In recreation and other sustained outdoor activities, a hydration system is an apparatus intended to promote its user's drinking enough liquid to support the physical effort involved in the activity. Such systems for consumers were first sold to cyclists, but by the 1990s also had found a substantial market among hikers. Familiar commercial models can also occasionally be recognized being worn by Western military personnel in southwest Asia.
In practice, such a system is almost always a commercially manufactured unit featuring at least
- a flexible bladder, of one or a few liters' (quarts') capacity, with a means (usually a screwtop) of filling and then reliably sealing it,
- a light hose to convey the beverage to the user's mouth, and
- a bite valve for starting and stopping the flow through the hose with minimum effort.
Designs that also include specific hands-free means for comfortably carrying are common.
The overall geometry of the bladder is nearly universal:
- The largest dimension is the vertical one, taking advantage of the long vertical dimension of the human torso.
- The hose joins the bladder very near the bottom of the bladder, to maximize the amount of accessible liquid.
- The bladder's change in volume as it is gradually emptied is reflected in the decrease of its smallest dimension (thickness); this lets the vertical and side-to-side dimensions stay fixed, thereby limiting the shifting of it and any items next to it, and minimizes the flexing of the bladder walls.
Typical commercial hydration systems are available with three fundamental approaches:
- A "bare bladder", intended to be carried in a matching pocket of a "hydration-system compatible" backpack. (Such a pack will usually also feature a hole through the pack, near the upper anchoring point of the pack's shoulder straps, sized for the hose and bite valve to pass through it, and often covered by a flap that prevents rain entering the pack through it.)
- As part of an integrated design incorporating both a backpack and the hydration system (usually sold together), usually with at least a few pockets-worth of storage space for other items. (Such a combination is likely to have a channel inside one or both shoulder straps, for the hose to pass through, preventing the hose from tangling with other gear or the environment, and ameliorating the annoyance of the liquid warming up while waiting in the hose to be drunk next, or the problem of it freezing.)
- A basic hydration system in a protective fabric shell, so that the bladder is unlikely to be damaged by other gear that it shares the interior of an ordinary pack with. (The CamelBak brand products of this kind are trademarked as "The Unbottle", for instance, and each looks a lot like a very small pack, with a hand-carrying strap at the top and four D-rings that could facilitate attaching it to the outside of a pack. However, they completely lack shoulder or waist straps.)
Some manufacturers offer parts for replacement or customization, whether compatible only with their own hydration systems, or usable also with others'.
- Shut-off valves: 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.
- Elbows: 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.
- Hose anchors: 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
- For the hose: plastic foam tubing to surround the hose may be used to reduce heat transfer between the environment and liquid in the hose, keeping a cold liquid refreshing in summer, or forestalling the system's becoming unusable due to freezing of the contents in the winter.
- For the bite valve: at least one manufacturer offers a tethered, slip-off, foam cowl that forestalls freezing of the bite valve, and also prevents contamination of the bite valve.
- Scrubbing systems: 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 removing bacterial colonies that may build up inside the system).
- Drying systems: 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.
- Clearing the hose between drinks. 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.
- Advocates point out that liquid in the hose (where it is all within 3/16 inch (5 mm) of the air) warms or cools relatively rapidly toward the ambient temperature, and value the reduction of this when nearly all the liquid is in the bladder. There it is as much as several inches (10 cm) from the bladder surface, and it may be still further insulated by a pack and its other contents.
- Detractors believe that blowing into the hose increases the amount oral bacteria inside the bladder, and requires more frequent scrubbing to keep the flavor and odor of the contents acceptable.
- Because the hose attaches to the bottom of the bladder, in order to let the last of the liquid into the hose, air that reaches the opening bubbles to the top of the bladder, and is trapped there until the liquid is nearly drained. Each time more air is blown into the bladder than the amount of liquid that was just drunk, the pressure inside the bladder increases, and the more effort blowing back involves. Therefore the technique profits from enough attention to develop a sense of when the hose is nearly full of air.
- Depending on the size of the opening for filling the bladder, it may be convenient to include, in hot weather, ice cubes or crushed ice when filling it. In theory too much ice means running out of liquid water while the bladder still has ice occupying space that could have been filled by more water (so it helps to take account of how temperature and humidity affect the balance between consumption rate and melting rate), but it helps that most people's tendency seems to be to put in less ice than would last through the hike.
- The use of liquids that provide calories (usually in combination with flavoring and perhaps rehydration salts) increases the cleaning burden by nourishing bacteria, and if the user neglects to empty and rinse the bladder, judging the cleanliness of the bladder at a later date involves more effort than with containers that are more transparent and in some cases more accessible. Some users carry plain water, with or without ice, in a bladder, but drink something else from Nalgene-style bottles during rests.
- On trips over a day, or in especially hot conditions, need for more water than is carried my require purification of local water.
- Attention to mechanical compatibility of a filtration system with the bladder, or with a Nalgene-style bottle that may be carried to fill the bladder, can be worthwhile.
- Use of chemical purification systems raises issues both of staining of the bladder and the possibility of weakening its material. Besides the aesthetic discomfort of drinking from a perfectly sterile but "dirty looking" bladder, staining can also interfere with judging later the cleanliness of a bladder.
U.S. manufacturers of ready-to-use hydration systems include: