be measured as a current, it would be significantly
more complex.
In this blog post [
http://masten-space.com/2009/04/17/post-space-access-technical-update/
] there is this picture [
http://masten-space.com/images/750-2_in_trailer.jpg ] of a Masten
engine. The mohawk along the top is thermocouples going through
Swagelok fittings measuring the chamber wall temperatures at a bunch
of stations, halfway between cooling channels. It made the assembly
and disassembly a bit more complex, but still easier than, say,
servicing a hybrid.
Ben
On Fri, Oct 18, 2013 at 12:59 PM, Norman Yarvin <yarvin@xxxxxxxxxxxx> wrote:
> 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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