[AR] Re: Best Practices for Measuring Engine Temps with a Thermocouple
- From: Pierce Nichols <piercenichols@xxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Mon, 21 Oct 2013 13:31:22 -0700
My google-fu is weak -- what's the curve for W/Re TCs?
-p
On Mon, Oct 21, 2013 at 1:27 PM, Eric Boyer <jeb19@xxxxxxx> wrote:
> At 01:22 PM 10/21/2013, you wrote:
>
>> On 10/21/2013 7:20 AM, Eric Boyer wrote:
>>
>>> At 07:36 PM 10/18/2013, Henry Vanderbilt wrote:
>>>
>>>> Yes, I was careful to weasel-word that and not say it was outright
>>>> impossible, because I *knew* someone would take that as a challenge
>>>> <grin>
>>>>
>>>> But until Omega lists W/Re TC's as a stock item, I think I'm fairly
>>>> safe in calling it thoroughly impractical.
>>>>
>>>
>>>
>>> http://www.omega.com/pptst/**www26_w5w26_w3w25.html<http://www.omega.com/pptst/www26_w5w26_w3w25.html>
>>> :-)
>>>
>>> We actually use W/Re TCs (Type C or D) fairly frequently in combustion
>>> research. We get them either pre-made or spark-weld them ourselves
>>> (especially the really small ones for better time/spatial resolution).
>>> Gotta make sure they don't get even a whiff of anything oxidizing when
>>> at high temperature, though--ever see how fast the filament of a broken
>>> incandescent light bulb disappears? Yeah, it's like that.
>>>
>>> Eric
>>>
>>
>> I'm having to back down from "just about impossible", through "thoroughly
>> impractical", to just "mostly impractical". Sounds like if you pick your
>> propellants, and accept low performance, and take a lot of care to run
>> reliably fuel-rich throughout, you could measure chamber gas temp with
>> off-the-shelf Omega Catalog hardware.
>>
>
>
> I'll definitely agree with "mostly impractical". Most of our applications
> for the W/Re TC measurements involve potting them in sort of ceramic
> (hafnia for the highest temps), using them to measure a subsurface
> temperature of something like graphite, or accepting that they're a
> sacrificial measurement device for a transient event (e.g.,
> temperature-distance profile of a solid propellant flame).
>
> Eric
>
>
>
>
> Dang, ain't modern industrial resources something else?
>>
>> Henry V
>>
>> On 10/18/2013 4:27 PM, George Herbert wrote:
>>>>
>>>>> Tungsten melts at 3,400 K; it looks like peroxide motors should stay
>>>>> under that, LOX/Kero at Pc of say 150-250 PSI might be under it (but
>>>>> not
>>>>> much), LOX/alcohol should stay under it.
>>>>>
>>>>> Rhenium is around 3,150 K, you could find propellant combinations that
>>>>> would stay under that as well, so perhaps a W/Re TC?
>>>>>
>>>>> But yes, this is edging into the "You just can't do that" trade space,
>>>>> when we're having to look at the lowest temp propellants people here
>>>>> might use and the highest temp metals available on the face of the
>>>>> earth...
>>>>>
>>>>>
>>>>>
>>>>> On Fri, Oct 18, 2013 at 3:08 PM, Henry Vanderbilt
>>>>> <hvanderbilt@xxxxxxxxxxxxxx
>>>>> <mailto:hvanderbilt@**mindspring.com<hvanderbilt@xxxxxxxxxxxxxx>>>
>>>>> wrote:
>>>>>
>>>>> I think the most ambitious thing we were talking about so far is
>>>>> directly TC measuring chamber wall temperature, which is merely
>>>>> really really hard. Direct measurement of chamber gas temperature
>>>>> with a TC is pretty much impossible, since the chamber gas in any
>>>>> halfway efficient rocket motor tends to be hotter than the melting
>>>>> temperature of just about any material you can name.
>>>>>
>>>>> Henry
>>>>>
>>>>>
>>>>> On 10/18/2013 1:54 PM, johndom@xxxxxxxxx <mailto:johndom@xxxxxxxxx
>>>>> >
>>>>> wrote:
>>>>>
>>>>> I wonder what commercial TC can measure the inside of a firing
>>>>> chamber where
>>>>> uncooled stainless sensor protection tubing simply melts. Yes
>>>>> soldering it
>>>>> to the regenatively cooled wall is an option, but that is not
>>>>> the chamber
>>>>> gas temperature at all.
>>>>>
>>>>> jd
>>>>>
>>>>> -----Oorspronkelijk bericht-----
>>>>> Van: arocket-bounce@xxxxxxxxxxxxx
>>>>>
>>>>> <mailto:arocket-bounce@**freelists.org<arocket-bounce@xxxxxxxxxxxxx>
>>>>> >
>>>>> [mailto:arocket-bounce@__freel**ists.org<//freelists.org>
>>>>>
>>>>> <mailto:arocket-bounce@**freelists.org<arocket-bounce@xxxxxxxxxxxxx>
>>>>> >]
>>>>> Namens Norman Yarvin
>>>>> Verzonden: vrijdag 18 oktober 2013 21:00
>>>>> Aan: arocket@xxxxxxxxxxxxx <mailto:arocket@xxxxxxxxxxxxx>
>>>>> Onderwerp: [AR] Re: Best Practices for Measuring Engine Temps
>>>>> with a
>>>>> Thermocouple
>>>>>
>>>>> 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.)
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> -george william herbert
>>>>> george.herbert@xxxxxxxxx
>>>>> <mailto:george.herbert@gmail.**com<george.herbert@xxxxxxxxx>
>>>>> >
>>>>>
>>>>
>>> ______________________________**______________________________**_
>>> Eric Boyer jeb19@xxxxxxx
>>>
>>>
>>
>
> ------------------------
> Eric Boyer Assistant Director, High Pressure
> Combustion Lab
> Research Associate Department of Mechanical and Nuclear
> Engineering
> (814) 863-2264 Penn State University
> jeb19@xxxxxxx University Park, PA 16802
>
>
>
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