temp gauge drop when termostat is opened

[JanDebL at aol.com]

I think you are both correct.  If you have a sensitive temp gauge mounted 
close to the radiator inlet, you will see a noticeable temperature decrease 
when the thermostat initially opens and the cold coolant enters the engine.  
The colder the outdoor temperatures, the more noticeable the drop is.  At - 
40 degrees, a 30 degree drop or more is common.  Once the coolant in the 
radiator is warm, the drop is much less noticeable on successive openings.  
Most factory gauges are not sensitive enough to detect this change - most 
factory gauges are not even graduated - but a good aftermarket Stewart 
Warner, VDO, etc., will provide a more accurate reading.
Jan Lahtonen