Minimum Diving Weight

[Akimbo]

Is safe to come out yet? Seriously this isn’t as scary as I thought.

First, let me state that my post is nothing profound and I have no "pride of authorship". I would like a good system, especially for new divers, to evolve even if my whole original post is trashed. I just figured that it was a good method to stimulate the conversation. I think that most experienced divers feel that overweighting is a common problem, especially among the newly trained. My take is it is harder in a class full of people for instructors to deal with under weighted students so they advise starting on the heavy side.

Weather this true or not, instructors and new divers need a fast and "low skill" method of finding the proper weight. New divers may not have developed the "feel" to accurately sense if they are at that sweet spot or not since it is all so new to them. I have read lots of threads that argue endlessly over every little detail without clearly defining what minimum is, let alone the "why" or "how". The salty dogs on the board "get it", but new kids on the block are lost by the third post.

Or set up per the OP, but put no weight on your rig.

Get a LONG 2" web belt and place weights at approx 6" lengths along the belt...

This is similar to the way I have used for years, but it is a little harder for new divers to get right, especially when most start the dive over-weighted instead of light. I was trying to choose a method that minimized getting in an out of the water more than necessary.

The advantage of using the scale is they can do the test quickly at the end of the dive when most of the entrapped air has escaped. It also can be a good drill to safely experience what really low tank pressure is like — assuming they purge gas off for the test just before getting back onboard/ashore.

Dress your tanks when full with all the do dads that you normally carry – regs, back plate, BC, light, …

Also a good system. I just thought the one-stop solution would work better for new divers, especially in a group.

I agree with knotical. Weighting for the safety stop can make for a very rapid ascent from the stop to the surface.

The proposed method is for the "minimum" weight, and is what I actually use. I wear a full 7mm farmer and don’t find buoyancy gain in the last 10' to cause anything near rapid ascents. Keep in mind, this may well be the only time most recreational divers are ever in the water with only 2-300 PSI so they will likely have more weight under real conditions anyway.

Maybe dive instructors should have students do the test in 3-5' in a pool instead? I couldn’t justify that as the "minimum" in the original discussion though. Just helping new divers get within a pound would be a huge success in my view.

I have to say I am rather amused by all the methods suggested to objectively measure your weight. Some are rather ingenious and have merit for a new diver, …

I was reluctant to prefix with "this is for new divers" because I wanted to hear from the old salts and instructors too.

…Like Peterbj mentioned, trapped air messes up a lot of methods done at the start of a dive. I would add with many thick wetsuits that they un-compress very slowly on ascent. This results in a thick wetsuit having more buoyancy at 10' on the way down than 10' on the way up. On top of that minor gear changes and the simple aging process of (cheaper) wetsuits continually changes ones buoyancy over the years so it is an ongoing process....

True. The suit compression issue is allowed for if the test is at the end of the dive. I agree that perfect weight is a moving target. It strikes me that there are a lot of new divers that have yet to experience neutral buoyancy so just stay heavy far longer than they should. Achieving good weight can be a tedious and time consuming process using the cave-man method, not that it isn't worth every bit of effort.

Thanks everyone so far for the constructive comments.

 

[Akimbo]

The best medicine I found on how to cure abusing a BC is to learn to dive without one. If you can do that then a BC is just icing on the cake.

Eric, I had you in mind when I added the "if you use one" comment.

…

• With a fully deflated BC (if you use one)

...

BTW, you are absolutely correct. But it ain't gonna happen any time soon.

 

[aquaregia]

Weather this true or not, instructors and new divers need a fast and "low skill" method of finding the proper weight. New divers may not have developed the "feel" to accurately sense if they are at that sweet spot or not since it is all so new to them. I have read lots of threads that argue endlessly over every little detail without clearly defining what minimum is, let alone the "why" or "how". The salty dogs on the board "get it", but new kids on the block are lost by the third post.

I agree that the benefit is to allow new divers to easily determine a good weighting. I was trained with 30# (7mm, salt), but looking for a better answer I chucked my gear into a barrel of water and "weighed" it at +17#, added 4# for the AL80 and I've been diving at 20# since. I called my body +0 because there is a lung volume at which I sink and one at which I float. On the other hand, my lungs have somewhere between a 1# and a 12# buoyancy swing, so that's not a great assumption.

Akimbo's method allows testing while actually in the water, so my neutral-body assumption wouldn't be needed. It seems like you could do this easily in a pool with the scale on a boom, although the idea of being in a pool for even five minutes with my 7mm sounds uncomfortable.

 

[synctek]

As a new diver, 1 year certified and only 30 dives, I have had to change my configuration from fresh water - no exposure suite to fresh water with a 7mil, and up to salt water and a 7mil. The advice given to me was (take the time) to rent a tank and have the dive shop fill or release the air till its at about 400lbs. Setup all your standard gear minus weights. Jump in and let all the air out of your BC then have someone hand you a weight belt. I add/remove weights til my reg is almost in the water. When I take a deep breath, the reg comes up above the water. When I exhale completely, I begin to sink. Admittedly, it's not much fun in 48 degree water, but at least the tank is lighter when you have to climb out.

I realized in all three configurations that I was overweighted by at least 3 lbs. One side note, as seasons change, body mass configurations do too. So if you become a body builder or shift into a couch potato between dives, you probably have to check your weight again - I speak from experience.

 

[Akimbo]

...It seems like you could do this easily in a pool with the scale on a boom, although the idea of being in a pool for even five minutes with my 7mm sounds uncomfortable.

Good point. I never considered it was an advantage until now, but the pool I trained in was unheated. We were used to full wetsuits and had our weight pretty close by our first ocean dive.

A submersible fish scale would be ideal because it would attach to the line near the diver who could read their plus or minus weight themselves. I wonder if such a scale exists?

 

[Lenaxia]

As a new diver I have a question for the OP or anyone else who cares to answer.

Should this be done with full lungs of air? Or with empty? Or with a half breath?

I find that when I dive, my rate of breathing slows naturally to half of what it is above the water, and considering I have huge lungs already (many years of competitive swimming in addition to other sports), I find that between breaths, my depth can vary over 2+ ft.

Since you're saying you should be in your most bouyant state, that would also imply full lungs, but I'm not sure how that would apply. I usually try to reach neutral with half lungs.

Should I start my own thread on this? It feels a bit relevant.

 

[Eric Sedletzky]

As a new diver I have a question for the OP or anyone else who cares to answer.

Should this be done with full lungs of air? Or with empty? Or with a half breath?

I find that when I dive, my rate of breathing slows naturally to half of what it is above the water, and considering I have huge lungs already (many years of competitive swimming in addition to other sports), I find that between breaths, my depth can vary over 2+ ft.

Since you're saying you should be in your most bouyant state, that would also imply full lungs, but I'm not sure how that would apply. I usually try to reach neutral with half lungs.

Should I start my own thread on this? It feels a bit relevant.

It means it should be done with your normal breathing cycle. Not with lungs mostly full to keep from sinking , and not lungs mostly empty to stay down.
So yes, lungs about half full on avarage.

 

[aquaregia]

I had a thread on this a month or two ago, but I can't find it right now. I was also seeing around a 2' change between breaths. Someone (perhaps TSandM) suggested I start breathing out as soon as I notice myself rise, and breath in as soon as I notice I'm falling. That lead to a considerably shorter breathing cycle, and got me down to like 1'. I just got back from a night dive where we burned the last ten minutes at 4-7' and I was starting to get the feel for the timing, although my lungs were pretty empty at 4' with an empty tank.

 

[spectrum]

You don't necessarily need to alter your breathing all the time due to porpoiseing. When you need to be spot on in position then you breathe as needed with precision. In general you can usually breathe at a relaxed pace. Timing of your breaths, your kicks and nuances in posture can all level out the flight path. Whatever variation remains is not a crime.

Spending time in the shallows figuring out just what you can achieve is a great way to get in touch with the secrets of buoyancy control.

Pete

 

[Akimbo]

…

• At the shallowest decompression or safety stop
• With a fully deflated BC (if you use one)
• With drysuit deflated to minimum without discomfort (if you use one)
• With nearly empty Tank(s), like 200-300 Lbs or 14-20 Bar
• With lungs comfortably inflated to your normal respiratory inhalation peak

...

There are really two steps here. First is to define the minimum amount of weight required. If you use the definition for minimum weight listed above from my OP, then you want lungs inflated to the full-end of your normal respiratory cycle.

The second step in buoyancy control is adjusting your BC — the vast majority of the time. You will be much heavier during most of your dive than for the test because you are usually deeper then your minimum stop and carrying more weight in gas in your cylinders. Adjusting your BC is when you want to shoot for the midpoint in your breathing cycle — which I am sure is the thinking behind ZKY's half full lung recommendation.

Think of it this way. Lets say you inhale 1 Liter of air during one of your normal breathing cycles. That equals 1 Kg, or 2.2 Lbs, of buoyancy change during that cycle. That would be about 1.1 Lbs difference above and below your half-way volume.

If you adjust your minimum weight at half a lung, you would be 1.1 Lbs buoyant about half the time at your stop on a nearly empty tank. Fighting positive buoyancy at a stop is more work than being neutral or negative, so you would use your low air supply even faster — unless you are hanging off a weighted decom line. Figuring that an un-weighted line is worse case, then it should not be considered for finding your minimum weight. You can easily add positive buoyancy, adding negative buoyancy is less convenient or reliable.

That is the logic behind the procedure listed in my OP. I put it up here for a review by my peers. I would rather have errors in my thinking pointed out than have a procedure that does not address the problem and never gets used.