Fitting the EGT Probe


During the construction of the Hodgson-9 radial engine, I became acquainted with thermocouples and their use in foundry work. Measuring high temperatures nearly always employs one of three methods... a thermocouple, an RTD probe, or a non-contact IR sensor. Of the three, the thermocouple is eminently more useful for our turbojets in that it is rugged, relatively cheap, and easy to use. RTD probes, while more accurate, are expensive and less durable. IR probes are useful for measuring the temperature of solid surfaces, but gasses are another matter entirely.

One of the best resources I have found on the web is at Omega Engineering. Not only do they sell probes and meters, they also have free literature which explains their uses.

The probe ordered is stock number HKMTIN-032G-4. This is a custom built probe which shortens the normal length from 6" to 4", and sheathes the probe in inconel rather than SS. The junction is a perfect .250" stainless round which lends itself well to clamping.
  Two aluminum clamps were engineered. The forward clamp grips the 1/4" junction, and allows movement of the probe fore and aft. The rear probe clamps a 316 SS sheath which protects most of the .032" inconel probe along the body of the engine.

Both clamps use the fore and aft 3-48 SHCS which secure the case outer to the case front and case rear respectively.

  The rear clamp is visible here, with the exposed inconel probe sheath still straight. I am going to drill a .040" hole in the outer cone, and bend the sheath into the hole, with an extension of .032" into the gas stream.

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