“Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.” --Archimedes of Syracuse
It’s the bane of all divers. We want to go down, but the inherent buoyancy in our wetsuits, our BCs, our lungs and our fat cells are all conspiring to keep that from happening.
To overcome the force of buoyancy you have to counterbalance it with ballast weight. The question is, how much? While the answer is different for every diver, the goal is the same: carry enough weight to enable you to function efficiently and safely at all depths, and not an ounce more.
Divers are generally taught to define this as being neutrally buoyant at 15 feet deep while wearing an empty BC and carrying a nearly empty tank. But how do you get there? There’s the basic ballpark method--carry 10 percent of your body weight in lead. Or there’s the surface float method—in full scuba gear, load enough weight to enable you to float with the water at eye level (some would say at the hairline).
But rather than just blindly piling on the lead, why not break it down to find out why you need to carry the weight you do, and what specifically you are counterbalancing. By deconstructing your buoyancy status, you know exactly where your counterweight needs are greatest, and that might reveal ways to reduce the amount of weight you ultimately have to carry. Here’s how:
STEP 1: Calculate for Your Body
How much weight do you need to make your body neutral? Take a few weights into the water wearing just a swimsuit. You will be perfectly weighted when you can hang motionless with half a breath, and sink when you exhale. (Using a snorkel can make this test easier.)
Tip for Shaving Ballast Weight: Lose weight. Also, work to turn your fat to muscle. Fat mass is a lot more buoyant than muscle mass, so any fat you can convert to muscle will lower your buoyancy deficit.
STEP 2: Calculate for Your Exposure Suit
Wearing your exposure suit, get into the water and repeat the procedure outlined in Step 1. Then take the total amount of weight required to get neutral, subtract Step 1’s total, and you’ll have the net buoyancy budget for your exposure suit.
Tip for Shaving Ballast Weight: If water conditions permit, cut down on the thickness of your wetsuit. A wetsuit can have two to three pounds of buoyancy for every millimeter of thickness. If you wear a neoprene drysuit, consider that compressed or crushed neoprene suits have much less buoyancy than standard neoprene. If you wear a fabric drysuit, remember that thinner undergarments have much less buoyancy than the puffy stuff.
STEP 3: Calculate for Your BC
BCs can be a huge source of inherent buoyancy, especially the older, full-featured models that have lots of traditional-style padding. It used to be common for BCs to carry upwards of four pounds-plus of inherent buoyancy, which means, of course, that you need four pounds-plus of extra lead on your weight belt to compensate for it. Fortunately, most modern BCs carry much less inherent buoyancy.
To test your BC’s inherent buoyancy, submerge it while venting all exhaust valves to bleed air from the bladder. Knead the padding in the shoulders and backpad and behind the pockets to release air bubbles. Slowly rotate the BC to enable any trapped air to escape. Be patient, allow plenty of time for water to displace the air in the material. When you stop seeing bubbles, release the BC into the water column. If it heads to the surface you’ve got some inherent buoyancy to deal with. Add weights until the BC will hang neutrally buoyant in the water. Then count up how many weights it took to get there and you’ll have your number.
Tip for Shaving Ballast Weight: Buy a modern BC. Models that have come onto the market within the last three or four years carry, on average, from one to 2.5 pounds of inherent buoyancy, and some carry none at all. Note: while most manufacturers don’t provide the inherent buoyancy of their BCs, you can always find that info in ScubaLab BC reviews.
STEP 4: Calculate for Your Tank
The buoyancy characteristics of tanks vary widely. For example, a standard aluminum 80 is 1.6 pounds negatively buoyant when topped off, and 2.8 pounds positively buoyant at 500 psi. That’s close to a four and a half pound buoyancy differential between the beginning of a dive and the end of a dive that, of course, needs to be dealt with by adding ballast weight.
A steel tank, on the other hand, tends to start off negatively buoyant and stay that way. For example, a high-pressure 80 is about nine pounds negative when full and three pounds negative when empty. That’s three pounds that can be removed from your weight system.
Tip for Shaving Ballast Weight: Switch from an aluminum cylinder to a steel cylinder. A properly-weighted diver who goes from an aluminum 80 to, say, a HP steel 80 could theoretically take six pounds off his weightbelt.
STEP 5: Calculate for Everything Else
Gather your reg, gauges, knife, fins and any other items you regularly dive with, place them in a neutrally buoyancy mesh bag, and submerge it. The goal here is primarily to see if the total package is positively buoyant. If it is, add some weight until it becomes neutral. If it’s negative it probably won’t be by much, so consider it a ballast slush fund. It’s not working against you, and that’s all that matters.
STEP 6: Put it All Together
Add it all up. This should be very close to your target ballast weight requirements, and it should also give you a clear picture of where your biggest buoyancy challenges lie. To double-check your calculations, gear up with all the components you measured separately, get back into the water and repeat Step 1. If the above scenario played out like it’s supposed to, you should be floating at eye or forehead level in a relaxed position. When you exhale you should start to slowly sink. If not, you couldn’t be more than a pound or so off your target. Make the final adjustment and go diving.
SALT WATER VS. FRESH?
If most of your diving is done in fresh water springs or lakes, then ballast calculations should be done in fresh water. If you dive mostly in the ocean, then do the calculations in salt water. If you switch back and forth, you’ll need to adjust your ballast needs as you go. Be prepared to add anywhere from 4 to 7 pounds going from fresh to salt water.