Hi,
The EGT is essentially just a mV meter and can easily be checked an calibrated with a simple millivolt source. The probe is a thermocouple that generates a very small voltage when heated.
You need an adjustable voltage source like the one found at
http://www.5bears.com/circuits.htm. Forget one turn potentiometers, you need a ten turn, knob- (expensive) or trim- (cheap) pot.
You also need a temperature – voltage table such as
http://www.carl-fh.com/carl/referenc...-ni-codes.php.
Finally, you need a DVM capable of reading with a resolution of at least one millivolt.
Remember, other than certified instrument shops cannot do work on normal category aircraft!!
Good luck!
/Sten S