Temperature sensors are by far the least reliable sensors on the engine= , and the TIT/EGT are the least reliable of temperature sensors due to the h= igh temperatures and vibration they must endure.

This is dredging back many years, but I'll give the old memory neurons a= try.

Modern instruments seem to use K type (chromel/alumel) thermocouples ev= erywhere when in years past they were only used for high temperatures. = They have poor output signals at lower temperatures (as with CHTs and Oil t= emp). Oil temperature used to be measured with Copper-Constantan therm= ocouples because of better output characteristics at lower tempera. tures.&n= bsp;

Now with sensitive digital electronics capable of handling non linear c= alibration curves and tiny input voltages, even these little little sig= nals are enough to be detected, corrected for non-linearities, and then used= for sufficiently accurate displays in our planes. So type K is being u= sed more and more.

However......

Measuring comparatively low temperatures (compared to EGT/TIT), these t= iny signals can be easily fouled up by dirty connectors, intermittent connec= tions caused by the small but stiff thermocouple wire wiggling and breaking,= and other mechanical and electrical resistance faults. It is just the= nature of the beast.

My MVP 50 engine monitor reads OAT from a thermocouple in my cabi= n vent NACA duct, and last flight decided to start indicating numbers way ab= ove and below the real OAT. Looks like a classic intermittent connecti= on problem. I checked the probe wires today (easy to get to) and wiggled the= connector on the data box (hard to get to) while a friend watched the reado= ut, but we could not induce an error. Also classic intermittent behavi= or. My bet: it will show up next flight, or the flight after that, aft= er a bit of vibration exposure. Or it could be a short in the thermocouple w= iring somewhere which creates a secondary thermocouple at the short (differe= nt location and temperature) that fouls the output of the primary therm= ocouple. As noted thermocouple wire is stiff and difficult material, n= ot forgiving like multi-strand copper.

Bottom line: suspect connections and wiring as much as the thermocouple= junction inside the sheath or fitting. Check for large en= ough service loops to absorb vibration, but not so long as to allow the wire= to flex between tie downs.

Fred Moreno