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Buoyancy Basics Part 4: Scuba diving and correct weight

2016-11-25 12:24:51

Buoyancy Basics Part 4: Scuba diving and correct weight
Be perfectly weighted in scuba

In the first and second part of the blog you read about why keeping neutral buoyancy while scuba diving underwater is so important, and how to do a quick buoyancy check underwater. In the third part you also learned how to adjust your buoyancy and body position simultaneously, and some troubleshooting tips.
Part 1: 6 reasons why buoyancy is important to scuba divers
Part 2: Buoyancy defined - positive or negative?
Part 3: How do I correct my buoyancy while scuba diving?

Now let's discuss another very important and mis-used factor: the amount of lead weight you put on your weight belt, or carry in your BCD's integrated weight system.

As you know from your PADI Open Water Diver Course and also from experience, your body and your diving equipment tend to float. Without a certain amount of additional weight you just cannot sink, no matter how much you deflate your BCD and exhale. You need weights to balance out the positive buoyancy of your equipment –especially of your wetsuit- and allow you to sink and stay underwater. The key words here are “balance out” and “allow”.
You only need as much weight as will allow you to stay underwater from the beginning to the end of the dive: you do not need the weight to make you sink, or make you negatively buoyant! This is a very common misconception, and is the reason why many divers dive with much more weight than they actually need.

“But wait! –you might say- Why is it bad to have more weight than the minimum necessary? After all, if I am too negatively buoyant I can simply inflate my BCD to compensate, can’t I?”
Yes you can, but it is not a good idea for two reasons:

1) It will make it very difficult to keep a horizontal body position. Your weights are usually positioned somewhere around your waist and you BCD is around your torso. So if you have to put a lot of air in your BCD to get neutrally buoyant, the weights will pull your hips down, and the air in your BCD will pull your shoulders up - resulting in a diagonal, head-up-feet-down position. And you already know that this can cause a lot of problems with your buoyancy, as well as unnecessary fatigue due to the increased water resistance while trying to move underwater.

2) It will make it very difficult to maintain neutral buoyancy as you go deeper or shallower.
You know that air expands as you ascend, and compresses as you descend; which is why you have to make adjustments to the amount of air in your BCD as you change your depth. But of course the bigger the initial volume of air, the bigger the adjustment you have to make. If you have a huge volume of air in your BCD to begin with, as you ascend a few meters / feet, the relative increase in volume will be much bigger, so it will have a much more noticeable effect on your buoyancy, forcing you to make bigger or more frequent adjustments.
Taking this a step further, this can even lead to an uncontrolled ascent, because if you fail to make the necessary adjustment fast enough, the extra air in your BCD will make you more positively buoyant and pull you up relatively faster.
On the contrary, if you have only a little bit of air in your BCD, the same change in depth will result in a relatively smaller change in the total volume of air in your BCD, so you might not even have to inflate or deflate.

So in essence, it is best to dive with as little air as possible in your BCD; and to achieve that you shouldn't carry too much weight on your dives. In fact, it would be best not to have any air in your BCD at all, but there are several reasons why this is usually not possible, even with correct weighting for diving. For example, as you descend, your wetsuit compresses and becomes less buoyant. This means you add air to your BCD to compensate.
Also, especially if you are diving with an aluminium scuba tank, you will become more buoyant towards the end of the dive due to the lesser weight of an empty scuba tank, versus the full one you started with.

Ultimately the question is, how do you know what is the correct amount of weight to take with you on a dive?

If you are unsure of the answer, a first step can be to ask your dive guide or instructor for guidance. At SUB AQUA DiveCenter, you will find our staff teams happy to help and offer guidance. Making an initial suggestion based on your equipment setup, wetsuit size and diving experience level. But even then, you should do a buoyancy check on your first dive in a new environment.

A recommended way to do this is at the end of your first dive, during your safety stop at 5 meters, when your tank is almost empty.

This is the moment in the dive when you will be most buoyant, or “lightest”, since your tank is lighter than at the beginning of the dive, and your wetsuit is not so compressed because you are in shallow water. So at this moment you should not have any or almost any air in your BCD to stay neutrally buoyant. If you do, it means that you can probably dive with less weight. Remember, your goal is to have as little air in your BCD during the whole dive as possible, so if you have to inflate your BCD to remain in neutral buoyancy during your safety stop, it means that during the rest of the dive you have to add even more.

So, to summarise:

1) Get to your safety stop depth, controlling your buoyancy so as not to ascend too fast.

2) Deflate your BCD completely and stop moving. (Kicking is cheating!)

3) See if you remain neutrally buoyant or if you tend to float up /sink down. If you tend to float up even with an empty BCD, you should probably take an extra weight on the next dive. If you sink, then maybe try one or two less weights.

Have you ever had a situation where too much or too little weight has caused you trouble with buoyancy during your dive?

For more information on PADI Peak Performance Buoyancy courses to help perfect your buoyancy and scuba skills, contact SUB AQUA DiveCenter.