The underwater world is fascinating to human visitors. There are
pictures to take,
fish to spear,
wrecks to explore. You might even find some
pearls,
gold or
plutonium. But more likely, you will find an experience worth remembering.
One of the critical skills any scuba diver must first master is buoyancy control. Remember the last helium balloon you saw as part of a gift basket. At the bottom of the string, there was probably a small decorative weight holding the balloon down. If you lifted the balloon and then released it, this weight would pull the balloon back to the ground. Since the weight exerts a greater force than the balloon’s lift, the total force is negative. If the weight wasn’t there, the balloon would rise and escape since the total force is positive.
Now imagine a weight just heavy enough to balance the balloon’s lifting force. This balanced weight is selected so that it’s downward force exactly matches the balloon’s upwards force. The total force is zero, or neutral buoyancy. If you let go of this balloon and weight, it would hover midair until something moved it. If you are patient, you can actually demonstrate this idea by tying pages of paper to a balloon’s string, and then tearing off small pieces until there is just enough paper to balance the balloon’s lifting force. The balloon won’t stay put for long, since even weak air currents will move it from rest.
Achieving neutral buoyancy is essential for a successful dive. Otherwise, a diver will expend their energy and attention with fin motion, or possibly even make an uncontrolled ascent. Flapping your feet frantically to stay in place eats up air, scares away fish, and kicks up sediment. Ray-finned fish are able to stay put at rest with an organ called a gas bladder, also known as a fish maw, swim bladder or air bladder. Through chemical reactions, they can add or remove gas from an enclosed bladder to adjust their total lift force.
Following the example of fish, a standard scuba set has a buoyancy control device (BCD) that allows a diver to adjust their buoyancy. In the balloon example, weights were added or subtracted to balance a system’s total lifting force. For both a fish swim bladder and a diver’s BCD, weights remain constant while air is added or released from a flexible air bag. Only during emergencies will divers release their lead weight belts, which are worn to counteract the combined lifting forces of an exposure suit and scuba tank volume.
Through practice during training, a diver will learn to achieve neutral buoyancy at a selected depth. However, divers need to change their depth during dives. For example, the bottom of a lake, river or lagoon is rarely at a constant depth over the distance a diver travels. When exploring a shipwreck or reef, a diver needs to be able to explore locations at different depths. If a diver ascends without adding air, they will still be balanced for their previous depth and will tend to sink. If a diver descends without releasing air, they will tend to rise. A successful dive demands careful attention to buoyancy, to make sure that a diver is always balanced for their present depth. This means that a diver will need to periodically add or remove air from their BCD when changing locations.
Depth changes are only one factor that changes buoyancy. Air inhaled or exhaled during a diver’s periodic respiration has the same effect of adding or removing air from a BCD bag. This adds a periodic, wavelike fluctuation to the total buoyancy, which a diver also needs to acknowledge. Divers insulate themselves by wearing an exposure suit to such as a wetsuit in mild waters or a drysuit for more frigid dives. Both of these will compress when exposed to increased pressure at depth. This decreased volume also decreases the diver’s total buoyant force, since they displace less water when their exposure suit compresses.
Wouldn’t it be great if someone invented an automatic system to control buoyancy so that divers could concentrate on enjoying their surroundings? It’s been done. And redone.
What are some qualities of a system with constant buoyancy at all depths? Starting out, the upwards forces need to balance the downwards forces. A system that displaces the same volume of water at all depths is the first step. However, both a diver’s lungs and exposure suit compress with depth. An early system (Fast, US patent 3,820,348, 1974) used a pair of air bladders connected to a feedback valve system that maintained their constant volume. The feedback valve added air to the bladders when they contracted, and released air when they expanded. To calibrate the system, the diver would manually achieve neutral buoyancy and then set a tensioning knob.
Similarly to the Fast system, a later mechanical system is based on the compression of a wetsuit wafer (Harrah US 4,324,507, 1982). A circular disc of wetsuit material is placed between two rigid plates. When the material compresses or expands from depth and pressure changes, it opens air inlet or outlet valves connected to a gas bladder.
Both of these systems are based on a two-way valve, operated similarly to the way a diver’s respirator works. When a diver inhales, they decrease the pressure in a small valve chamber that opens a valve to release air. When they exhale, a release valve opens to let gas escape.
Over the course of a dive, a diver will descend, spend some time at a selected depth, and finally ascend. For safety and convenience, a rope is usually provided to guide the diver in both directions. Ascending slowly is critical to prevent decompression sickness, also known as the bends. Without a rope, a diver has only their depth gauge to guide their downwards or upwards motion. A computerized system (Egan, US Patent 5,496,136, 1996) automates a diver’s descent and ascent.
Instead of a two-way pressure sensitive valve, Egan uses a computerized metering system to track the air volume in a gas bladder over a dive. An external pressure sensor monitors the surrounding depth. To compensate for suit compression, Egan requires that a diver enter the size, thickness and type of exposure suit along with a freshwater or saltwater selection and their weight before the dive.
For these extra preparation steps, Egan allows a diver to automatically ascend at a safe rate (about 1 foot per second) and remain at safety stops if required. These stops are published as a recreational dive table, which Egan stores electronically in the computer system. Egan’s system also ignores small depth changes (+/- 1 foot) to conserve air.
For all of their complexity, automatic BCD’s make great engineering school design projects. In 2001, a MIT student built a computerized system that uses a feedback controller to maintain neutral buoyancy as part of his thesis. However, this system was limited to maintaining neutral buoyancy and did not provide the ability for a diver to automate their descent or ascent. In 2008, a group of engineering students from the University of Auckland built a computerized system similar to the Egan system called the “Electronic Dive Buddy.”
Even in a field full of expensive esoteric gadgetry, automatic BCD’s have not had much commercial success. Only a few systems have been commercially marketed to recreational divers. A German company, Gesellschaft für Tauchtechnik (GfT), sold a computerized system called “Aquapilot” (see Dive Magazine article, 2003). A dive table, rope and manually operated BCD remains the standard for most divers.
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Automatic BCD systems
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System
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Sensor and control
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Required calibration parameters and
adjustment
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Harrah (US Patent 4,324,507), 1982
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Mechanical
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Wetsuit elasticity set by control knob
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Fast (US patent 3,820,348), 1974
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Mechanical
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Control knob
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Egan (US Patent 5,496,136), 1996
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Electronic
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Freshwater / saltwater selection, dive table
selection, diver’s weight, exposure suit size, thickness and
type
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Dyer MIT thesis, 2001
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Electronic
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Computer interface
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Electronic Dive Buddy, 2008
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Electronic
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Computer interface
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Aquapilot
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Electronic
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Computer interface
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References