Abstract: A liquid meter including a freely rotatable impeller driven by a constant speed drive. Liquid to be metered is introduced as a flow against, and accelerated by, the impeller. The energy necessary to accelerate the liquid is monitored to determine liquid flow. Preferably, the impeller is driven through a magnetic coupling which facilitates isolation of at least large portions of the drive from the liquid flow. One preferred embodiment employs a pair of spring coupled shutters with one shutter being magnetically coupled to the impeller. The displacement of the spring is dependent on liquid flow with that displacement being determined by photoelectrically monitoring the relative positions of the shutters. The power necessary to maintain the drive at constant speed may also, or alternatively, be monitored to determine liquid flow.