Parabolic (orifice) resistor pressure drops selected from ASTM F1100 (K values), for inspiratory flows at 2, 1, and 0.5 l/s, respectively Rp5 K 2.70 ± 20 % (equivalent orifice size = 8.48 mm) pressure drop 10.80 cmH2O at 2 l/s Rp20 K 17.61 ± 20 % (equ