Tanks and air consumption

[Jerrod]

Hi,

I am a new divers with only 10 dives. I had the pleasure of working with a good instructor for the last five and got some great tips on reducing my air consumption. I have reduced my weight (went from 24 lbs to 12), and stopped fiddling with adding air and letting it out of my BC for neutral buoyancy, reduced my drag and am more "tighter", and most of all am way more confident.

Taken as a whole, all of the above have helped me reduce my air consumption by about 200 psi per 30 minute dive. I think only more experience and other tips along the way will really help reduce my air consumption.

With that in mind, I am going on a trip to a lake this weekend. There will be 3 or 4 official dives and I hope to convince someone to do 2 or 3 shore dives. Air there is 10.00 a fill, and one must drop the tank off and hope that it's done when you come back in 4 or 5 hours. So since I am an air hog and want to do many dives, I am wondering what tanks I should rent. The tanks are 15.00 regardless of the size, so is bigger better?

We will be doing probably 3 boat dives and the rest shore.

I think for the 3 boat dives, I should get 3 super80 because it might be hard for me to get back on the boat. But for the shore dives, what about these bigger tanks? Are they just bigger and contain more air? Will they mess with my new found neutral buoyancy and require more weights? I tend to get very light at about 1,000 PSI on the 80. How much more time should I get (I think the biggest is 119 cu. ft)? They don't look to be much heavier 31 vs 40 lbs.

Any insight or advise would be appreciated. Google searches did not reveal much for beginners.

 

[ben_wilson3301]

They can mess with buoyancy. Id take 3-4 tanks then dive and fill as needed. You almost never end up diving as many times as you hope to.

 

[TSandM]

Are the 80's you are using aluminum? In that case, there will be a significant difference in switching to bigger tanks which are steel. Steel tanks are negative when empty, and aluminum tanks are positive when empty. So when you switch from Al to steel, you need to remove some weight. In addition, aluminum 80s get butt-light when empty, and most steel tanks don't undergo that buoyancy change, so WHERE you carry your weights can vary a little, too.

If you are getting light at 1000 psi, it's likely you've cut your weight down a bit TOO much. There's no benefit to being too light -- it makes you work harder at the end of the dive, and can make a too-rapid ascent much too easy.

If you are diving with people who have much better gas consumption than you do, and you don't like being the gas-limiting diver, then renting bigger tanks is a great idea. Otherwise, stick with what you know. One of the biggest challenges new divers face in trying to get stable and comfortable in the water, is that they are frequently changing configurations on every dive or set of dives. Stick with one thing until you have it sorted; it will then be much easier to sort out a new arrangement.

 

[Jerrod]

Yes they are aluminum. I think all we use here in KY is aluminum. This might be wrong, but I thought I heard someone say that.

I am basically diving with advanced divers. Of the 15 going, 5 are doing some NAUI master diver dives and 6 are doing NAUI advanced dives. I think there are a few beginners going though. This is my first open water dives (all have been in a rock quarry) and this will be the only chance I get this year to do any more diving outside of quarries, so I really need to get as much experience as possible while I have the chance. (Also leaving the wife and 3 year old at home, so I am going to hit the diving hard .

I think the bigger tanks, the 119 or 110 are aluminum too. With my consumption rate of 1,000 PSI per 30 mins, how much will I actually benefit from the larger tank? Especially if my buddy is a beginner anyway (I think I will get paired with someone of similar background)?

Thanks,

Jerrod

---------- Post Merged at 10:20 PM ---------- Previous Post was at 10:17 PM ----------

Also, while I agree that I should not be making that many configuration changes, I want to try different equipment so I have enough experience to make wiser equipment purchases.

 

[spectrum]

There can be a significant difference in buoyancy and balance of different cylinders.

The super 80 I think is sometimes called a neutral 80 is a 3300 PSI fill which will make a perfect fill harder to come by. The classic AL80 is 77.4 Cf so gas capacity is a wash. The Super 80 has more aluminum used in the construction and just moves about 4 pounds from your weight belt to the cylinder. There's no real advantage to these beasts.

If you can get into larger steels, probably high pressure then you can boost your bottom time and you will need to adjust your weighting. Adjust according to the difference in "Empty Buoyancy" values in this chart.

Trim will be a much more iterative adjustment. I'd suggesting limiting the variety of cylinders in play so you can walk away with some good conclusions and not be a basket case diver all weekend.

Be careful about pushing to hard to keep up with the veteran divers, i would hope that they accommodate you as a novice and adjust plans accordingly.

I also agree that dive count very rarely exceeds what one hopes to pack in and often falls far short. This is a time consuming social sport that can eat up a day in a hurry. If you need to add the logistics of chasing air then I rest my case.

Pete

 

[jar546]

Changing tank size will throw off the buoyancy you just figured out. Get use to it first then worry about bottom time. You will enjoy yourself whether you get in a 30 minute dive or a 60 minute dive. Take another day with different tanks to figure the new weighting and trim of steel or larger AL such as a 100. Since this will be your first dive outside of a quarry, stick with what you are comfortable. You will have enough on your mind. Enjoy the diving for now.

 

[Rogersea]

Now that you know how to figure out bouyancy and proper weighting, you should have confidence that you can do with any size/style tank you use. If it is my money and I had what sounds like fairly "high" air consumption, I would opt for the larger tank and spend the first dive figuring out my bouyancy, then enjoying the rest of the trip....

Have Fun!!!

Cheers,
Roger

 

[Colliam7]

Now that you know how to figure out bouyancy and proper weighting, you should have confidence that you can do with any size/style tank you use. If it is my money and I had what sounds like fairly "high" air consumption, I would opt for the larger tank and spend the first dive figuring out my bouyancy, then enjoying the rest of the trip....

Good advice. And, based on 'average' bouyancy characteristics of cylinders, you should be able t0 come up with at least a rough starting point for weghting. For example, if a 'standard' aluminum 80 is ~4 lbs positive when empty, and a HP steel 119 is ~2 pounds negative when empty, to go from the 80 to the 119, drop 6 pounds. Obviously, this is a very coarse adjustment, bouyancy characteristics vary somewhat according to cylinder manufacturer, and a proper weight check is needed. But, it gives you a starting point. (I also agree with Lynne, it sounds like you may be 1 or 2 lbs light, given your description of feeling 'very light at around 1000 PSI'.

I think the bigger tanks, the 119 or 110 are aluminum too.

I suspect the 119 is not an AL cylinder. And I suspect that you may be referring to an AL100, rather than a 110 cf aluminum (or possibly a steel HP100 or LP112).

Given your desire to gain experience, there is nothing wrong with trying out a larger steel cylinder. YES, your bouyancy / trim will be different, compared to your 80cf aluminum. Yes, you will (probably) require less weight. If you have the option of trying a HP steel (100 or 119, for example) by all means do it. I personally prefer the HP100 to a 119, but either one is worth trying out to see how you like it. The one cylinder I would not waste time with is an AL100 - it is not a commonly encountered cylinder, and many people find it somewhat ungainly (heavy) for the modest extra gas.

Cheers,
Roger[/QUOTE]

 

[Dive the World]

Don't worry too much about your "new found buoyancy" - everything time you add gear, remove gear or swap one equipment for another you buoyancy will need to be adjusted accordingly.

So if you take 15L steel - you will not need any weights, if you take 80s with a 3mm shorty you will be need weights, you take the same tank with 7mm suit you will need to take more weight.

Sounds like you are conscious about it - which is 80-90% of the battle - rest will come with more dives.

If air consumption is your biggest concern than just take it easy and breathe nice and slow - take 5 seconds to breathe in pause and take 5 seconds to breathe out. Remember to open up your diaphragm and take nice and slow breaths in.

Diving is a lazy sport - so be as lazy as you can underwater and save that energy.

When I started I would consider my self lucky if a tank would give me a 30 min dive at 20meters - now I could do the same dive for 50mins and probably come out with 80 bar; which I consider it to be great.