Hydration Packs 101
So what exactly are hydration packs? Hydration packs and backpacks differ in the built in features that vary from your traditional backpack. Basically all hydration packs center completely around a built in pouch and water bladder. So in a nut shell a pack and a water reservoir is makes up a hydration pack.
But when it comes to modern hydration packs this doesn't simply mean stuffing a plastic pouch full of water inside of a pack where you have to keep pulling it out to access it.
Modern hydration packs are designed so the tube from the water bladder feeds outside of the pack with loops and snaps that run down the arm straps where you can easily grab ahold of the tube so you can drink while on the go without having to stop, pull the reservoir out and then put it back when you are finish. 
Better yet the straps and loops are designed so that they keep the drinking tube from flopping around or getting in your way when it's not in use.
The water bladders in hydration packs are made from soft and durable plastic materials. This has the added benefit of being very lite weight and fits nicely into a pack without taking up extra room like bottles can. But they are made out of plastic and some people frown on that because of the chemical or taste that they can impart to water stored in them compared to stainless steel bottles.
However anymore almost all manufacturers of hydration packs and water bladders use BPA free materials. This is a good thing. Take caution though and be watching out for this with cheaply made or bargain hydration packs. They are cheaper than the name brands for a reason and it's because they use cheaper parts.
Now when it comes to functionality, essentially a hydration pack or backpack comes in two distinct styles: one designed just to carry water in an insulated water bladder made from plastic material and one designed to carry the water bladder and cargo.
Taking it one step further, some manufacturers like Camelbak design packs specifically to suite certain types of sports or activities. There are even some models specifically designed for the shape and contour that better fit a womans body.
Choosing the right hydration pack completely depends on the type of outdoor activity or sport you plan to partake in as there are models that have features designed for different sports.
Generally you will find that models fall under hiking, cycling, running, snow sports and hunting types each with there own unique set of features.
As previously mentioned all models of hydration packs are centered around the water bladder. When you are outdoors you need to carry around a good amount of water to stay hydrated. Factor this is as a good starting place when picking out your pack.
If you plan for short outings then a 70 or 100 ounce bladder should hold enough water for a few hours. You can also get packs that carry 120 ounces or more but of course you have weight factors to consider with carrying higher amounts of fluid. For very day long journeys or overnighters you will need a pack with a large reservoir, tons of storage and probably an additional water bottle or two.
Don't skimp on the water bladder size. When you are exercising or just going on an easy hike your body loses water and you need to replenish it. Dehydration is a serious condition you do not want to find yourself in.

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