To be honest, I would start with the 80s and see how you go. I suspect you will find your bottom time extends naturally as you get more comfortable, even over the course of a week.
100cf tanks?
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emttim
Contributor
The wife and I are going to Cozumel in June for our first true dives after certification.
Someone suggested that as "newbs" we are probably going to suck a LOT of airand that we should ask for 100cf tanks instead of the 80cf standard ones.
The $10 a day extra for the tanks do not bother me one bit, but I am curious as to what others have to say. I want to ensure we enjoy our dives and get maximum SAFE time (we will have either a Suunto or Oceanic computer, I am still trying to figure out which one) and do not want us being "newbie breathers" to cut short our time.
Personally, I went the HP100 route as a new diver for precisely that reason and I was indeed happy with the bottom times I was able to get with it. My original dive buddy sucked down air equally as badly so he got an HP100 as well, and fresh out of OW, we were able to go from 30 minute bottom times to 45 minute bottom times, sometimes 50 minutes.
One thing you will want to keep in mind if you did your OW class with say an AL80 is that with a steel tank, especially a HP100, you will need far less weight in your BCD. Make sure that you do a proper weight check after each dive until you get your weight adjusted for the different tank.
Pretty much exactly how me and my original dive buddy's experiences went.
Are their typical divers blessed with the physique of Arnold? HP120s weigh around 42 pounds don't they?
This is probably the best advice. Plus Cozumel is obviously warm water so I would imagine air consumption rates will improve if the OP got certified in cold water.
I'm not sure if air consumption rates will decrease a remarkable amount over the course of a week, unless there's an enormous amount of diving taking place in that week, but they will definitely go down with time so it's a self-correcting problem.
eric gundrum
Contributor
Looking in my log book, I see that over several dive days I was using 20# with HP100s. My use of AL80s was less frequent. For most AL80 dives I used 28#, but dropped to 26# on my last trip. All the other gear was the same. Doing the math, you might start by dropping 6# from your normal weight and see how that goes. Then see if you can trim a few more pounds.
An AL 80 is about 4 lbs positive when near empty (4.4 lbs positive when totoally empty).
In comparision, a Luxfer AL 100 is 3 pounds positive when empty but 9 pounds heavier in terms of overall weight compared to an AL 80. Despite being 9 pounds heavier the buoyancy is a wash and it is more or interchangeable with an AL 80 in terms of lead you need to carry.
A Catalina AL 100 is neutral when empty but weighs 46 pounds, so you save 3 pounds of lead off the weight belt - but the tank itself is 15 pounds heavier than an AL 80.
PST steel HP 100's are -1.3 pounds negative when empty and weigh 33 pounds - only 2 pounds heavier than an AL80 but they require 4 pounds less weight to be neutral at the end of the dive so you end up with a net savings in weight of two pounds.
Worthington 3442 psi X-7-100s weigh 33 pounds and are -2.5 pounds buoyant when empty. So you they need 6 pounds less weight than an AL 80 with a 4 pound net savings in weight. The qualifiyer here is that it does not really help you unless you need at least 6 pounds of weight.
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High air consumption tends to go hand in hand with over weighting. Excessive weight at best requires more work and more gas to maintain neutral buoyancy. For newer divers less skilled at buoyancy control excess weight usually results in much more difficulty controlling buoyancy do to the larger gas bubble being managed and many new divers expend large amounts of energy swimming up or down throughout the dive to compensate for less than perfect buoyancy.
My advice is to ensure you have your weight exactly right before you go on the trip and make adjustments on each dive on the trip until you get it exactly right (neutral at 15' with absolutely no air in the BC and 500 psi in the tank. In that condition you should be floating about eyeball level at the surface (with absolutely no finning or hand skulling) with full lungs and you shoudl sink when you fully exhale.
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If you have the chance, do a dive to determine your normal SAC. Basically swim around just under the surface (10-20 ft) at a normal speed for five minutes, then go to a moderate depth (30 to 60 ft), establish neutral buoyancy, note the time and your spg reading and the time (if using a computer start when the computer kicks over from one minute to the next) and then swim at that exact depth at the same normal steady swimming speed for 5 or 10 minutes (longer is better, especially at shallower depths). Then at the 10 minute mark note your spg reading.
You will need to know how much gas you used. If you used an AL 80 it has 77 cu ft at 3000 psi. If you used 500 psi during the test you figure the volume used as follows:
77/3000= .0257, .0257*500 psi = 12.85 cu ft.
You also need to know the atmospheres at the depth you were at during the test. If you were at 40 ft you figure the atm as follows:
(40/33)+1= 2.21
And you need to remember how mnay minutes you swam. (10)
Then just divide the 12.85 cu ft by 10 minutes to get 1.28 cu ft per minute and divide that by the ATM to get the SAC rate:
1.28/2.21= .58 cu ft per minute
A SAC of .6 would be really good for this type of test, and a tank larger than an AL80 would not be needed. If you used 800 psi on the same test the SAC would increase to .93 and that is getting toward the high end of average so a 100 cu ft tank would make sense, even if you have to go with an AL 100.
If steel HP 100's are available, go with the steel tank anyway.
In comparision, a Luxfer AL 100 is 3 pounds positive when empty but 9 pounds heavier in terms of overall weight compared to an AL 80. Despite being 9 pounds heavier the buoyancy is a wash and it is more or interchangeable with an AL 80 in terms of lead you need to carry.
A Catalina AL 100 is neutral when empty but weighs 46 pounds, so you save 3 pounds of lead off the weight belt - but the tank itself is 15 pounds heavier than an AL 80.
PST steel HP 100's are -1.3 pounds negative when empty and weigh 33 pounds - only 2 pounds heavier than an AL80 but they require 4 pounds less weight to be neutral at the end of the dive so you end up with a net savings in weight of two pounds.
Worthington 3442 psi X-7-100s weigh 33 pounds and are -2.5 pounds buoyant when empty. So you they need 6 pounds less weight than an AL 80 with a 4 pound net savings in weight. The qualifiyer here is that it does not really help you unless you need at least 6 pounds of weight.
----
High air consumption tends to go hand in hand with over weighting. Excessive weight at best requires more work and more gas to maintain neutral buoyancy. For newer divers less skilled at buoyancy control excess weight usually results in much more difficulty controlling buoyancy do to the larger gas bubble being managed and many new divers expend large amounts of energy swimming up or down throughout the dive to compensate for less than perfect buoyancy.
My advice is to ensure you have your weight exactly right before you go on the trip and make adjustments on each dive on the trip until you get it exactly right (neutral at 15' with absolutely no air in the BC and 500 psi in the tank. In that condition you should be floating about eyeball level at the surface (with absolutely no finning or hand skulling) with full lungs and you shoudl sink when you fully exhale.
----
If you have the chance, do a dive to determine your normal SAC. Basically swim around just under the surface (10-20 ft) at a normal speed for five minutes, then go to a moderate depth (30 to 60 ft), establish neutral buoyancy, note the time and your spg reading and the time (if using a computer start when the computer kicks over from one minute to the next) and then swim at that exact depth at the same normal steady swimming speed for 5 or 10 minutes (longer is better, especially at shallower depths). Then at the 10 minute mark note your spg reading.
You will need to know how much gas you used. If you used an AL 80 it has 77 cu ft at 3000 psi. If you used 500 psi during the test you figure the volume used as follows:
77/3000= .0257, .0257*500 psi = 12.85 cu ft.
You also need to know the atmospheres at the depth you were at during the test. If you were at 40 ft you figure the atm as follows:
(40/33)+1= 2.21
And you need to remember how mnay minutes you swam. (10)
Then just divide the 12.85 cu ft by 10 minutes to get 1.28 cu ft per minute and divide that by the ATM to get the SAC rate:
1.28/2.21= .58 cu ft per minute
A SAC of .6 would be really good for this type of test, and a tank larger than an AL80 would not be needed. If you used 800 psi on the same test the SAC would increase to .93 and that is getting toward the high end of average so a 100 cu ft tank would make sense, even if you have to go with an AL 100.
If steel HP 100's are available, go with the steel tank anyway.
The Kraken
He Who Glows in the Dark Waters (ADVISOR)
ScubaBoard Supporter
Scuba Instructor
Divemaster
Just throwing out some basic numbers, but if the two of you are doing dives averaging say around 55' and having SAC rates somewhere around 0.60 cfm, you'll have about 38 minutes bottom time (with 500 psi reserve) on an aluminum 80 and about 53 minutes on a HP 100 (with 500 psi reserve).
the K
the K
The wife and I are going to Cozumel in June for our first true dives after certification.
Someone suggested that as "newbs" we are probably going to suck a LOT of air
The $10 a day extra for the tanks do not bother me one bit, but I am curious as to what others have to say. I want to ensure we enjoy our dives and get maximum SAFE time (we will have either a Suunto or Oceanic computer, I am still trying to figure out which one) and do not want us being "newbie breathers" to cut short our time.
If you are going to pay extra to use AL 100's make sure they fill them correctly the working pressure for an AL100 is 3300 psi not 3000 as with most AL tanks
The Kraken
He Who Glows in the Dark Waters (ADVISOR)
ScubaBoard Supporter
Scuba Instructor
Divemaster
Actually, I think it's 3442 psi, or basically 3500 psi.
the K
the K
That's a good point as an AL100 with only 3000 psi is really only a 91 cu ft tank and would give you about 48 minutes bottom time compared to 40 with an AL 80 at 55' with a SAC of .6.
A steel 100 underfilled to the same pressure is only 87 cu ft, reducing the bottom time by 2 more minutes but at least you are not carrying an extra 10-15 pounds of tank weight.
A steel 100 underfilled to the same pressure is only 87 cu ft, reducing the bottom time by 2 more minutes but at least you are not carrying an extra 10-15 pounds of tank weight.
hudson
Contributor
Regarding the overall tank weight (not buoyancy), I wouldn't be too concerned about it in Cozumel if you are diving from six-packs (as you should). You don't need to walk about the boat prior to entry and you can simply hand up your BC before climbing in.
Reg Braithwaite
Contributor
The wife and I are going to Cozumel in June for our first true dives after certification.
Someone suggested that as "newbs" we are probably going to suck a LOT of air
The $10 a day extra for the tanks do not bother me one bit, but I am curious as to what others have to say. I want to ensure we enjoy our dives and get maximum SAFE time (we will have either a Suunto or Oceanic computer, I am still trying to figure out which one) and do not want us being "newbie breathers" to cut short our time.
I have just returned from Cozumel, and here are my thoughts, such as they are:
- if you are diving with an operator that routinely offers 100cf tanks to its customers, you probably want to go with that. If you go with 80cf tanks and others on the boat are diving 100cf tanks, you will be shortening their dives due to your high SAC rate and smaller tank. Go big as a courtesy to them. When you are a better breather, you can go smaller and you will no longer be the "limiting factor" on the dive, it will be some other Hoover
- Do not worry about whether your tank will be so large that your gas supply will outlast your minimal deco or no-stop deco limits. Cozumel diving is multi-level diving, so you typically start deep and work your way up to shallow. Your DM will plan and lead the dive such that your air will run out long before you run into deco. FWIW, I was diving 120s and never ran into deco.
- $10 is nothing in exchange for longer bottom times. Others have done the math here, but presuming you could get 77cf out of an AL80 and insist on a full 100cf fill for the money, you are still getting 30% more dive for $10. If you are paying $65 for a two-tank dive (the cheapest I found), then you are paying $32 per dive. So 30% more dive for 30% more money is a wash, you are not really "paying more." And if you are paying more than $32 per dive, $10 becomes a bargain.
- Consider diving with Aldora Divers. It will cost you $89 for a two tank dive, but you will get a 120cf tank for that price. That's 55% more dive for 38% more money :-D
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