Air Pressure Switch for Hour Meter

Hello all, can somebody suggest a good place to get an air pressure
switch to hook into the Pieto line of an aircraft which will then in
turn operate the hourmeter when the aircraft is flying.

I have a friend who has a really long taxi and warmup time and he tells
me he is putting too many hours on these aircraft which are not from
actually flying. He wants to install a switch which will give him more
accurate times.

I have had a quick search of the Internet but unfortunately cannot find
exactly what we need (i think). Any help appreciated, regards Michael