Engine should be warmed up to operating temperature so the voltages remain constant. Otherwise they would contiue to gain as engine warms causing some discrepency as you moved down the glow plugs. This is a very sensitive test.
Thermocouple action is responsible for the voltage. Heated dissimular metals generate voltage. I suspect the glow plug filament is an alloy mix and is the predominate factor.
The important thing though is the glow plug does generate a changing voltage with heat. Pizzo effect is probably not there.
It is still early to be absolutly certain of much yet. I only wish there were a detectable peak at moment of ignition. As it is an indirect injection engine it might be harder to detect but one never knows until trying. Really though I suspect not. I will have a more careful look one day though.
When this all started there were so many possibilities against it working at all it was not funny. Slowly it came together. It is not new by any means. By hearsay I think it has pattents by bosh but time has passed them by with newer diesels technology I suspect.