Scuba Tank economics and buoyancy

[dbs129]

I realize that this is more of a post for the Scuba Tank section. However, I decided to post it here under general discussions to try and generate a bit more discussion/feedback.

Being fairly new to scuba diving, I have purchased most of the required equipment. However, I still rent aluminum 80 tanks from my LDS. I am currently contemplating the purchase of my own tanks and would like to get a few opinions.

For my current diving with the aluminum 80, I carry 40 lbs of lead (10 lbs in each ditchable weight pocket on my BCD, 2 lbs in each trim pocket, and 16 lbs on a weight belt. I would really like to get rid of the weight belt. Would the purchase of the steel 80 or 100 tanks allow me to lose the 16 lbs lead on the weight belt?

I am also looking from the perspective of the cost of purchasing my own tanks and the required maintenance (yearly visual inspection and five year hydrostatic testing) costs as opposed to keep renting. Yes, my own tanks would be more convenient, but as a recereational diver who forecasts approximately 40 - 50 dives a season, does it make economic sense to purchase my own tanks?

In addition, what is the general feeling of aluminum and steel tanks when it comes to maintenance and longevity of either material? Benefits?

Any information would be greatly appreciated.

 

[doctormike]

I realize that this is more of a post for the Scuba Tank section. However, I decided to post it here under general discussions to try and generate a bit more discussion/feedback.

Being fairly new to scuba diving, I have purchased most of the required equipment. However, I still rent aluminum 80 tanks from my LDS. I am currently contemplating the purchase of my own tanks and would like to get a few opinions.

For my current diving with the aluminum 80, I carry 40 lbs of lead (10 lbs in each ditchable weight pocket on my BCD, 2 lbs in each trim pocket, and 16 lbs on a weight belt. I would really like to get rid of the weight belt. Would the purchase of the steel 80 or 100 tanks allow me to lose the 16 lbs lead on the weight belt?

I am also looking from the perspective of the cost of purchasing my own tanks and the required maintenance (yearly visual inspection and five year hydrostatic testing) costs as opposed to keep renting. Yes, my own tanks would be more convenient, but as a recereational diver who forecasts approximately 40 - 50 dives a season, does it make economic sense to purchase my own tanks?

In addition, what is the general feeling of aluminum and steel tanks when it comes to maintenance and longevity of either material? Benefits?

Any information would be greatly appreciated.

Hey, DBS... thanks for writing..!

If you are doing 40-50 local dives in a season (that is, without flying anywhere), buying tanks is definitely the way to go, even just from a financial viewpoint. Furthermore, it is very nice not to have to rush back after every dive to return your tanks or pay for extra days. If you have your own tanks, you can keep them filled so that you can dive on a moment's notice, when the opportunity arises. The only downside is the need for a visual inspection once a year (industry standard) and a hydrostatic pressure test every five years (Dept. of Transportation requirement), plus whatever maintenance you need to do (not too much apart from the inspections).

As far as buoyancy issues, you will definitely appreciate the difference with steel tanks. Rental tanks are aluminum primarily because of cost issues. Steel has the advantage of much less of a swing in buoyancy between full and empty (making it easier to hold a safety stop with an empty tank). Also, as you correctly surmised, you are shifting some of the weight off of your weight belt, which makes for better trim and more comfort, IMHO.

Here are some tables:

Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan

Rodale's Scuba Diving Tank Chart


The amount that you can shift to the tank varies depending on which steel tanks you use, but comparing empty buoyancy of Luxfor aluminum 80s (+4.4 lbs) with a low pressure Faber steel 100 (-7.26), you can see that you can move over 11 lbs off of your weight belt and still have the same buoyancy at the end of the dive...

Best,

Mike

 

[Scared Silly]

Steel has the advantage of much less of a swing in buoyancy between full and empty

Just to clarify. The buoyancy swing is the exact same amount in a HP80 and AL80 as the air capacity is the same. What is different is the initial buoyancy. One is relative (swing) the other is absolute (initial).

Steel cylinders are typically negative when empty while aluminum cylinders are positive when empty. As such, when diving in cold water with buoyant exposure protection it is preferable to dive with steel cylinders so you do not need as much lead.

For your diving I would certainly look at nothing but steel cylinders. Besides the buoyancy characteristics there are more capacity options. We have LP95s and LP108s for singles.

There are some MP3180 made by Faber that are quite negative and are great as singles for cold water diving. (I would not double them).

 

[a22shady]

I would go with steel cylinders. The nice thing is that when empy they are negative. You dont need to worry about the tank becoming positive as you do with the AL. I dive the Worthington HP100 and love them. Provide me enough gas to do the dives I am currently doing. And for me they had the best bouyancy / weight characteristics. And When I go to doubles the 2 100's will be great.

40lbs of lead that is A lot. Not sure how that comes to be but you gotta be very negative. I dive drysuit and HP100 and I use a total 16lbs. 6 SS backplate / 6 STA / 2 2lbs in each trim pocket. BUt even when using the AL80 I would only use 20lbs total.

 

[ssturges]

For me having my own tanks is more about convenience, they are alway filled and ready to go. I never have to worry about getting to the shop to have them filled or returned. I can also have tanks that suit me best.

Steel tanks in general hold resale value better in the long term but cost a lot more up front. Steel tanks if you take care of them will last a lifetime.

As people bring out above it a lot better to have the heavier steel distrubted on your back vs in pockets or a belt. Not sure what kind of suit you have but what amount of weight is a huge risk.

If you really do need this much weight you should look at changing your suit. In your current state should you accidently drop that belt you are going to be in a world of hurt. I fear for you.

 

[stakanak]

DBS, I just bought an extra heavy steel 100 Faber 3180 and went from 28lbs with an aluminum 80 down to 16lbs...that is a lot of weight to drop off. I wear an 8/7 Aqualung wetsuit (attached 7mm hood)with 5mm gloves and booties. I think I could probably go down to 14lbs but 16 seems just right. It was well worth the purchase.
I no longer have to live and dive by the hours of the LDS, which stokes me out!
Get Wet!

 

[vinegarbiscuit]

You've received some great advice so far. If you want to play around with numbers, check out the buoyancy calculator that an enterprising SB member created and posted just over a year ago. It was created to allow divers to calculate how much lift is needed to offset their rig's ballast, but you can also determine how much ballast you can remove from your belt by using different tanks. See http://www.scubaboard.com/forums/bu...ems/158370-ultimate-wing-lift-calculator.html . You can use doctormike's tank characteristic's spreadsheet to play around with with different tanks, if you're so inclined. As you can see, you'll certainly take off a fairly hefty whack of weight by employing a steel tank.

 

[doctormike]

Just to clarify. The buoyancy swing is the exact same amount in a HP80 and AL80 as the air capacity is the same. What is different is the initial buoyancy. One is relative (swing) the other is absolute (initial).

Excellent point, and I did phrase that in a somewhat oversimplified and misleading way. But while the weight change is the same, the buoyancy swing is determined by more than just the tank volume.

Air has mass, and the weight of 80 cubic feet of air is about 6.5 lbs (0.081 lbs per cubic foot), whether it is at surface pressure or compressed into a tank. Tank volume ratings tell you the number of air molecules in the tank, since they refer to the volume that the air in the tank would fill at sea level, when the tank is filled to its service pressure.

So if you are looking at the difference of the weight of an empty or full tank on dry land, you will find a change of 6.5 lbs. Therefore, the weight swing of all 80 CUF tanks is the same (the weight of 80 CUF of air at sea level). However, the weight swing is less relevant to diving (more relevant to the poor divemaster who has to carry tanks to and from the dive shop!)

Buoyancy, on the other hand, is dependent on the volume of surrounding fluid displaced, and therefore it does change with pressure. So the higher the pressure of the gas, the less buoyancy for a given number of air molecules, as they are squeezed into a smaller space, which would displace less water, which would result in less buoyancy. That is where those tables come in handy.

If this is hard to visualize, imagine a hypothetical scuba tank with a service pressure of 14.7 PSI (sea level pressure) - basically a balloon or something with very thin walls. An 80 CUF tank of this design would be 80 cubic feet in physical volume, and would have tremendous buoyancy - it would be basically an enormous lift bag. You would need over two tons of lead to sink that tank, since you would have to equal the weight of the water (62 lbs per cubic foot) that it displaced (80 x 62=4960 lbs).

Mike

 

[dbs129]

Just a follow-up my original post and some of the concerns regarding the amount of lead I am wearing when using the aluminum 80 tank.

Unfortunately, in the environment where I dive (North Atlantic) I am required to wear a two piece 7 mm wetsuit, boot, gloves, and hood. Thus, the need for the current 40 lbs of lead due to the buoyancy of the suit (I am still convinced that the zipper flap alone on the Pinnacle Escape step-in jacket accounts for at least 2 lbs of lead as it appears to be at least 8 - 10 mm thick- much thicker than other suits I have seen).

During my openwater certification, I was initially carrying 42 lbs of lead. But with experience in many dives since certification, I have been able to drop a couple of pounds. I feel that I could possible drop another couple of pounds as during a recent dive I was still a bit negative when I did a buoyancy test at the end of the dive with approx 400 PSI remaining in the tank. In addition, during the test I took a larger than normal breath prior to dumping the air from my BCD. Therefore, being still negative definitely indicates I am still carrying too much lead.

From a review of some of the information I was able to obtain from soem of the links in your replies, it should not be a problem for me to get rid of the 16 lbs of lead on the weight belt if I purchase steel 100 tanks. Even if I have to max out the lead in the BCD trim pockets to 5 lbs each with the 10 lbs in each dumpable weight pockets, it would still allow me to carry 30 lbs of lead maximum (if required) without the weight belt.

Thanks for all the replies and the great information.

 

[fdog]

Hello DBS - I'm not sure what rentals cost in your area. Here they're roughly US$7 to $10 per cylinder per day.

Balance that against the cost of air fills. Again using Reno as an example, simple air is $5 to $7 per fill.

Most folks do two dives at a whack, so you'd most likely need two cylinders.

That's a price savings of about $2.50 per dive. Since aluminum cylinders are about $150 to $200 delivered, and you'd need two, it would take you roughly 150 dives to to have the purchase price become cheaper than renting.

That's 3 years at the rate you indicate. For steel cylinders, 6 years. It's tough to make a good case for buying cylinders purely on price alone.

Where it becomes worthwhile is the freedom. Phone rings, it's your buddy - "Hey, the vis is great, want to hit the water?" Instead of running down to the dive shop, you just toss your gear into your vehicle and go. And when it becomes easy, you'll probably dive more, too.

If you have a track record over the last year of diving consistently, this has become more than a passing fad. Buy some cylinders.


All the best, James