50 replies to this topic

[intotheblue]

Not necessarily...

Most River Caver's here are diving Sidemount steelie dubs WET, not dry...supposedly no issues...

nhb

So, from what I'm reading (thanks for all the comments - they are great!)

if I decide to go with steel tanks, then I'd most likely need to dive dry to have any positive lift. Correct?

again Thanks!!

True... I have begun replacing my LP steel cylinders with Fabres as they are much better for sidemount cave diving. They come closer to "floating" next to the body instead of dragging downward destroying your profile.

FG... I had a female student that we set up with a set of Fabre 85's in doubles configuration that worked great for her personally. You have more to worry about when you dive with someone that uses larger tanks as you have to compute your tank turn pressures based upon your partner's need for your extra 1/3 of gas... if you have smaller capacity, or the other way around if the role's reversed. I may not have made this perfectly clear, but point is that you are diving doubles and probably will be diving in situations where you follow rule of thirds. Your last third MUST be enough and preserved for your buddy. I'm simplifying this... but someone can add to this that has more time. Sometimes, getting small tanks that work for you and your SAC rate, may not be suitable for diving with someone else if they will need a much greater volume of gas in an emergency air sharing situation when breathing off your tanks along with you.

[PerroneFord]

Diving doubles, especially diving them in scenarios where they are intended, really begins to focus your attention on you BUDDY. Or at least it should.

Gas planning is not simply concern for yourself as is so common in the recreational arena, but you are looking at having a reserve for you AND your buddy to get home safely. The concept of "available gas" becomes important.

Assume for a moment that you are going to do a basic dive and call on thirds. You have twin 80s and so does your partner. The one with the highest SAC rate becomes the limiting factor if you turn the dive on thirds and not on time. Assuming gas failure at the point you turn the dive, there needs to be enough gas available for the person with the HIGHEST SAC rate to get back safely. Where I think people go wrong is that they account for THEIR OWN high-stress SAC rate, but NOT the combination high stress SAC rates of both divers. Which is what you will need.

So in gas planning, we were taught to work with a reserve. Essentially saying I want to be back at point of entry with 500 psi. So if my buddy is the heaviest breather, then I subtract my 500spi from my initial gas (3000psi) and now we calculate the dive based on 2500psi. We turn the dive on thirds or 1700psi. Now the question is, is that 2700psi enough to get BOTH of you back under stress. And is that 500psi "emergency cache" enough to deal with entanglements, a lost buddy search, a lost line search, etc.

To my mind, you can NEVER have too much gas available. This can be taken to the extreme of course, but I would CERTAINLY not be opposed to laying an emergency bottle down in a cave, or tying one off to a wreck if there was a high potential for silt-out, or other issue. I would also not worry so much about carrying heavier, larger tanks, for a few moments on the surface if it would in any way compromise the dive.

Again, these are just the thoughts and ramblings of someone new to doubles diving, and without a lot of experience. Differing thoughts are more than welcome.

[Diverbrian]

Not necessarily...

Most River Caver's here are diving Sidemount steelie dubs WET, not dry...supposedly no issues...

nhb

So, from what I'm reading (thanks for all the comments - they are great!)

if I decide to go with steel tanks, then I'd most likely need to dive dry to have any positive lift. Correct?

again Thanks!!

True... I have begun replacing my LP steel cylinders with Fabres as they are much better for sidemount cave diving. They come closer to "floating" next to the body instead of dragging downward destroying your profile.

FG... I had a female student that we set up with a set of Fabre 85's in doubles configuration that worked great for her personally. You have more to worry about when you dive with someone that uses larger tanks as you have to compute your tank turn pressures based upon your partner's need for your extra 1/3 of gas... if you have smaller capacity, or the other way around if the role's reversed. I may not have made this perfectly clear, but point is that you are diving doubles and probably will be diving in situations where you follow rule of thirds. Your last third MUST be enough and preserved for your buddy. I'm simplifying this... but someone can add to this that has more time. Sometimes, getting small tanks that work for you and your SAC rate, may not be suitable for diving with someone else if they will need a much greater volume of gas in an emergency air sharing situation when breathing off your tanks along with you.

Thanks for reminding of the reason that I have that Gas Management turn pressure table. I HATED those calculations, LOL.

Norm's point is that if you have a diver with double 130's who breathes heavy and a diver with double 80's who has lungs like a bird, the diver with more gas will have less to think about. With the light breather (call diver A), diver B's 1/3 will get both diver A and diver B out as long as they maintain the same swim pace and breathing rate (or some facsimile thereof). If diver B has the OOG and both are diving straight 1/3's, then it is unlikely that diver A's extra 1/3 can support their bird lungs and the air sucking dog on the way back out. They are likely to have to turn the dive before diver A reaches 1/3. I have a table that figures in an RMV conversion factor for this type of situation. For instance, diver A may only be able to use 0.6 times their third before turning the dive to be sure to have enough gas to get both divers out in an OOG situation.

All in all, fun stuff to think about on a Thursday morning and a major reason that I dive double 130's on most "serious" dives.

[intotheblue]

Not necessarily...

Most River Caver's here are diving Sidemount steelie dubs WET, not dry...supposedly no issues...

nhb

So, from what I'm reading (thanks for all the comments - they are great!)

if I decide to go with steel tanks, then I'd most likely need to dive dry to have any positive lift. Correct?

again Thanks!!

True... I have begun replacing my LP steel cylinders with Fabres as they are much better for sidemount cave diving. They come closer to "floating" next to the body instead of dragging downward destroying your profile.

FG... I had a female student that we set up with a set of Fabre 85's in doubles configuration that worked great for her personally. You have more to worry about when you dive with someone that uses larger tanks as you have to compute your tank turn pressures based upon your partner's need for your extra 1/3 of gas... if you have smaller capacity, or the other way around if the role's reversed. I may not have made this perfectly clear, but point is that you are diving doubles and probably will be diving in situations where you follow rule of thirds. Your last third MUST be enough and preserved for your buddy. I'm simplifying this... but someone can add to this that has more time. Sometimes, getting small tanks that work for you and your SAC rate, may not be suitable for diving with someone else if they will need a much greater volume of gas in an emergency air sharing situation when breathing off your tanks along with you.

Thanks for reminding of the reason that I have that Gas Management turn pressure table. I HATED those calculations, LOL.

Norm's point is that if you have a diver with double 130's who breathes heavy and a diver with double 80's who has lungs like a bird, the diver with more gas will have less to think about. With the light breather (call diver A), diver B's 1/3 will get both diver A and diver B out as long as they maintain the same swim pace and breathing rate (or some facsimile thereof). If diver B has the OOG and both are diving straight 1/3's, then it is unlikely that diver A's extra 1/3 can support their bird lungs and the air sucking dog on the way back out. They are likely to have to turn the dive before diver A reaches 1/3. I have a table that figures in an RMV conversion factor for this type of situation. For instance, diver A may only be able to use 0.6 times their third before turning the dive to be sure to have enough gas to get both divers out in an OOG situation.

All in all, fun stuff to think about on a Thursday morning and a major reason that I dive double 130's on most "serious" dives.

and the nice thing about normal cave diving (not sump diving), you start against the flow and let the flow carry you back out, hopefully making it easier and giving more margin of safety in oog emergencies. One must be aware, it is possible for a cave to reverse flow, and there are some caves that have little to no flow, and some applications where you may continually be in the same direction in a cave going from point A to point B, not from A and returning to A. A novice in a cave would do well to avoid "sump" diving and low flow "silty" caves (especially tight passages that can become difficult to handle in siltout). Wrecks present different concerns, but this all comes back to my original point of knowing what you are interested in Tech diving for. Some people are really only interested because it hones their skills in a new way and teaches them a new skillset they are not exposed to in rec. diving. For me, I loved diving with o/w divers and new students, but I was also looking for diving with people that had their "F. M." together... and tech diving used to be pretty synonomous with that. You may not be sure what your end goal is at this point in "learning tech", but if you know where you want to head... it will help you in determining gear, etc. Tech diving can be done with singles as well. There are applications for a good cylinder with a redundant regulator set-up... and then there are "side mount" applications too. I use mostly single 120's with double regs and deco stage for much of my offshore dives. It's not elaborate tech diving, but this configuration gives me a safer way to perform the types of dives I do there and is easier to manage on the boat.

[Trimix2dive]

should open water be taught in duals and a dry suit? JK - starting a new thread

[Trimix2dive]

This tread needed revived. I just doubled a set of alum 80s, their OK and great for a run to a local watering hole. I'll try them out out again in a few weeks, and keep ya updated. "My little pony" came in yesterday. (another thread)