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 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] Namens Norman Yarvin Verzonden: vrijdag 18 oktober 2013 21:00 Aan: 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.)
