Apparatus and method for detecting obstructions in pipes or channels

Apparatus and method of monitoring for obstructions in a pipe or channel. The method comprising the steps of: (1) transmitting a wave into a first end of the pipe or channel from a transmitting transducer at a first known location relative to the pipe or channel, wherein the frequency f of the transmitted wave (13) varies in a predictable manner governed by the function f=f(t), where t is time; (2) noting the frequency f tr of the transmitted wave; (3) detecting any reflections of the transmitted wave travelling towards the first end of the pipe or channel at a receiving transducer at a second known location relative to the pipe or channel; (4) determining the frequency f rec of the reflected wave (14); (5) comparing the frequency f tr of the transmitted wave and the frequency f rec of the reflected wave, being transmitted from and being received by the respective transducers at the same time t; and 6)determining the distance L from the transmitting transducer to an obstruction and back to the receiving transducer from the difference f between the frequency f tr and the frequency f rec .