The easiest and safest way to figure out whether your filter needs to be replaced or not is through a pressure drop indicator (or manometer). The higher the pressure drop after the air filter, the worse is the filtration and energy efficiency.
The dimensioning and the operation of almost every air handling unit depends on the air flow rate. It is unusual to put ventilation systems into service without an effective control of the flow rate during operation. This would be comparable to driving a car without a speedometer, fully relying on your instincts.
An air flow transmitter is showing the current flow of a ventilation and air conditioning system to the operator. Additionally, the air flow value is transmitted to higher-level control technology. The result is a demand-controlled, more energy-efficient and healthy unit.