How do you assess your students weight and buoyancy?

[freewillie]

Last weekend I had the pleasure of taking my 13 yo daughter diving down in San Diego. She has previously done cold water dives with a dive master in Catalina at Casino Point. This trip she was with me as her dive buddy. Personally, I think she was overweighted in Catalina but we can't remember how much lead she used and she didn't record the amount in her log book.

For this dive I calculated about 14 lbs for her using the formula 10% body weight (she' 86 lbs) + 5 lbs for AL tank. First dive she complained that she was 'heavy' and had a tendency to sink. During the dive she was constantly playing with her inflator. At one point she was trying to clear her mask, and when I was watching her she started to rise so much so I had to grab her and pull her back down. From that point on I was within arms reach during the dive to help keep her from floating too far up or down.

The second dive I took off 4 lbs. When we tried to descend we got about 2-3 feet under and she just stopped. I motioned for her to give a big exhale to see if she could descend but we wound up going back to the boat and I put the 4 lbs back in her weight belt (she had the kind with pockets - made if very easy add the weight back). I could have tried her at 12 total but didn't want go back again if she couldn't sink to used the 4 lbs. But she still complained she was little heavy.

So, for all of you instructors out there, when your students complain that they are heavy how do you adjust their weights? How to you know it is really the weight issue or just that she (my dtr - or your student) is actually properly weighted but simply needs more experience diving and managing their buoyancy.

I know, do a formal weight check but we sort of did that. She couldn't sink at the weight she thought she should have been using.

Thanks.

 

[TSandM]

Well, the description of a diver that he is "too heavy" or "too light" can be very misleading, especially in someone who is a novice. Basically, unless someone is grossly overweighted and has a very small lift BC, it's possible to get neutral no matter how you are weighted. People who complain of being "heavy" are generally afraid to put enough air in the BC to become neutral, and this is often because they are not horizontal and have learned that any kicking will drive them upwards.

If your daughter could get 2 - 3 feet under the water, she'd done the hard part, and she was carrying enough weight to sink.

A formal weight check works, it really does. It's what we do with our students. Weight yourself so that, with an empty BC, you float with the water at eye level. Then add a pound for every 13 cubic feet of gas in the tank that you intend to use -- this will be proper weighting. (Tobin George of DSS would argue that, where you are diving, you may not even need that much, since the thick neoprene doesn't rebound instantly on ascent.)

Many novices have behaviors that impede descent. Kicking is a big one -- if you aren't balancing nicely at the surface, you are probably kicking, and will continue to kick in an effort to stabilize yourself as you begin to sink. Unfortunately, with your fins beneath you, kicking drives you upwards. Crossing the legs at the ankles can help remind someone not to do this, and bending the knees a little will get the diver out of the vertical position sooner, where an errant kick won't immediately result in an ascent.

Breathing is the other big one. NW Grateful Diver taught me that, rather than exhaling when I pushed the button to vent my BC, I should INHALE, and hold that breath as I emptied the BC. As my face got to the water, I was to EXHALE forcefully, which would continue my descent. Adding that to crossing the ankles resulted in huge improvement in my ability to sink.

 

[DivemasterDennis]

TSandM has posted a clear, concise and spot on statement. I wish I'd said it. I have students complain and I watch them. I am sensitive to volume of air in their BC at depth, and base wight adjustments on that as well as on a weight check before each dive on an open water weekend class. Often times students think too much. They have been told to be aware of every little thing, and they are. I confess we often overweight students a pound or 2 on their first dive ( fresh water reservoir, no entanglements, bottom 35 feet or less, training on a platform at 20 to 25 feet.) Many need the extra weight from the hyperventilating excitement of their first real plunge. But as we know, proper weighting is the key to proper buoyancy throughout a dive. TSandM, thanks. I printed your post for reference.
DivemasterDennis

 

[Blue Sparkle]

Agreed - I like TSandM's description. I know that when I did my OW dives I was grossly overweighted, but the thing is, I couldn't sink with less weight. And while the instructor told me I was overweighted and should not need that much, he never explained why I wasn't "sinking" or how to overcome it. I think I probably would have been fine at the right weight if I had known about the breathing/not-kicking tips. (Also we never did a weight check, but I still know that I was overweighted because I have since done one.)

 

[freewillie]

Thanks for all the input so far.

On our second dive with her diving lighter whe was actually more horizontal than vertical. She was making motions like she was having a hard time sinking. I actually took her inflator, held it up and tried to vent her BC. When it didn't vent I even undid the inflator hose from the BC to hold it up over her shoulder and did not get any venting so I know her BC was not inflated. It was weird, she was 2-3 feet under and just hovering.

I suspect she needed to exhale some more and relax to get under, but this is only her 4th dive since certification. In general I think she is doing really well. She monitors her guages carefully. I suspect with more experience we will dial in her buoyancy better. We discussed her difficulties sinking and I advised her about breathing out. We'll see if she has better luck next dive.

 

[TSandM]

One tip is to tell her to breathe out, and when she's done breathing out, to breathe out some more -- people are often very much surprised at how much more air there is to exhale!

 

[griffon]

It's pretty common with wet diving for new divers to have a big air pocket, burping the suit can really help and can make a several pound difference. Typically it's better when new to be slightly heavy rather then slightly light though, so I would have put those numbers at about right (assuming 7mm).

 

[DevonDiver]

In addition to that already posted, I always inform students to visually confirm that air is being dumped - i.e. watch the LPI and see that air is leaving the BCD when they try and dump it. It's amazing how many students vaguely wave the LPI above their heads with the button depressed and don't actually manage to get any air out (usually due to body and/or LPI position in relation to air in their BCD).

I also teach students to check their BCD for those 'air pockets'. A quick tactile check can confirm if and how much air remains in their BCD.

As for weighting - I just start with experience... pretty accurate with guesstimating a student's needs now. On a first dive, I carry extra weights just in case my estimate is wrong. At the end of that first dive, I do a full weight check (empty BCD, hover at 5m... progressively remove weight until optimal is found).

Of course - some vigilance is needed to ensure that student isn't making a 'technical mistake' with their initial descent procedure - I check that BCD is deflated/ing, student is exhaling... and that instinctive 'finning' isn't giving them upwards propulsion. Those 3 issues account for most student mis-appreciation of their weighting needs and the consequent assumption that they need more weight.

 

[halocline]

Weight yourself so that, with an empty BC, you float with the water at eye level. Then add a pound for every 13 cubic feet of gas in the tank that you intend to use -- this will be proper weighting.

Two quick points, the weight check as described is with an empty tank. Actually, and this one is REALLY nit-picking, it's not how much gas you intend to use, it's how much you are actually carrying. If you only intend to use half your tank and weighted accordingly, but due to a leak or an unexpected air-sharing, you'd end up using most o the tank, you'd be a little light, at least in theory. Otherwise, just right!

Getting a new diver's weight dialed in precisely is not easy; there's usually some experimentation to do, and of course with a new diver's inconsistency in breathing control and sub-conscious kicking, there can easily be a few lbs difference over a couple of days' diving that seems to work best.

 

[--tom--]

Halocline: I think TS&M's reference to "Then add a pound for every 13 cubic feet of gas in the tank that you intend to use" is referring the size of the tank the person has chosen to use. A 13 yr. girl will probably use a 50cf tank, so ~4 lbs swing, dad would probably use an 80AL closer so ~ 6lbs...