Principle of BQ-OCFM Open Channel Flow Meter The physical quantity directly measured by the open channel flow meter is the liquid level. Water measuring weir will be installed at open channel and it’s for measure water flow.The measuring water weir converts the flow rate in the open channel into the level of the liquid level.The meter measures the water level in the water tank, and then calculates the instantaneous flow rate according to the relationship between the water level and the flow rate of the corresponding water tank.