SAC change with doubles?

[Spg01]

Has anybody had a drastic change in SAC rate when first moving into doubles?

I have only been on 3 dives with them, so I am guessing (hoping) that I just need to get more dives in with them. With a single tank I am ~.53 cuft/min normally, but my last dive with the doubles was ~.95 cuft/min. I thought I was pretty relaxed on that last dive so I was surprised to see it that high. I also don't remember a lot of inflating/deflating of my wing or drysuit.

I could understand the 1st dive in them to be high, but it was ~.80 cuft/min and my 2nd dive was ~.85 cuft/min. So I am hoping it will come back down soon.

 

[Divin'Papaw]

Yup. Doubles can take quite a long time to dial in. I certainly experienced a long transition period. It easily took me 15-20 dives to get comfortable. Your SAC will come back down with more practice.

 

[TSandM]

Have somebody look at your trim. The most common thing I've seen in people who transition into doubles is that they immediately go about 45 degrees head up, because the rig is head-heavy and they're trying to balance it. That puts the diver into the "swimming uphill and having to stay negative" situation, which is very wasteful of gas. It's also much harder to push yourself forward in the water when you're out of trim.

In my admittedly limited experience, a lot of people don't weight themselves properly with doubles, and even when they carry weight, they may not put it in the right place. A lot of sets benefit from some kind of tail weight.

 

[flots am]

Has anybody had a drastic change in SAC rate when first moving into doubles?

I have only been on 3 dives with them, so I am guessing (hoping) that I just need to get more dives in with them. With a single tank I am ~.53 cuft/min normally, but my last dive with the doubles was ~.95 cuft/min. I thought I was pretty relaxed on that last dive so I was surprised to see it that high. I also don't remember a lot of inflating/deflating of my wing or drysuit.

I could understand the 1st dive in them to be high, but it was ~.80 cuft/min and my 2nd dive was ~.85 cuft/min. So I am hoping it will come back down soon.

Doubles have more drag than a single tank. Also they contain more air, which means that at the beginning of the dive you'll have more air in your wing to offset the weight of the air in the tanks, which also increases your drag. If you're overweighted, this also adds more air to your wing and increases drag even more.

If you can tune your weighting to be correct, and adjust your trim to get horizontal again, your air consumption will drop, but it will never be quite as low as with a single tank, simply because you're moving an extra tank through the water.

flots.

 

[Superlyte27]

You'll get there with practice. It takes time to get your buoyancy, trim and propulsion perfect when moving to doubles. Remember you are moving more than twice as much size and weight through the water. Take is nice and easy. You'll see a drastic decrease soon.

 

[Spg01]

Thanks for the replies. I just figured I would ask since I did not find any similar threads when searching around earlier.

I am sure with a few more tweaks here and there it will work itself out. As long as I don't die lugging them around first.

 

[ADeadlierSnake]

I also have a drastic increase in my SAC when diving doubles. I switched from diving a single Worthington LP 77 to diving double Faber LP 108s. Needless to say, it takes way way more effort to push those things through the water. I have had plenty of dives to get comfortable with them at this point, so I consider the issue resolved, and filed under the "it's normal" category. And on dives when I switch back to singles, I notice my SAC drop back down. I feel like I have nothing on my back on those dives!

 

[irishsquid]

Thanks for this thread! I'll be going to doubles soon and this will help when I obviously will likely see the same. With that said, I didn't really see much of an increase in my SM dives. I will admit I didn't check my SAC while dialing my tanks/rig in, only later; after getting it close and comfortable diving in it. Still have more tweaks to go.

 

[Spg01]

I actually went over the numbers again and it looks like I messed up on the last dive. I copied my pressures from the dive before for some reason. Good thing I saved the real numbers on my phone after the dive,

So instead of 0.95 cuft/min it was actually 0.74 cuft/min.

 

[TSandM]

I actually have very little change nowadays between single tank and doubles. My SAC rate in the caves, where I am swimming constantly, is only very slightly higher than my SAC rate at home, where I mostly dive singles and rarely swim steadily.