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HomeScuba DivingI Had Perfect Buoyancy What Happened

I Had Perfect Buoyancy What Happened

I like to think that I have great buoyancy control, very nice trim and am properly weighted. It was not easy to get to that standard, however, it was well worth the efforts.

Recently, I came to the decision that it was time to retire my BCD. It was still in very good shape after 20 years. It looked better than most six-month-old rental equipment and was still as safe as the day I bought it. The BCD fit well and was comfortable. That has also become its shortcoming. Things have changed in the last 20 years. For me, one of the changes was more frequent dive travel and that mostly included air travel. Another is that it becomes more expensive flying with scuba gear. My BCD has a nice amount of padding and that took up space and added weight.

I bought a new BCD to replace my first. It does not fit in the “travel” BCD category, but it is considered travel-friendly. It is a back-mounted BCD when folded it takes up about half the space as my old one and saves me almost 6 pounds (2.72 kg) of weight for my flights.

How will it impact my buoyancy?

Test Dive

Changing equipment can change the amount of additional weight you need. I started to consider how would the new BCD impact the amount of weight I would now carry. I carry 20 pounds (ca. 9 kilograms), and before you all comment it is a lot it is within the range of basic calculations for saltwater, 80 al tank and 3 mm shorty at my weight. My first thought was if I lessen 6 pounds (2.72 kilograms) of weight from my BCD, would I need to add that amount to my added weight.

I arranged a dive to check this out and to get used to the new gear. The dive site started on a reef at 10 meters and had a wall that dropped to 25 meters. The dive plan was to suit up with my normal weights, make some surface adjustments and then descend to the reef. On the reef, I would “play” with my BCD, getting used to the fit of the BCD, differences in the air release and adding air. Then it was down the wall to use some air. Back to the reef to play with the weights.

What I found out was that my thinking about compensating for the new BCD, I forgot a few items to consider. It was not the change in weight I needed to consider but the change in the inherent buoyancy. My new BCD weights about 8 pounds (3.63 kilograms) on the surface but near neutral buoyancy in water. My old BCD weights just over 14 pounds (6.35 kilogram) but because of all the cushioning and large jacket it was positive buoyant by about 6 pounds (2.72 kilograms). To bring my old BCD to neutral I had added 6 pounds (2.72 kilogram) of weight. That could be removed.

I also found that my trim was slightly impacted as well. The tank will need to be strapped lower so a little more weight is above the fulcrum point.

My experience is an extreme example because of the age of my equipment. However, divers who rent equipment may find that renting a different BCD may cause some difference in weight requirement and impact the trim.

Changing Tank Sizes?

When I was a “air hog”, I would sometimes dive with a larger steel tank. It influenced how much weight I needed to carry. To make it simpler instead of comparing apple (AL 80) to oranges (Steel 100 liters), lets compare apples to apples. Aluminum 80 to Steel 80 both at 3000 PSI or 200 Bar. An aluminum tank will go from a negative buoyancy to a positive buoyancy as you reach 750 PSI/ 50 bar. Roughly the gas you used weighted about 4 pounds (ca. 1,814 g). While there are differences between manufacturers on average the tank is 1.5 pounds (ca. 680 g) negative at the start and when at your exit pressure about 2.5 pounds (ca. 1,134 g) positive. When you look at the status of a Faber 80 HP steel, it is 8 lbs negative when full. Substrate the 4 pounds (ca. 1,814 g) of air and it is 4 pounds (ca. 1,814 g) negative buoyant at the end of the dive. That is the weight you do not need on your weight belt on in your integrated weight pouches. Do not believe it? The next time you see the dive staff floating aluminum tanks to shore, throw in a steel.

Wet Suits

As you change the thickness of your wet suit, you change the buoyancy. Generally, you need to add a pound for each mm of change. So going from a 3 mm suit to 5 mm suit will add a two pound of weights. You might need another pound if you add a hood and gloves. Changing your wet suit might also change your trim.

Also, the longer you use your wet suit and the deeper you dive the suit will lose buoyancy. Also, it you do not properly clean and store it you will accelerate the buoyancy change. When I retired my last wet suit, I had to add 3 pounds (ca. 1,361 g).

Fresh or Salt Water

First not all salt water is created equal but unless you are talking the Dead Sea you will not like to be able to tell the difference. Your buoyancy is measured by the weight of the water you displaced. A cubic foot of salt water weights about 64.1 pounds (ca. 29 kg), while the same volume of freshwater is only 62.4 pounds (ca. 28 kg). The dissolved salts are the main reason for the additional weight of the salt water. Even without dive equipment, most people will notice how much easier it is to float in salt water. How their heads are higher out of the water.

The diver will find that the difference between the 54.1 and the 62.4 per cubic foot translates to a five pounds to eight pounds difference on the belt.

Body Fat

I am going to add this in here because if I don’t people will comment on it. Fat is more buoyant than muscle. If you increase your muscle mass you will become less buoyant. If you are overweight due to fat, then by all means try to lose weight. However, losing weight will not have a great impact on your weight belt requirement. Losing 10% of your body fat while maintaining your muscle mass may allow you to drop a pound from your weight belt.

Losing that weight will show up better in longer dives and a better fitness level.

The amount of weight you carry while you dive is not as important as having the right amount and having it in the right places

Charles Davis
Charles Davis
Charles Davis is an active diver for over 19 years who enjoys writing about his favorite activities, Scuba Diving and Travel. Also known as the Scuba Diving Nomad


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