The only place I have seen specs for the mini-tanks is in the Basis Scuba
book. Here's what I learned:
38s - These are 1800 psi USD tanks. Fill them 10% over (1980 psi) and they become 42s.
42s- These are 1880 psi Sportsways tanks. Actually 38 cf at 1880 and filling 10% over (2068 psi) gives 42 cf.
Discussion 1 - USD started with 1800 psi tanks and called them 38s. Sportways were 1880 psi and they advertised them as "42 cf". BOTH were 42 cf when filled 10% over. USD then changed their advertising to say "42 cf" to compete with Sportsways (IMHO).
Discussion 2 - Sportsways 1880 psi 42s have a little thicker steel which makes them heavier in the water. USD 1800 psi 38s are lighter and were actually advertised as being positively buoyant when empty... An advantage to the diver swimming home after his dive. I've heard 1800 psi 38s referred to in a negative light as "Butt Floaters".
Voit Steel 50s - These are about the same size but thicker steel at 2250 psi. Also, they aren't true 50 cf unless filled to 10% over (2475 psi). These have a reputation for being quite heavy in the water. But modern style divers seem to prefer heavy tanks.
All three of these "mini tank" types are about 19" tall.
Discussion 3 - My personal favorites are the 1800 psi 38s. These are buoyant enough that I have to wear a weight belt even when diving without a wet suit. As for the "butt floater" reputation... The bottom of the tanks are more buoyant than the top because the top has the heavier metal valve manifold. For me, a weight belt balances this and I swim in a level attitude.
Discussion 4 - My Sportsways 1880 psi 42s are a close second. I can't dive them without a wetsuit or horse collar BC, however. The make me negatively buoyant. I wear 6-8 lbs less on the weight belt (or no weight belt) when diving these tanks with a wet suit on.
Discussion 5 - These mini-tank twin setups are THE most perfect tank configuration. They are comfortable to wear. They position your double hose regulator perfectly on your back. And they hold PLENTY of air for most sport diving.
Discussion 6 - There seem to be a variety of sources for the 1800 psi 38s. I have one set made from war surplus Walter Kidd "Shatterproof USN" tanks. I have one set from USD AquaLung, And I just made up a recent set from two different brands of fire department tanks. One is Walter Kidde and the other is MSA. All three sets of 1800 psi 38s seem to have the same buoyancy characteristics.
l-r: WK 38s, USD 38s, Sportsways 38s
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