Weighting Going From Single To Double Steels

[cool_hardware52]

Nice discussion but shouldn't a diver plan a dive anyways whether it be single 80 or double 149? Thereby eliminating the new double diver from trouble because he is a proficient dive planner. Planning isn't something that takes years to accomplish it's done on your open water dives.

I am a certified gold air hog. My 100cu will get me into trouble if I exceed my dive plan. In the great Lakes none the less

Every dive should be planned. Can a diver using a 100 cu ft cylinder get into deco? Of course. Are the chances of deco, and *longer deco obligations* increased with greater gas volumes? Few would argue against that.

Having said that let's remember the original topic of the thread, Weighting for doubles vs singles.

Here is the reality, with normal single cylinders, (72's - ~120's) divers using thicker exposure suits seldom need to start the dive negative for the simple reason that their exposure suit will be less buoyant at their shallow stop depth (~15 ft) by approximately the weight of the gas in their *single* cylinder.

For example will a 7mm wetsuit that is ~+20 lbs at the surface be ~13-14 lbs buoyant at 15 ft? Yup. 15 ft is almost 1.5 ata....... Same for a drysuit and compressed undergarments.

( For those in doubt try starting neutral at the surface and then swim down to 15 without adding any gas to your bc.)

That changes with doubles. 200 cuft of gas represents ~16 lbs, and suit compression isn't going to provide that.

Additionally the gas volume provided by 2 x 100 cuft is enough to generate a bit more obligation than a 3 minute safety stop.

That makes being weighted so the diver can maintain a long(er) shallow stop with near empty cylinders without shrink wrapping their drysuit a bit more important than it would be for the typical single tank diver.

I have always advocated using the least ballast possible. Having said that the added volume of gas 2 x 100 cylinders provides justifies the added risk of additional ballast to offset the added risk of not being able to hold a shallow stop.

Tobin