Constant flow control in a personal monitoring pump works like the cruise control feature in a car. When the pump senses that the flow rate is beginning to fall off (such as the sample filter loading with dust), it will automatically speed up the motor in order to keep the flow rate constant. This is similar to how the cruise control on a car speeds up the engine when the car starts an uphill climb.
Pump manufacturers have taken different approaches in monitoring and controlling a constant air flow rate. Some monitor motor speed, while others monitor the flow rate directly with pressure or flow measurements, but they all accomplish the same task in maintaining a constant air flow.