In Reply to: Dry suit diving posted by Frank Farmer on September 15, 2006 at 22:47:38:

I really liked the Zodiac analogy about trim.

Here is a repost from a discussion earlier this year which lays out some additional points on the topic:

In Reply to: Infinite argument - drysuit or BCD for buoyancy... posted by jlyle on February 20, 2006 at 14:59:29:

Sorry, but can't agree on the notion that this is the same as over or under toilet paper. That analogy implies one is the same as the next with only personal preference as the distinction. I'd like share some thoughts about why using the drysuit for buoyancy is flawed and why a better analogy might be driving on bald tires.

Sure, there are circumstances where use of the drysuit as a buoyancy device is less of a problem e.g. when the full versus empty gas buoyancy differential is low. In that circumstance the amount of excess gas you need to maintain neutrality with a full tank is relatively small. But the practice in itself is flawed and those flaws become truly dangerous as the gas buoyancy differential between full and empty gets bigger-when you carry bigger tanks.

First a couple of concepts/definitions: 1) Your weighting should be such that you can hover at 15 feet with an empty cylinder. This allows you to finish a shallow stop even if you're buddy breathing without having to scoop up rocks, hold on to kelp or otherwise depend on solutions which might not be available. 2) Your drysuit buoyancy should not change with depth. That is, you're not using a neoprene drysuit. 3) The amount by which gas in the suit exceeds the minimum necessary to allow you to swim comfortably and not get cold when swimming I'll call "excess gas." With no excess gas in the suit you should be slightly to very negative-depending on volume and composition of breathing gas-and otherwise have a "shrink wrapped" effect with the suit.

OK, now why is using the drysuit as a buoyancy compensator a bad idea?

1) The more excess gas in the suit the more difficult it is to maintain proper trim. With lots of gas "sloshing for and aft" the tendency to get head down or feet down is constant. Maintaining proper trim is a continuous excercise in countering this tendency. This uses energy and is distracting.

2) The likelihood of getting head down and out of control rises with the amount of excess gas. This can be extremely dangerous in precipitating an out of control ascent.

3) With excess gas in the suit the likelihood of popping off a fin rises. Losing a fin in this situation makes recovery of control close to impossible.

4) Getting feet down is also no good.

5) The more excess gas in the suit the more micro adjusting you need to do as the suit naturally tries to vent this excess gas. You're constantly adding and venting gas in the suit. This wastes gas and forces your body to expend energy, in the form of heat, to warm the newly introduced gas. It becomes a cycle. Solving this problem by closing the valve creates problems in the event you have a stuck valve and a runnaway.

6) The more you're injecting gas into the suit the greater the liklelihood of a stuck valve and a runnaway. If you do have a runnaway in the suit you're more likely to get out of trim faster because you've already got a lot of gas sloshing around the suit. Once again the potential for a cascade of problems leading to out of control ascent.

7) The more excess gas in the suit the less streamlined you are. The less streamlined the less efficient you are in swimming and the more gas you use.

How does using the BC as the buoyancy device change this?

1) A good BC, like a wing, is designed to float you in a properly trimmed position. A drysuit is not.

2) The wing is designed to be streamlined even when full. A drysuit is not.

3) A properly designed BC can be vented even if you're in a head down position. A dry suit cannot. Ankle dumps are idiotic.

4) The issue of task loading is overblown. It's the only reason to not use the BC but is flawed logic on its face. The idea that someone whould be diving, much less in a drysuit, if they can't handle a simple task like managing a BC and a drysuit raises the question of diver training and whether drysuit manufacturers should be selling suits to incompetents in the first place. How much "task loading" is involved? If you keep the drysuit valve open, excess gas in the suit resulting from ascent takes care of itself. If you close the valve on ascent to allow for some excess gas during stops, you simply dump gas from the BC until it's empty and then bleed gas from the drysuit on the shallow stops. Since you're not putting any new gas into the suit at this point, a stuck valve is a non issue. The ascent is slow and controlled by its nature and you're typically not swimming so the tendency to tip one way or the other is reduced. Obviously you don't allow so much excess gas into the suit as to compromise trim, anyway. If you get cold a lot use thicker underwear or switch to argon.

Like so many of these types of discussions, the degree of the problem increases as the amount of gas you're carrying goes up. What is a minor issue with a steel 72 is a major issue with double steel 104's. Like driving on bald tires, the flaw evolves from insidious to overtly hazardous depending on the circumstances. Nevertheless, driving on bald tires is a bad habit no matter what.

I'm not looking to start an argument or tell anyone how to dive. It just confounds me that DUI would promote using the drysuit for buoyancy when there are so many obvious reasons why this can create problems.