Unless you can ensure that your inserted-into-the-chamber thermocouple ends up flush and smooth with the rest of the chamber wall you're going to change the local temperature anyway, by introducing local turbulence. Probably increasing local temperature, quite possibly enough so to destroy the thermocouple, if not the local chamber wall.
Even if you can reliably install it flush and smooth, you also need to make sure the installation isn't significantly different in thermal conductivity than the rest of the chamber wall. Differences will of course change local temperature and again give inaccurate readings, and if you're depending significantly on radiative or active cooling, can potentially interfere with cooling enough to lead to a local burn-through.
Not to discourage experimenting, but don't let it distract you too much from the basic job of making the rocket work. Accurate internal chamber wall temperature measurement is really hard. Mostly people settle for an empirically measured wall temperature of "cool enough not to destroy the chamber".
Henry On 10/18/2013 4:34 AM, Graham Sortino wrote:
I really like your idea of using a screw to hold it in place. I was wondering if you ever considered taking this a step further and drilling a hole to access the chamber so that you could measure the internal wall temp. You might need to use a NPT or AN thread for this to prevent leaks but would it be a clever way to measure the internal wall temp? ------------------------------------------------------------------------ *From: * Robert Watzlavick <rocket@xxxxxxxxxxxxxx>; *To: * <arocket@xxxxxxxxxxxxx>; *Cc: * Graham Sortino <gnsortino@xxxxxxxxx>; *Subject: * Re: [AR] Best Practices for Measuring Engine Temps with a Thermocouple *Sent: * Thu, Oct 17, 2013 3:19:54 AM For the exterior temperature, as long as it doesn't get too hot, I used stick-on TCs from Omega. For the temperature somewhat inside the wall (halfway between the inner and outer walls), I drilled and tapped a hole halfway. Then I drilled out a #6-32 aluminum socket set screw and used it to clamp the TC to the chamber (also aluminum). Since the screw and parent material are the same material, they should theoretically be the same temperature without any gradients (the hole has been filled by the screw). I have no idea how much error it has though. Probably at least some due to the conduction of the TC wires. See this pic: http://www.watzlavick.com/robert/rocket/testStand/dsc_4842m.jpg -Bob On 10/16/2013 06:25 PM, Graham Sortino wrote:I’ve been testing out some thermocouples on my revised test stand and I was wondering if there is a best practice for connecting the thermocouple to the object** being measured? I’m assuming the entire surface area of the thermocouple tip (I’m using a type K insulated wire) should be touching the metal in order to get an accurate measurement but what is the best way to do this. Should I tape it to the object, clamp it, etc... /** Note that in this case I’m trying to measure the outer wall temperature of the engine./ // For example, one of the tests I did was to heat up a piece of stainless steel using a propane/air mixture. After about 5 minutes I removed it from the heat and quickly touched the tip of the thermocouple wire to my piece of metal. The propane/air mixture is supposed to have a flame temperature of about 2000 Celsius; however, the metal only measured about 400 Celsius. I'm not necessarily saying the results are wrong but I had assumed the temperature of the metal would have been a bit higher as the piece was fairly small and fully engulfed in the flame. Many thanks in advance. Graham
