"Correct Weighting" Identified as #1 Needed Improvement in SCUBA Diving

[The Chairman]

So, one misconception I've seen more than once around here

You missed your own misconceptions which are egregious.

The actual shift in buoyancy you plan on an 80 for is 4 pounds.

Please show the math for this. Better yet, I'll do it for you:

• Air weighs 0.0807 lbs/cf
• If you drain an 80cf tank from 3,000psi to 500psi, then you've used 5/6 of the available air.
• 0.0807lb/cf x 80cf x 5/6= 5.38 pounds.

Many women have a small tidal volume with little extra capacity and so can't inhale enough to accommodate a 5 pound swing. Are you wearing a wetsuit? Now let's complicate it further with a 7 mil wetsuit that has about 20 pounds of lift on the surface. According to Mr Boyle, at 99 ft, you'll lose about 75% of your buoyancy which is another 15 pounds. So at the beginning of your dive, at 99 ft, you have an insurmountable 20 + pounds to compensate with your lungs... or keep swimming like a shark which is what we did before BCs became main stream.

 

[Joneill]

Right. I was only quibbling with this statement.



It seems like I often see posts that are somehow implying that using an AL80 makes you light or a steel tank makes you heavier. Which is wrong. If you are properly weighted, then you will start the dive negative by the amount of gas you're carrying. It has nothing to do with aluminum vs steel tanks.

Similarly, if you use an AL80 and 3 pounds of lead or an HP80 that has the 3 pounds of "lead" built in, none of that is what makes you negatively buoyant at the start of a dive. It's only the gas in the cylinder that makes you negative. You use it during the dive, it's gone, and you're neutral at the end.

Whether you understood that or not, it seems like statements like the one you made lead some people into confusion about weighting and cylinder materials.

I dived with someone once who was using an HP100 for the first time. She'd always used AL80s before. She blamed her feeling of being heavy and struggling to control her buoyancy on the tank being heavy and steel. She didn't get that that had nothing to do with it. Being heavier than she was used to was simply from carrying more gas. She was used to starting 6# negative, but with a 100 (whether it's steel or aluminum), she was starting 8# negative. And that means it required more gas in her BCD at the start of the dive than she was used to putting in. And more gas in your BCD makes it harder to control your buoyancy.

Anyway, my apologies for being didactic. I'll shut up now.

Understood - but determining "proper weighting" needs take into account the amount of gas you are carrying (and the amount of gas you expect to consume) and the buoyancy characteristics of the tank - no? What I'm trying to say is that in determining the proper weighting, the buoyancy properties of the tank itself as well as the amount of gas factor in. For example, I see that a Luxfer 80 is 4.4 lb positively buoyant when empty while a Luxfer S80 is only 2.26 lb positively buoyant - but both have ~78 cu.ft. of air when filled to 3000 psi. So proper weighting with those 2 different AL80's will be a little different to account for the difference in buoyancy when empty/low on gas?

In your example of the HP100's, I looked up a Faber HP100 and, in addition to carrying more gas than an AL80, the Faber is 7.26 lb negative when empty versus a Catalina S80 that is 4 lb positive when empty. So, don't both of those items need to be taken into consideration in determining proper weighting? The Faber is an anchor when empty while the Catalina will float you.

At 50 dives, I'm a relative newbie, but improving with every dive and trying to learn - so please let me know if I have this wrong.

 

[TMHeimer]

Lot's of informative posts here. If we're just talking about the correct AMOUNT of weight (rather than trim, ditchable weight, etc.) I always figured you just do the simple weight check--doesn't matter what tank, air, exposure suit, etc. Someone somewhere said it's not as simple as that and that there is a better way. It's worked for me.

 

[The Chairman]

Lot's of informative posts here. If we're just talking about the correct AMOUNT of weight (rather than trim, ditchable weight, etc.) I always figured you just do the simple weight check--doesn't matter what tank, air, exposure suit, etc.

Bingo! Yahtzee! Winner, winner, chicken dinner!

Someone somewhere said it's not as simple as that and that there is a better way. It's worked for me.

Trim is the fly in the ointment. If you can get flat, then your propulsion will be straight behind you, with no upward or downward component that will affect your buoyancy. In other words, if your feet are pointed a little down, then you're creating lift with your kicking and you must compensate for that lift with more weight. When you stop, you'll find yourself suddenly negative, having to adjust your breathing or more likely putting more air in your BC. Start moving and you have to vent the excess air and the cycle chews through your air pell-mell.

 

[KenGordon]

However, that 5.7 pounds is to completely empty the tank (like for doing a VIP). This is not something you are EVER going to do while underwater.

I was not going to bother but then that was said. Nonsense, being polite.

When I were a lad a BCD (Fenzy) was an unreachable thing. Wet suits were cut from a pattern, then glued and taped at home. Direct feeds were the height of decadence. You had to get weighting right and pick up rocks if you did not.

Having a BCD makes life better for the diver and better for the environment.

The 6lb for an 80 is to cover contingencies, proper planning means that your buddy may use that gas. You may have a free flow not want an uncontrolled buoyant ascent. You may fail to pay attention and use too much gas etc etc.

The idea of contingency gas is to prevent these minor problems drowning you. But they are foreseeable events. So the gas may be used. If the quotes claim were true we could just all have smaller ranks or longer dives.

 

[REVAN]

You can plan for 4 and still use the whole 5.7 pounds in an emergency guys. Please don't tell me that you are to prissy to be inconvenienced to kick down a whopping 2 pounds if things go off plan. If it's an emergency, you do what you have to.

I once lost a 12 pound weight pouch on a fairly deep deco dive. It sucked, but I swam it out, completed my deco and surfaced okay. Fortunately, I had only accumulated about 6 minutes of deco when it happened, but I'd have done 20 if that's what was required. It could have been worse, but then it could have been better also. I stopped using weight pouches after that.

Anyway, this talk is about recreational diving not tech and deco stuff.

Edit - The difference between an HP80 and the theoretical 80 based on the 72 is that the theoretical tank could be filled to capacity in a low tech destination that can't do high pressure fills and the tank is smaller in diameter, so less drag.

 

[The Chairman]

Please don't tell me that you are to prissy to be inconvenienced to kick down a whopping 2 pounds if things go off plan.

Prissy? No. I've never been accused of being prissy. Lazy as ____??? Oh, hell yeah. Ask @sphyon how effortlesslyand efficiently I dive.

I'm not a fighter: I'm a diver. My diving isn't an adventure in minimalist gear: it's an adventure in being an underwater tourist. I don't want those white knuckle moments on the surface, trying to keep my head up while waiting on the boat. I relish being uber relaxed and in the zen. My heart rate drops to under fifty and time stands still. I am in control and completely relaxed about it.

 

[REVAN]

You missed your own misconceptions which are egregious.


Please show the math for this. Better yet, I'll do it for you:

• Air weighs 0.0807 lbs/cf
• If you drain an 80cf tank from 3,000psi to 500psi, then you've used 5/6 of the available air.
• 0.0807lb/cf x 80cf x 5/6= 5.38 pounds.

Many women have a small tidal volume with little extra capacity and so can't inhale enough to accommodate a 5 pound swing. Are you wearing a wetsuit? Now let's complicate it further with a 7 mil wetsuit that has about 20 pounds of lift on the surface. According to Mr Boyle, at 99 ft, you'll lose about 75% of your buoyancy which is another 15 pounds. So at the beginning of your dive, at 99 ft, you have an insurmountable 20 + pounds to compensate with your lungs... or keep swimming like a shark which is what we did before BCs became main stream.

I vaguely remember getting into this with you before. Maybe we can make a game of this and it will stick in people's memory. Anyone else want to take a stab at where @The Chairman may have gone a little astray with this and over predicted the weight for a typical recreational dive?

 

[wetb4igetinthewater]

I used a bungee system I learned from Pete (formerly NetDoc) to attach trim weights to my students' cam straps. The shop's instructional staff was completely mystified. Why would you do that? Why not just put it all on the belt? When I explained the concept of trim, they clearly had never heard of it. When I told them that they would soon be required to include it in their instruction, they were amazed.

Would you or @The Chairman care to share this? I'd be interested in trying this with students (as the BCD's my students use don't always have trim pockets).

 

[The Chairman]

Would you or @The Chairman care to share this? I'd be interested in trying this with students (as the BCD's my students use don't always have trim pockets).

Second request for this today...

3/16" bungee with opposing grape knots (prusik style).
The various weights need differing lengths. Experiment!

Thread from the front to the back through one of the stops.
The knot will stop it from slipping through.

Pull the loop under the strap and up through the other slot.

Pull the clean side over the edge first.
Again, the knot will keep the other side from sliding through the slot.

It will get tight here, so the knot helps you to pull this side over the other edge.
If the bungee is a bit loose, cross them over to opposite sides.​