Want to buy my own tank, but I know very little...need advice!

[WillyCham]

I regularly rent 80cf HP steel tanks when I go diving, but I want to buy my own tank now.

I am considering just buying exactly what I've been renting, however, I am curious about the difference if I bought a 100cf HP steel tank instead of the 80cf? I assume I'd get more air (possibly more bottom time, too) with a 100cf HP steel? What would be the major difference that I would notice...is the size the only difference? Would you recommend a 100cf as my first purchase over the 80cf? Would I have to make any gear adjustments based on the fact I always rent 80cf? thank you

 

[tplyons]

Are you sure you're normally renting steel 80cf tanks instead of aluminum 80cf? Since you don't have a location, I'm not sure what is standard in your area.

Around here the aluminum 80 is most common. I bought two because they're cheap, but also have two steel 100's that are a little bit smaller, but hold 29% more air. That, and I can do so wearing less weight on my belt.

But there are other things to keep in mind, i.e. fill pressure. If you can regularly get fills to 3442 psi, a steel 100 will be just fine. But if you can only get fills up to 3000 psi, you wouldn't get any significant benefit from having the tank ~80% full.

 

[spectrum]

Hi WillyCham,

In terms of capacity 100CF will be about 22 cubic feet more than your rental Al80's since they are not truly 80CF. On the surface it sounds like just over 25% more air but in actuality it is a little more since you get to breathe it all. A basic reserve is already built into your expectation of a 80 CF dive. Depending on your profile that will more or less translate into additional bottom time of a similar percentage.

The size will be comparable to the AL80 but the buoyancy properties will have you wearing less lead and net out lighter on your feet as well. Here is how it works staying with an 80 and you can calculate the bottom line of the 100 by looking up the properties and following the calculations.

All in all the HP100 is a hightly desirable cylinder and unless you are very stout it's hard to imagine going wrong.

One thing to question is how much air do you need? Are you the one coming up short in the group for instance. Your dive count is relatively low so improvement probably still awaits you. I have HP80 and HP100 and the 80's are my cylinder of choice, when needed the 100's are great too.

On occasion the HP80 can be found attractively priced. The typical list price make going for the 100 a no brainer unless you really really want something more compact.

The availability of 3442 PSI fills is occasionally a challenge but ony in odd remote situations these days. The challenge of getting a perfect fill increases with pressure but I see plenty of AL80 divers get short sheeted too. At worst a soft fill on the 100 is still a good deal better than an 80.

I just recognized the OP mention of presently using steel. Much of the above is still relevant regardless of material.

Pete

 

[Scared Silly]

HP80 are stubby cylinders and popular with many "shorter" divers. In steels the HP 100 is the de-facto standard cylinder, as opposed to the AL80 which is de-facto standard aluminum cylinder. Many like having the 100 which gives them enough gas for deeper dives. There is an old rule about 1 cuft of gas for each foot in 1 depth.

Exempt HP cylinders are 3442 psi when full. So make sure you can get full fills. A 3300 psi fill may cool to 3000 psi which means instead of 100 cu you have 87 cuft which is still better than a std. Al80 (which is really 78 cuft).

As for gear adjustments, you will be able to shed a pound or two from weight belt - depends on which mfg Faber or Worthington.

 

[MalibuJerry]

You need to find out the following:
1. What are the buoyancy characteristics of the tanks you are using and the ones you are considering
2. How much does the tank weigh when it is full and you are out of the water. If you have long beach or rock crossings to your entries this might be relevant.
3. What size is the tank and how will it sit on your back.
4. Can you get fills to the rated pressure of the tank. If you have an LDS that will do cold fills, you will get the additional air. Typically on a boat, if they even fill to the higher pressure, they will give you a hot fill as they have lots of other tanks to fill during surface intervals.
5. Do you need the extra air?
6. Price

Once you have all of that data, you should find that there is an obvious choice for your needs.

 

[lowviz]

MalibuJerry, great list. Might add trim. Buoyancy characteristics don't directly equate to trim. Still comes down to the old adage "try before you buy".

WillyCham, also try an HP120 (long tank) if you can find someone who will lend you one. It is at the other size/dimension extreme of the HP100 so diving it will help you make a better decision between the two.

Perspective: Diving thirds, a 120 is an 80 that you can suck dry and still get a 'high five' from your instructor when you surface. 2/3 x 120 = 80.

 

[Rusty Nail]

I too have been searching for new tanks. After researching and taking into consideration the advice I have received from SB, I have decided to rent a few different sizes of tanks and make sure i'm comfortable with what I buy and not wind up with something that is not practical for me. With that said, the next tanks I'm renting are gonna be hp steel 100's. On paper, they seem to be what i'm looking for. I run across this info, maybe it will help you out some.

Steel Tanks - Steel has a higher density rate than aluminum, which allows the tank to be negatively buoyant when your dive is completed. Steel tanks are available either in high pressure (HP) or low pressure (LP). High pressure tanks are smaller and contain a higher volume of air, and also require a regulator set up for DIN. A HP 100 is slightly smaller than an AL 80. LP tanks are much easier to fill, but are also quite large for the volume of air that they hold.

The difference between Pressure & Volume - It should be noted that the pressure of a tank is not a measure of the size or volume of a tank. For example, most aluminum tanks, from the smallest 6 cuft pony bottle to an 80 cuft tank, have a fill pressure of 3000 psi. For a 6 cuft bottle, this means at 3000 psi the tank contains 6 cuft of air. For an 80 cuft bottle this means at 3000 psi the tank contains 80 cuft of air. Tanks come in a variety of combinations of volumes and fill pressures but the cubic foot (cuft) volume is the measure of how much air a tank holds.

Valves - A K-valve, which comes on all aluminum tanks and low pressure steel tanks, do nothing other than release or stop the flow of air from the cylinder.

DIN valve - These valves are used on all high pressure tanks - 3300 PSI and above. Regulators used on HP tanks require that your regulator be converted to DIN.

 

[drrich2]

High pressure tanks are smaller and contain a higher volume of air, and also require a regulator set up for DIN.

Not always. I've got a 130 cf HP steel tank with a roughly 3400 PSI rated fill pressure and I only use yoke regulators.

Richard.

 

[eelnoraa]

DIN valve - These valves are used on all high pressure tanks - 3300 PSI and above. Regulators used on HP tanks require that your regulator be converted to DIN.

This is inaccurate. 3442PSI HP steel tank can be equiped with either DIN or Yoke (K) value, or convertable valve.

 

[Rusty Nail]

Here is the the link i got that info from. I suppose there are variables. I should have worded my post to not appear that I had taken a hard stance on that information, for I am learning too.


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