A stationary liquid oxygen system contains 80 lb of oxygen when full. A home care patient is using it 8 hours each day, during sleep, at a rate of 3 L/min. At this usage, how many days will the liquid oxygen last?

a. 6 c. 19
b. 8 d. 27

ANS: C
Use the liquid oxygen duration formula as given previously.
A liter of liquid oxygen weighs 2.5 lb, so: 80 lb/2.5 = 32 L.
Gaseous oxygen occupies a volume that is 860 times the volume of liquid oxygen, so: liters of liquid ? 860 = liters of gas.
Duration of supply (minutes) = gas supply remaining (in liters) ÷ flow (liters/minute).
Once you get the answer in minutes, you divide by 60 to obtain the number of hours. Then di-vide the number of hours by the usage, which is 8 hr/day; that gives you 19 days.