A hydration system is an apparatus used in recreation and other sustained outdoor activities. It is intended to help its user carry liquid to support the physical effort involved in the activity. Such systems for consumers were first sold to cyclists, and by the 1990s had also found a substantial market among hikers. Familiar commercial models can also be recognized occasionally worn by western military personnel in southwest Asia.
In practice, such a system is almost always a commercially manufactured unit that features at least
Also common are designs that include specific hands-free means to comfortably carry the hydration system.
Hydration systems first appeared commercially in the late 1980s or early 1990s, at backpacking stores. They were adopted by US special operations troops in the early 1990s, and became standard issue for all US troops in the later 1990s. The concept of the hydration system appeared in Robert A. Heinlein's 1955 novel, Tunnel in the Sky, where the main character has "a belt canteen of flexible synthetic divided into half-litre pockets. The weight was taken by shoulder straps and a tube ran up the left suspender, ending in a nipple near his mouth, so that he might drink without taking it off."
The overall geometry of the bladder is nearly universal:
Typical commercial hydration systems are available with three fundamental approaches:
Some manufacturers offer parts for replacement or customization, whether compatible only with their own hydration systems, or usable also with others'.
Especially while a hydration system is being carried in a vehicle, there is some danger of the bite valve being squeezed, opening it to leakage or a steady flow; this can be guarded against with an additional valve, usually installed between the bite valve and the hose, that stays open or closed according to the position of a lever.
The bite valve may be installed with a right-angle extension between it and the hose, to achieve a preferred positioning and angle of the valve relative to the user's mouth.
Position and angle may also be adjusted by clips that clip usually to a pack strap and (for instance by clipping to the hose, or looping around it and snapping to themselves to close the loop) control the hose's path
Plastic foam surrounding the hose can be used to reduce heat transfer between the environment and liquid in the hose: keeping a cold liquid cool longer in summer and slows a liquid freezing in winter.
At least one manufacturer offers a tethered, slip-off, foam cowl that slows freezing of the liquid in the bite valve, and also reduces contamination of the bite valve.
At least one manufacturer, MSR, offers a in-line ceramic based filter, allowing use of local water supplies for refill. Ceramic filters can be cleaned at home by back flushing with clean water so no replacements are needed.
Sets of cylindrical brushes (e.g., a large one for the inside of the bladder, and a thin one with a flexible handle long enough to run at least halfway through the hose) permit mechanical removal of Biofilm that will grow on the surface inside the system.
Means of holding the bladder walls apart to encourage drying between uses are available, such as a plastic frame that collapses to pass through the fill opening, but expands inside the bladder to hold the sides apart even near the corners.
Various specialized practices may be applied in using a hydration system.
Two views exist on the practice of blowing air back through the bite valve, to force the liquid out of the hose and back into the bladder.
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