GRT Avionics

What type of thermocouples are the long-life versions (Type K, J) ? What heat can they withstand?

The EGT probes are Type K. The CHT are Type J. See page 5b of the manual where it talks about extension wires.

Jim Butcher

Hi, Thanks Jim.

Just wondering what makes the 'long-life versions' different?
If GRT have the supplier spec sheet on them, might list the heat ranges.

The actual thermocouple is a junction of two different metals (google type J, K, ... for details). But these materials would quickly be chemically destroyed in the hot, toxic exhaust, so they are protected by a cover layer. Stainless steel is pretty robust, but a poor (for a metal) conductor of heat. So a thick covering of SS gIves longer life, but now you get a slower response (when you change the EGT, it takes 3 to 15 seconds or more, depending on the SS thickness, for the thermocouple to get close to the actual EGT). So fast response probes have a thin protective layer and a shorter lifetime, while long life probes tend to have a slower response time. Of course expensive engineering and exotic materials can be used, at an increase in cost, to try for the best possible compromise. Some readouts also use software to attempt to extrapolate out to the final temperature. But all EGT probes are designed for the 1500 F or so EGTs.

To help with the direct heat, I was thinking of adding Thermowells.
Yes it will have a longer response time, but with a Rotary engine producing very hot exhaust, this might be a way to extend their life.