US Patent 7343794 Weir box and sensor

The flow rate of water passing into a weir box and over a weir that is mounted on the weir box is determined using a sensor to continuously or intermittently obtain a measure of the water level within the weir box relative to the bottom of the weir notch to thereby determine the head of the water flowing over the weir which, in turn, translates into flow rate. The sensor is located in the interior of the sump which is in fluid communication with water within the weir box. The sump is positioned deeply enough within the ground so that the sensor is below the frost line and is not damaged upon freezing of the water within the weir box.