Pulse output meters
The most common type of volume meter has a pulse output.
Each pulse will correspond to a certain volume passing through the meter. The meter will be marked with either the volume per pulse (for example 1G/p), or pulses per some volume (100p/m³).
This type of meter is usually designed to measure the volume, although in many cases we can assume that the flow is relative to the rate of pulses.
Note that some electronic flow meters count pulses internally and will send out bursts of pulses every few minutes to save battery. This type of meter can’t be used to monitor flow.