How much effort does it take to suck water through the FilterPen?
By sucking on the mouthpiece an under pressure is created which pulls water through the microfiltration membranes. Under pressure is expressed in bar or psi (or pounds per square inch). An average individual can comfortably create an underpressure of 0.1 bar (1.5 psi) by sucking. The flow rate is the amount of water which will pass through the FilterPen at a given under pressure. It is usually expressed in gallons per minute or liters per minute. At 0.1 bar the flow rate through the FilterPen will be about 0.5 l/min (0.013 GPM).
