What Size Doubles?

[rivers]

I've been doing the dive planning for my AN/DP checkout dives. My instructor told me to plan 2 dives, on successive days. Both to 120' for 35 minutes. I'll be using double HP120s and an AL40 for deco gas.

After going through this exercise, I am very glad I did not get doubles that are any smaller.

I ended up going back to my instructor and proposing we shorten the dives to 30 minutes because, essentially, I was gas limited by the 120s. The dives are long/deep enough that my planning showed I would need well more than half the gas in my deco bottle. That, in turn, means I have to accommodate a lost deco gas scenario in my planning - i.e. have enough back gas to do all my deco on that, if necessary.

If I were diving, for example, double 100s, I would have needed to shorten the dive even more. I think 30 minutes at 120' isn't really that long, so I am definitely glad I don't need to shorten the dive any further.

Of course, if everyone took an AL80 for deco gas, instead of a 40, it wouldn't be an issue. Then I could probably do the 35 minute dive on double 100s. But I don't really want to step up to carrying a deco bottle that big just yet.

And, yes, my planning is using very conservative numbers for consumption. These will be my first real deco dives, so I don't mind being extra conservative in my planning and having 5 minutes less bottom time as a result.

Just another thought on the size of doubles...

An ali 80 really isn't a big deal in the water. Out of the water, it's more cumbersome, but in the water, i hardly notice the difference between it and my 7l (roughly an ali 50).

 

[Reku]

Here's my 2 psi ... again

instead of carrying monster tanks:

When I have my double 85's and I need more gas I just sling an AL80 stage - I start the dive on the stage and dive it until 1/3 then I switch to my 85's for the rest of the dive and ascend then switch to deco gas at target depths.

start -> descend on stage to 1/3 -> backgas 1/3 -> ascend on backgas - > lean 50@70ft -> rich o2@20ft -> finish

if its REALLY deep then add a travel gas to descend and ascend on and use stage then backgas at target max depth.

carrying a bunch of stages and travel gas is no problem - carrying super heavy tanks sucks though. steel backgas, stages and deco is always AL

ymmv

 

[Chris H]

If any of your buddies are diving doubles, find out what they are using. It may make dive planning easier in the future. At one point, I had double 130s, 95s, LP80s, and LP50, all at the same time. I now have a KISS Classic rebreather, 104s, and 50s. I love diving the 50s, but they aren't ideal for an only set. Nothing beats a carefree scooter dive in baby doubles! As others have mentioned, the 104/130 are brutal out of the water, but they trim out great. The 95s were too short for me. I'm about 6'2". The 80s were very comfortable, but weren't great for cave dives when my buddies had 104/130s so I traded my 80s for PST104s. Carrying them does suck. 98% of my dives are in a Drysuit. YMMV

 

[wetb4igetinthewater]

I wonder why HP-117s are not more popular than HP-100s. They are not longer than HP-100s but they are fatter and do not weigh much more than 100s. If anyone has experience with 117s then I would like to hear your complaints.

I wish they were a bit longer. I'm 5'11". I find it easier to rest my 100's on a wall than my 117's. I have 4 117's for rec and 2 100's for tec

 

[stuartv]

I love diving the 50s, but they aren't ideal for an only set. Nothing beats a carefree scooter dive in baby doubles!

Just curious, would you rather dive double 50s than a single HP100 or 117 or 120? If so, why?

Like I said, I'm just curious and it seems (to me, being totally inexperienced) like a single tank with the same amount of gas (but possibly with an H valve, if you're worried about having redundant 1st stages) would pretty much always be preferable to doubles.

 

[matt_w]

Just curious, would you rather dive double 50s than a single HP100 or 117 or 120? If so, why?
.

You were on to part of it with the H-valve. Redundancy. Using an h-valve does little for redundancy. 50s sidemount or backmount with an isolator are redundant. Double 50s are also very streamlined (in either configuration). Finally, they are LP tanks so getting to a full 100 ft^3 is easy. If you were to fill them to 3200 they hold more than the 120 at full pressure (not that I am recommending that).

For me, diving double 50s backmount is much more comfortable than even a single 80. Diving them sidemount is like diving without tanks.

 

[Reku]

You were on to part of it with the H-valve. Redundancy. Using an h-valve does little for redundancy. 50s sidemount or backmount with an isolator are redundant. Double 50s are also very streamlined (in either configuration). Finally, they are LP tanks so getting to a full 100 ft^3 is easy. If you were to fill them to 3200 they hold more than the 120 at full pressure (not that I am recommending that).

For me, diving double 50s backmount is much more comfortable than even a single 80. Diving them sidemount is like diving without tanks.

lp50's are SO FUN. Side mounted 50's are like you said - nonexistent.

 

[stuartv]

If you dive double 50s in back mount are they mounted where you can reach the valves? Valves up? Doesn't that position the tanks pretty high up on your back? It seems like that would make it harder to trim out well.

 

[cool_hardware52]

If you dive double 50s in back mount are they mounted where you can reach the valves? Valves up? Doesn't that position the tanks pretty high up on your back? It seems like that would make it harder to trim out well.

I have several sets of LP45's doubled and valve access is no problem. As the tank diameter decreases the valves move closer to the diver......


Tobin

 

[Doby45]

My understanding is the use of 50s, backmount or sidemount are a recreational riot. They are on my list of dive gear that I eventually want.