Abstract: An open channel flow monitoring apparatus for measuring a fluid level is disclosed having a first sensor configured to obtain data indicative of a fluid level below a first threshold level, a second sensor configured to obtain data indicative of fluid level above a second threshold level, which is lower than the first threshold level, and both the first sensor and the second sensor are configured to obtain data indicative of the fluid level when the fluid level is between the first threshold level and the second threshold.

Abstract: A method of measuring the speed of flow of a liquid along a conduit by omitting a continuous acoustic signal of known frequency through a wall of the conduit and receiving the signal reflected, from the liquid. This reflected signal is of increased frequency due to the Doppler effect, and the received signal is demodulated to give a signal at the Doppler difference frequency which is a function of the liquid speed. The Doppler difference frequency is divided by a divisor of known numerical value and the quotient frequency counted. This quotient frequency is a function of the speed of flow or the flow rate, and one or more indicator devices utilize this count either as analogue signal or integrated with respect to time to indicate the instantaneous speed or flow rate or the total volume or weight of liquid which has passed.