[pct-l] how altimeters work

[Lenny Leum]

I don't recall any discussion of altimeters in the last few months, just GPS / journal gizmos and other silly electronics, so let me break the ice here.  I went down to Outdoor World and asked if they had any altimeter watches and picked up a Timex gizmo.  

My primary conundrum concerns how this thing works!  As I have found, all altimeters also have barometers incorporated into the same gizmo.  Aparently, both programs use air pressure measurements to operate.  If you know your correct altitude and set the watch, then the barometer program will have correct readings.  Similarily, if you set the barometer, then the altimeter will also be set correctly.  

My question is...... how does the watch determine which pressure change is due to elevation and which pressure change is due to changes in weather?????

I have done some loose testing of the altimeter, including taking it to the beach and setting the altimeter to zero.  It seems to work fairly well, when I climb up a hill it goes up, down goes down, but there have been some serious disconnects (right now it says I am 70 feet bellow sea level and I am definitely not).  I was thinking that the problems I encountered with the altimeter may be related to the changing weather, and I was hoping that a better understanding of how these work might lead to improved performance.  Thanks for any imput or ideas you have.
Lenny