The present invention is a instrument used for the measurement of water level variation over a wide range. The water level sensor is a float-type transmission system, the float drives a rotary disk through the float drum line, to the other end of which is attached a weight; a potentiometer is mounted coaxially to said rotary disk. When the float rises and falls along with the water level said rotary disk is turned accordingly and puts the sliding arm of the potentiometer in motion. The water level variation is then converted into a variable voltage signal to be employed in water level measurement. Microcomputer software is used to realize the functions of: automatic measurement, recording, alarm and interactive dialogue etc..