The Hobbs Meter records the time a device is used. It is frequently used in aviation applications to record the time that the electrical power (battery master switch) is "on". Hobbs is a genericized trademark for products generically called "Engine Hour Meters". The meters run electrically, indicating hours and tenths of an hour, but there are several ways a meter may record the "Hobbs Time":
It can measure the time that a master switch is on. This tends to overstate Hobbs Time.
It can be activated by oil pressure running into a pressure switch, and therefore only runs while an engine is running. Many rental aircraft use this method to preclude flying with the master switch off to improperly reduce Hobbs Time.
It can be activated by another switch, either an airspeed sensing vane under a wing (as in the Cessna Caravan) or a pressure switch attached to the landing gear (as in many twin engine planes). In these cases, the Hobbs Meter only measures the time the aircraft is actually flying. This is for those who wish to record Time In Service. Metrics such as Turbine Actual Runtime are kept to monitor overhaul cycles, and are usually used by commercial operators under Federal Aviation Regulations Parts 135, 121, or 125.
It can be activated when the engine alternators are online (as in the Cirrus SR series).
For general aviation, Hobbs Time is usually recorded in the pilot's log book, and many Fixed-Base Operations that rent airplanes charge an hourly rate based on Hobbs Time. Tach Time is recorded in the engine's log books and is used, for example, determining when the oil should be changed and the time between overhauls. Tach (tachometer) time differs from Hobbs Time in that it is linked to engine revolutions per minute (RPM). Tach Time records the time at some specific RPM. It is most accurate at cruise RPM, and least accurate while taxiing or stationary with the engine running. At these times, the clock runs slower. Depending on the type of flight, Tach Time can be 10–20% less than Hobbs Time. Many organizations such as flying clubs charge by Tach Time so as to differentiate themselves from Fixed-Base Operations by the fact that 10-20% less time recorded makes it 10-20% cheaper to fly (if the hourly rate is the same).