Series 9146 Ex i frequency transmitters monitor the speed of rotating parts on one or two channels, e. g. the speed of fans or centrifuges. The frequency measured at the intrinsically safe input (between 0. 001 and 20 kHz) is issued as a unit signal from 0/4 to 20 mA or processed by a frequency divider. In one-channel devices, these frequency transmitters check whether speeds have exceeded or fallen below the limiting values.