A few newbie tank questions

[The Chairman]

HP tanks often don't get fully filled. They get filled to 3000 more often than not. Why? Shops don't always look to see if it's an HP tank, they're too lazy to fill it beyond the other tanks being filled or they have some phobia about going above 3000. IOW, that HP100 might only have 80cf of gas and its frustrating . However, I've never seen an LP tank under filled. Ever. I have seen them over filled, but never under filled.

 

[DavidFL]

^This. My local LDS fills my AL80s to 3300psi....the classic "10% overfill" since it makes people happy. They also fill my HP Steel 100s to 3300psi. I keep telling them that 3442+10% is 3786.2, but I think there's a big RED line on a gauge somewhere at 3300. So if more of you buy and use HP100s....maybe we can get a new "make the customer happy" setpoint! But a 3300 fill in an HP100 is still ~96 cu. ft.

 

[RhodeDiver]

I would go with a HP80 if you want short and light. Otherwise I would go for Aluminum.

 

[2airishuman]

HP tanks often don't get fully filled. They get filled to 3000 more often than not. Why? Shops don't always look to see if it's an HP tank, they're too lazy to fill it beyond the other tanks being filled or they have some phobia about going above 3000. IOW, that HP100 might only have 80cf of gas and its frustrating.

Shops underfill cylinders for all kinds of reasons. I agree that it is more of a problem with HP steels than with other cylinders. There are also a handful of shops that charge a few dollars extra for HP fills, although this is rare.

There are three reasons for short fills that @The Chairman does not mention.

1. The shop may fill from a cascade that is operated at 3500 PSI. This is common, and it is usually not possible to fill a cylinder to 3442 PSI from such a setup without starting the compressor. Many of these shops really prefer to operate the compressor only at particular times of the day (usually early in the day when The Boss is there and customer traffic is limited).
   
   
2. HP cylinders take time to cool off, and many shops will not adjust the fill pressure upwards to compensate for the effects of temperature.
   
   
3. There are still some shops out there that have compressors with a 3000 PSI maximum output, such as the Ingersoll Rand Type 30 223x5, which is a great compressor in many ways but can't be pushed beyond 3000 PSI without significantly shortening its service life.

Here's what I do to be sure I get good fills. I think this is all important regardless of what kind of cylinders you have:

1. Choose a dive shop that has an approach to fills that you like, considering price, location, hours, air quality, VIPs, Nitrox availability, etc. as well as availability of HP fills.
2. Cultivate a relationship with the dive shop.
3. Make it clear that you want good fills when dropping off the cylinders, until such time as they remember who you are and what you expect.
4. Try to give them as much time as they want. If they are busy, have classes going, it's the weekend, the compressor is due for maintenance, etc., they may want an extra day or two.
5. Ask for a tank checker and check the cylinder pressure before you leave the shop.

A good shop will have a tank checker for you to use, and will not be the least bit offended when you use it. A good shop will respect you more because you do it. I expect my cylinders to be filled within 100 PSI of their capacity and will ask for a top off if they are below that. I have run into dive shop employees once or twice who felt put upon to give me a tank checker but they are not the people in charge and they will quickly learn the way the world works.

Even if you get a short fill, in an HP100 you'll still have more gas than you would have in a properly filled AL80, and with an HP80 you'll have far more gas than you would in a properly filled AL63.

However, I've never seen an LP tank under filled. Ever.

I've had problems with some places underfilling LP steels because the fill operator either doesn't see the + or doesn't understand its significance.

 

[scbaNE2179]

Steel tanks only for local diving here. Sure they require more maintenance but you cannot beat the fact that it will reduce the amount of lead you have to carry during your dive. Sorry but AL80 tanks are not my thing as when empty, it's crazy buoyant. I'm a big believer of reducing the overall amount of lead to use during dives as I often hear way too many divers: "I like diving heavy".

 

[Bob DBF]

If there is a constraint on money, and there was for me when I bought my first tanks, I chose to buy two AL tanks instead of one steel tank because refilling a single for a 2 dive day was time prohibitive, even though I had to deal with more lead for my 7mm wetsuit. And at the time a steel tank ment a 72, so the AL was also larger.

Now, in the age of Craig's list and a variety of used tanks available, I would look for a couple of old steel 72's, which are widely available and inexpensive. That is if you don't find a good deal on larger tanks along the way. I was always more interested in getting underwater than the length of the dive.

The HP 100 is a great tank, the old steel 72 of today, and is a great choice. I have not had a problem getting them filled, but check your local shops. As a reference, a 3000# fill is still about 87cuft. LP tanks are good for the reasons given above, but check if the local hydro facility does + hydros, ours doesn't, which means you get fills at service pressure which decreases the amount of gas you can carry from stated tank size.


Bob

 

[MrVegas]

Thanks for all the great advice. One last question -- there seems to be an "FX" steel tank and a regular HP tank. (Is hot dip galvanized a third choice?) Not sure if there is much difference, but it seems like the FX series is a little lighter, but maybe a little more buoyant at the end of the dive. Just curious if there was any reason to prefer one over the other.

 

[2airishuman]

FX is the Faber/blue steel branding for their exempt high pressure cylinders. They are available painted and galvanized. Galvanized cylinders are usually slightly heavier than otherwise identical painted cylinders because the galvanized coating is thicker and denser, but they last longer.

"Regular HP cylinders" may be similar to FX cylinders, or not, depending. There are some older 3AA-type (non-exempt) HP cylinders out there that are usually quite heavy and also very (about 8-10 pounds) negatively buoyant.

 

[MrVegas]

I am not sure about bringing an ancient thread back to life, but for those that have similar questions, I thought at would at least end the story of my search for tanks. I would not have guessed it this time last year, but I in the end I wound up with 7 used tanks:

2 steel LP72s with standard threads
2 steel LP72s with half inch tapered threads
2 LP 108s
1 LP 95

My son has pretty much decided the two standard thread LP72s are his, and by the end of the season, I was mostly diving with the LP108s. Even as singles, the LP108s were kind of heavy, but I liked them a lot under the water.

The tapered thread 72s will need hydros over the winter, so we'll see how that goes. I have dived with all of the tanks, though, and they all worked well, and I am very grateful to those that helped me out with these. Much less expensive than buying new. If I had to buy new, I'd probably go with LP85s for the price\capacity tradeoff and the fact that they are pretty close to the 72s in size and shape. If I were diving salt water with my own tanks, I'd probably just get AL80s.

The 72s are really great for starting out, and I would recommend them to anyone if you can find them in good shape for a good price. Thanks to everyone on this thread for their advice.

 

[Bob DBF]

Glad to see everything is going well.

The tapered thread 72s will need hydros over the winter, so we'll see how that goes

From my experience, you can't kill them. As a note, if you are not rebuilding the valve and haven't changed out the burst disk's when you first got them, it might be a good idea to change them at hydro. Who knows when it was done last and could give you quite a supprise at an inopportune moment.


Bob