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Originally Posted by vidiots
With the air powered dosing pumps you can't get a pressure buildup in the bottle because you have a leak designed into the system. That leak is where the fluid comes out. The maxium pressure you can get above atmospheric with the pump off is equal to the head height of the output hose, which only changes with the fluid level in the bottle from one dose to the next unless you move the bottle or hose. If the pressure in the bottle were to exceed atmospheric pressure plus head height, fluid would flow.
What you do have increasing from one dose to the next is the volume that the pump has to fill with the pressurized air. Remember the pump is pumping air at a relatively consistant volume per minute. You use a timer to make sure your timing is consistant. At the start if you only have one cubic inch of air at the top of the bottle, then use the pump to add another cubic inch of air you have doubled the pressure inside the bottle which causes fluid to flow at a good speed. Now after several doses you have 10 cubic inches of air at the top. It now takes the pump 10 times as long to pump the bottle up to the same pressure/flow rate. What you tend to end up with is a time delay before fluid flows after the pump comes on, a slower flow rate as the bottle empties, but it also continues flowing for a longer time after the pump shuts off as this larger volume of air at a lower pressure equalizes with atmospheric. This is eventhough you have the timer on for a fixed amount of time.
As I was saying more head height on the output hose, a large diameter container, and a stronger pump should minimize these effects.
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If you could find a thick walled balloon of the proper size to fit in the bottle (those kid's balloons lose too much air due to leakage), then the balloon would expand as the fluid was used, keeping pressure (and volume) in the bottle the same, and fluid delivery the same over the whole dosing period.