Recirculating Air Ballast System
In the late 1990’s Dr. Art Broder developed an alternative ballast system using the bladder and pump from a blood pressure measuring machine. Building on the work first begun by John Darnell many years before, he created a closed ballast system requiring no consumables. He dubbed it RCABS for ReCirculating Air Ballast System.
The RCABS system consists of a small air pump mounted inside the pressure hull connected by tubing to an external bladder. A valve connects to the tube running from the pump to the bladder. The other side of the valve is open to the pressure hull. At rest, the bladder is deflated, the valve is closed and the air in the pressure hull is at one atmosphere. In this condition, the sub is trimmed to have a slightly negative buoyancy.
Just prior to operations, the bladder is inflated with air pulled from the pressure hull. This adds enough buoyancy to the sub to keep it on the surface. With the pump off, the valve remains closed and the air inside the pressure hull is under a partial vacuum. When the command to dive is given, the valve is opened and the vacuum draws the air back out of the bladder. The bladder deflates, buoyancy is reduced and the sub submerges. To surface, the valve is closed, the pump is turned on and the cycle repeats.
As a closed system, RCABS has no gas bottles to charge, no outside air to draw through a snorkel, and normally draws less than a quarter amp of current. It was quickly embraced by the sub modeling community.
A subsequent modification of the system was developed a few years later called RCABS-R (or Reverse-RCABS). It was designed as a means to maintain the air in the WTC at a constant ambient pressure. Instead of drawing air from the watertight spaces, the bladder is connected to an air storage vessel. The cycle begins with the bladder inflated and the air storage vessel at ambient pressure. To dive, the blow valve is closed and the air pump pulls air from the ballast bladder and compresses it into the air storage vessel. Once the bladder is deflated, the boat submerges and the pump is shut off. To surface, the valve to the air storage vessel is opened and the bladder automatically inflates creating positive buoyancy.
The RCABS-R has several inherent advantages. If there is a loss of power, or a low battery condition, the boat will not dive. In addition, in both versions the pump will allow a very slow leak over time. As a result, air will naturally return to the ballast bladder and the boat will surface. The system serves as its own failsafe.
Contributors: George Protchenko, BD Welch, Bob Martin.
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