The metering unit does just that, it meters the amount of air entering the vsi so that a predetermined lag exists between the pressure inside the VSI and the ambient pressure. In this way the capsule inside the VSI expands or contracts in a measureable way and therefore its output can be calibrated.
The metering unit comprises a number of restrictions in its construction, the airflow increasing then slowing down, becoming turbulent, then laminar. These have been incorporated to ensure that the lag is more or less constant for varying pressure/temperature variations.
The next problem is that with a deliberate lag it is possible to climb tens, or even hundreds of feet before the instrument indicates a climb/descent. This problem is solved by incorporating an inertial dashpot inside the (I)VSI, after the metering unit which acts on the capsule such that the inertial forces acting on the a/c will cause an almost instant indication of climb or descent, after which the normal pressure differential action provides the indication. Hence the term instantaneous (or inertial) VSI (IVSI).
