The tach(ometer) timer is an instrument used in aviation to accumulate the total number of revolutions performed by the engine. The unit of measure is equivalent to the number of hours of running at a certain, specific reference speed of rotation. If the reference speed of rotation is 2400 RPM then the timer runs in real time when the engine is running at 2400 RPM, half speed while the engine is run at 1200 RPM (a fast idle for some aviation engines) or at 5/6ths real time at 2000 RPM (a slow cruise speed). The tach timer integrates over time the instantaneous rotation speed displayed by the tachometer. The displayed number is incremented by one if the engine is run at its reference speed for one hour. The quantity recorded is referred to as tach(ometer) hours. If the reference rotation speed is 2400 RPM then the tach timer records

${\displaystyle {\frac {{\text{total}}\ {\text{revolutions}}}{2400\cdot 60}}}$