Please notice this! When having thick neoprene suit (7=>) there's a risk of uncontrollable decsend. When your ballast (weights, regs, knife, steel botles etc) comes close or exceeds the volume of the BC it's possible to get into situation where there's negative boyancy even with fully inflated BC and after some 80'-120' you sink................................Condolences to family.
Descending Vertically Vs Horizontally
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Cold_H2O
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I always have trouble decending on my first dive. It doesn't matter what I am wearing and has nothing to do with my being over or underweighted.
I am fine once my gear has had enough time to take on some water.
I start in the verticle postion but after a few feet shift to a horizontal position. I like seeing where I am going and find that it is easier to slow down or stop in this position if I need to.
I am fine once my gear has had enough time to take on some water.
I start in the verticle postion but after a few feet shift to a horizontal position. I like seeing where I am going and find that it is easier to slow down or stop in this position if I need to.
AquaHump:
Whether you are decending feet first or horizontally, you need a balanced rig which includes proper weighting. You should be weighted (depending on your rig, wetsuit thickness, tank choice) so that you can maintain neutral buoyancy at 15-20 feet with 500 psi in the tank(s). As far as how much air to let out at the surface; you need to let out enough, so that when you exhale completely, you make a slow descent so that equalization isn't a problem...period. If you have to let all the air out fine. If you don't fine. If you are properly weighted and balanced you'll find that you are overweighted a little at the beginning of the dive to account for the positive swing of the tank (if you have a tank that swings negative to positive as the air is depleted). I never let all the air out; just enough to start the descent...
You add no air to get neutral...so what do you do when you go deeper? Do you add air then? How about when you come up to a shallower depth? I'm sorry but if you are nuetral at 60 ft with no air in your BC then there is no way in the word that you can be nuetral at 15 ft. Not happening in this world.ScubaTwo:I guess Im a freak of nature cause I do let all the air out and I dont add air to get neutral. But as far as how slow I descend I thank you all for your suggestions and will work to improve on it. Happy and safe diving
Charlie99
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Air is air and weighs the same no matter what type of tank it is in.P_Abercrombie:
Doesn't matter whether the tank is positive or negative when empty, the difference between full and empty just depends upon how many cubic feet of air it holds.
You will always be a bit heavy at the beginning of the dive due to extra gas, but offset by extra buoyancy of your wetsuit or other gear that hasn't yet gotten saturated with water. With a dry 3/2 or 5mm suit and 80 cubic feet of gas I'm pretty close to neutral on the surface with no air in my BCD. I'll have to add extra air over the first 2 or 3 minutes of the dive (beyond that needed to compensate for wetsuit compression) as my wetsuit gains weight as it saturates.
Charlie99:
This is simply not true. Tanks, whether aluminum or steek, have different buoyancy characteristics depending the size of the tank and how much gas is in them. Check the websites for the various manufacturers and the information is readily available. An AL80 swings negative to positive in the water when the pressure runs down. Steel tanks tend to stay on the negative side even when empty. This is a fact; no debate necessary.
You guys are both right..P_Abercrombie:
What I think you're missing is that 80cf of air weighs the same whether it's in an Al80 or an HP80. While the HP80 may be negative or neutral at the end, both of the tanks have gained about 6 pounds of buoyancy going from full to empty. All the HP80 allows you to do is take a little lead off your belt, because it's built into your tank.
Properly weighted, you will be overweight by 6 pounds at the beginning of a dive if you have 80cf on your back.. whether that 80cf is 3100 psi in an Al80, 3500 psi in an HP80, or 2000 psi in an LP104.
dbulmer
Contributor
Plus the weight of gas in your cylinder is also affected by the ambient temperature if I'm not mistaken?
P_Abercrombie:
What Charlie99 was addressing is this statement....
What Charlie said is true. All tanks swing from a negative weight to a less negative (or more positive) as the air is depleted from said tank. The fact that a steel tank is still negative when sucked down to 500 psi does not discount the fact that it was more negative at the beginning of the dive.P_Abercrombie:
Let's see it in numbers....
Let's say you are using a steel tank that when full is negative 8lbs and when empty is negative 1lbs. Let's also say that you need 5 lbs to be neutral on your safety stop. Okay, your tank will be negative 1lbs when doing your safety stop so that means you need to add 4 more lbs on your belt to get the 5lbs need to be neutral at the safety stop. So now let's look at the beginning of the dive and how much weight you were carrying. 8lbs (from a full tank) and the other 4 you put on a belt. So you see from this example that you are still overweighed at the beginning of the dive the same as if you had been diving one of your Al80's.
pants!:
Nope. While the amount of air weighs the same it does depend on the type and size of tank you put it in. That is the difference. An AL80 and a HP steel 80 might both contain 80cf but the buoyancy characteristics of the tanks they come in are different. An 80cf steel tank will always be more negative than an AL80 even if they have the same pressure in them.
See http://www.luxfercylinders.com/products/scuba/specifications/us_imperial.shtml
for an example of what I mean. They list the tank sizes and the shifts of the tanks from negative to positive based on the amount of air in the tanks. Here is a link for PST's (steel tanks) http://www.pstscuba.com/specsheet.pdf
Notice the steel PST 80cf tank swings from -8.5 lbs full to -2.5 lbs full.
Notice that the Luxfer AL80 swings from -1.4 to +4.40! Same amount of air in the tanks when filled to the rated pressure but look at the buoyancy swings. That has to be accounted for when balancing your rig and determining how much weight you need.
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