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The In’s and Out’s of Air in the BCD
Last month we discussed the general concepts of what your Buoyancy Compensation Device – BCD – does and some of its history and evolution. This month, we’ll learn ways to get air into and out of the BCD in order to adjust your buoyancy.

Positive Buoyancy
When we're on the surface, we generally want to be positively buoyant so that we float. By putting air into the BCD, we do that without having to work very hard finning. In the early days, the only way to get air into the BCD was to manually blow it in, like blowing up a raft. At the end of the inflator hose - usually a corrugated rubber hose on the left shoulder - there's a set of buttons. On most BCD’s the button on the very end of the attachment is used to manually open the valve. To manually inflate the BCD, push the button in, place the mouthpiece in your mouth, and blow. After each breath, don’t forget to release the button so the air doesn't escape.

While that method works fine (and is the only method available if you have no air in your scuba cylinder!) there's an easier way. Most BCD’s allow a low pressure hose from your regulator first stage to be attached directly to the buttons at the end of your inflation hose. Typically pushing the button on the side of the control attachment, (blue button marked P, above) allows air to go directly from your cylinder into your BCD. Varying the force applied and the length of your push will control the amount and rate of air going into the BCD.

Negative Buoyancy

When we're ready to descend from the surface, we want to be negatively buoyant, so we can sink to our desired depth. To do this, we need to let the air out of our BCD. How? By releasing air from the BCD through an exhaust port. A very simple BCD may only have one exhaust port, while a higher quality BCD could have as many as five. While one exhaust port will do the job, having more makes diving more convenient, as we'll see in a moment.

When using any exhaust port, it's important to remember one basic rule of physics: air goes up. In other words, no matter which exhaust port you use, it must be the highest point on the BCD at that time in order to allow air to escape. For instance, most everyone learned to vent air from the BCD by holding the inflator hose as high as possible over your head and pushing the button on the end of the controls (the same one you pushed to manually inflate the BCD). This allowed air to exit the BCD, you became negatively buoyant and began to descend. This usually works great if you're in an upright position on the surface and the left shoulder is close to the highest point on the BCD.

But what happens if we're swimming horizontally underwater and want to vent air? With only one exhaust port on the left shoulder, you'd have to stop and roll upright to release the air. Otherwise, no air exits no matter how long or hard you press the button. To make it more convenient to vent air from the BCD during the dive, more exhaust ports can be added in various places, usually on some or all of the other three “corners” of the BCD – the right shoulder, the lower right hip, and the lower left hip. Having exhaust ports on all four of these “corners" allows the diver to vent air from the BCD from just about any position, even upside down. Easy! These additional exhaust ports are usually opened by pulling a short cord attached to the individual ports.

But wait! That’s only four ports and I said there could be as many as five. In most BCD’s (but not all) there's a cable that runs inside the corrugated inflator hose from the plastic activator buttons directly to the exhaust port valve on the left shoulder. Instead of raising the hose above your head, simply grasp the hard plastic piece and pull it down to open the exhaust valve. This is usually easier and more convenient than raising the hose over you head, and, in fact, will vent air faster when you’re ready to descend.

Neutral Buoyancy

Most of the time while swimming under water, we want to have neutral buoyancy – not sinking or rising. The best way to do this is to use a combination of inflating and deflating the BCD to reach an approximate neutral buoyancy. Then, use your lung volume and breath control to fine tune it.

Newfangled Stuff

Here's a variation that you may see - combining the BCD inflation/deflation system with the alternate air source from the regulator. This works the same way as the standard system but has an extra button or two for the regulator functions.  The convenience is having one less hose from your regulator first stage.

Look ma - no hoses! This fairly recent development eliminates the standard corrugated low pressure inflator hose entirely and replaces it with a control module mounted on the lower front left hip of the BCD. You’ll find the same buttons in the same general layout as before. Pushing the inflate button on the control panel will put air from the cylinder into the BCD. Pushing the deflate button will open all of the exhaust ports to vent air, regardless of the diver’s position in the water. An additional hose is tucked away somewhere on the BCD to manually inflate it if there's no air in the cylinder.

Coming Up

Next month, we’ll take a look at a few more features you may find on a BCD. Plus, how to clean and lovingly care for your BCD.