cooperscuba
Contributor
If your properly weighted at the start of a dive (using ALU not steel cylinders) and 1-2 KG is to counter the air you're going to breathe during the dive, are you, in fact, over-weighted?
If you're properly weighted at the start of a dive; are you over-weighted?
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Nirvana
Contributor
I'd say you are properly weighted and a little negative.
Whats important is the situation at the end of the dive.
If you are underweighted at the start of the dive then when you reach the safety stop you will have trouble holding at 5 metres (15 ft), possibly requiring you to skip the stop and go directly to the surface..
That is much worse than having to add a puff of air to your BCD at the start of the dive to counter the weight of the air in your tank.
If you are underweighted at the start of the dive then when you reach the safety stop you will have trouble holding at 5 metres (15 ft), possibly requiring you to skip the stop and go directly to the surface..
That is much worse than having to add a puff of air to your BCD at the start of the dive to counter the weight of the air in your tank.
You are properly weighted but let me correct you on one item. For whatever reason many people think there is a weight/bouyancy SHIFT difference between steel and aluminum tanks, there is not. The weight or more correctly bouyancy shift of a tank soley depends on it's volume and has nothing at all to do with the material it is made of. An 80 cf/12L tank made of any material will have the exact same shift, assuming you use the same amount of gas out of it. You will need more weight (lead) to offset your overall positive bouyancy with an aluminum tank as opposed to a steel one but shift in bouyancy, which is what you need to be concerned about, is no different as long as the tank volume is the same.
underH2Oman
Contributor
If you weight yourself at the beginning of the dive without taking into account for the loss of weight from gas consumption you may find it difficult to complete a safety stop at the end of a dive. You should be able to stay neutral buoyant in 10 ft of water with very minimal gas in your tank (500psi or even a little less).
You would be slightly more negative at the beginning of the dive than at the end, but proper weighting requires you to consider the entire dive and weight yourself accordingly. You would be a little more negative but not over weighted. You can increase your buoyancy quite easily by adding a little air to the BC zt the begining of the dive, but reducing buoyancy when your tanks get lighter by removing gas from an empty BC is not possible.
You would be slightly more negative at the beginning of the dive than at the end, but proper weighting requires you to consider the entire dive and weight yourself accordingly. You would be a little more negative but not over weighted. You can increase your buoyancy quite easily by adding a little air to the BC zt the begining of the dive, but reducing buoyancy when your tanks get lighter by removing gas from an empty BC is not possible.
In some aspects I am lucky here in Kwajalein. When I work with new Open Water students our confined area is a swimming pool with sea water. So we are able to fine tune their weight prior to getting into the ocean. The description the OP mentions is pretty close to what we end up. Being a PADI OW course I use the weight check system of enough weight to float at eye level with a full tank and a "normal" breath. If they start to descend when they exhale we have our starting point. Each student diver will be a little different so I make my fine weight adjustments base on their personal characteristics. My goal is NOT to over weight them to make my life as an instructor easier, but to provide them the tools to make adjustments as their diving parameters change. Based on their body's composition and the size of their tanks some need a little more weight and some a little less to hold a safety stop and continue into the beach with 600-800 psi / 40-50 bar while diving in control in less than 10' / 3 m of water which is a typical way to end our beach dives.
oldschoolto
Contributor
You need to be very slightly "NEG" at the end of your dive with 300 to 500psi in the tank and no air in your lungs.. ... I see people so concerned about getting every bit of weight off that they can't control themselves at the end of the dive... Better to have 2lbs more lead then corking when it's "DECO" time..
Jim....
Jim....
You are about right in your assumption. Like a few others, I would take for a first dive, a bit too much lead and fine tune/check my buyancy at the end of the dive 
. You want to be slighty negative with no air in your lung. Please make sure that your fins are TOTALLY inactive. I see too many people finning slowly when checking buyancy at the surface 
Your assumption that you check your buyancy at the surface + add a few pounds for the air you are going to consume is correct in a lot of cases. BUT, in cooler waters, for a first dive with very thick but dry wetsuits, you can have A LOT of air trapped in your wetsuit at the surface. When you are diving, your wetsuit will compress and you will loose all pockets of air that you had between you boddy and your wetsuit. Therefore a few pounds extra may be too many.
. You want to be slighty negative with no air in your lung. Please make sure that your fins are TOTALLY inactive. I see too many people finning slowly when checking buyancy at the surface Your assumption that you check your buyancy at the surface + add a few pounds for the air you are going to consume is correct in a lot of cases. BUT, in cooler waters, for a first dive with very thick but dry wetsuits, you can have A LOT of air trapped in your wetsuit at the surface. When you are diving, your wetsuit will compress and you will loose all pockets of air that you had between you boddy and your wetsuit. Therefore a few pounds extra may be too many.
I would just add this: that the decreased weight of the depleted tank is set off a little bit by the complete saturation of the wet suit ( especially on a 1st dive) . Still, a proper weight check has no substitute when it comes to proper weighting.
DivemasterDennis
DivemasterDennis
It is quite common to see people confuse two concepts. One is proper weighting, and the other is the diver's relationship to neutral buoyancy.
When the dive is begun, the diver is wearing a bunch of equipment, some of which floats and some of which sinks. The "hard parts" of the equipment won't change buoyancy through the dive, but the "soft parts" (which include exposure protection and any padding) will. In addition, the diver is carrying somewhere between 5 and 10 pounds of breathing gas that he intends to exhaust into the water as the dive progresses.
So let's assume we have a diver in a 7 mil wetsuit, carrying a 100 cubic foot scuba tank, and wearing 20 pounds of lead. When the tank is empty and the diver is at the surface or in very shallow water, this diver, with his equipment, is cycling right around neutral while breathing normally. When the tank is full, he is about 7 pounds negative under the same conditions. Negative -- but not overweighted, because if you took off any of the lead he is wearing, he would be POSITIVE at the end of the dive with the air all used, and he would have difficulties holding a safety stop.
Now let's take this diver down to 100 feet at the beginning of his dive. His 7 mil wetsuit is going to lose a lot of its lift. I know some folks did an experiment a number of years ago, and took a fairly new 7 mil suit down to 100 feet to measure how much buoyancy it lost, and they came up with over 20 lbs. So our diver, who started the dive 7 pounds negative, is now 27 pounds negative! Without air in his BC, he will sink like a rock; but he still isn't overweighted. He needs quite a bit of lift in his BC to counter all the flotation he's lost from neoprene compression, and he might be in quite a bit of trouble if that BC fails, but he isn't improperly weighted. (He may be unwise, though . . . )
"Negative" and "overweighted" are commonly confused, but they are not the same.
When the dive is begun, the diver is wearing a bunch of equipment, some of which floats and some of which sinks. The "hard parts" of the equipment won't change buoyancy through the dive, but the "soft parts" (which include exposure protection and any padding) will. In addition, the diver is carrying somewhere between 5 and 10 pounds of breathing gas that he intends to exhaust into the water as the dive progresses.
So let's assume we have a diver in a 7 mil wetsuit, carrying a 100 cubic foot scuba tank, and wearing 20 pounds of lead. When the tank is empty and the diver is at the surface or in very shallow water, this diver, with his equipment, is cycling right around neutral while breathing normally. When the tank is full, he is about 7 pounds negative under the same conditions. Negative -- but not overweighted, because if you took off any of the lead he is wearing, he would be POSITIVE at the end of the dive with the air all used, and he would have difficulties holding a safety stop.
Now let's take this diver down to 100 feet at the beginning of his dive. His 7 mil wetsuit is going to lose a lot of its lift. I know some folks did an experiment a number of years ago, and took a fairly new 7 mil suit down to 100 feet to measure how much buoyancy it lost, and they came up with over 20 lbs. So our diver, who started the dive 7 pounds negative, is now 27 pounds negative! Without air in his BC, he will sink like a rock; but he still isn't overweighted. He needs quite a bit of lift in his BC to counter all the flotation he's lost from neoprene compression, and he might be in quite a bit of trouble if that BC fails, but he isn't improperly weighted. (He may be unwise, though . . .
)"Negative" and "overweighted" are commonly confused, but they are not the same.
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