| 摘要 |
A method of detecting water level in an elongate cavity comprises emitting an alternating signal into the cavity, varying the frequency of said signal to provide a specific response from the cavity, detecting said response, and comparing said outgoing signal and response. In a preferred embodiment the response is a returning signal reflected from the surface of the water. The frequency of the outgoing signal may be varied continuously to establish a substantially steady state difference frequency between the outgoing and returning signal.
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