Confusion

[Jeff Toorish]

Dori Fish --what kind of diving are you planning to do primarily? That will go a long way to determing what tanks will be best for you.

If you want to become a cave diver, one type of tank would fit the bill. If you are going to limit yourself to shore dives in warm water, another tank will do.

Is air consumption an inordinate concern for you? Or do you use less air than most divers?

Discussions of fills and tank weight are really less important than what you plan to do with those tanks once they are strapped to your back (unless you are considering becoming a sidemount diver, which would also impact your decision).

Jeff

 

[WD8CDH]

<snip>It is pretty common for 3442 psi tanks to get filled to 3600-3700 psi in cave country, but no one is filling them much over that. Most people understand that the test pressure is 3/2 rather than 5/3rds and that the safety margin on HP tanks is not as great (and they lack the service history of the LP tanks), so I don't forsee shops that cater to cave divers pushing them more than a few hundred psi over the 3442 psi service pressure.

The safety factor of Exemption or Special Permit tanks is actually almost exactly the same as 3AA tanks if you consider the allowed 10% over fill of 3AA tanks. i.e. 3AA tanks are hydro'd at about 3/2 of the 110% fill pressure.

That is also why there are some "exemption" LP tanks (made by Faber and others). They effectively are made the same way as a 3AA tank but they are stamped at 110% of what they would be if they were certified under 3AA but they don't need the extra REE calculation to keep the "plus" rating.

 

[DA Aquamaster]

Note the "and". Test pressures are different as well as the engineering standards. Like most structurally engineered products of the era, 3A and 3AA tanks were overengineered with a very large safety factor to take into account what engineers did not know or could not control in the manufacturing process.

It's harder to say that with the same degeee of confidence about current special permit steel tanks especially when you consider they are certified for higher pressures yet weight less for a given physical tank size (i.e. thinner walls).

 

[Leadking]

When looking for a tank consider the following:

• buoyancy characteristics
• price
• amount of air needed
  • 
    
    
    Prioritize to your needs. Pay particular attention to buoyancy. A steel tank may seem attractive if you do not have to wear a weight belt but remember you WANT to have ditch-able weight in an emergency. I would rather ditch a belt than a tank.
    
    If price is not a major consideration and buoyancy is not an issue, as a sport diver, it is virtually impossible to go wrong with an HP steel 100. Even with a short (3000psi) fill you have more air than an aluminium 80 in a smaller, less buoyant package.

 

[DA Aquamaster]

Prioritize to your needs. Pay particular attention to buoyancy. A steel tank may seem attractive if you do not have to wear a weight belt but remember you WANT to have ditch-able weight in an emergency. I would rather ditch a belt than a tank.

Ditchable weight gets excessive emphasis based mostly on outmoded agency dogma.

In 25 years of diving and maybe 2000 dives I have only encountered one diver who ever dumped weight in an "emergency" underwater and frankly there was no real need to do it then (he was upside down in a dry suit in a current and had better options than dropping weight).

In contrast I have long since lost count of the divers who have unintentionally lost ditchable weight underwater and then struggled to stay down, control their ascent rate, hold a safety stop, etc. In effect, assuming a diver is properly weighted rather than over weighted to start with, ditchable weight creates more emergencies than it resolves.

If ditchable weight has a role, it is only to establish buoyancy in an emergency on the surface when a need exists to immediately establish positive buoyancy. That was a much bigger issue in the pre-BC power inflator era, but now for the need to exist you need an emergency on the surface plus an OOA or BC failure scencario on top of the initial emergency - and those are very rare. In most cases, dumping the BC/tank, leaves the diver positively bouyant in their exposre suit and it is only slightly more difficult to ditch the BC, tank, etc than a weight belt or integrated weight pouches.

Again the important thing is not ditchable weight but rather that the diver understand proper weighting and not be over weighted (neutral with no air in the BC, 500 psi in the tank, at 15').

 

[RJP]

Remember a one pound of feather weighs just as much as one pound of lead. So a 80cft tank at 3500psi has just as much air as an 80cft tank at 3000psi.

Also remember that there are many people on the internet who have no earthly idea what they're talking about!

 

[WD8CDH]

<snip> Like most structurally engineered products of the era, 3A and 3AA tanks were overengineered with a very large safety factor to take into account what engineers did not know or could not control in the manufacturing process.<snip>

That is why the + rating came about on the 3AA tanks (way back around WWII). Most newer standards have about 10% less safety factor (related to stamped service pressure) but don't allow the 10% overfill. So the normal operating safety factor as about the same.

 

[Leadking]

Ditchable weight gets excessive emphasis based mostly on outmoded agency dogma.

In 25 years of diving and maybe 2000 dives I have only encountered one diver who ever dumped weight in an "emergency" underwater and frankly there was no real need to do it then (he was upside down in a dry suit in a current and had better options than dropping weight).

In contrast I have long since lost count of the divers who have unintentionally lost ditchable weight underwater and then struggled to stay down, control their ascent rate, hold a safety stop, etc. In effect, assuming a diver is properly weighted rather than over weighted to start with, ditchable weight creates more emergencies than it resolves.

If ditchable weight has a role, it is only to establish buoyancy in an emergency on the surface when a need exists to immediately establish positive buoyancy. That was a much bigger issue in the pre-BC power inflator era, but now for the need to exist you need an emergency on the surface plus an OOA or BC failure scencario on top of the initial emergency - and those are very rare. In most cases, dumping the BC/tank, leaves the diver positively bouyant in their exposre suit and it is only slightly more difficult to ditch the BC, tank, etc than a weight belt or integrated weight pouches.

Again the important thing is not ditchable weight but rather that the diver understand proper weighting and not be over weighted (neutral with no air in the BC, 500 psi in the tank, at 15').

In the sport there are multiple instances of divers drowning because of the negative buoyancy of their tanks. In a desire to get rid of a weight belt or to rely on thier BC to gain positive buoyancy is foolish. BC's fail, divers make mistakes. Multiple cases of divers entering the water forgetting to turn on their air and pounding themselves into the bottom and drowning as well as BC failures are well documented in the sport. To mislead a new diver into not understanding the risks involved in buoyancy characteristics is irresponsible. A diver floating on the surface is an inconvenience not a fatality.

 

[DA Aquamaster]

In the sport there are multiple instances of divers drowning because of the negative buoyancy of their tanks. In a desire to get rid of a weight belt or to rely on thier BC to gain positive buoyancy is foolish. BC's fail, divers make mistakes. Multiple cases of divers entering the water forgetting to turn on their air and pounding themselves into the bottom and drowning as well as BC failures are well documented in the sport. To mislead a new diver into not understanding the risks involved in buoyancy characteristics is irresponsible. A diver floating on the surface is an inconvenience not a fatality.

I don't entirely disagree, but I think we would all be much better served addressing the underlying problems than using ditchable weight as a crutch.

The reason we have so many "new" divers doing stupid stuff and sometimes dying is directly related to watered down OW courses, a move away from solid swimming skills and a failure to screen out those divers who frankly shoudl just not be diving. The dive training industry as a whole is not about safety, it is about separating as many new students from their money as possible in the shortest possible time. Ditchable weight just makes all that easier to do while ignoring basic skills.

 

[Leadking]

I agree on the water skills. I have long advised potential divers, who are looking for a quickie certification, with the following;

If I took you 25 miles offshore and left you in a wetsuit and told you I would be back in an hour, would you be comfortable with that? If the answer was no, I would advise them to take the longest certification course possible.

Unfortunately skills come over time, and usually are the result of things going wrong, and learning from one's mistakes. In a rebreather seminar with the EDU (Experimental Dive Unit of the USN) their analysis of rebreather incidences stated that the had never seen a catastrophic failure of a rebreather. The statistics show that rebreather divers are dying at a rate higher than open circuit. I think we can safely assume that rebreather divers, as a group, are experienced beyond the average diver. So what has all this experience delivered? Divers make mistakes. The system needs to try to make sure it's not a fatality