HOBBS METER

The 'Hobbs Meter', as described by Ed Gerkin of Honeywell, is a "DC powered hour meter that is very straight forward in operation. When DC power is applied to the appropriate +/- terminals the meters will start to accumulate elapsed time." Hobbs meters are frequently used in aviation applications to record the time that the electrical power (battery master switch) is 'on'. Note that 'Hobbs' like 'Kleenex' has become a de facto name for a class of products generically called 'Engine Hour Meters.' Hobbs is made by Honeywell, but there are others such as ENM, and Datcon. They all run electrically and indicate in hours and tenths of an hour (1 tenth = 6 minutes,) but there are several ways that a meter can record the time:
1) It can simply measure the length of time that the master switch has been on. This tends to overstate Hobbs time.
2) It can be activated by oil pressure running into a pressure switch and therefore only runs while the engine is running. Many rental aircraft use this method to preclude the possibility of flying with the master switch off to improperly reduce Hobbs time.
3) 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 twins.) In these cases, the Hobbs only measures the length of time the aircraft is actually flying. This is for those who wish to record 'Time In Service.' Things like 'Turbine Actual Runtime' are kept like this for purposes of overhaul cycles, and are usually kept by commercial operators under FAR Parts 135, 121, or 125.
4) It can be activated when the engine alternators are online (as in the Cirrus SR series).
For purposes of GA (general aviation,) Hobbs time is usually recorded in the pilot's log book, and many FBOs that rent airplanes charge their hourly rate based on Hobbs time. Tach Time is recorded in the engine's log books and is used for such things as determining when the oil should be changed and TBO (time between overhauls.) Tach (tachometer) time differs from Hobbs time in that it is linked to the RPM of the engine. Tach Time records the length of time as some specific RPM - 2400 RPM for example. That means that 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 FBOs by the fact that 10-20% less time recorded makes it 10-20% cheaper to fly (if the hourly rate is the same.)