Weighting for descent vs bottom

[Diver0001]

Beau,

Proper planning involves proper contingency planning. It may sound all nice and clean on the innerwebs when you say that a diver should never end their dive with less than 500psi but that is (a) incredibly naive and (b) does not address the *function* of that 500psi, which is for contingency at the tail end of the dive. If you are setting your ballast for 15ft AND 500# then you are compounding bad advice with bad planning.

R..

 

[Beau640]

It's a good discussion to have.

I am talking about weighting yourself for contingency. I'm talking about finding the minimal weight you can carry for an essentially empty tank (the buyouncy change from 500psi to 0psi is less than 0.5lbs) to hold your stop near the surface at 10-15 feet. that means in your worst nightmare of breathing your tank to near 0, you should still be comfortable between 10-15 feet with your buddy's hose and not be struggling. Now you're talking about ascending less than the distance of most indoor pool bottoms if you can hover at 10.

 

[gopbroek]

I am also in the camp of weighting for zero air. You can plan for 500lbs all you want but when the plan goes south and you are in a real or percieved emergency event you really do not want the extra task loading of fighting bouancy. I have been under weighted and then needed to loan air to another diver to clear Deco. I was upside down finning and on an octo inflater while the other diver cleared 11 minutes of Deco while breathing off my short 26" primary, what a PITA.

 

[mi000ke]

Reasons for inability to sink initially may include not completely exhaling, unconscious finning, air pockets in the BC, and air pockets inside the wetsuit.

Well as I mentioned I'm pretty good at purging my lungs, and my instructor verified I was not finning, but it was the first dive of the day in a dry wetsuit and probably had air pockets. so that makes sense.

How much air was in your tank at the end of the dive with 12lbs?

I had 500 at my safety stop

I have to agree with BRT. I even like to ascend from my 15' SS by inhaling a little deeper and then hovering at 7-10 feet for another 15 seconds. It is tough for me to ascend that last 15' in 30 seconds; so breaking it up helps.

I was able to hover at 15' using normal breathing with no problem, and then was able to make a controlled ascent that last 15' by taking a bit larger breath and slowly finning up. So, putting aside the discussion of whether to weight for the safety stop w/500 psi or for the surface with empty tank, it seems like 12# accomplishes what I needed. I just need to work on my descent technique as suggested by a couple of folks. Any lighter and I may have to really struggle (or work really hard on my duck dive technique) to get down, and may have a hard time holding the safety stop. But I have pretty good lung capacity and breath control (40 years of sax breathing exercises) so may even try 10 lb at some point.

You can easily control your ascent from 15ft up with your lungs/breathing. I've never had an uncontrolled ascent from my safety stop solely because of weighting for 15ft.

Agree - see above

Thanks to all for the input. It seems that 12 lb (and maybe less - we'll see) is my correct weight using the "safety stop with 500 psi" guideline and the ability to ascend the last 15' under control. So looks like I just have to work on my duck dive descent technique, as feet first just does not seem to work for me at that weight. (Is there a way to get air pockets out of a dry wetsuit?)

I did a couple more 12 lb dives yesterday (last two of my AOW), and one thing that helped was to do a stride entry with no air in my bcd and the momentum carried me a few feet below the surface, which made it a bit easier to get down.

Man, it's great trying to figure out this diving thing!

 

[halocline]

Personally I go for neutral or very slightly negative at 10ft with a near-empty tank. I usually do a stop at 10ft before surfacing; this mimics deco diving and I find I often feel better, depending on the dive profile, making a short stop at 20 ft and a longer one at 10 instead of a single stop at 15.

Diver0001's argument is a good one, you don't want to ever be in a situation where you are fighting to stay submerged. However, it's not so cut and dry that weighting for the safety stop will cause an uncontrolled ascent. It depends on your wetsuit, technique, and even BC. I can easily weight for the last stop, find myself just a bit positive on the surface at the end of a dive, and everything is fine with the last few feet of ascent. Someone else, using a thicker suit, spongier BC, maybe a little more "finnish" at a stop (no offense to the finns!) might need a bit more weight to stay down.

What we're essentially talking about here is a pound or two in most cases. The bottom line is you want to carry the minimal weight that need to stay down at any depth.

I would not worry about needing to swim down at the beginning of a dive, especially if you're using a padded BC. Those things frequently make the initial descent more difficult.

 

[Beau640]

Agree

In terms of air pockets in the wet suit - you can attempt to do the dry suit purge method of curling into a ball and hold the neck out to attempt to get some air out but that won't do too much for a wet suit. The giant stride into the water so you initially completely flood your wet suit will help with the descent when ready. And as you dive your wet suit more and more the neoprene will become more permanently crushed and it will be less of an issue.

What halocline said is true - padded BCDs have lots of air pockets. I wouldn't worry about the descent as much as control during the rest of the dive with the least amount of weight you can get away with. Besides the headache of carrying around the weight on land, with less weight under water you have less exertion and better air consumption.

 

[Diver0001]

Ok, I think the issue of how much ballast to carry has passed the review. I will just add one more thing to this, which is that we're talking about a minimal amount of weight here. In some sense we are splitting hairs.

Air has a mass of 1.2kg per cubic meter (1000 litres) so the difference between a full tank and an empty tank using an AL-80 or something similar is about 2.6kg (say 6lbs) during the entire dive. If you are talking about the buoyancy swing from 500# to zero then we are talking about a minimal amount of weight. 1/2kg or 1lb. for a typical recreational diver.

The issue at hand isn't even really the issue of 1 or 2lbs more or less, the issue is about the principle of control. If you believe that you should be entirely in control of your dive from the moment you submerge to the last moment before you surface then you will take the extra ... say... 2lbs/1kg.

If you don't, you will probably not die from it, but the end phase of every dive will entail some kind of buoyancy issue that could have been avoided with a minimal amount of extra ballast. In my personal case, I'm an instructor instructing (mostly) newbie divers and also a highly active technical diver so to me, relinquishing *any* amount of control over my dive is anathema to me. Clearly, as we can see from this thread, there are two common paradigms.

=======================

In case anyone is interested, this is what I teach in my OW course:

First of all, your ballast needs to be right. We do multiple buoyancy checks during OW and we experiment with placing weights to get the right amount of weight but also the right placement of weight. In terms of total ballast we do it using a "practical" technique in OW which involves being able to descend and maintain a fin pivot in shallow water. If we do it with a full tank then we adjust it for the weight of the air, which is why I know how much air weighs per 1000 litres.

Officially we use the PADI technique in the course because I teach a PADI course but I'm quite happy to show students the difference between theory and practice.

The reason we do multiple buoyancy checks is because new divers normally relax a lot over the first, say, 50 dives or so. We tell them this. We train them to understand that doing a buoyancy check is something that you don't do once, but that you do it every season. New gear, gaining/losing weight and so forth can make a difference, so this needs to be part of your "routine".

As for descending (addressing the OP) we teach students to do a 5pt descent and as part of that to vent the BCD until your head is under water and then to STOP venting and roll forward into what we call the "diving position" and to descend the rest of the way as a sky jumper would. We also train students to STOP descending after a few metres and hover. Even in the pool we do this. The reason for that in the OW course has to do with training students to "feel" neutral buoyancy but to also learn that both your ascent and your descent should be under total diver control from the surface to the surface.

R..

 

[guruboy]

I had 500 at my safety stop

I was able to hover at 15' using normal breathing with no problem, and then was able to make a controlled ascent that last 15' by taking a bit larger breath and slowly finning up.

Why wouldn't you just fin up with a normal breath? Why use a larger breath?

 

[guruboy]

You're still talking about an emergency situation. People shouldn't be ending their dives with less than 500psi. Weighting for an essentially empty tank which is an emergency situation and shouldn't be happening means 99/100 other dives you have absolutely no weighting or ascent or circus act issues because you'll have the extra weight from ending the dive with enough air in your tank.

I don't plan to crash my car but I wear my seatbelt still.

500psi is often the min gas for ending recreational dives. Getting to your safety stop at 15' with 500psi is not an emergency.

 

[Bob DBF]

k, I think the issue of how much ballast to carry has passed the review. I will just add one more thing to this, which is that we're talking about a minimal amount of weight here. In some sense we are splitting hairs.

Exactly! Everyone is finding the perfect weight and it is all quite close.

Air has a mass of 1.2kg per cubic meter (1000 litres) so the difference between a full tank and an empty tank using an AL-80 or something similar is about 2.6kg (say 6lbs) during the entire dive. If you are talking about the buoyancy swing from 500# to zero then we are talking about a minimal amount of weight. 1/2kg or 1lb. for a typical recreational diver.

1.38# for my 120. I can easily compensate for this at the safety stop and above by keeping my wing empty and using lung volume.

The issue at hand isn't even really the issue of 1 or 2lbs more or less, the issue is about the principle of control. If you believe that you should be entirely in control of your dive from the moment you submerge to the last moment before you surface then you will take the extra ... say... 2lbs/1kg.

If you don't, you will probably not die from it, but the end phase of every dive will entail some kind of buoyancy issue that could have been avoided with a minimal amount of extra ballast. In my personal case, I'm an instructor instructing (mostly) newbie divers and also a highly active technical diver so to me, relinquishing *any* amount of control over my dive is anathema to me. Clearly, as we can see from this thread, there are two common paradigms.

I'm of the 15', empty BC, and 300 to 500# to find your "ideal weight". However, this is a number and you are a diver, so it is up to you to decide how to use the information. Weight is a function of the dive plan, not just an ideal number. I add weight when carrying a pony on a deep dive because if the shtf I might wind up with a ceiling and ultimately have two empty tanks.

As an aside, I use old weights and have found the weight stamped on them seems to be a suggestion rather than a fact, with very very few being over so I use a couple of "extra" pounds to make up for the lies.

Clearly, as we can see from this thread, there are two common paradigms.

I think the two different paradigms are those who argue about what is the ideal weight, and those that have no idea how to weight themselves properly.


Bob