Most divers never give much thought to what their air cylinder is made of. Even so, it can affect aspects of your dive. Besides, it never hurts to know a little about the equipment you’re diving with.

Material

Cylinders are made out of two types of material: steel and aluminum (actually an aluminum alloy). Each have different properties that make it appealing for different types of construction and use.

Steel cylinders are tough, making them resistant to damage. They also tend to support higher capacities, because of their increased strength. The downside to steel is that it can rust if not properly cared for.

Aluminum cylinders are softer than steel, so they’re not as tough, although they do just fine for general use. To compensate, the walls of aluminum cylinders are thicker than for steel. For this reason, an aluminum cylinder is larger and heavier than a similar capacity steel cylinder. They also don’t handle overfilling nearly as well.

Despite this, aluminum is the dominate choice of material for cylinders in many parts of the world. The upside is that aluminum tanks are far more tolerant of corrosion from salt water. As opposed to steel cylinders, when a layer of aluminum oxide, or “rust”, forms, it acts as a barrier to prevent further oxidation. 80 cubic feet aluminum cylinders are probably the most common type encountered in tropical dive destinations.

Identification

Unless you’re into metalworking, it can be hard to distinguish between steel and aluminum tanks. It’s even worse when the cylinders are painted for enriched air diving. Fortunately, manufacturers in North America are required to stamp certain information on tanks they produce, including the type of material used.

You can find this stamp at the top, rounded part of the cylinder. It is a sequence of letters and numbers stamped into the metal, arranged into two rows.

To determine the type of metal, look in the middle of the first row. These days, you will most likely see either “3AA” or “3AL”. “3AA” is the markings for chrome-molybdenum steel, which is practically all steel cylinders made today. “3AL” is the designation for the aluminum alloy used in cylinder manufacturing.

Buoyancy

The type of cylinder you use only has one major effect on your diving: your buoyancy. Aluminum tanks are more buoyant than steel, and thus you will require more weight when diving with them. It is worth knowing what you usually dive with in order to compensate one way or the other when diving with a different cylinder.

The usual recommendation is to add about 5 lbs / 2 kg to your base weighting you get from a weighting guide to compensate for an aluminum cylinder.

Aluminum has a particularly annoying characteristic. A full steel tank is negatively buoyant. An empty steel tank is also negatively buoyant, but less so. This is why during a proper buoyancy check it is recommended to use a near-empty cylinder, or add weight to compensate. Aluminum cylinders also become more buoyant as air is consumed, but they change from being negatively buoyant to positively buoyant. This means a full aluminum cylinder will sink while an empty will float. This makes it harder to pin down a perfect weighting for the entire length of a dive.

If you’re curious, a standard steel cylinder weights about 30 lbs, while an aluminum tank weighs about 35 lbs. If aluminum tanks are heavier, then how can they be more buoyant? For the same reason they’re not as strong as steel: aluminum is less dense than steel, and thus has a lower specific weight.

I hope you learned something about cylinders today. Next time you go diving, take a quick second to identify what type of cylinder you’re using. Use this information to adjust your weighting, instead of defaulting to being overweighted. Keeping note of the tank type (when different than what you usually use) in your log book could also be useful.

There are many tell-tale signs that your weighting is off underwater. For instance, you may balloon to the surface near the end of the dive. There are other, more subtle clues that your weights could use some fine tuning. One of these is how streamlined you are in the water.

Streamlining refers to your ability to maintain a horizontal position in the water. When you’re overweighted, you have to compensate by inflating your BCD. This alters your center of gravity to the point that you are swimming with your chest high in the water (and your legs low).

When you’re underweighted, you have to continuously kick down to keep yourself at depth, resulting in a profile position with your legs high in the water.

See the figure below for how each of these looks in the water. Also, take a moment to make the mental connection of how you might feel in each of these positions. This way, if it happens you become aware of it. Like I said, it can be subtle.
Click to enlarge

Correcting this problem has many great effects. First of all, you can more easily enjoy your dive when you are perfectly streamlined. Secondly, it’ll improve your air consumption. It’ll also reduce the likelihood that you inadvertently kick any aquatic life.

Fixing the problem may not be so easy as putting more weights on your belt (or taking them off), although you will want to try this at first. Play around with the location of your weights and see how it affects your balance underwater. You may find putting weights in trim pockets is more helpful.

For example, I use a weight-integrated BCD with around 10 lbs of weight (in tropical water). Most people might put 5 lbs in each weight pocket. However, I prefer 3 lbs in each trim pocket on my back, and 2 lbs in each weight pocket. This took experimentation and about four dives for me to fine tune, but it was well worth the minimal effort.

Also, if you find your legs are especially buoyant, you may want to consider using ankle weights to bring them horizontal. Basically, it comes down to observing your body underwater. Remain still for a moment and feel where your balance is, then think about how you can shift weight to make yourself centered. You will be amply rewarded in future dives.

Yesterday I overhead a man at a dive shop. He was complaining to his wife that he felt he was carrying too much weight on his dives. In his words, “I’m overweighted for the beginning of my dive, but if I carry less I’ll balloon to the surface near the end of my dive. I start to feel it when I’m down to about 1200 PSI.”

I say he’s not overweighted, but rather weighted just right. If you remember how to do a proper buoyancy check, you’ll recall that you do it with a near empty tank. Your weights should be able to keep you underwater with an empty cylinder.

Weights have two purposes: to get you underwater at the beginning of the dive and to keep you underwater until the end, when you decide it’s time to come up. The correct weighting is the smallest amount that satisfies these requirements—something this guy forgot.

Last week we looked at how to choose a wetsuit. Today we’ll cover guidelines for choosing weights. These numbers aren’t set in stone, but should act as a general guide to start your proper buoyancy check. This is the first step towards perfect buoyancy control.

Naturally, if you are leaner or, um, less leaner you should adjust accordingly.

Women should add 4-5 lbs (about 2 kg) if diving in salt water, or subtract 4-5 lbs (about 2 kg) if diving in fresh water. Men should add 6-7 lbs (about 3 kg) if diving in salt water, or subtract 6-7 lbs (about 3 kg) if diving in fresh water.

I’ve also heard an additional 5 lbs / 2 kg recommended if diving with an aluminum cylinder, due to how buoyant they become when near empty.

Buoyancy is determined by a lot of competing factors on a scuba diver. The net effect, however, is that you are either positively, negatively, or neutrally buoyant. For recreational divers, you are usually positively buoyant (at the surface). Weights are used to offset this positive buoyancy and make you slightly negatively buoyant, in order to sink to depth. Choosing weights may feel largely like guesswork, but there are physical principle at work as you dial in to your ideal weighting.

Archimedes’ principle

The most basic principle at work is Archimedes’ principle, which states that an object in a liquid is buoyed up by a force equal to the weight of the liquid displaced by the object. Imagine we put a ball in a pool, and the ball is 5 liters in size. The ball will displace 5 liters of water, since the water has to move when the ball is placed in it. 5 liters of water weights 5 kg, so the ball feels like it weights 5 kg less when in the water. If the ball is less than 5 kg, then it becomes positively buoyant. If it is more than 5 kg, it is negatively buoyant, because an upward force of 5 kg is not enough to counteract the weight of the ball. If the ball is exactly 5 kg, it is neutrally buoyant. Archimedes’ principle relates the amount of a liquid displaced to the buoyancy provided.

Specific gravity

We have all we need to know to do computations in fresh water, but not for sea water (or for the general case of any fluid). The missing factor is how much the liquid weights per unit volume. This is the density of the liquid. Rather than dealing with the density of the liquid directly, we can just use the ratio of the fluid’s density to the density of fresh water, which weights 1 kg / 1 liter. This number is called the specific gravity¹ of the liquid.

The specific gravity of fresh water is exactly 1, because it’s the ratio of a number (the density of fresh water) to itself. The specific gravity of salt water is 1.03. This means a liter of salt water weights about 1.03 kg. This is enough for scuba diving. If you ever find yourself diving in anything else, you may need to look up the specific gravity of that liquid.

Example 1

Quick recap: Archimedes’ principle tells us that an object (including you!) in water is buoyed by the weight of the displaced water. Specific gravity tells us for a given volume of displaced water (either sea or fresh), how much it weights. We can use these to compute the exact amounts needed to make something neutrally buoyant, or positively / negatively buoyant by any amount.

Let’s say a diver weights 80 kg with all their gear on, and the volume taken by them (and all the gear) is 90 liters. We can ask the question

Is the diver positively, neutrally, or negatively buoyant in the ocean?

To answer this question we need to know how much the displaced water weights. The specific gravity tells us that 1 liter of sea water weights 1.03 kg. The problem says that 90 liters of sea water is displaced, so 90 liters of sea water weights 90 l * 1.03 kg / l = 92.7 kg. This means the water is “pushing up” on the diver with a force equivalent to 92.7 kg. However, the diver weighs 80 kg, so this is more than enough to stop them from sinking. Thus, the diver is positively buoyant.

How much weight is required to make the diver neutrally buoyant?

We know the diver is positively buoyant, but by how much? 92.7 kg – 80kg = 12.7 kg. If we put 12.7 kg of weights inside their pockets, they would be neutrally buoyant. Anything more than that would make them negatively buoyant, and they would sink.

Example 2

Example 1 showed how we can compute exact weighting requirements, but we don’t do this in real life. In real life finding the weight and volume of the diver would be too cumbersome. Instead, the first time with a gear setup in certain water we find the correct weighting through trial and error. We then make modifications in future dives based on that amount of weight.

Here’s a common question seen with these types of problems. Imagine we lose something heavy off the boat into the ocean, such as an untied anchor. The anchor weighs 100 kg and is 50 liters in volume. A person can’t lift the anchor from the bottom, so we want to attach a lift bag filled with air to buoy it to the surface.

How big of a lift bag do we need?

To answer the question, we first need to know how negatively buoyant the anchor is (if it sank it’s clearly negatively buoyant). It displaces 50 l * 1.03 kg / l = 51.5 kg of sea water, but the anchor weighs 100 kg. This means we need at least 100 kg – 51.5 kg = 48.5 kg worth of additional buoyancy from the lift bag to make the anchor float.

The problem asks for the size of the lift bag, which is in liters, but we only know the weight needed. This means we need to displace more sea water using the lift bag. How much sea water? 48.5 kg. How much volume will displace 48.5 kg? 1.03 kg of sea water is 1 liter, so we can convert. 48.5 kg / 1.03 kg / l = 47.09 liters. This is all air, which weights so little we can practically ignore it.

The answer then is that we need a lift bag that can hold at least 47.09 liters. By the way, this is how your BCD works. When you inflate it, your volume becomes bigger without changing your weight, so you displace more water and become more positively buoyant.

Another side question we can ask is

If the anchor is in 10 m (33 ft) of sea water, how big does the lift bag really need to be?

The lift bag needs to be 47.09 liters to make the anchor positively buoyant, but there is another principle at work here: as the lift bag moves towards the surface, the volume will expand. For a depth of 10 m, the volume will double as it reaches the surface. We not only need a lift bag that holds at least 47.09 liters to begin ascending the anchor, but it also must be able to hold at least 2 * 47.09 = 94.17 liters so that it doesn’t explode before reaching the surface.

These problems may seem confusing at first, but are easy after working through a couple. Even if you never find a use for them in real life, they are required for most divemaster-level examinations.

1. Modern science prefers the more descriptive term relative density over specific gravity, but many textbooks (including diving references) still use specific gravity, so we use it here.

Skip-breathing is briefly holding your breath between inhales / exhales. Theoretically, it could cause hypercapnia, or excess carbon dioxide in your blood. Serious complications from skip-breathing seem unlikely, nevertheless it is a bad habit that should be avoided. Why would someone do it?

• Unconscious activity

  Sometimes we do things without even thinking about it. Any habit formed during basic certification could easily persist without any conscious effort. Force yourself to become conscious and it won’t take long to break the habit.

• Trying to save air

  The less you actively breath the less air you use, right? Wrong. Holding your breath, even momentarily, raises the amount of carbon dioxide in your body, which requires more oxygen to flush. The net effect is that any savings in air usage are cancelled, or become negligible.

• Trying to control buoyancy

  I’m guilty of this one, especially during fin pivot exercises. Fine tuning your buoyancy through breath control is natural, and expected. However, breath control does not equal breath holding. Rather than holding your breath, practice achieving the same effect through shallower breathing.

Slow, controlled breathing is best, while any form of breath holding, even skip-breathing, is not recommended. Try to keep your breathing non-stop throughout, and watch how quickly good habits are formed.

Previous posts have discussed the importance of proper buoyancy. Here I describe how to do a simple weight check in the water.

1. Initial weight

   If you don’t know where to begin, take about 10% of your body weight. If diving in tropical waters with a thin wetsuit, subtract 4-6 pounds; if diving in cold water with lots of exposure protection, add 4-6. This will give you a starting weight to tweak.

2. Enter the water

   Begin at the water surface with full diving equipment and an inflated BCD.

3. Hold a normal breath and deflate your BCD

   At this point you should find out if you are properly weighted. If you sink, you are overweighted, if you bob out of the water, you are underweighted. An ideal weight will keep you approximately eye-level.

4. Repeat

   Based on feedback from the previous step, adjust your weight accordingly and repeat until you float at eye-level.

5. Compensate for your cylinder

   If you are doing this check with a full cylinder, you should add about 4 pounds to compensate for the end-of-dive when the cylinder will be more buoyant. Trust me, it won’t be fun kicking to stay underwater during the safety stop.

That’s it! Once you have a proper weight, note it in your dive log to save time in similar diving environments with the same exposure protection.

We all know the importance of a buoyancy check. Even if you manage to descend, poor buoyancy affects your body’s profile in the water, decreasing the overall quality of your dive. Effective air consumption, good ascent / descent control, and an effortless dive all depend on a streamlined position–which can only be achieved with proper weighting.

Despite these facts, rarely do we check our buoyancy. Divers are not entirely to blame, most dive outfits never give us the opportunity in the water, instead rushing the group to depth as quickly as possible.

Next time you are in strange waters, or have unfamiliar equipment and need to perform a weight check, try to be first in the water, either by sitting near the back of the boat or assembling your gear quickly. Then, while waiting for others to splash, quickly check that you are properly weighted. It only takes a moment, and you will still have a chance to adjust weights if necessary.