[AR] Re: Best Practices for Measuring Engine Temps with a Thermocouple
- From: Norman Yarvin <yarvin@xxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Sat, 19 Oct 2013 02:57:15 -0400
On Fri, Oct 18, 2013 at 08:26:40PM -0700, David Weinshenker wrote:
>David W. Schultz wrote:
>> On 10/18/2013 09:08 PM, Norman Yarvin wrote:
>>> Nevertheless, that voltage source doesn't have infinite ability to
>>> source current; it's not quite _that_ magical. And if you have a
>>> situation where two nominal "voltage sources" are fighting each other
>>> (as you would if you had one junction near the inner wall and one near
>>> the outer), their relative ability to source current is what
>>> determines the resulting voltage (and thus the measured temperature).
>>>
>>
>> The junctions do not source the current. The current is the result of
>> heat flow in the conductor. You get a voltage output because the
>> thermocouple wires have slightly different properties and generate
>> slightly different voltages.
>
>As I understand it the actual voltage is developed in the thermal
>gradient along the wires. The junction itself is an electrical
>connection to force the "far" ends of the wires to be at the same
>voltage.
Right; that's why the whole wire has to be of the thermocouple
materials, not just the bit at the end. (Well, perhaps not precisely
"has to", but close enough for that to be the standard engineering
practice.) It still usually is good enough for the purposes of
analysis to regard the junctions as being voltage sources, though.
In any case, it's true that the current flow is always accompanied by
heat flow; it has to be, from a thermodynamic point of view. This is
a heat engine, creating electrical energy from heat, and thus has its
maximum efficiency bounded by the same formula that bounds the
efficiency of heat engines that create mechanical energy from heat.
(Both electrical and mechanical energy are classed as "work" in
thermodynamics terms.) So in addition to converting some of the
thermal energy to electrical energy, it also has to just move a lot of
heat from the hot junction to the cold junction. Or in the case of a
rocket with two junctions at the hot end, one inside the chamber and
one outside, heat would flow between those two junctions, heating up
the outer one and cooling the inner one. (And that'd be in addition
to the heat flow due to the normal thermal conductivity of the wires
-- and then on top of that there would be I^2*R heating.)
This raises an interesting question: suppose you had an engine made of
dissimilar metals -- an aluminum injector plate fitted to a stainless
chamber, say, or vice versa; or perhaps a brazed engine like the
RL-10. You'd get this sort of increased heat transfer along the
joints between dissimilar metals; would it be big enough to notice?
--
Norman Yarvin http://yarchive.net/blog
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