A Hobbs meter is an actual "instrument" in and of itself. They've been around forever... but today, Hobbs meters are quartz clocks... and fairly accurate at measuring time (not accurate enough to have one strapped to your wrist). I do not think there is any such way to have them measure engine rpm.The ways you mention Fr. Bill, are how typical Hobbs are activated. In fact, I have one installed in my F-250HD (in a sense, similar to Method #1). They have many applications... people use for generators, trucks, riding lawn mowers, boats etc... (so one can know when to do engine maint.).Method #2 is typically used by FBOs that rent aircraft. Prevents people from (easily) disconnecting the Hobbs and getting "free" (stolen imo) flight time as it "switches on" as soon as receiving idle oil pressure.The hour meter in the tachometer is designed to measure engine hours at cruise rpm.You will see both in rental aircraft for example... they are distinctly different instruments. This is why I was saying I couldn't find the Hobbs.Hobbs is used for Flight Time.Engine Hour meter used for the logging of Engine Time in the aircraft logs. Sometimes Tach Time is used for the figuring of Flight Time (when there is no Hobbs or when it breaks) by applying some type of multiplier... which will be up to the specific FBO or whoever owns the plane.You can do a check here... run the Maule at idle and see how long it takes to tick off a tenth of an hour.Then go bore a hole in the sky at full power and see how long that takes (to tick off a tenth).

I heard after a certain number of hours of engine hours the plane will be locked in the hanger until you pay Microsoft a fee for maintenence and certification. Just kidding!