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A hydration pack is a type of hydration system built as a backpack or waist pack containing a reservoir or "bladder" commonly made of rubber or flexible plastic. The reservoir contains a capped mouth for filling with liquid and a hose that allows the wearer to drink hands-free. Most hoses end with a "bite valve" that opens when the user bites down on it; the valve may be protected by a dust cover. Some hydration packs are insulated to keep water from freezing or becoming warm.
Reservoir sizes commonly range from 1.5 to 3.0 liters. Some of the latest developments include hydration packs that have pressurized hydration bladders  which will allow the water to spray as with a hose or to be force water through an in-line-filter allowing the user access to clean water on the go. 
Hydration packs are commonly used for outdoor recreational activities, such as hiking, bicycling, and kayaking, as well as for military maneuvers, often dubbed the Camelback hydration system or camel bag in this context. Hydration Packs are used by various militaries; the British Army, for example, has been attempting to incorporate water filtration into the system.
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