A Hobbs meter measures the total number of hours that a machine or a device has run since it was put into operation. It is typical for an electromechanical Hobbs meter to be working on an AC mains supply of 230 volts, which corresponds to 50 to 60 Hertz. In this case, the constant mains frequency is used as a drive for an analogue counter driven by a synchronous motor. In this way, the operating time in hours is displayed with a resolution of 0.01 to 1 hour. A Hobbs meter that operates on a DC voltage have counters with a stepping mechanism and a timer. In the electronic version of a Hobbs meter, an oscillating quartz crystal as well as an electronic circuit which realizes counting, are installed as timers.
A Hobbs meter for the aviation, agriculture and construction industry
Within the aviation industry, Hobbs meters are used on an aircraft in order to determine the operating time of the engine. For example, they can be used to calculate the maintenance intervals of the airplane or to calculate the rental period. The Tach Timer is especially relevant for engine maintenance as it indicates the engine revolutions divided by the nominal speed. The Hard Time, on the other hand, indicates the pure flight time.
Agricultural and construction machines usually use a Hobbs meter which either starts counting when the ignition is switched on or is connected to terminal D+ of the alternator and only shows the operating hours when the engine is running.