Army, before you spend too much time on that deal... have you visualized what you would actually be measuring and if that would be of any value ? Even if the rise time was very fast..
You do not have a static heat source which the glowplug is in contact with... out of the total 720 crank rotation only a split second is the hot air in contact with the end of the glowplug hot. The amount of heat it will be exposed to will thus be affected by the rpm in addition to all the other stuff....and I can not see how even if everything were in your favor with the physics of the ' thermocouple' your readings would be anything other than a ' dulled' average of what was happening... I think it is more likely you will be measuring convection derived head temperature more than combustion temperature.. or at the very least an unkown ratio of the two..... which itself will probably a ' moving target' as the engine heats up...

__________________
1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/showthread.php?threadid=10414

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html