measurement of rotational speed of the dynamometer is measured either by a system using a toothed wheel and a pulse sensor within its associated electronics and display or, more recently, by use of an optical encoder system. While the pulse pick-up system is robust and, providing the wheel to transducer gap is correctly set and maintained, reliable, the optical encoders, which use the sensing of very fine lines etched on a small disk, need more care in mounting and operation. Since the commonly used optical encoders transmit over 1000 pulses per revolution, misalignment of its drive may show up as a sinusoidal speed change, therefore they are normally mounted as part of an accurately machined assembly forming part of the machine housing.
It should be remembered that with bidirectional dynamometers and modern electrical machines operating in four quadrants , it is necessary to measure not only speed but also direction of rotation. Encoder systems can use separate tracks of their engraved disks to sense rotational direction. It is extremely important that the operator uses a common and clearly understood convention describing direction of rotation throughout the facility, particularly in laboratories operating reversible prime movers.
As with torque measurement, specialized instrumentation systems may use separate transducers for the measurement of speed or for the control of the dynamometer. In many cases, engine speed is monitored separately and in addition to dynamometer speed. The control system can use these two signals to shut down automatically in the case of a shaft failure.
Measurement of power, which is the product of torque and speed, raises the important question of sampling time. Engines never run totally steadily and the torque transducer and speed signals invariably fluctuate.An instantaneous reading of speed will not necessarily, or even probably, be identical with a longer-term average. Choice of sampling time and of the number of samples to be averaged is a matter of experimental design and
compromise in measurement of rotational speed.