From 1 to 2 tanks
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Blackwood
Contributor
If you are talking doubles, they'll weight a little more than double (heh) the single tank due to the bands and, if included, manifold. From a buoyancy perspective, those additions are primarily negative (they can be considered weight).
I used my Ranger with 45#'s of lift for a few dives with Steel 100's and it worked good. The 65# wing will be plenty of lift. Are you useing steel or alum tanks? What size? I now use a BP/W set up and it is better than the ranger for doubles. I use the range now for Pool dives and traveling.
Luxfer AL 100's weight 41 lbs and are 3.1 lbs positive when empty and - 4.3 lbs negative when full.
Catalina AL 100's weight 46 lbs and are .4 lbs positive when empty and -7.8 lbs negative when full.
So it makes a difference whether they are Catalina or Luxfer 100's.
If you have Catalina tanks, there is no need to change the weight at all - you will just need to be aware that you have another -7.8 lbs of negatve buoyancy with a second full tank.
If you have Luxfer tanks, you'll need to add 2 to 3 lbs of lead to ensure you can maintain your safety stop with near empty tanks.
Catalina AL 100's weight 46 lbs and are .4 lbs positive when empty and -7.8 lbs negative when full.
So it makes a difference whether they are Catalina or Luxfer 100's.
If you have Catalina tanks, there is no need to change the weight at all - you will just need to be aware that you have another -7.8 lbs of negatve buoyancy with a second full tank.
If you have Luxfer tanks, you'll need to add 2 to 3 lbs of lead to ensure you can maintain your safety stop with near empty tanks.
Reg Braithwaite
Contributor
Just a question to the OP from a n00b who has never dived with anything more complicated than a pony: What did your instructor say about handling a BCD failure at depth when carrying a pair of full 100s?
Back in the day, diving fairly buoyant doubles like AL 80's and Luxfer AL 100's was pretty common and the big issue was the trouble you could get into carrying all that gas.
A BC failure with an addtional AL 100, Luxfer or Catlina, will require the ability to swim up 7 lbs more weight than with a single AL 100. That was not a big deal then, but I suppose with the lower swimming standards for OW and AOW certs today, it might be more of an issue.
The potential BC failure issues get more pronounced with a thick 7mm wet suit that suffers more compression and buoyancy loss at depth. It is consequently a good idea to look at your whole configuration to ensure you are diving what some divers refer to as a "balanced rig" that you can indeed swim up from depth in the event you have a BC or wing failure.
One major issue is the 15 lbs of swing weight from the 200 cu ft of gas you are carrying. If you are only doing recreational dives you do have the option of dropping some weight to help you swim up. Assuming you are weighted so that you are neutral at 15' at the end of the dive with both tanks and you have a failure in the first third of the dive, you will in effect still be carrying 10 lbs of gas and could drop 10 lbs of weight, swim up and still hold at 15' for a safety stop. Later in the dive, you could not drop the weight and still hold the stop, but then that's ok as you are not all that negative and can swim it up anyway.
So be aware of it and weight yourself accordingly, but don't get too hung up on a BC failure, especially if you wear a dry suit or a 3mm wetsuit.
A BC failure with an addtional AL 100, Luxfer or Catlina, will require the ability to swim up 7 lbs more weight than with a single AL 100. That was not a big deal then, but I suppose with the lower swimming standards for OW and AOW certs today, it might be more of an issue.
The potential BC failure issues get more pronounced with a thick 7mm wet suit that suffers more compression and buoyancy loss at depth. It is consequently a good idea to look at your whole configuration to ensure you are diving what some divers refer to as a "balanced rig" that you can indeed swim up from depth in the event you have a BC or wing failure.
One major issue is the 15 lbs of swing weight from the 200 cu ft of gas you are carrying. If you are only doing recreational dives you do have the option of dropping some weight to help you swim up. Assuming you are weighted so that you are neutral at 15' at the end of the dive with both tanks and you have a failure in the first third of the dive, you will in effect still be carrying 10 lbs of gas and could drop 10 lbs of weight, swim up and still hold at 15' for a safety stop. Later in the dive, you could not drop the weight and still hold the stop, but then that's ok as you are not all that negative and can swim it up anyway.
So be aware of it and weight yourself accordingly, but don't get too hung up on a BC failure, especially if you wear a dry suit or a 3mm wetsuit.
hudson
Contributor
For peace of mind, establish what weight you can comfortably swim up in the pool. Then if you are diving wet with doubles, see if you can devise a situation where you are heavier than that, figuring in complete BC failure, wetsuit buoyancy loss and weight of backgas. Then make sure your ditchable weight covers the difference without making you dangerously buoyant. The only unworkable situation would come up if you can't swim up just the weight of doubles+air and are wearing a thinner wetsuit which means you are overweighted in the first place just by wearing the tanks, therefore carry no weight.
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