Flowmeter

A bypass flowmeter assembly has a plurality of branch pipes (11) connected in parallel with a main pipe (10), with at least one of the branch pipes having a respective flowmeter (14), and at least another of the branch pipes having respective selectively operable pipe closure means (12, 13). Preferably each branch pipe has a flowmeter and closure means. Preferably also control means (30) are provided to operate the closure means through a routine of successive phases of respectively different patterns of branch pipe flow. This allows derivation of main pipe flow rate on the basis of branch pipe flowmeter measurements with comparison of flowmeter outputs between different phases to eliminate high order measurement error, averaging of such outputs to reduce low order measurement error, or by computation to eliminate unknown factors. The assembly can be connected serially with a similar assembly (20) to monitor relative variations in the total flow of interest and adjust the actual measurement of this flow.