Weekend OW class. Pool

[Warren_L]

With breathing. If he is truly neutral and the only device that can change volume are his lungs, then on every inhale he would ascend a little. This because if there is no trapped air (as inside a BC) no air will expand, per Boyles law, therefore he will not displace more water during ascent, thus leaving him neutral through the entire ascent.

Or of course swimming. But that doesn't require formulas.

Hmm... I would have never thought of that.

In theory this could work. However, this assumes also that the diver in question is not wearing anything in way of exposure protection like a wetsuit or a drysuit. If there is no exposure protection in the equation, the diver would have to be weighted so that he would be negative at the beginning of the dive by the amount of air that would be used during the dive. In the case of an 80 cu ft tank then, approximately 5 lbs negative. Towards the end of the dive, the diver would then be close to neutral at any depth, not just at 20'.

But this would not work well if the diver was wearing a wetsuit or drysuit. If he was neutral at the end of the dive at 20', lung volume would be able to control buoyancy up to a certain point. As the diver ascends from 20', the neoprene wetsuit will expand and at some point there would not be enough air in the lungs to vent to counteract the increased buoyancy of the wetsuit which could potentially result in not being able to hold depth.

As well, in both cases, if the diver is neutral at 20' and wanting to do a stop there, keep in mind that the tank is continuously becoming more buoyant as air is drawn down.

 

[Ayisha]

...then on every inhale he would ascend a little.

Assuming that the inhale and exhale are not of the same volume, or he would also descend on the exhale.
The trick to that is to take a long, deep inhale, and just a short quick exhale, and repeat.
The converse is true of descending. If you exhale with a LOONG SLOOW breath, you will start descending faster and break that first 5 foot "barrier"; then take a quick shallow inhale, and repeat as necessary.
Whichever breath is longer and deeper will gain you the most footage. The short breath will just maintain your depth.

...the environment we teach open water in is typically fairly cold and students wear full 7mm wetsuits.

Fairly cold? Warren, you often teach at Tobermory where the bottom temp is typically 38F in mid-summer. Granted, your O/W students can have as balmy a temp as 50F... See you in Toby!

 

[Warren_L]

Fairly cold? Warren, you often teach at Tobermory where the bottom temp is typically 38F in mid-summer. Granted, your O/W students can have as balmy a temp as 50F... See you in Toby!

We actually got up over 60F last summer! What a treat. This past weekend I had a young lad who is trying to enrol in a commercial diving program which starts next month - but a prerequisite is o/w certification. So we were out in Humber bay doing dives in 37F water. But I am looking forward to the warmer temperatures, that's for sure. See you down below!

 

[GeekDiver]

Wow, I wish I would have kept up with this thread when it was current. I've never heard such long winded explanations. To go from questions about a pool training session, to how to start an accent, compairing alm 80 vs steel 80 and even comments about being overweighted because someone kicked or sculled to maintain depth. Since I chimed in I early on I thought I would put my 2 cents in even if though the thread is very old. Staying with being long winded my post is a bit long.

All padi students do fin pivots which introduces bouyancy. This also demonstrates how a single breath can affect bouyancy and the need to maintain a normal breathing pattern. Any one who has done a fin pivot let alone actualy having well defined bouyancy skills can see that you can easly start an accent just buy taking a breath; provided your neutral to begin with. You shouldn't need to kick or add air, take a breath, just a little deaper than you normaly do or hold a breath just a little long than normal and your accent will begin, if your neutral. JBD I glad you explained your position. I was with an OW class the other weekend and 4 students on the first dive used the inflater on the way up and I know the instructor had explained it wasn't an elevator button in class but the students all still thought they needed to use it that way despite training to the contrary.

As to alm 80 vs steel 80. Yes the air will weigh the same. Ever look at at an alm 80 tank beside a steel 80 tank. The same cu ft size steel tank will be smaller and weigh less. So the air swing may be the same but they have different bouyancy traits and it is like compairing apples to oranges. If you compaired a steel tank with the same diminsions and displacement of an alm 80 the steel tank would be capable of holding a greater supply of air thus would have a greater swing rate due to weight of the additional air. My E8-130 is the same ht as an alum 80 but has an air swing of about 10lbs ending up being -2 bouyant. I know I being petty but it's a pet peve of mine.

Someone made a point about having to kick or scull to maintain depth was because they were overweighted but that doesn't mean that a person is overweighted. If you were perfectly weighted with an alu tank at the begining of a dive then you would be 4-5 lbs light at the end of the dive due to the air swing. Kicking or sculling might mean that your neg bouyant and need to add a touch of air to your BC to get neutral. Now if someone had clairified that statement buy stating that with an empty tank and no air in the BC you had to kick or scull a bit, well then they may need to take some lead off for ther next dive. Me I like to be neutral at the end of a dive with less than 500 psi and no air in the BC, but I would also rather be a tad heavy than a tad light at the end of a dive. I wouldn't sug making a blanket statement like a persons overweighted without knowing more details.

I'm not a dive god nor will I pretend to be, I'm a diver who likes to dive and enjoys the sport plain and simple. I would think it's best to refer them back to the instructor and keep our answers simple. I think the best advice I ever received from an instructor was to be a silent but good role model. New divers listen and watch and pick up lots of info, some good, some bad but may not know the differance. Eplain what you need to when you need to, to much info can overload them with info but they may not have enough knowledge or understanding yet to put the information in the proper context or use.

Final food for thought on bouyancy. A week or so ago I made a fun dive after class with two newly cert ow divers. After the dive we talked and they asked me about why I never use my inflater under water or dump air when coming up but could still hover at will. My answer was that they needed to learn how to breath.

Geek