Earlier in this forum I started a thread discussing the flow rate of peristaltic pumps. However, after doing some calculations I've decided to go with a gear pump instead. After locating a suitable unit, I ran a test today with a blood pressure cuff bladder:
Since for this test I did not use a check valve between the pump and the bladder, after shutting the pump off I expected water to come gushing out due to the bladder's elasticity wanting to it deflate but to my surprise, it stayed in this state for a quite a long time... Examining the pump intake and the inner diameter of the rubber hoses, the latter being about 1/8'' (3mm), it seems that water is leaking very slowly out of the hose.
I realize that this is a pretty nice built-in safety system since it will cause the sub to eventually rise to the surface, if no pumping input is applied for a long time. The question is, if the ballast intake is to be located about 1 inch below the water surface through the WTC endcap, will I even need to install a valve to prevent water from entering the bladder when it is empty? The inside of the WTC will be at standard atmospheric pressure (101.3 kPa), will that be enough to counter the outside water pressure at 1 inch below the surface?