Browsing Tag backplates
Differences between a jacket and a backplate and wing system
Looking at BCD systems and their merits will require an understanding of what they are there to do while you are diving. Obviously, a BCD is there to help control your buoyancy, but in terms of function, how does this actually work?
All BCDs have a way to inflate and deflate the air bladder. More gas in the bladder equals more positive buoyancy, and less gas equals less positive buoyancy. Simple!
However, how these devices are built and designed play a huge role in how easy it is to manage the gas within the BCD, and thus manage your buoyancy as a result.
Let’s look at jacket BCDs, which are the most common type of BCD found in almost every dive centre the world over. This is shaped like a vest or jacket, with arm holes and an air bladder that goes from the front to the back, with additional pockets and clips and adjustable straps for convenience.
The backplate and wing system, is a completely different looking piece of equipment, with a 5 point harness and crotch strap, attached to a rigid metal or plastic back plate, which in turn is bolted onto a round donut shaped air bladder.
Functionally, both types of BCD will allow inflation of gas into the air bladder through the inflator mechanism, and both will allow deflation of the gas from either the deflate button, or one or more dump valves.
However, the relative positions of the dump valves, and how the gas moves in the air bladder plays a critical role in how the BCD performs while diving. Ultimately, we need to know where the gas is located in the bladder while underwater, and that is how we can use these devices effectively.
Now if we can agree that a good diving position in the water would be a flat, horizontal trim to aid in streamlining, then where would the gas be in the BCD in that position? In a jacket BCD, the gas would be at the top, near where the tank strap is. Depending on the exact trim of the diver, this gas can be near the shoulders, or near the bottom of the tank. There could also be some gas trapped in the front pocket areas as well.
Most new divers learn to dump gas from their BCDs by extending the inflator hose upwards and pressing the deflate button. The diver would need to come into a head up position, to allow the gas to flow to the deflate button to be released.
This additional movement is not ideal, firstly because in that position, the diver would then need to come back to a horizontal position again, wasting energy and effort to move into and out of position.
Secondly, if the diver was in a runaway ascent, the best way to mitigate that would be to position head down and kick downwards, which would be really hard to do if the diver is angled upwards with their legs beneath them. Any additional movement in the legs would only exacerbate the problem by bringing them closer to the surface and causing the gas in the bladder to expand more.
Let’s contrast this with a backplate and wing system. While diving, the donut shaped wing will fold upwards, and wrap the tank on both sides. The gas in the bladder in this position then can only be in one position, at the highest point of the wing on both sides of the tank.
In this position, getting the gas out is relatively easy, as the dump valve on the wing is located near the bottom of the wing and close to the top edge. In most cases, the diver would not need to adjust his trim by a lot in order to remove the gas from the wing. He would just need to pull on the dump valve string upwards, trapping the gas between the dump valve and the tank. In this case, there isn’t anywhere for the gas to travel to, and will be easily dumped from the BCD.
BCDs should also minimize the amount of trapped gas that cannot be removed from the bladder, in terms of corners which can create pockets of gas that are not easily moved to the dump valves. The dump valve position should also allow for the maximum amount of gas to be removed from the bladder, so that every bit of gas is removed and won’t cause any additional positive buoyancy due to gas expansion on ascent.
If those are the key traits that make for an ideal BCD, then I feel a wing design makes the most sense. It’s round and doesn’t have any corners to trap gas, has a smaller internal surface area, the dump valve is located close to the edge of the wing, and it’s design prevents catastrophic failure in the event of a dump valve breakage.
The backplate system also has other benefits in terms of fit and streamlining, which ultimately leads to better control in the water.
BCDs these days come in all types and flavours, but choose wisely! It needs to be effective in the water, and not cause any additional complications.
