GAS METERING

A gas meter comprises a conduit for passage of a gas flow, in use; an ioniser arranged to ionise the gas flow in the conduit, in use; a modulating electrode structure downstream of the ioniser arranged for modulating the ion distribution in the ionised gas flow; and at least a first detecting electrode structure downstream of the modulating electrode structure arranged for detecting the modulated ion distribution in the ionised gas flow. The modulating electrode structure comprises an upstream electrode and a downstream electrode, and a respective modulating potential is applied, in use, to each electrode to modulate the ion distribution in the ionised gas flow. The modulating potential applied to the downstream electrode is of the opposite polarity to the modulating potential applied to the upstream electrode and of a magnitude selected such that, downstream of the modulating electrode structure, the electric field due to the upstream electrode is cancelled by the electric field due to the downstream electrode.