Non resuscitator demand valve units operate much like your scuba regulator. Oxygen flows only when the patient inhales, so it minimizes waste, and with a proper mask it can deliver nearly 100 percent oxygen. In addition, a rescuer can inhale from a non resuscitator demand valve unit and ventilate a non breathing patient with a high oxygen concentration (the body only consumes a small fraction of the oxygen in each breath), Bali Scuba Dive accident first aids calls for delivering the highest oxygen concentration possible for as long as possible, making the non resuscitator demand valve the best choice for rescue Scuba Diver.

Continuous flow units release oxygen continuously, so they're more wasteful than non resuscitator demand valve units. Fixed continuous flow units usually deliver six or 10 liters per minute; adjustable usually deliver up to 25 liters per minute. With the proper flow (15liters per minute recommended) and a non rebreather mask with reservoir bag, continuous flow units can deliver more than 90 percent oxygen, but with low flow rates and/or an improper mask, the concentration may remain below 60 percent. 

By using a pocket mask, you can ventilate a non breathing patient with partially oxygenated air using a continuous flow unit(more about pocket masks and rescue breathing later). Most non resuscitator demand valve systems have multifunction regulators that can be used continuous flow so you dont sacrifice this benefit.