Weighty thoughts

[Se7en]

Been thinking about a couple of items of recieved knowledge, that don't make much sense to me.

Firstly - 'neutral at 15 ft with 500 psi and no air in BC'

The more I think about this, the more I dislike it. By definition, to be neutral means that you rise as you breath in. With nice slow breaths, the breath in and hold part of the cycle will be 10 to 15 seconds - during which time you have become positively boyant. In this time frame, you may rise 6+ feet - a pressure change of at least 15% at these depths.

Wearing a 7mm wetsuit, that will act to make you more positively boyant - to the extent that you now can't get back down, and you have just blown off your safety stop - and had an uncontrolled ascent through the greatest percentage pressure change of the whole dive.

No Thanks.

The second issue is that with less than 500 psi in your tank, you will again be positive. (12 litres of air at one atm weighs approximately 10 grams, so 500 psi in a 12l tank is approx 350 grams, or 1 pound - not huge, but significant) If I ever was in a situation where I was worried about DCI, I'd like to be able to breath my tank dry at a safety stop, and the final ascent. That 500 psi represents about 10min of relaxed breathing at that depth - 10minutes of decompressing that I may oneday want. In that situation, I really don't want to be fighting positive boyancy.

Alternately, what happens if you have a complete loss of air from a tank? You do a safe ascent with your buddy, but find that you are positive from around 20 - 25 ft? So now you drag your buddy up on an uncontrolled ascent as well?

Surely being neutral at the surface with an empty tank is a much better goal? So you can maintain position at any depth, with any amount of air in your tank?

My other question is with the DIR notion of not wearing steel tanks and backplates when diving wet - as you will be overweighted. Wearing a 7mm, I wear 25lbs of weight with a steel tank, so say 28 lbs negative. A SS backplate is around 5 to 7 lbs negative, Each tank around 5 lbs negative full - giving a total of 17 lbs negative.

That would mean wearing an 11 pound weightbelt to achieve neutral boyancy - Which is ditchable weight.

Now, I have never actualy measured, but I don't believe that I put 11 pounds worth of air / boyancy in my BC at 40 m. Therefore, even at this depth, ditching my weightbelt leaves me neutral to positive.

So what's the problem?

ciao
Mike

 

[The Chairman]

Mike,

I follow your line of reasoning completely and even agree with you. I configure my neutral boyancy at the surface with little/no aire in my tank... I am talking in the 100# range. I just set it up with the boat ahead of time. I get as neutral as I can and then add a pound ar two. With my PST hp 120s I only need 4 pounds. I can live with that! I can still fin easily to the surface from most any depth, and my 100# lift OMS can rocket me to the surface if I want an embolism!

 

[Warhammer]

First the being neutral at 15ft thing is a matter of personal preference more than anything. Different people do it differently. And either way is fine as far as I'm concerned. I do it both ways, but as I get more experience, and my confidence grows, I tend to weight myself neutral at 15ft, depending on the conditions. If you choose to be neutral at 15ft, then that's what you want to be, neutral not postive. In the average person, lung volume causes a 2lb shift in bouyancy. That's very little, enough to fine tune, but not enough to cause an uncontrolled ascent. Mastering bouyancy is about maintaining a constant depth, not rising and falling. To be perfect, you'd want that 2lbs split, whereas at half lung volume, you'd be perfectly neutral. As you inhaled the 1lb difference in bouyancy would not have time to take affect because it would have already been countered by the 1lb decrease in bouyancy from your exhale. Therefore, your depth would be constant. I haven't mastered it yet, but I have dove with several that have. And even if you were slightly off, a 1 or 2 lb shift in bouyancy is easily countered by a kick or two in the right direction.

The wetsuit & steel tank issue is a heated one. I personally dive HP100s with a wetsuit all the time, almost exclusively. I see nothing wrong with it at all. DIR's standpoint is aimed more at the tech community where doubles are involved. In the event that you had a BC failure and you were wearing double steels and a wetsuit, you might get in serious trouble rather quickly. Depending on the person, it would be difficult to swim that rig to the surface. You wouldn't want to ditch your weights, because if you did, then you'd become extremely positive on your way up as the wetsuit uncompressed. If you were a tech diver, with an hour deco obligation, the outcome would be catastrophic. So you can see the reasoning behind their theory.

 

[Se7en]

Originally posted by Warhammer

In the average person, lung volume causes a 2lb shift in bouyancy.
Mastering bouyancy is about maintaining a constant depth, not rising and falling.

The only people I've seen that are actually able to do this were on rebreathers - however when diving open circuit the norm is to sway up and down as you breath. If you think you have it under control, try doing it with eyes shut and no physical reference...

I'd also question the average of 2lb shift in boyancy... that's less than a litre - which really isn't long deep breaths.
Splitting hairs here I know.

2 lb shift in bouyancy is easily countered by a kick or two in the right direction.

Hmmm - not so easy to kick back down if you are in a heads up position and positively boyant.

I guess my main question to you is - if you are neutral at 15 ft, this implies you are positive above that depth. How do you then do a controlled ascent to the surface?
(I'm thinking about people wearing 5mm + wetsuits here, as with a 2/3mm or less I agree that the boyancy does not change. I have dived in shorts to 40m. and not put air in my BC at any point.)

Depending on the person, it would be difficult to swim that rig to the surface. You wouldn't want to ditch your weights, because if you did, then you'd become extremely positive on your way up as the wetsuit uncompressed.

The example I gave was for twin steel tanks, as that was the config I was interested in. I also use a very negative 12L tank for most OW diving, so am quite happy with that config.

As for the weight ditching, you would actually be worse with AL tanks. In both instances (twin AL or Twin Steel), in the event of a BC failure, you would be negative at depth, and negative to the same extent (assuming that you were correctly weighted at the start of the dive) The only changes in boyancy are from air use, and wetsuit compression.
With the twin steels (as per my first post) I'd be wearing an 11 pound weight belt. With twin AL, you would have to wear at least a 20 lbs weight belt (assuming neutral AL tanks).

Which would you prefer to drop?

ciao
Mike

 

[Warhammer]

Like I said, I've yet to master bouyancy, but I have witnessed it mastered by several people. I witnessed the one guy in particular that I'm refering too hover inside a tight wreck for close to 5 minutes and if he moved at all, I sure didn't see him. But I can maintain a pretty much constant depth, even under blind conditions. For example, we rountinely turn off our lights on safety stops during night dives when diving in saltwater, to observe the incandescent creatures. While I don't close my eyes, it is rather dark and when the lights go back on, I'll be very close to my target depth of 15ft.

As for the kicking back down bit. Guess that's one point we differ on. I'm rarely in a vertical postion while at a safety stop. I'm usually horizontal or have my head just slightly upward. I usually decend horizontal as well.

When I weight myself neutral at 15ft, which is more often than not, I've never had any problems maintaining a safe ascent speed above that depth. Why? Don't know for sure, maybe because the shift in bouyancy just hasn't been great enough. But since you mentioned it, yes, the overwhelming majority of my dives are either in a 3mm full, 3mm shorty, or shorts. So I really haven't tried it in anything more than that, where I was paying any attention to it.

I see your point about the steel/wetsuit issue, but DIRs philosophy, as I understand it, is that you should be able to swim the entire rig up unassisted by your BC. With an aluminum tank, as you use air from it, it will become positive aiding in you in your ascent. With a steel, it remains negative no matter what, requiring you to have to swim it up as well. I don't think it's an issue with singles, but certain steel doubles, I think it could become one. Let's look at the extreme of this for an example. Take the Faber HP100, while empty it is still -7.5Lbs x 2 = 15Lbs of negative weight. Add your backplate's weight to that and you quickly mount up the unditchable weight. Would you have enough ditchable weight to still get positive, or be able to swim it up, with your BC gone and your wetsuit compressed?

Granted there are people on this board better suited at answering your question than me, as I'm not a DIR follower myself. I'm just stating it as I understand it. But obviously with your current rig as you state it, that requires you still carry 11Lbs of lead, you could ditch weights discriminately in order to be able to swim the rig up and probably still maintain somewhat of control on the ascent. So I'd think you'd be fine. But do a little experiment next time you dive the rig. See if you can swim it up without aid from your BC at several stages in your dive. If you can, great, if you can't, then maybe you should re-evaluate your rig.

 

[Lost Yooper]

Hey Mike,

Warhammer has basically outlined the DIR thing pretty good regarding double steels and wetsuits. It's basically a rule of thumb that states one should be able to swim their stuff up in the event of a BC failure at the worst possible time -- at depth with full tanks.

Here's an example of what not to do:

Double LP PST 104's are -25lbs FULL.
SS Plate is -6lbs.
A couple AL 80 stages are -2lbs each full -- some even use steel stages .
-6lb canister light
Wet Suit

If you can't swim up your stuff, then that's bad.

Mike

PS. I go after 1lb negative on the surface with empty tanks, empty BC and empty dry suit. The negativity at 15' allows me to put air in the suit for warmth during deco.

 

[Se7en]

Seems the boyancy question just highlights the difference between minimal neprene (ie up to full 3mm) 7mm two piece, and drysuits. With minimal wetsuit, the boyancy change is SFA between surface and depth, so there is no significant difference between being neutral at the surface, and neutral at 5m.
With a drysuit, you maintain boyancy such that there is no difference between surface and depth. In both these cases, the BC is really only there to hold the tank to your body.

With a two piece 7mm with integral hood, plus a 3mm hooded vest, there is 17mm of neoprene on your chest to compress - so there is a significant difference between 5m depth and the surface, and a much bigger difference between the surface and 40m.
So you have to pay a bit more attention to boyancy. I'll stick with being neutral at the surface with everything empty I think - which means somewhat negative at 5m.

Warhammer - anyone who can hold 15ft with no tactile or visual reference qualifies as having excellent boyancy, so I think you are selling yourself short there
Just for fun, try it eyes shut sometime - light's out at night still gives a fairly good reference (In Sydney, we quite often do night dives by moonlight only for the majority of the dive - in 10m of water though)

As for the doubles set up - I currently don't own a doubles set up, but will be getting my own for upcoming cavern course, so was just trying to work out the logic.
(Logic says replace my 8 year old 7mm with a drysuit - But I'd prefer to spend the cash on a good doubles setup)

I'm still not convinced by the DIR logic on steel vs Al.
Consider a basic doubles rig - twin 12l tanks. Dived with a 7mm, which means between 24 and 34 pounds weight required (I've seen people with 40 lbs on boats) - Say 28 lbs cause that's what I use

I'll allow 10 lbs in either config for backplate, regs lights etc. I'll also assume that I want to be neutral at the surface, with 400 litres of air / ~ 15 cuft in each tank (with 12l tanks this equates to 33 bar ~ 490 PSI) and weighs about a pound per tank.

Twin HP Steel Fabers - 15lbs neg full, 1 neg at 500 PSI
Required weightbelt: 17 lbs.

Twin Luxfers AL - 9lbs neg full, 5lb pos at 500 PSI
Required weightbelt: 23 lbs
(Note that these are the only tanks I could find that are more buoyant than Steel tanks.)

In both cases I'm 14lbs negative at the start of the dive, and neutral at the end, and my buoyancy change will be identical in both cases. Therefore, with weightbelt on there is no difference between Steel and Al tanks

The bit that I've missed out here is wetsuit compressibility. My memory tells me that my wetsuit is 24 lbs buoyant at the surface. A bit of research leads me to believe that neoprene compresses linearly with pressure gradient change (not depth change), such that each doubling of pressure reduces buoyancy by 1/3rd.

Therefore wetsuit boyancy changes as such;
Surface: 24 lbs pos
10m: 16 lbs pos (reduction of 8 lbs)
30m: 10.6 lbs pos (reduction of 14.6 lbs)
70m: 7lbs pos (reduction of 17 lbs)

so what does this tell me?
Worst case, with full tanks at 70m, I'll need 17 + 14 = 31 lbs of lift to be neutral. No problem, I get a wing with this much lift.
If the wing becomes non functional, I can ditch weight belt. Neither the Steel weight belt, nor the AL weight belt puts me positive, so I need redundant lift.

Hmmm - seems that diving doubles requires redundant lift whether Al or Steel. Therefore, it really doesn't matter which you use...
Hey - there's my excuse for a drysuit...

ciao
Mike




Reference info:
Boyancy figures for some 100 cubic foot tanks Tank Full Empty
Catalina Cylinders Aluminuim -7.5 0.0
Luxfer Aluminium -4.3 +3.2
OMS Low pressure Steel -7.73 0.00
Faber HP Steel -7.5 +0.5
PST HP Steel 0.0

(sources
http://www.fifthd.com/gear/tankspecs.html
http://www.paradisedive.com/tspecs.htm
Rodales
Divernet
ScubaSource)

 

[Warhammer]

Thanks for the compliment, Se7en. I haven't quiet perfected it yet, but I am getting better. Alot of the guys I dive with have vastly more experience than do I and I do alot of diving with them in relaxed environments, that has a lot to do with it.

As far as the steel/wetsuit issue goes, you come across as a very intelligent diver, so I'm sure you'll work something out that works for you. I'm virtually the same way in that I do what works for me. That's why I said I wasn't a DIR follower. While I agree with a lot of their ideas, I tend to make up my own mind. However, if I were engageing in the "high" risk type diving that they cater too, I'd most likely follow their lead.

 

[Butch103]

guys.....I can't really add anything to the conversation. I am a new diver and still trying to master the art of bouyancy.

WarHammer....great to see you back on board and adding to the confusion....

 

[Lost Yooper]

Hey Mike,

Since you asked about DIR, let me fill you in on a little more. Redundant buoyancy isn't a part of DIR. Some people think so, but that is not the case. DIR says that one should be able to weight themselves in such a way that they are able to swim their gear up without the BC.

Now, I'm not a wetsuit diver, and I have a bit of trouble conveying that aspect. Here's what I know: wet DIR divers in the warm waters of Florida, using relatively thin wet suits, use double 80's and they are able to swim their gear up. Dry suit divers are the same way. They dive with double LP PST 104's, SS plate, 6lb light, and stages and they can swim their stuff up without ditching anything.

Redundant lift isn't the answer as far as DIR is concerned -- which is partially the reason why double bladders aren't needed. A dry suit can act as a redundant source of buoyancy, but that's not what it's all about. Will a dry suit diver kick up if he has a functioning dry suit in a real life situation? Nope, he'll use the dry suit and kick because it's easier.

I also keep an 80lb Halcyon lift bag with me to be used for emergency ascents. Yes, the bag could get me up, but that's not what it's all about. On top of that, my buddy is a dry suit diver wearing a BC who could get me up and he also has a lift bag.

You need to be able to swim your stuff up – a balanced rig. A balanced rig is optimal, not only in allowing you to swim up, but in streamlining and therefore energy usage and gas consumption. We’re talking about a package deal here – one thing affects another especially in DIR.

Do as you will, but it is important that you keep doing what you’re doing – asking around. Most people seem “disturbed” about DIR at first, but when they understand it and give it an honest chance; they see what all the talk is about.

Good luck.

Mike