Hi all, I'm new to this forum... all good stuff. I am what you call a "turbine nut" in that my R/C skills are only good enough to fly an electric foamie about the park, but I have machined an MW54 and a turboprop, just to enjoy running them on the bench. I also developed an ECU, which is successful and which I use on my own turbines. Some of you may have seen my hobby web site at 5bears.com. Nothing commercial, just someone who loves jets.

Anyway, over the years, I have struggled with thermocouple placement and the theory behind the EGT's we so carefully track. Obviously, EGT is critical. Those of you like myself who have done a LOT of bench testing know that minute variations in T/C placement can cause MAJOR changes in displayed EGT. For example, the extension of a 1/16" type K probe through a cone into the exhaust stream, if varied by as little as a millimeter or two, may cause an EGT swing of 50 to 100 degrees. Likewise, radial position can have an equal effect. Manufacturers often give rough ideas of location, but the result isn't satisfactory. It is too easy to deceive ourselves with a bit of fiddling to measure the lower EGT.

Initially, I would locate the hottest portion of the exhaust and locate the probe there. This is conservative but limiting in that it is possible that in doing so you are essentially "derating" the turbine. Likewise, finding the coolest isn't smart. One could, in an extreme example, locate the probe 8" downstream of the turbine wheel and find a nice cool EGT but the critical components are certainly seeing higher.

My final theory is this... given that the turbine wheel is rotating, if the "hot" spot is at 7:00 looking up the pipe, each blade will pass through this hot zone, and likewise rotate through a cooler zone, and thus see an average temp. Probe tip extension in the stream should be tested, finding the hottest, then the coolest, and basically locating it in the middle of this range. Since the wheel is a rotating metallic mass, it will ultimately absorb an average amount of "work" from the hot gasses, and I think a probe placed in this manner more closely measures the true thermal load of the wheel.

Has anyone else given this thought after noticing how sensitive probe placement actually is?