Recently pulled from my personal files- written by Him!
"The combination of the ever increasing vacuum within the WTC and the outside pressure dictates how deep the RCABS can still put air in the bag. It's that differential pressure, outside pressure/inside pressure, that dictates system maximum depth.
Let's say the RCABS pump stops moving air at 15" of Mercury: That's equivalent of 15' of fresh water. That means the pump will not push air into the bag if the outside depth is in excess of 15' ... but wait a minute: that works only if there is an unlimited WTC volume. No such thing. What really happens is that a vacuum is created within the WTC as air is pulled out of it and pushed into the bag. You can start to get air into the bag at 15', but the differential across the pump intake and discharge will quickly rise and the pump will stall long before any significant amount of air gets into the bag.
And that's the little secret: as the air-pump pumps air out of the WTC and into the bag, a vacuum is created within the WTC against which the pump has push, in addition to having to push against outside hydrostatic force.
The maximum depth to which the RCABS on-board air-pump can push is related to the degree of vacuum it can suck from the available WTC volume -- the lower the volume, the quicker the pump gets into the critical vacuum number.
To answer your question: The maximum depth an RCABS system can START to get air into the bag is dictated by the maximum differential pressure the air-pump can reach before stalling. (Remember: one atmosphere is equal to about 30 feet of water).
Finally, The maximum depth capability of the system depends on the stall differential pressure of the air-pump/compressor. Each motor-pump combination has a different stall point."