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Bcd Buoyancy
Uses of Dry Suits
Dry suits are often worn for boating, especially sailing, and on personal water craft in the winter months. The primary uses are for protection from spray, and in case of accidental short-term immersion in cold water if the user falls overboard. Dry suits only intended for temporary immersion protection are less rugged than diving dry suits. They are usually made of a breathable membrane material to let sweat permeate, keeping the wearer dry and comfortable all day. Membrane type surface dry suits only keep the user dry, and have little thermal insulating properties. Most users will wear a thin thermal undersuit, or street clothes, for warmth; but wearing ordinary fabrics can be dangerous if the suit leaks in cold water because they will lose all insulating properties.
radical">http://www.himfr.com/buy-radical_islam/">radical islamDry suits are used for windsurfing, kitesurfing, kayaking, water skiing and other surface water sports where the user is frequently immersed in cold water. These suits are often made from very lightweight material for high flexibility. Membrane type suits are commonly used in the spring and fall with moderate water temperatures, but Neoprene and hybrid dry suits for surface sports are preferred in cold water. These provide greater thermal protection in the event of a leak. The ability to swim for self-rescue in these types of suits is important to water sports users that do not use a boat. A neoprene bottom also is less likely to allow trapped air to collect in the legs, causing the wearer to tend to float head down in the water.
Crew members who must work on the decks of commercial ships wear a type of dry suit also known as an immersion survival work suit. Single engine aircraft ferry pilots flying between North America and Europe, and helicopter pilots that must fly over the open ocean, must wear a survival suit in the cockpit, so they can continue to fly the aircraft, then exit immediately if the aircraft is ditched in cold water after an engine failure. These suits are also used on shore when working on docks, bridges, or other areas where cold water immersion is a safety risk. They are usually a three-part system consisting of:
A warm undersuit made of synthetic fabric designed to wick moisture from sweat generated by physical exertion away from the user’s skin.
A dry suit made with a waterproof breathable membrane to let moisture permeate out of the suit.
A durable outer shell, designed to protect the dry suit, and to carry tools and survival gear. The outer shell may also be equipped with an inflatable bladder to give the wearer additional flotation and freeboard when immersed.
Immersion survival suits are dry suits carried for use by ship and aircraft crew who will be immersed in cold water if the craft must be abandoned. Unlike immersion survival work suits, these are not intended to be worn all the time, and are only to be used in an emergency. Survival suits will typically be a one-piece design made of fire-retardant neoprene, and optimized with quick donning features.
Dry suits are also worn by rescue personnel who must enter, or may accidentally enter, cold water. Features of dry suits designed for rescue may be a hybrid of the immersion survival and work suits, since the wearer is not expected to be working in the suit for an extended time. They may also be optimized for a specific task such as ice rescue, or helicopter rescue swimmer.
Drysuits for sport diving are made in both membrane and neoprene, and primarily differ from surface drysuits in that they have inflation and deflation air valves to maintain neutral buoyancy, and are slightly more durable.
Drysuits for commercial and military diving tend to be much heavier and even more durable than sport drysuits because they will endure a harsh and abrasive environment, especially if being used for heavy labor work such as underwater welding. Some commercial drysuits are rated for hazardous-environment diving, and when combined with a full-face helmet can completely isolate and protect the diver from hazardous environments such as sewage pits and chemical storage tanks.
Drysuit donning is usually more difficult than with a wetsuit and often requires the assistance of another diver or person. Drysuits pose their own unique problems compared to wetsuit diving, due to the complex construction and since a diver needs to constantly manage and adjust the air volume inside the suit. During descent, air must be added to maintain constant volume. This prevents suit squeeze, loss of neutral buoyancy, and potential uncontrolled descent. During ascent, air must be removed to prevent ballooning, loss of neutral buoyancy, and potential uncontrolled ascent. A drysuit can be equipped with an automatic spring-loaded exhaust valve, which can assist with this problem.
Latex seals are easily pierced by sharp objects. Gripping the seal with long fingernails to pull it on or off can cut through the material, while long toenails can damage thin rubber booties when the foot is pushed inside tight-fitting fins.
Latex seals are somewhat elastic, but can be easily torn if overstretched. Powdered talc can help the seals slide on easier.
Neoprene seals are often used. They are less waterthight than latex, but can be repaired easily by the user.
Waterproof zippers need the two rows of open teeth to be reasonably lined up in front of the pull, for the zipper to slide without excessive effort. (Because of their construction waterproof zippers require two or three times as much pull as regular zippers to close.) It is best to hold the opening together as the zipper is pulled shut to prevent misalignment that can permanently damage the sealing edge. For this reason zippers across the back of the shoulders or down the back of the suit are almost impossible for one person to close properly by themselves, and yanking harder to try and force the unreachable zipper closed often just results in misalignment and permanent zipper damage.
Damage to the lower part of the suit can cause a sudden inrush of very cold water for winter users, or an inrush of hazardous chemicals for commercial inspection divers.
Damage to the upper part of the suit can cause a sudden venting of the air, resulting in a total loss of thermal insulation in membrane suits and sudden uncontrolled descent, followed by water/chemicals seeping in.
Since the drysuit can contain air, some divers control their buoyancy with the drysuit and dive without the usual BCD / buoyancy control vest that is commonly worn by wetsuit divers. Although it is possible to dive like this, the risks are higher than when using a buoyancy compensator, Drysuits generally are more easily damaged and prone to failure. Buoyancy compensators generally are more robust and reliable.
If there is more air in the drysuit than is needed to counteract “squeeze” on the undersuit, that excess air creates a "bubble" which moves to the highest point of the suit, which in an upright wearer is the shoulders.
Drysuit wearers wearing loose baggy suits need to keep their legs at level or below their waist. When inverted, with the legs above the waist, the bubble quickly moves top the highest point, the legs.
If the suit is being used correctly, the bubble is small and its movement is not important. The bubble may be large if a diver has ascended without venting the suit or the diver is over-weighted and extra air has been put in the suit to make the diver neutrally buoyant. The movement of a large bubble can be a problem; it balloons the legs and it may inflate the thin rubber booties causing the fins to pop off, losing them in the water. Also, as the drysuit vents are most often situated at the top half of the diver, it is impossible to vent the suit while inverted. If the diver is positively buoyant, there is an increased risk of a fast ascent to the surface.
The size of the bubble can be minimised by being correctly weighted and venting the suit on ascent. Some divers ensure that the bubble remains at the top of their body by using the buoyancy compensator to counteract any excess weighting and keeping the minimum air, to avoid squeeze, in the suit.
For an inexperienced diver, ballooning of the legs can cause a loss of control that may to lead to panic and an inability to flip upright again. The recommended solution is for the wearer to bend at the knees, reach up and grab the legs, do a somersault to flip upright again and vent the suit if needed by opening the neck seal.
Surface drysuit users can face a similar inversion problem. The problem is more acute when not wearing a personal flotation device (life vest) over the drysuit. For surface drysuit users, the inversion situation can be much more critical if no one is nearby to assist, since the wearer may be held upside down and unable to breathe, and may also have water run down into their nose while inverted.
It is not a problem for close-fitting neoprene suits, or hybrid suits with neoprene bottoms, which prevent air from easily moving into the legs of the suit. Wearers of baggy surface drysuits can mitigate the problem by venting out as much excess air as possible before entering the water. This is typically done by crouching down and leaning forward, wrapping the arms around the knees, and then having an assistant zip the suit shut while it is stretched out tightly. Excess air can also be "burped" out of the neck seal. Some baggy suits have elastic "gaiters" that can be pulled snug around the legs to help prevent this inversion event from happening.
About the Author
Himfr is a scholar, focusing his research on Chinese cultures. If you are interested in purchasing China goods, please visit www.himfr.com
How does a SCUBA BCD work?
OK, so you shoot some air into the BCD, and it makes the diver more buoyant, right? But aren't we talking about the same amount of air...just moved from the cylinder to the BCD? Does buoyancy change that dramatically when the air is no longer compressed?
Archimede's Principle says that when a body is immersed in a fluid, it's subject to a force up-warding. The intensity of this force is given by the volume of the body and an equivalent mass of water.
Let's make an example.
For simplicity I am using the metric system.
I'll measure the volume with liters (a liter is the volume of a cube with an edge of 10cm=4inches), and weight with kg (a kg is the weight of 1 liter of water).
Everything has a volume and a weight. If the ratio of volume and weight in liters and kg is 1, then we have something that weights like water. If it is less than one, it means that it's heavier than water, if greater than 1 it is lighter.
A fully equipped scuba diver (suit,weight, tank, bcd,regulators) can weigh about 100 kg (more or less 200 pounds). That means that there is a force that pulls him downwards with a 100kg strenght
If it's volume is 100 lt (more or less 25 gallons), according to Archimedes, there is a force of 100lt of water that is pushing him up. What is the force of 100lt of water: exactly 100kg.
Given this, inflating the bcd will raise the volume of the diver, that is the pushing up force is larger than the pulling down: the diver go to the surface. Viceversa, deflating the bcd will decrease the volume so the pulling down force is stronger: the diver goes deeper.
Now... how come the volume increase when you inflate the bcd: you get the air from the tank and move it to the bcd, so should be the same. The difference is that air is a gas, and gases can be compressed: you can see it using a empty (i.e. full of air) plastic bottle. Even if it is well sealed, you can squeeze it. If you try the same with the bottle full of water, you will see that you can't squeeze it. That is because only gases can be compressed, while liquidsa and solids not.
But what it means to compress a gas: it means that you can reduce its volume, without loosing any gas: you can decrease the volume of a bottle of milk, by drinking it... with air or any other gas, you can decrease the volume, without breathing the gas.
When you compress a gas, you raise its pressure: the higher the pressure, the lower the volume. So what happens: in the tank we have highly compressed air: at least 50 to 200 bars. In the bcd we have low compressed air, usually 10 bars more than the ambient pressure (at 50mt depth, it means that the air pressure is 15 bars).
So the air in the bcd, being at lower pressure than in the tank, is larger (in volume) and you get morepush upp force...
A final consideration: remember that the tank is in metal, not fabric. When you take air out of it it doesn't change its shape (and consequently its volume) , while the bcd, being soft, will change its shape and its volume.
Luca
Bare Black Jack BCD