What size of steel tank for beginner doubles?

[TSandM]

The most widely excepted general use cylinder size are LP 95 steel.

I don't know where you dive, but I don't know anybody diving doubled LP95s. Around here, you see 85s, HP100s, and LP104s, as well as HP130s. In Mexico, it's mostly Al80's. I don't think you can say that any one tank selection is the most common.

Further, your volume analysis is faulty. Yes, the tank factor between the LP and HP tanks is different. But a tank contains its rated volume at its rated pressure. HP100s hold more gas than LP95s, if both are filled to legal limits. The difference is that some shops WILL fill LP tanks above their rated pressure, in which case they hold more than HP tanks of the same rated volume. And some people have trouble finding shops that will fill HP tanks to their rated pressure, in which case they hold less than their stated volume.

But a bigger tank is a bigger tank, if rated pressures are respected.

 

[DA Aquamaster]

The reason you want LP is because they will give you a greater volume of gas than HP cylinders. Here is the formula for comparing the difference between LP vs HP. Take 95 divided by the working pressure or 2640 psi and you will get .0359 then multiply x100 and you will get 3.59 or in other words 3.59 cubic feet of gas for every one hundred psi. If you compare a HP steel 100 with a working pressure of 3442 you will get 100 divided by 3442=.029x100=2.9 cf of gas per 100 psi. with this formula it is easy to see that the LP cylinder wins out with greater volume everytime. I hope this has been of some help to you.

I agree that a low pressure tank will have a larger internal volume than a high pressure tank of similar capacity (95 vs 100 cu ft). That's just basic math - and also very obvious for anyone with a basic understanding of basic gas laws.

But...unless you filll them to equal pressure (ie: a massive overfill for the LP tank) it makes no difference - which brings us back to the cave country argument where the percent of an overfill you can stuff into an LP tank is much higher than the percent overfill you can stuff into a 3442 psi tank with a given compressor. (25% at 3000 psi in the LP tank and 43% at 3442 psi versus 0% in the 3442 psi tank at 3442 psi) So if you plan on over filling beyond the legal 10% and live in an area where you can actually get illegal overfills, the LP tanks still make sense. For the other 99% of us, they don't.

In the world of tech diving, both divers need to have similar size cylinders so each diver has the same amount of breathable gas. If you dive with to small of cylinders you will have trouble finding other divers willing to dive with you. The same problem happens with cylinders that are to large for you. You will end up with buoyancy control problems and bad trim.

I also agree with you in regards to gas matching, but to be most useful, you need to gas match based on the gas consumption rate of the biggest hoover on your team as you need to plan to get him or her out on your reserve gas, not just yourself and too small a tank among anyone in the groups means you turn the dive sooner. I also agree that in some situations that could put yourself into a situation where you have too large a tank for your height/build and that will affect you on eacvh and every dive as well. So go as large as you can tolerate size wise, but don't get carried away with it or feel pressured to use a bigger tank just to allow a little more dive time with the hoover buddy.

The most widely accepted general use cylinder size are LP 95 steel.

That's because doubled steel 95's are HEAVY and anything larger in an LP tank is UNGODLY HEAVY.

Because of that (outside cave country and/or in cave country if all those long abused 95's start blowing up during overfills) in 10 years, if not already, I suspect 3442 psi steel 120's and 130's in either E7 or E8 flavors will be considered to be the common tanks for a technical diver.

 

[PerroneFord]

I suspect 3442 psi steel 120's and 130's in either E7 or E8 flavors will be considered to be the common tanks for a technical diver.

I have seen the rise of 19cuft tanks, and I suspect in 10 years they will be MUCH more prevalent than they are now among technical divers.

 

[Jimmer]

I know around here for the Great Lakes wreck divers, the guys on lp's are very often on 95 or 108's and the guys on hp are often on 119's or 130's. Equalivalent size and weight, just lp versus hp.

P.S. Double 108's/130's are a pain in the butt for boat diving, but some boats are ok. My shop's ladder can support me walking up with my double 108's with a 40 slung, with my fins on, and it is rock solid.

 

[texdiveguy]

I prefer to use double lp85's myself....for an older guy whom is not as big and strong as you young divers they are a great match....even with a 'standard' fill they hold ample gas and are not to heavy to manage....many times for longer duration diving I will 'over' fill them a tad and can always opt. to sling a back gas stage bottle as needed. LP85's are easy for trimming out also. Much prefer lp85's over double Al80's both for wet and dry diving.

 

[Jay Roy]

I always recommend starting with AL 80s for just the reasons you mention. They make the best starter set and have the best all around usage. The only exception would be if you're going straight to cave diving. That would be the only case where I would recommend starting with steel tanks.

What about cave diving in Mexico?

 

[TSandM]

Mexico seems to use mostly Al80's. But they can, because the caves are so much shallower. Sixths on Al80's in Florida would be some REALLY short dives.

 

[CamG]

First of all thank you for your response. I am 5'10 190 lbs and accustomed to a fair amount of lifting. My dive buddies and myself are interested in some deeper wreck dives and doubles go with the territory. First thoughts were in the 95's Lp to 119's Hp? What do you think?
keep diving....keep training....keep learning!

 

[WD8CDH]

To see the true advantage of HP tanks over LP tanks is not to compare tanks with equal cubic foot capacity at rated pressure but to compare tanks with equal internal volume. For example, the OMS LP-98 and the OMS HP-119 both have an internal volume of 915 cubic inches, an outer diameter of 8.02 inches and both are within 1/4" of 24" long. The LP tank weighs about 1.5 pounds less than the HP tank but needs 1.5 pounds more weight on the belt to be neutral buoyant so the weight is a wash.

So these two tanks are almost identical in size, bulk, balance, buoyancy and at the same fill pressure, hold exactly the same amount of air.

At 2640psi, it would be almost impossible to tell them apart in either diving or carrying them to the site. The advantage between these two otherwise identical tanks is the LP tank would be limited to 95 cubic feet of air unless you fill it beyond it's rated pressure but the HP tank (the one that is no harder to dive with or carry to the site) could be filled to hold 22 more cubic feet of air without exceeding it's pressure rating. If you were limited in compressor pressure, there would be no advantage to the HP tank, but there would be no disadvantage either.

LP-120 to HP149, LP-108 to HP-130, LP 85 to HP-100 show the same advantages of HP over LP without the disadvantages so other than cost, I see absolutly no reason to get a current LP tank. You can pretty much consider an under filled HP tank as a LP tank that can legally be over filled.

 

[WD8CDH]

CamG,

One other thing to consider is will the doubles be your only tanks or will you have other tanks?

If it is your only set, you may want to limit yourself to double HP-80s or whatever the maximum size that you want to carry to every single dive no matter what kind of dive. If you have other tanks, then you could go with a larger set of doubles and pick and chose what you want based on the dive plan and site accessability.

Personally, I have so many tanks, that I have many weight and volume choices to chose from. Some of my sets are rather unusual in that the multiple tank sets are smaller, lighter and hold the same or more air than singles. I have one set of quad 20s that weighs 6 pounds less than a single HP80.