Cold Junction and CHT/EGT

[N541X]

Winter cold is starting to set in and it got me thinking about the accuracy of my thermocouple instruments like the CHT and EGT. With my AreoVee 2.1 engine I am using Westach analog gauges which are calibrated at 75 degF. If the "cold junction" (the place where the thermocouple leads attach to other wiring or the instrument) temperature is above or below 75 degF, the instrument will read high or low by one degree for each degree the cold junction is above or below 75 degF. My cold junction is located in my unheated cockpit. If I'm at 35 degF in the cockpit, Westach instrument instructions say my gauge will be reading 40 degF high (subtract the cold junction temp if below 75f for corrected temperature). On a recent flight in 35f weather I was seeing about 400f on my cht and was running rich to try and keep things cool. In fact, if I applied the correction, I was running a comfortable 360f. For those with an efis system, the cold junction correction may be applied by the instrument to display a correct reading, but that correction would likely only be valid if the thermocouple leads are connected to the same place as the instrument is sensing the cold junction temperature, such as perhaps in an RDAC module or a direct connection on the efis display unit. If copper wire is used to extend the thermocouple leads to the RDAC or display unit, the cold junction location is at the point of the wire extension, not the RDAC location. If that connection is in an environment different than the rdac, them some additional correction may be necessary. If thermocouple wire of the same type as the sensor is used as an extension, then the cold junction point should remain at the final termination, and no additional correction should be necessary.

Just some food for thought about the accuracy of our instruments as we start to fly outside a comfortable 65-85 degF environment. :roll:

[GordonTurner]

I did not know this. Thanks for writing.

Gordon