Abstract: An acoustic flow meter in the form of a valve key has an acoustic sensor 12 for measuring the peak sound level which occurs when the key is used to close a valve in a fluid pipeline. The peak sound level can be used to accurately calculate the flow through the valve and provide the flow information on a display unit 11. Parameters for the upstream supply pressure, pipe diameter and valve characteristics can be entered to ensure the accuracy of the flow calculations. The key therefore provides a way of determining fluid flow in a pipeline to a reasonable level of accuracy without having to install in-line flow meters.