7.25" versus 8" tanks (new PST E Series)

[Scubaroo]

Are there any disadvantages to using 7.25" tanks in a doubles setup, versus 8" tanks?

All of my diving is salt water, either shore or boat. I'm happy to stick to single tank diving for shore dives, but would like to get my own doubles setup for boat diving so that I'm not reliant on borrowing doubles from a buddy (mostly because we rarely dive together, and he's my only doubles buddy for now). I also do some occasional wreck diving, which is off the Lois Ann in San Diego (which can pump nitrox to 4000psi - relevant to this thread). When wreck diving, I dive with my doubles buddy.

Before the PST E series was announced, I was looking at picking up a set of LP 95cf tanks. These would be 8" tanks, and weigh something in the order of 105lb, filled, on land! But would be around -8 or -9lb near empty in the water. A good thing seeing as I am now diving dry.

But now I am looking hard at the new E7-100 tanks. Filled on land, these would only weigh around 83lb or so (22lb less than 95's), and still be about -4lb or so near empty in the water.

So does the line of thought that double E7-100s would be preferable to double LP-95 tanks "hold water"? I'm guessing that there would be some benefits from the smaller tanks having less drag as well, due to the reduced surface area, although whether or not this would be actually noticeable in diving conditions is another story. The fact that I can get a 3442(ish) psi nitrox fill means that being able to get full HP fills won't be an issue. I'm not mixing my own gas, and don't intend to in the next few years while living in the US.

Here's a comparison chart from Lloyd Bailey's. Basically the E7-100 tank has the same specs as the old LP-80 tank.

thanks

 

[pescador775]

Depends, if you're sized similar to magilla gorilla on Smackdown, the 8's may be OK. Personally, I can tell you that a pair of 6.75" doubles have noticably more drag than a single 7.25. So, double up those 8" monsters and it will be an issue if you do any swimming in current. You will use more air. If you only go up and down a line and crawl around on the bottom and there is little current, may not be a difference. On the surface, the larger dia tanks will try to hold you under water(they don't like being out of water).

 

[Scubaroo]

pescador775 once bubbled...
(they don't like being out of water)

That's another consideration I thought of - less ditchable weight on the surface with these particular 8" tanks, and having to drag another 22lb up the swimstep into a boat.

 

[MechDiver]

Scubaroo once bubbled...
That's another consideration I thought of - less ditchable weight on the surface with these particular 8" tanks, and having to drag another 22lb up the swimstep into a boat.

You don't configure scuba equipment to be optimal on land.

To answer your original question, yes, there is a difference in that you will have less air capability and different buoyancy characteristics. Alum 80 have been doubled for a long time, so diameter isn't the issue.

I would question your figures on buoyancy also. My dbl PST 95's are about 1 lb negative when empty, not 8 or 9 as you state when "near empty". That would be pretty accurate when full though.

Ditchable weight is not really an issue when diving heavy doubles dry. Or shouldn't be anyway.

The tanks you are looking at should do fine for what you want.

Phil

 

[MechDiver]

pescador775 once bubbled...
Depends, if you're sized similar to magilla gorilla on Smackdown, the 8's may be OK. Personally, I can tell you that a pair of 6.75" doubles have noticably more drag than a single 7.25. So, double up those 8" monsters and it will be an issue if you do any swimming in current. You will use more air. If you only go up and down a line and crawl around on the bottom and there is little current, may not be a difference. On the surface, the larger dia tanks will try to hold you under water(they don't like being out of water).

You don't configure scuba equipment to be optimal on land.

You do not need to be "large" to handle steel doubles. I am 54, 190 and 5'11. I carry double lp95s with V-weight, canister light and two alum 40 deco tanks up two flights of stairs from the water. I also know a gal quite a bit smaller than I who dives lp 104s.

And, of course steel tanks "don't like being out of water". Thats why we use them.

 

[chickdiver]

I concur on Mechdiver's statement that you dont need to be "large" to dive steel doubles. I am 5'3", 135lbs, and I dive double 104's all the time, I also load and unload them from my truck, carry them to the fill station etc. Granted, doing this requires me to spend some time in the gym "jacking steel" as a friend of mine phrases it, but that has it's rewards aa well.

 

[JamesK]

Just to further talk about the "on land" issue and being a Gorilla to carry them, it ain't so. With my 104's I can still crawl up ladders, and stairs with ease. I have seen women about chickdivers size do this with 120's!!!!!

Personally, I would go with the 95's. If you can get "cave fills" (3500psi) you will have more available air in those 95's anyway.

If you are worried about weight still, check out OMS tanks.

 

[Scubaroo]

Personally, I would go with the 95's. If you can get "cave fills" (3500psi) you will have more available air in those 95's anyway.

Not an option where I dive.

I don't have any concerns about lifting the weight out of water - I'm 6'3", 220lb, and grew up on a farm schlepping 50kg (110lb) bags of fertilizer and feed around.

I also have a father who is a cripple (literally) from a lifting related spinal injury and subsequent spinal operations that went CF. He's got a nice collection of x-rays with enough metal screws and braces fused to vertebrae in them to make a Terminator look human.

I would question your figures on buoyancy also. My dbl PST 95's are about 1 lb negative when empty, not 8 or 9 as you state when "near empty". That would be pretty accurate when full though.

I came to these figures from the table at Lloyd Bailey's Scuba website, a link to which I've already posted in this thread. A single PST LP-95 is quoted at -3.3 empty in the water. Doubled up, that would be -6.6, to which you have the additional weight of steel bands and the manifold, plus any gas remaining in the tanks. Air weighs *about* 0.075lb per cubic foot, so if I had 500psi left in my tanks (to pick a figure), that would be about 36cf of air, which would be -2.7lb. So about -9lb, plus any weight from the manifold and bands. I used similar guesstimate math to come to the figure for the E7-100 tanks.

So far I haven't heard anything in favor of the LP-95s except that they weigh more on land??? The boats and shops I dive on/at will NOT overfill, and home mixing is not an option for little old apartment dwelling me.

If you are worried about weight still, check out OMS tanks.

I've dived with double OMS 85's, and found them really comfortable. The E7-100 tanks have very similar characteristics, with the benefit of the better galvanising finish - these tanks are going to be used in salt water, so withstanding corrosion is a must.

 

[JamesK]

Well then, it looks like those new PST's are perfect for you. I say go for it!

 

[MechDiver]

Scubaroo once bubbled...
I came to these figures from the table at Lloyd Bailey's Scuba website, a link to which I've already posted in this thread. A single PST LP-95 is quoted at -3.3 empty in the water

If you read 4 different tables for tank weights you will have 4 different results. The -3.3 is wrong. I have the tanks and weighed them.

And I agree with JamesK. The 100's sound like the tanks for you.

You may also want to educate those poor unfortunate shop owners in your area who don't know correct fill pressure for LP steel tanks :bonk:

Phil