38's vs. 42's

[SurfLung]

- Reading in the Basic Scuba book, there are two types of Scuba tanks referred to as "38's". I've pictured both in the photo above.
- On the left are U. S. Divers 38s. These have a working pressure of 1800 psi with 38 cf of air per tank. Filled 10% over they are about 41.9 cf.
- On the right are Sportsways 42s. These have a working pressure of 1880 psi with 38 cf of air per tank. Filled 10% over they are about 42 cf.
Big Buoyancy Difference
- My first dive with the Sportsways tanks was a surprise after diving with seemingly identical set up from USD. I wear 8 lbs of weight with the USD 38s. With the Sportsways tanks,I sank pretty good with the same weights. Took off two lbs and still sank heavily. And it got me to thinking.
- Basic Scuba lists the internal volume of these tanks. The USD 38s are 535 cu inches. The Sportsways 42s are 505 cu inches. External dimensions are awfully close and I don't think a difference in water displacement accounts for the difference in buoyancy. I think the Sportsways just has more steel.
- I did a little test to see how much they weigh in water. I used a very nice digital fishing scale and suspended each set of tanks underwater full of air. Basic Scuba says 84 cf of air weighs 7.2 lbs.
- Sportsways weighed 7.9 lbs which would be -.7 lbs when empty
- USD weighed 2.4 lbs which would be +4.8 lbs when empty. I also noticed that the USD tanks floated bottom up.
The REAL 38s...- The USD 1800 psi 38s are truly unique in being a twin set of tanks that won't sink you to the bottom if you dive without a wet suit. The Sea Hunt episodes where Mike Nelson wears twin tanks plus a weight belt... They could only be with the buoyancy profile of these type of 38s.
- I know with modern dry suits, many divers prefer tanks that are heavier in the water but, I prefer the USD 38s buoyancy. I like the idea of having the only thing that makes me sink be the weight belt.

[Herman]

I have a set of the USD ones with a "2 in 1" harness (metal tubing with nylon and plastic straps). Full they are about 2 lbs negative and empty about 2 positive, a very nice combintion. I dive them with no extra weights when the water is warm here.

[SurfLung]

- I'm just realizing how lucky I am to have the USD 38s. I dove the Sportsways 42s three times with less weight each dive. The last one was with zero weights and I was still sinking... I dive a 3mm suit but I'm kind of 50 lbs overweight... These 42s are little sinkers.

[SurfLung]

An Acquired Taste?
- I'm beginning to like these 42s... I keep trying them out and I seem to be getting them squared away. Last night after work I dove them again for 29 minutes at 16-18 ft deep in the lake. Started with 2000 psi and ended with 1400 psi... LOTs of air in these little buggers! And, I like that I don't have to baby the galvanized finish. With the USD 38s I have to be careful of the yellow finish.
- This time I wore my 3mm hooded vest under my 3mm suit and I was finally a little bit too buoyant... controlled it with shallow breathing (like a good vintage diver) and I had a wonderful time with my little twin 42s. This is the wetsuit configuration I usually wear so, I may have things zeroed with the addition of 2-4 lbs of weight... Just like the USD 38's they do get more buoyant as the air gets used up.
- With the smooth round bottoms and close fitting vintage harness, these things ride on your back perfectly. The regulator diaphragm is not only at shoulder level, it is pressed flat against your back. And they're comfortable. The round bottoms rest in the small of your back when standing. The twin tank set-up rides evenly when walking and more importantly when swimming.
- Oh and it's worth mentioning that last Saturday, I was able to connect a banjo SPG without a long yoke... That surprised me. I was using my West Delhi DAAM and forgot it had a short yoke... I just gave it a try and darned if it didn't (just barely) allow the banjo into the connection. Last night I just dove my Voit 50 Fathom with no SPG and flipped up the reserve (just in case).
- My friend Tom McClusky is a Sportsways fan... He has these twin 42s in both galvanized and orange and he asked me why I don't put Sportsways decals on them. Well, it's because they're my simulated Mike Nelson tanks!

[Vintagediver]

Boy do I hear you Eben about liking those smaller twin tanks. Last year I got lucky at Portage and scored some Voit twin galvanized 50's with the round bottoms, and as I'm getting more vintage myself I find that my body is really liking these shorter tanks and the way they ride on my back. I have mine set up with a Voit snugpack and I love diving them. I'm also discovering the more that I dive them that I'm going to be able to lose some weight off that weight belt. I'm currently using 8 lbs with my 5mm JMJ vintage beavertail wetsuit, and during the last two dives I've noticed that I felt a bit heavy. In fact on the dive I did Sunday at Crystal Lake while I was at 35' I got comfortable in a prone position on the bottom and carefully removed my weight belt. At first with it laying on the lake bottom I held on to the end of my belt just in case I found myself too positive to prevent an uncontrolled ascent, and as it turned out I felt perfectly neutral. So it looks like with a bit more experimenting I'll be definitely shedding some weight. I'm really liking that! Terry

[SurfLung]

- Hi Terry, I've been thinking a 5mm suit might be the better choice for these twin 42s but I've got them pretty much dialed in with 2 lbs of weight belt and diving my 3mm suit with 3mm hooded vest. Your twin 50's must have a similar buoyancy characteristic. With the USD Twin 38s I have to wear 8 lbs to get to the same buoyancy as I do with 2 lbs and the 42s. See the picture of my two different weight belts above.
- My thinking on the 3mm is that it will compress less and therefore change buoyancy less. I've had the 42s down to 50 ft and the 38s down to 100 feet. And while I was "Sinky" at depth, I was still able to rise with a full breath of air.

[frogman63]

After reading all of the forum discussions this past season on 38's and 42's, I was motivated to give it a try. Luck came my way and I was able to purchase an old (1962) set of rusty USD's double 38's with original bands and harness. After a lot of sanding and a little help from Allan and Karl, and from Craig and James from the LDS, and from Mike from our dive club, I finally finished the project. I was able to get them out diving this fall and had some fun. Thanks to all of you who posted on the other 38 and 42 rebuild projects; the info was very helpful.





Brian

[SurfLung]

- Those USD Twin 38's are beautiful. I don't think they're all that easy to come by either. Congratulations on finding them and on your successful restoration. They really look exceptional.

[frogman63]

Thanks and thanks for all of your posts on this topic.

Brian

[eskimo3883]

Hi Frogman,

I am inspired by the quality of your restoration. The paint job looks like a very good match to USD in terms of color as well as that smooth thick look. Do you recall the brand and color of your paint and the the surface treatment before the firt coat of paint?

[frogman63]

For the surface prep, I used a couple of solvents to get all of the stickers, decals, and residual glue off. I then hand sanded the entire tank with automotive Emery cloth. I started with a course grit paper, then worked my way to a medium grit, and then finished with a fine grit. The original paint was the USD yellow enamel. I wanted to preserve as much of it as I could; that is why I hand sanded them (it seemed to take forever). I was able to sand off the rust and any remaining evidence of the decals and stickers. The bottoms were in the worst condition and I had to sand down to bare metal. Finally, I used a damp cloth to clean off the tanks before spray painting.
For the painting, I first spray painted the bottoms with Rust-oleum Rust Reformer; that filled in the small amount of pitting. I waited a couple of weeks and then sprayed the entire tank with their Clean Metal Prlmer. I then waited a couple of days and then sprayed the entire tank with their Sunburst Yellow Gloss Enamel. After two weeks, I sprayed the entire tank with their Clear Coat Enamel. After two more weeks, I reassembled the manifold and bands, and then added the USD decals from Karl. The wait time was to let the paint cure. I am pleased with the match of the yellow in the decals and the Sunburst Yellow spray paint.
Brian