Originally Posted by
jsigone
Let throw jr high science class out the window here and visually think this out. Yes the bladder is on the bottom, yes there is LESS air in the bladder then the hose forcing the water up through the tube. So picture this, the standard camelbak is on your back, full of 100oz. The hose is route back up through the pack, over your shoulders and bite valve is hanging roughly at belly level. Right? The the valve is lower then the top of the bag pressurizing the entire system and water is in a closed system at the disposal of the bite valve as the only exit. The first time used, you might suck on air for a second or two to get the pressure going in the positive direction. After that the water is right at the valve for the remainder of the 100oz. Now you ask why hasn't the system lost pressure as less water is in the bladder, well there is still no air in the bladder so the system is flowing in the positive direction.
You've never used a hydration system, have you? You seem to be a bit confused about how they operate. In particular, you seem to think that a hydration bladder
must be siphoning to work. In fact, that isn't the case: these systems work perfectly well when the bite valve is positioned above bladder. In that case, it operates like a gravity-assisted straw.
Hip bags work the same way, but the hose doesn't go up your back, is stays wrapped around the bladder to keep the positive pressure.
Camelbak makes
nine models that feature lumbar reservoirs. Only
two of those models wrap the drinking tube around the bladder. Why would seven of the nine models position the bite valve significantly higher than the bladder if what you're saying is true?!?
If you extend the hose to say a pannier pack, you now added cubic volume of air to the hose. And more volume of air that your mouth holds. Meaning it will take a few hard sucks on the valve to even TRY to get water but your fighting gravity and physics here
Sure you can drop the bite valve lower to get higher pressure for an instant fix but over few minutes the pressure will fix itself and putting water in half the hose between the bladder and the bite valve.
I'll grant that you might have to suck once or twice to get water to fill the tube. Luckily, gravity is actually
helping you in this case: gravity is pushing down on the water, which forces water into the drinking tube, which means you have to suck much less than you would if you just stuck a tube into the top of the bladder. Once the drinking tube is filled with water, the bite valve serves to keep water in the tube similar to what happens when you put your finger over the end of a straw.