[AR] Re: Best Practices for Measuring Engine Temps with a Thermocouple
- From: Ben Brockert <wikkit@xxxxxxxxx>
- To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx>
- Date: Mon, 21 Oct 2013 11:34:40 -0600
Eric,
Does "spark-weld" have a specific technical meaning or are you using
it to refer to electrically welding in a generic way?
Ben
On Mon, Oct 21, 2013 at 8:20 AM, Eric Boyer <jeb19@xxxxxxx> 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
> :-)
>
> 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
>
>
>
>> 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@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@xxxxxxxxxxxxx>
>>> [mailto:arocket-bounce@__freelists.org
>>> <mailto: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@xxxxxxxxx>
>>
>>
>
> _____________________________________________________________
> Eric Boyer jeb19@xxxxxxx
>
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