Tank size and air consumption

[bullfrog]

Has anyone here done a careful study of their air consumption while using 108's vs 85s? How much did your sac rate or penetration change with different tanks. It seems that with big tanks you would have to add more air to your bc and because of tank size your profile would be larger, causing more resistance to swimming.

[jcook]

SAC rate went down with the LP85's. I am consistently getting further in the cave with less gas compared to when I dove LP95's. There are a few things to take into account when looking at it, the wing isn't inflated as much hence the less drag from it, less drag from a smaller diameter tank, these equate to better efficiency while swimming, so SAC rate will go down.

[whodunit68]

this is well known, though I haven't seen official documentation. my SAC improved because of drag/resistance, air in the BC, and I was becoming more and more comfortable in the water. It improved by 1.2 over the course of the transition, though it wasn't a "careful study," per se.
You live in an amazing city!

[PfcAJ]

Here's a thing to consider. You might get farther on some smaller tanks, but then you're farther in with less total reserve volume.

[whodunit68]

Here's a thing to consider. You might get farther on some smaller tanks, but then you're farther in with less total reserve volume.

it's a good, valid point....I had this same discussion with a shop person not long ago stating a similar point. so to take away, just because you *can* go farther doesn't mean you *should* go farther. plan your dive.

[speleodiver]

When I started diving sidemount I was using 108's, 95's and 85's. Looking at the 8' diameter of the 108's and 95's and the fact of a 38 cf gas difference in the two, I discontinued use of the 95's and strictly dove 108's and 85's. There is a gas difference of 66cf between the two. 85's are 7.25" diameter so profile does come into play proportionate to the diver size and type of thermal protection being used. My sac rate is way lower then most and equal to those I dive with. We have found that 108's give us about 20 minutes more bottom time at a given depth then the 85's. We were diving a Texas cave most of the time when this was being conducted so run times and distance were easily measured. Since that time, sac rates have improved even better and I now pretty much use 85's for eveyday diving and only bring out the 108's and sometimes 120's on the big push dives. Most of the time they come out when the scooter does.
Overall, I do not see enough difference in the two to consider profile an issue. During our swims, we never really had issue with one burning more gas then the other or having a difficult time keeping up the pace in the larger tanks. We did both switch many times and the results were always the same. I would have to say the 66cf of gas difference made the biggest impact and not the tank size or weight. Basically, the 108's are near the same as 85's with a stage. The stage does make a big difference. Which is why I use bigger tanks on longer pushes and try to leave the stages out.

[DA Aquamaster]

Crap. I agree with AJ. I really like my SMS-50 compared to my Nomad as it is a lot smaller and a lot cleaner - and with a set of small tanks it's a joy to dive with far less effort required to maintain the same speed with a commensurately lower SAC. Tank buoyancy is not the major issue, as the total difference in cave filled LP85s and cave filled LP 95s is about 6 pounds more negative buoyancy with the LP 95s and the added gas in the wing is pretty minimal in terms of drag.

Similarly, I don't think an increase of 6 square inches in frontal area makes much difference given the already poorly streamlined diver and equipment and the low swim speeds involved. If a diver really worried about drag, they'd ditch the dry suit as those are very draggy.

That additional lift requirement for the extra gas is significant however as the SMS 50 is lift challenged with steel tanks and that additional 6 pounds pushes it over the edge, leaving with the need for a larger rig with more drag.

But even though the SMS 50 is a lot cleaner, I also have a lot less gas and the improved SAC and/or the improved speed at a similar level of effort and SAC that comes from the SMS 50 and smaller tanks does not offset the lower volume of gas.

What's worse is that while I only lose 11 cu ft in the penetration portion of the gas with LP 85s compared to LP 95s, I am also losing 11 cu ft of exit gas and 11 cu ft of reserve gas. So even if I could go farther, there is a real disincentive to do that.

[RN]

About 5 years ago I did a couple of comparison dives with 108s and 85s. My RMV was 25% lower in the 85s versus the 108s. Pretty significant. As for the amount of gas I'm carrying, I don't dive to thirds as it is so I always have about 1.5 times the gas I use to penetrate in reserve. If I am doing a lengthy penetration then I add an AL80 stage. That with 85s gives me about the same amount of gas as a set of 108s with a lot less weight and drag so my RMV still remains lower.

[DA Aquamaster]

We very seldom dive actual thirds either, we're old and pretty unexciting people who want to get a lot older.

I really liked diving a pair of AL 80s with an AL 80 stage in MX, and I've been playing with the idea of bringing a stage with AL 80s, Steel 72s get gas capacity close to the same ballpark as a pair of LP 95s - but without some of the downsides.

The steel 72s and AL 80s lose between 10 cu ft and 28 cu ft compared to a pair of LP 95s, depending on how much you pump the steel 72s. However they are very nice to dive in the water with smaller wing requirements that make the SMS-50 a do-able proposition in a drysuit.

I'm going to play with it some more this weekend as well as on the next cave trip, after I consider some of the lost gas scenarios in both SM and SM/BM mixed teams.

Rob, I'm interested on your gas plan and gas switch thoughts in that configuration.

[a64pilot]

While not being any kind of scientific study, I dive HP 100's , HP 130's and LP 108's. Penetration wise I don't get much further at all with the bigger tanks, but I do get longer dive times. My SAC rate stays about the same, I'm just slower with the bigger tanks, I usually dive alone so I'm swimming at my speed, not someone elses
But, scootering is different, the scooter can easily pull me as fast as I want to go, so I think I have ended up doing swim dives with the smaller tanks, but using the big ones with a scooter. Scooters complicate the gas planning thing considerably, I plan on having enough gas so that if I lose the scooter and a tank at the furthest point of penetration, I can swim out, and that is a whole lot of gas you have to carry, so big tanks make more sense to me with a scooter than without.