A fuel injection has to know the amount of air consumed by the engine in order to add the correct amount of fuel to the air. Therefore the air must be metered. This can be done by the following methods:

HOT WIRE AIR MASS METER
A wire is heated, and when air flows over it, the wire is cooled. This is measured. The wire is subject to vibration, shock, and a coating of dust.

AIR FLAP
A spring loaded door is placed in the air stream, which is pushed open by the airflow. The amount of opening is measured and can be related to airflow if the spring tension is known. A very popular system, it automatically compensates for air density (altitude, temperature). There are problems with fitting it to superchargers.

ABSOLUTE MANIFOLD PRESSURE
It is solid state, no moving parts. The pressure in the manifold is measured and at a certain RPM and butterfly position, it relates to airflow. This is the method Perfect Power is using on all products.