A great video! This video by Steve Martin (as recommended by
@Cyborg Pirate) is free.
Thank you!
Since this is the Basic forum, let's look at this a little further, and see why
@Cyborg Pirate and I are actually not disagreeing!
First, his observation that many divers are significantly overweighted is absolutely correct! Overweighting has contributed to many diving mishaps, and not a few deaths. At the very least, excess weight makes for difficult buoyancy control, especially in the final 15 feet.
And what happens in the last 15 feet is the key subtlety to why a diver may or may not float with his/her head out of water with a full breath and full tanks at the beginning of a dive. What is the only difference between
@Cyborg Pirate 's comment and mine? Wetsuit thickness.
Cold water divers and warm water divers are often taught to do their buoyancy checks differently. The key difference (after agreeing to float at eye level with a mid-breath lung volume and an EMPTY bcd) is whether or not one's tanks are at 500 psi, or full. In my example (and what only seemed to be a disagreement with
@Cyborg Pirate ) I supposed a 5mm wetsuit. With that configuration, it is better to do a buoyancy check with a near-empty tank (or to do it with a full tank and then ADD the amount of air that will be breathed off before you splash). Why? Because the objective is to be neutrally buoyant at the end of a dive at 15 feet, when the wetsuit is somewhat compressed by the extra 0.5 atm of water pressure.
In the video above, we have a colder water diver, wearing a 7mm wetsuit, hood and gloves. With this equipment configuration, there is more buoyancy loss due to wetsuit compression at 15 feet. Therefore, cold water divers often do their weight check with FULL tanks as shown in the video. In the video above, using a 12L tank, floating at eye-level with an 80% breath (three fingers displayed in the video) means that you will start to sink with a mid-breath (two fingers displayed), as the video shows. That means that at the end of a dive, with 5lb of air breathed off, you will float high at the surface, which is higher than you would with the 5mm weight check I described in my post. That is appropriate, because at 15 feet, with more relative wetsuit compression than with a thinner suit, there will be more buoyancy loss.
This all translates out to exactly the same objective: being neutral at 15 feet with your tank nearing its end-of-dive pressure, with NO air in your bcd.
So
@Cyborg Pirate and I didn't differ about wanting to be neutral. I assumed with the term "recreational" a thinner wetsuit. The excellent video was in conditions requiring thicker exposure protection, and requiring a slight adjustment to PADI's neutral buoyancy check: do your check with a FULL tank.
To examine this issue in more detail, and see how 0# buoyancy at 15 feet at the end of a dive yields eye-level with a thin wetsuit, but head high in a thick one, take a peak at the numbers in this spreadsheet:
Buoyancy, Balanced Rigs, Failures and Ditching – a comprehensive tool
The buoyancy change between 15 feet and the surface can be an eye-opener, depending upon your exposure suit.
No matter how you attack the issue, we agree on one thing: DON'T BE OVERWEIGHTED. It hurts your trim, increases your air consumption due to increased drag from the gas you need to carry in your bcd, and poses a safety risk in the event of equipment failure.
Dive Safe!
