As per USP guideline the normal healthy person is having

Lungs Air volume Capacity is 4lit. and when we measure the

air flow at that litre,so flow metre will show the air flow

60.

thus it mean 1 second per 1 Litre, so for the 4 litre is

required 4 second , that's why we performed the Andeson

Cascade at 60LPM/MIN and at 4 second