Encoders can detect speed when output pulses are counted in a specified time span. The number of pulses in one revolution must also be known. In the equation given here, S represents speed, C represents the number of pulses counted, PPR the resolution of the encoder, and t represents the time interval, in seconds, during which the pulses were counted. The second equation shows that if sixty pulses are counted in a time interval of ten seconds using a 250 CPR encoder, the shaft speed is 1.44 RPM.