Weight check at start of a dive??

[miked]

I just returned from a trip. The diving was good, but I have a question regarding how this op approached weight checks (6 different DMs, said the same thing, so it probably is the op’s idea).

They asked the divers to please do a weight check at the start of the dive. The DM then explained how (this is word for word as I copied it down after hearing it 3 times.

“ To do the weight check, float at the surface, completely empty your BC, and exhale slightly. If you sink, you have enough weight. If you do not sink, come back to the boat and the crew will give you a few pounds more.”

This seemed to work for most of the divers (not all), but my questions are:

1) Does this method fail to take into account the difference in buoyancy between a full tank, and a tank at the end of the dive?

2) Could this method result in divers being significantly overweighted?

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On a similar note:
What about the old “if you float at eye level with a full tank, empty BC , and holding a “normal” breath you’ll be neutral at 15 ft. with a near empty (500 psi) tank.”


I realize that the better methods would involve a near empty tank, and being at depth,but what is the best way (if there is one) to do a surface weight check with a full tank?
[I recall the "eye level" method, but then add 5 lbs for the weight of air (AL 80).]

 

[dumpsterDiver]

It is a very good way to flag divers who are underweighted. They may actually need MORE lead than this technique indicates, but it is a good first cut and can be fine tuned on subsequent dives when the diver finds out how he feels with a tank that is close to empty.

 

[Insta-Gator]

Normally (according to the method I use) if the diver is holding their breath when the air is released from the BCD, they should sink to the point where the water line is mid way across the mask. Once the diver exhales they should sink if they are properly weighted. If they do not sink, add 2 or 4 pounds (in salt water.) As dumpstgerDiver said, weighting can be tweaked for subsequent dives.

My guess the dive op would rather the divers be slightly over weighted than under weighted. Seems like an expedient way to check/adjust weight on a boat.

 

[chrpai]

It's not the optimal way of doing an NB check for sure.

 

[OldNSalty]

It is not uncommon for divers to show up and not have enough lead to sink. Does having enough lead to sink mean you are properly weighted? No, but they are trying to avoid the problem of people jumping in, floundering around on the surface a bit (and holding up other divers) and then needing to climb back on the boat to get more lead.

 

[eelnoraa]

what wetsuit? If it is 5mm or 7mm, I often heard the argument that by the end of the dive, your wetuit will be compressed enough that it won't fully rebounce to the level at the beginning of the dive. Added 15ft of depth, the wetsuit may lose enough buiyance to compensate for the weight of the gas. With thich wetsuit, I found this work pretty well. With thin suit for drysuit, this can be a problem.

 

[spectrum]

NO that check is almost worthless. It won't detect marginally under weighted or over weighted.

A better version is to bob in and out of eyeball level and then add 5 to account for use of air. Some of this may get backed out latter to account for trapped air early on and variable neoprene rebound.

More here.

Pete

 

[RonFrank]

I tell folks to do a standard check. Once they find an optimal weight just add 5lbs for the full tank.

 

[TSandM]

Where was this taking place? If this was in the tropics, with thin or no exposure protection, this was not correct and will result in divers being light at the end of the dive.

If it was in colder water with wetsuits, it might work just fine; with the small amounts of air trapped in the wetsuit, plus the fact that it hasn't been compressed, result in divers being a bit more positive than they are at the end of the dive. If they are using very large tanks, though, the difference may not be enough to compensate for the weight lost through exhaled gas.

With a dry suit, I'd say this isn't going to work at all.

 

[supergaijin]

I realize that the better methods would involve a near empty tank, and being at depth,but what is the best way (if there is one) to do a surface weight check with a full tank?
[I recall the "eye level" method, but then add 5 lbs for the weight of air (AL 80).]

Assuming that this was a tropical dive, then the 'exhale to get down' approach would probably work for the vast majority of people who don't know how much weight they need.

My reasoning for this is that IME beginner divers enter the water without properly slowing themselves down first ie. increased heart rate and breathing rate. If they can sink themselves in this state, then by the end of the dive (when breathing rates are reduced due to increased relaxation) they will be able to hold their SS.

BTW I have used this method in both tropical (3mm) and not so tropical (2 piece 7mm) waters and it's pretty accurate for the beginner divers who don't know how much weight they need. In the colder water, the shock of the entry also makes people breathe more rapidly and shallow- whatever the reasons, it seemed to work in both climes.

So as a general way of guesstimating weight requirements for beginner divers (who rightly or wrongly often have no clue as to their weight requirements for a given dive), the 'breathe out and sink' technique works pretty well IME.

I personally dislike the PADI way of weighting- floating at eye level, holding a normal? breath, exhaling to get down and then adding 1 or 2 kg (2.2-4.4lb) to compensate for the tank. Looks really nice in a swimming pool or in Confined Water classes but simply doesn't work out in the field- especially in divesites exposed to waves, current, outboard engines, groups of divers etc.

It is not the end of the world to be overweighted during a dive- an extra kg or 2 doesn't require you to plow through a divesite. I regularly dive up to 4kg overweighted on the job and can still hover horizontally even in the clearly inferior Jacket BCD

Another way of looking at it is to consideer that all divers wearing a 2 piece 7mm suit, will be severely overweighted 100' down which is why they require BCDs- at these depths you're adding massive amounts of air, not just a couple of puffs and that'll be about right like in the tropics.

Your personal weight requirements will change fairly often if you're an active diver traveling to various places to dive- but the upside is that, as an active diver, these weight changes are easier to estimate and compensate for.

Another reason why new (<50 dives) divers should use a logbook for more than "Saw a red fish, a blue fish and a yellow fish" and getting a stamp.