Over vs underweighted

[String]

However, the number you mentioned is kind of meaningless in itself as it doesn't say anything about what that number means in relation to weight required to be neutral in that water.

It does because its directly related to density which is directly related to the weight needed

Fresh water is 1.00kg/m3. Salt water ranges from 1.020 - 1.030kg/m3. So the difference between the red sea and the pacific (the lightest waters) is still only a fraction of 1% of the total mass in terms of weighting (even factoring different temperatures) - thats insignificant especially when your weights come in 1kg chunks.

The DEAD sea on the other hand does make a substantial difference where its 33% salt content is 10 times that of the ocean.

 

[IceIce]

Over weight, especially early in the trip when wetsuit is still dry.
Puff some gas in bcd and it will work out fine.

Underweight is dangerous especially for new divers when they can flot up to surface anytime. Combined with depth, current and boat passing by, it can be very very dangerous, esp night dive.


Cheers,
Ice

 

[Phil G]

Someone said that being underweighted, even by just a few pounds, is not really dangerous, because you slowly rise to the surface.

I disagree.

The real problems starts at the end of the dive, when you carry less air, so less weight and you're starting to go back to the surface. By then, you're even lighter and more underweighted than at the start of the dive and you're equipment starts to expand back due to the lesser pressure.

All this doesnt cause you to slowly rise to the surface, but to accelerate when rising, unless you act fast.

Phil


"Fresh water is 1.00kg/m3." ----> might be gassy water. Shouldnt it be 1.00 kg/Liter ?

 

[String]

The real problems starts at the end of the dive, when you carry less air, so less weight and you're starting to go back to the surface. By then, you're even lighter and more underweighted

No you aren't. As weighting is measured or should be on an empty tank. If someone is more than 5-6lbs underweight they wont even start the dive. At absolute worst case assuming they descend they're going to be 4-5lbs positive and that IS possible to swim down/control against. That is not going to cause a rapid ascent.

Its rapid expansion of large amounts of gas in jackets/suits that causes accelerating rapid ascents and that is an overweighting issue.

"Fresh water is 1.00kg/m3." ----> might be gassy water. Shouldnt it be 1.00 kg/Liter ?

SI unit for density is kg per cubic metre (density is mass/volume).

 

[InTheDrink]

It does because its directly related to density which is directly related to the weight needed

Fresh water is 1.00kg/m3. Salt water ranges from 1.020 - 1.030kg/m3. So the difference between the red sea and the pacific (the lightest waters) is still only a fraction of 1% of the total mass in terms of weighting (even factoring different temperatures) - thats insignificant especially when your weights come in 1kg chunks.

The DEAD sea on the other hand does make a substantial difference where its 33% salt content is 10 times that of the ocean.

Ok, so you're absolutely right. I won't try to work out the maths in detail only to find I'm wrong: a quick google backs you up. Another myth busted! Cheers, J

 

[InTheDrink]

Its rapid expansion of large amounts of gas in jackets/suits that causes accelerating rapid ascents and that is an overweighting issue.

I very much agree with this statement. The less air in your BC to keep you neutral the better - particularly for new divers.

 

[boulderjohn]

Why would you ever HAVE to be either one?

When you go to a resort, hop on the boat, and head for the weights they supply, you may find that you have to make some unpleasant choices because of what they have in stock. I have been on boats where the only weight options were frankly ridiculous. In one such recent case, I made a decision that will seem wrong to some, but worked really well, and it brings me to a point that has not been made yet in this thread.

I think WHERE you put the weight is at least as important as how much weight you have. For example, if I am diving steel double 120's with a BP/W, I am usually very much overweighted at the beginning of a dive, even without adding another ounce of lead. The fact that the tanks distribute that weight over my entire back helps me deal with that extra weighting effectively.

I was recently on a boat in Mexico with my ScubaPro Nighthawk, a back inflate BCD with characteristics similar to a BP/W. The boat only carried 4 pound weights. With that setup, an aluminum 80 tank, and my 3 mm wet suit, 8 pounds is about perfect, meaning it is the least weight I want. I therefore had to choose whether to go in with those 8 pounds only or double it to retain some sense of balance. I really wanted to have some of the weight in the trim pockets by my shoulders, so the weight would be distributed, as it is with steel doubles. I decided to try a dive with 16 pounds--way more than I would normally use--with 4 in each of the regular pockets and 4 in each of the trim pockets.

It was just fine. In fact, I thoroughly enjoyed the dive. I was able to maintain horizontal trim effortlessly. I have enough experience to avoid the overweighted/overinflated problem that String very accurately describes.

On another trip, with a similar what-do-they-have-on-the-boat dilemma, I ended up taking a weight and strapping it with a weight belt to the top of my tank, and it worked great.

 

[boulderjohn]

Its rapid expansion of large amounts of gas in jackets/suits that causes accelerating rapid ascents and that is an overweighting issue.

This very accurately describes a very common problem with beginning divers and (frankly) DM's on a lot of commercial operations.

Here's the situation:

A diver is doing a relatively shallow dive, such as one would do on Molasses Reef in Key Largo, Florida. The diver keeps finding himself/herself heading toward the surface during the dive. The diver vents air to get back down each time it happens, then reinflates at depth to get relatively neutral.

So, for the next dive, the diver asks for more weight, or the DM offers more weight. It only makes things worse.

The diver was going to the surface not because of not enough weight but rather because at that shallow depth, the air in the BCD is very reactive to changes in pressure caused by the diver's change in depth during routine swimming. The diver goes up a little, the air starts expanding, and the diver is on the way to the surface. If you add more weight, then you have to add more air to the BCD at depth to get neutral. More air means more reaction to those changes in depth.

If you have enough weight so that you have to add air to your BCD to get neutral, then you don't need any more weight (at least until your tank empties).

A more experienced diver will anticipate those changes in depth and adjust air volume in the lungs to compensate.

 

[gotthebug]

I very much agree with this statement. The less air in your BC to keep you neutral the better - particularly for new divers.

I guess the answer is to be weighted just right. I just adjust mine based upon conditions and experience and that seems to work fine for me.

 

[Twistedarts]

Good luck trying to swim a rig up carrying a few tanks and/or stages! I wouldn't fancy the constant effort maintaing a depth swimming simply to hold a stop either.

I'm just a rec diver. I do not carry stages and have not dove doubles (although I would like to someday). Maybe swim was a poor choice of words but I do not "float" my way up, I try to stay as close to neutral as I can so that I'm in control of the ascent and only rise when I kick. At depth my wing is usually almost empty. Back in the day, and if I remember correctly they taught us to weight for depth not the surface. It seems that has been reversed in current times/teachings.