Single vs. double tanks

[ERP]

OK so here's my calculations, did these in my head so they could be off

Airssharing ascent from 100ft assuming 2 divers SAC rate of 1cuft (stressed), 1 minute on the bottom to resolve the issue, 8 minutes total ascent time.

so roughy 32cuft for the ascent.

so at 100ft on air NDL is roughly 20 minutes so with a SAC of 0.75 thats 60cuft of gas, so I'd need a total 92cuft

on EAN32 roughly 30 minute NDL so thats 90cuft used for a total of 122cuft

Seems to me that 72's are a pretty good choice.

Has anyone weighed their 72's against a single large tank, say an LP104 or a HP120, including the lead worn, I'de be interested in the difference?

 

[DA Aquamaster]

I purchased four Faber 71.4's for my spouse to use with her drysuit. She likes the short overall lenght. The -11 to -6 lbs bouyancy works well with her neoprene drysuit and minimizes the weight she needs to carry on a belt.

I have not considered using them for doubles as the -12 lbs bouyancy when empty would leave me with no ditchable weight and they add about 12 lbs of weight out of the water compared to generic low pressure steel 72's.

 

[Karl_in_Calif]

It is overkill to wear twin 72s for NDL diving if the only justification is in case you have an OOA emergency with a buddy on a deep dive.

Twin 72s are a deco diving configuration.

NDL diving does not warrant twin 72s, unless you are staying shallow for a very long time.

Having said all that, twin 72s are still a great way to dive, for the pure pleasure of it (except when you are out of the water!)

 

[Dryglove]

It is overkill to wear twin 72s for NDL diving if the only justification is in case you have an OOA emergency with a buddy on a deep dive.

Since when is carrying extra gas for emergencies a bad thing especially on a deep dive. You obviously think its over kill while a lot of us dont.

Twin 72s are a deco diving configuration.

I wouldnt use 72's for deco diving myself. I would use 95's or 104's. What would happen if you couldnt use your deco gas and had to rely on your backgas for deco. 72's would be marginal at best depending on needed deco time.

 

[Don Burke]

Karl_in_Calif once bubbled... It is overkill to wear twin 72s for NDL diving if the only justification is in case you have an OOA emergency with a buddy on a deep dive.

Here are the figures I come up with for no-stop dives with EAN 32 and an SAC of .75:

30 minutes at 100 feet consumes 91scft which will take a set of 72s filled to 2250psig to 686psig.

35 minutes at 90 feet consumes 98scft which will take a set of 72s filled to 2250psig to 568psig.

45 minutes at 80 feet consumes 115scft which will take a set of 72s filled to 2250psig to 264psig.

I don't see any overkill here. Please point it out if I have missed it.

If anything, I see them not carrying enough gas for a safety margin on these dives.

Karl_in_Calif once bubbled... Twin 72s are a deco diving configuration.

If I can't get into staged decompression diving with them at 100 feet, have a really tough time calling them a "deco configuration". I do use them for deco diving and for no-stop diving. I consider them to be right on the edge of how small a set of doubles can be and still be useful.

95s or 104s are more of a deco-only configuration.

Karl_in_Calif once bubbled... NDL diving does not warrant twin 72s, unless you are staying shallow for a very long time.

Unless you consider 30 minutes at 100 feet to be "shallow for a very long time", I don't follow you.

Karl_in_Calif once bubbled... Having said all that, twin 72s are still a great way to dive, for the pure pleasure of it (except when you are out of the water!)

I'm not much of a physical specimen and I can carry two sets of 72s for quite some distance.

 

[Doc Intrepid]

I like'em. Been diving them over 20 years. Never messed with twin steel 45s or 72s, but dove a set of twin aluminum 50s for awhile, then a set of twin aluminum 63s. They balance well and are comfortable. They enhance horizontal trim. They don't knock me either on the back of the head or the base of the spine. They may offer a bit more drag, but I'm not particularly in a hurry underwater anyway. The twin manifold lets me use two regulators in case one decides to have a problem at depth. I don't use them for decompression diving, but did a lot of NDL dives spearfishing, sometimes at night, at depths from 60' to 90' feet of warm clear seawater. For NDL diving, small twin tanks provide plenty of air - especially for longer shallow dives, and they allow for an adequate reserve. They may not be good for decompression diving, but I never considered them adequate for longer, deeper dives. For recreational NDL diving, they're great. I haven't dove a single tank configuration in many years.

Doc

 

[Karl_in_Calif]

In a perfect world we would all be diving small doubles for NDL diving.

Too bad its not a perfect world, and we are saddled with these single tanks.

 

[roakey]

Karl_in_Calif once bubbled...
I wish the equipment makers would figure this out for themselves, and then stop making B/Cs with lift LESS than 45 lbs.

That would elminate the sweetest setup that I have which is an 18 pound lift wing. Wouldn't trade it in for nutt'n.

Everywhere is not California, remember...

Roak

 

[Don Burke]

Karl_in_Calif once bubbled... In a perfect world we would all be diving small doubles for NDL diving.

Too bad its not a perfect world, and we are saddled with these single tanks.

What is keeping you from diving doubles? You can double up anything with a manifold. Custom bands are actually pretty cheap. Getting really short tanks to fit a backplate may present a challenge, but hardly an insurmountable one.

 

[Karl_in_Calif]

I can invision a future where single tanks have fallen from favour, and everyone is wearing doubles. The single tank may then be viewed like the old two-hose regulator.

The high pressure technology in Europe currently could make double 55s almost as light as a current single DOT tank.

Rigid backplates, whether composite or metal (aluminum or steel) would then also be the norm.

We are looking 50 to 100 years into the future here, however.

There was a time when the LP steel 72 was the norm. It was a great tank, and still is. A fair number of low-RMV tech divers still band them together as twin 72s for many tech applications.

Faber 72s (71.4s) with a "+" can be filled to 3300 for 79 cu ft of capacity. That is probably still the current measure of a perfect single tank.