DIR and traveling

[FreeFloat]

Ideally you could swim up without having to drop the weight, but that may not be possible.

However, if you do, you're an additional 12 pounds more positive than usual by the time you get to 15', and you will most likely *not* be able to stay down. Ditching your weight in a thick wetsuit means you're surfacing whether you like it or not, I believe

Once you get going from the bottom of course. There is such a buoyancy swing inherent in coming up from any reasonable depth with a thick wetsuit - you start the dive with a heavy weight belt required to get you under, then at the bottom must compensate with air in the wing (adding to drag while swimming and affecting your SAC). If you were to have a wing failure you would have no choice but to ditch your weightbelt. Even so, you would likely be somewhat negative until you were able to swim up to a point where your wetsuit expanded to make you neutral, then as you ascended further you'd become progressively more buoyant.

This is coming from a cold-weather reformed wetsuit diver. I've dived both AL80 and LP80 (PST steel) tanks and frankly I prefer the steel as I require less initial ballast to sink me. I have also been able to hold my 20' and 10' stops while underweighted (such as having passed a weight block to another diver etc) although it really isn't much fun. I've also swam my rig up from depth (not very deep mind you, in case 'anything' happened) so that I know I can. With steel tank I was wearing 6lb on the belt and 10lb with the AL80.

I say "reformed" as I've switched to diving dry for my last 12 or 20 dives now....

 

[Hank49]

Go to this thread ... http://scubaboard.com/showthread.php?t=68203&page=4&pp=10 ... start on reply 33. Lamont went through the calculations rather well.

... Bob (Grateful Diver)

Thanks for your help Bob. I've read through that long, descriptive post, but I still don't understand why an al 80 doesn't have enough gas for a dive to 30 meters. Following is a quote from that post. Diving Preacher described his dive as similar I believe. ?? Thanks again. Hank

"If we're doing a wall dive at 100 fsw on a single AL80 our rock bottom will
be 1300 psi. If the plan is to descend, travel the wall, return on the
same path and ascend, then the turn pressure will be:

usable gas = 3000 psi - 1300 psi = 1700 psi
gas used on swim out = 1700 psi / 2 = 800 psi
turn pressure = 3000 psi - 800 psi = 2200 psi"

 

[cornfed]

Thanks for your help Bob. I've read through that long, descriptive post, but I still don't understand why an al 80 doesn't have enough gas for a dive to 30 meters.

I'll admit I didn't look at the numbers you posted very closely but when people say "an AL80 doesn't have enough gas for a 30m dive" they don't mean it's impossible to do. Instead they generally mean, "leaving enough reserve in an AL80 to safely make a 30m dive leaves you with so little usable gas the dive isn't worth the effort." Do you see the difference?

 

[Hank49]

I'll admit I didn't look at the numbers you posted very closely but when people say "an AL80 doesn't have enough gas for a 30m dive" they don't mean it's impossible to do. Instead they generally mean, "leaving enough reserve in an AL80 to safely make a 30m dive leaves you with so little usable gas the dive isn't worth the effort." Do you see the difference?

I get it. It depends on the 30 meter dive site as to whether it's worth it.

 

[lamont]

Thanks for your help Bob. I've read through that long, descriptive post, but I still don't understand why an al 80 doesn't have enough gas for a dive to 30 meters. Following is a quote from that post. Diving Preacher described his dive as similar I believe. ?? Thanks again. Hank

"If we're doing a wall dive at 100 fsw on a single AL80 our rock bottom will
be 1300 psi. If the plan is to descend, travel the wall, return on the
same path and ascend, then the turn pressure will be:

usable gas = 3000 psi - 1300 psi = 1700 psi
gas used on swim out = 1700 psi / 2 = 800 psi
turn pressure = 3000 psi - 800 psi = 2200 psi"

Weither or not an AL80 works for a 30msw/100fsw dive is kind of subjective. I've probably got a dozen dives down to 90-100fsw at the I-beams on rental AL80s that I racked up before I got my E8-130s. I didn't stay down there long (10 mins or so) before heading back up. That's long enough for me -- after 10 mins at 100 fsw in the cold dark water up here it gets me plenty narc'd.

If there was any current down there or anything else making the I-beams more complicated, then I think an AL80 would be cutting it close. The example of diving halves at 100fsw on an AL80 probably isn't a good one -- I'd normally get down to 100fsw at the I-beams with about 2500 psi which is pretty much the turn pressure for halves...

 

[MXGratefulDiver]

I'll admit I didn't look at the numbers you posted very closely but when people say "an AL80 doesn't have enough gas for a 30m dive" they don't mean it's impossible to do. Instead they generally mean, "leaving enough reserve in an AL80 to safely make a 30m dive leaves you with so little usable gas the dive isn't worth the effort." Do you see the difference?

It also depends on your air consumption rate (SAC).

One of my AOW exercises is, after taking consumption measurements, to have the student calculate their SAC and RMV rates, and based on those figures determine how much gas they would need to do a dive to a depth of 100 fsw, with a specified bottom time at that depth and planned stops at specified depths on the return to shore.

One student ... a fairly new diver with a fairly high SAC rate ... calculated he'd need 94 cubic feet of air for the dive. I, on the other hand, would use about 62 cubic feet of air for the same dive.

That's why it's important to know how much gas you consume ... it can vary wildly from one diver to the next.

It's also why it's important to have some idea of the difference between your air consumption rate and your buddy's.

... Bob (Grateful Diver)

 

[MonkSeal]

It also depends on your air consumption rate (SAC).
... Bob (Grateful Diver)

SAC finaly. When doing the gas management calculations one have to include the SAC but not his/her common SAC but increased (e.g. metric: if normal SAC is 20 l/min, 30 l/min should be used). Why ? Because if you have sharing gas scenario it's little stressful situation so you have to count on increased SAC.

 

[MXGratefulDiver]

SAC finaly. When doing the gas management calculations one have to include the SAC but not his/her common SAC but increased (e.g. metric: if normal SAC is 20 l/min, 30 l/min should be used). Why ? Because if you have sharing gas scenario it's little stressful situation so you have to count on increased SAC.

I have my students take consumption measurements under two conditions ... first under "normal" diving conditions, where they are moving at normal pace ... then under "stressed" conditions,where they are finning furiously and continuously (preferably into a current) for several minutes to simulate stress conditions. Sometimes the latter measurement is more than twice the former.

You use the "normal" consumption rate (converted to RMV) for planning how much gas you'll need for a given dive plan.

You use the "stressed" consumption rate (yours and your buddy's) for calculating rock bottom.

We've steered a bit off-course here ... if someone objects, say so and we can move this to a different forum/thread.

... Bob (Grateful Diver)

 

[O-ring]

What do you DIR divers do when traveling? Well, more directly, those who use primarily double tanks. I am considering purchasing a set but for the majority of the diving I do the extra isn't necessary. I am diving with a 15L single tank now and as long as I stay within no deco limits to 30-35 meters I have enough air, (in my limited knowledge) I surface from a dive to 30 meters to within 1-2 minutes of no deco with 50-60 bar remaining in the tank. Even if I buy a double DIR style valve for reg redundancy, what do you others do when you fly somewhere to dive? Only single tanks available, no double valves available, How many carry things like tanks in checked luggage and how many use a different configuration when traveling?

Perhaps I have two issues here but look for some advice on both,
thanks,
DP

This might not be DIR, but I'll tell you what I do. I have a DiveRite stage strap and an extra cam band that I take with me (along with one of my deco/stage regs). Using that setup with my bp/27# pioneer singles wing, I can dive one back tank and one stage/deco tank. You can do it almost as easily with a halcyon stage kit or a stage kit of your own manufacture and a cam band or an SS hose clamp, but I got the DiveRite strap pretty cheap and it works. Strap one single tank on your back, rig the other rental tank (or 2 if you are also carrying a deco mix), and jump in. You do have to watch your gas consumption in each tank because you are basically diving independents, but I have found that a much easier solution than bringing a manifold and bands, etc. with me and trying to talk some dive op that doesn't speak English as a primary language into letting me knock the valves off a couple of their tanks.

Alternatively, there are "soft bands" that you can use (basically a two way STA with bolts coming out of the junction of the two STAs). You strap a tank on each side of the thing with the included cam bands and bolt the thing to your bp. Voila! Instant independent doubles..