Ballast bag

just a quickie if I fit a water ballast bag inside my wtc will it empty with just the pressure build up in the wtc or will the water need to be pumped out?

Hello,

that's just a question of the pressures inside and outside the WTC and depends also on the relationship between the total inner volume of your WTC and the volume of your ballast bag.

Imagine you are at the surface. Then you have to pump water into your ballast bag to overcome the increasing inner air pressure of the residual WTC volume and you must shut a ballast valve to keep the water in your ballst bag as long you are close to the surface and water pressure is lower than the inner air pressure of the WTC. Otherwise the ballast water will be pressed out and the model will surface.
But with increasing diving depth the water pressure rises and in a certain depth it will be higher than the inner air pressure so when opening the valve even more water will enter the bag and surfacing is impossible. That certain depth where the water pressure equals the inner pressure depends upon the volume ratio of the WTC volume and the bag volume. If the WTC volume is relativly large compared to the bag volume the inner pressure will rise only for a small percentage of the normal atmospheric pressure, so the critical limit is already reached in a small depth (rough exaple: ratio WTC:bag = 10:1 will raise the inner pressure approx. by 10 percent. A water pressure of 10 percent more than normal atmospheric pressure that equals will be reached in a depth of about 1 metre (or 3 ft) that is not much.)
Large ratios resulting in greater depths are feasible but that method is not recommended in general. You should always provide a pump that is also able to get the water out against a higher water pressure (at least half an atmosphere) to bring your model home.

By the way - perhaps you have already experienced a model with a pressure hull or WTC that is not really tight. When pumping water in bubbles will come out due to the increasing inner pressure. When the model submerges you will see that the bubbles will become less and from a certain depth on there will be no more bubbles coming out (than the water pressure is higher than the inner pressure). When lifting the model bubbles will start again.

Kind regards
Hermann

Thank you Hermman for your concise reply I can now say that I fully understand and will have to rethink my ballast plumbing
thank you