than is carried may 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. Another way to ensure the bladder does not start a breakout of microbes is to let out as much water as possible from it, and freeze it as soon as possible. This technique makes use of the property that microbes cannot propagate under freezing conditions. 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. When the air temperature is below freezing if the hose isn't cleared it can rapidly freeze blocking the hose. This often occurs skiing where high wind speed and low temperatures are combined. Detractors believe that blowing into the hose increases the amount of 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 contained inside the hose, 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. See also Hydration (disambiguation) Hydrational fluid References Camping equipment Military personal