34 replies to this topic

[lv2dive70]

Not sure if this is the right place for this but it seems like it might fit, so here goes:

First, lots of background...

This past weekend a dive buddy from SD.com and I went to Key Largo. I did quite a bit of research on line and we made reservations for specific dives with a shop ahead of time:

Friday night: City of Washington (shallow reef dive)
Saturday AM: Double Dip Spiegel Grove (my max depth was 94)
Saturday PM: Supposed to be Bibb/Duane (ended up as Bibb/Benwood; my max depth was 111)
Sunday AM: Double Dip Spiegel Grove (my max depth was 99)
Sunday PM: South Ledge, Mike's Wreck (not what was planned at all but whatever)

Spiegel Grove, Bibb and Duane all sit well over 100ft deep. Others are mostly less than 30ft. I had contacted the dive shop and asked for 100's or 120's since my DB doesn't consume gas like I do and I thought I might run into gas issues before I ran into NDL's even though we were diving on Nitrox. They told me no problem but when they brought out the tanks they had a DIN connection... and no adaptor (I don't know if such a thing exists but anyway). So I ended up diving on 80's and I didn't have gas issues so it was all good in the end.


And now to the question...

I have since been reading some posts on ScubaBoard about this idea of not going deeper than one's tank's capacity in Cubic feet of gas (not sure if I am saying that right). So - no deeper than 80 ft on an AL80. Which would pretty much limit these recreational dives on the Spiegel Grove and Duane or Bibb to 120's - which I could not even rent even though I tried!

There are quite a few people who seem to feel very strongly about this. I am curious as to people's thoughts on this board.

[Hipshot]

A correlation between tank volume and depth may sound good, but different divers have different air consumption rates. One foot depth/cubic foot of tank capacity is arbitrary and capricious to say the least. Also, always distrust ROUND numbers--the math may be easy, but the caveats tend to get lost.

Rick



'No man is free who is not master of himself. ' -Epictetus

Edited by Hipshot, 20 August 2010 - 05:42 AM.

[peterbj7]

I agree with Rick. I often hear simplistic rules quoted as mantra, whereas in reality they are rough guides for early provisional planning. If you really want to get into the subject of gas planning then do it properly - work out your normal SAC from measurements on a number of dives, learn and understand the (IMO very unhelpful) US way of measuring tank volume so you are able to calculate how much air has been consumed from a tank. Then study the theory of redundancy and reserves and you will be able to say with some degree of precision how much air you need to take down with you for a particular dive. It isn't actually very complicated, just rather painstaking and monotonous, but learning it will really help you to advance your diving. I can't recommend any particular book, but the into-tech books from any tech agencies should do the trick. Focus on understanding, not learning a raft of recommendations.

[Landlocked Dive Nut]

Indeed, everyone is different!

To take the easiest way: you dived the Speigel Grove 4x over the weekend, so you now should have a pretty good idea of your gas consumption for a dive of that depth & duration. That dive is a straight up & down profile, since you descend & ascend on the mooring lines. You did not give your bottom time, but I doubt you went into deco.

I dived that wreck twice in July on AL80's with EAN 31, max depth 100 (my computer is programmed to beep annoyingly when I hit 100 feet). The first time was an easy dive once you got beneath the surface, and the second was rougher current below. I used slightly more gas on the second dive, mainly because of surface conditions (I breathed on my tank until I was back on the boat). Neither dive got me anywhere close to deco, the dives were very similar bottom times, and I came up with 1000 and 900 psi. Gives me a pretty good baseline for future 100' dive planning in those conditions!

[lv2dive70]

I should be more specific with my question - I was meaning in terms of a limit (vs. a planning technique). I guess the thing that surprised me was sort of an attitude that I saw that I interpreted as "I'd never go deeper in ft than the cubic ft of my gas supply" (regardless of that person's SAC). It was just something I thought was kind of extreme.

I wouldn't use something so trite as a planning technique, and I do appreciate the points made above. Although I do plan my dive including elements related to gas consumption, I haven't started tracking my SAC yet so that hasn't been an element to incorporate. It *is* something I've been thinking I need to do since I've started to have a consistent and slightly more reasonable rate of gas consumption. I'm glad you brought that up, it'll push me over the edge to start doing the math!

Thanks for your thoughts!

[secretsea18]

On Nitrox, your maximum depth is determined by the EAN mix in your tank. The MOD (max operating depth) will vary depending upon the pp of O2 that you use. Most folks use btwn 1.4-1.6 bar for this setting.

So your MOD with on 32 % O2 mix and set at 1.6 (like I use) would be a MOD of 127 feet, for example.

The amount of air in your tank can hold is irrelevant to your MOD.

[scubaski]

"about this idea of not going deeper than one's tank's capacity in Cubic feet of gas", I have never heard of this rule of thumb.

[lv2dive70]

On Nitrox, your maximum depth is determined by the EAN mix in your tank. The MOD (max operating depth) will vary depending upon the pp of O2 that you use. Most folks use btwn 1.4-1.6 bar for this setting.

So your MOD with on 32 % O2 mix and set at 1.6 (like I use) would be a MOD of 127 feet, for example.

The amount of air in your tank can hold is irrelevant to your MOD.

Totally understand MOD.
Totally understand that tank volume has nothing to do with it MOD.

Wanted to know what you all thought about the comments that people were making in relation to the fact that they WOULDN'T (not COULDN'T) dive deeper than their corresponding ratio eg 80 cu ft meant a floor of 80.

Apologize for not being clear.

I've spent half an hour trying to find the posts and had no luck so I'm giving up for now. I was surfing quite a bit earlier so they could be anywhere.

Edited by Kate P, 19 August 2010 - 08:16 PM.

[Parrotman]

My thoughts are that it does not even make sense. What does tank size have to do with depth? I am very good on air consumption, at times I have used an alum 63 and done an hour dive to 110 feet. I typically use an alum 80 and rarely come up with less than 1000psi regardless of my depth considering that I follow recreational limits.

I think it is kinda of wierd.

Just my Opininon.

J

[Capn Jack]

The only published rule I've ever seen regarding depth and tank size is the Navy dive manual, Figure 6‑14. Normal and Maximum Limits for Air Diving - which limits diving to 100' on tanks of 100cf or less. Paragraph 6-6.1 explicitly states "All open-circuit SCUBA dives deeper than 100 fsw shall employ cylinders having a capacity of at least 100 cubic feet."

I hate rules of thumb - especially about gas management. PADI only says OW divers are limited to 60 feet, 100 feet for advanced and 130 if you bought the deep card.

FYI - I can suck through a 120 CF HP steel in about 15 minutes in shallow water if I'm totally freaked out about losing contact with my novice-diver daughter, kicking my ass off swimming against a Gulf current and hoping I can finger walk to the mooring line....

I can also stay with secretsea on a relaxing muck dive, lay there and take 8000 pics of nudibranchs - so go figure...

So, all I'm saying is know what you're doing, plan the dive, dive the plan - and don't run out of gas and ideas at the same time.

Edited by Capn Jack, 20 August 2010 - 01:29 PM.

[peterbj7]

Most folks use btwn 1.4-1.6 bar for this setting. So your MOD with on 32 % O2 mix and set at 1.6 (like I use) would be a MOD of 127 feet

Agree with the logic, but I work it out at 131.2 feet in sea water.

[secretsea18]

Most folks use btwn 1.4-1.6 bar for this setting. So your MOD with on 32 % O2 mix and set at 1.6 (like I use) would be a MOD of 127 feet

Agree with the logic, but I work it out at 131.2 feet in sea water.

Just set my Suunto computer to dive at 32% O2 = 127 feet
........... 31% .... = 132 feet


I use the 1.6 setting.

I go from what my computer says is the MOD. I suppose the "never get wet" and Never get to dive" computer is also never "you can't go that deep".

Edited by secretsea18, 20 August 2010 - 04:52 AM.

[WreckWench]

Kate I am guessing that the zealots you were reading posts from adapted the Navy Dive Guidelines Mark quoted to correlate depth to tank size.

IF and I say IF a rough rule of thumb were being used by men diving with a mission i.e. Navy divers then they *might* be served by a quick guide of 80ft on an 80cf and 100 ft on a 100cf. One could surmise that Navy divers mostly male would be working and therefore have a higher SAC rate so this guideline you've been reading about could have come from that scenario.

The crux of the matter is that as others have said...it is not a general rule nor principal in mainstream diving. Nor does it seem practical to follow unless your SAC rate happens to be pretty high and therefore these tank sizes also happen to correspond to your ability to safely dive these depths and have enough gas.

If you find the posts you were referring to, ask where this rule comes from and let us know as I like others have never heard of it in my 15 years of diving.

[lv2dive70]

The only published rule I've ever seen regarding depth and tank size is the Navy dive manual, Figure 6‑14. Normal and Maximum Limits for Air Diving - which limits diving to 100' on tanks of 100cf or less. Paragraph 6-6.1 explicitly states "All open-circuit SCUBA dives deeper than 100 fsw shall employ cylinders having a capacity of at least 100 cubic feet."

I hate rules of thumb - especially about gas management. PADI only says OW divers are limited to 60 feet, 100 feet for advanced and 130 if you bought the deep card.

FYI - I can suck through a 120 CF HP steel in about 15 minutes in shallow water if I'm totally freaked out about losing contact with my novice-diver daughter, kicking my ass off swimming against a Gulf current and hoping I can finger walk to the mooring line....

I can also stay with secretsea on a relaxing lay there and take 8000 pics of nudibranchs - so go figure...

So, all I'm saying is know what you're doing, plan the dive, dive the plan - and don't run out of gas and ideas at the same time.

Glad to know where it might have come from, at least! Thanks for that info! I'm also glad to know that others haven't heard of it / don't give it much credence. I really wish I had bookmarked the thread where I found the discussion, as there were multiple people agreeing with the sentiment and I was just extremely surprised bc I had never heard of it before.

[secretsea18]

PADI only says OW divers are limited to 60 feet, 100 feet for advanced and 130 if you bought the deep card.

Good thing that I am OW1 with NAUI, and they told me when I got it that the depth limit was ~ 140 feet. And that my AOW did not say any thing about depths.

I can also stay with secretsea on a relaxing lay there and take 8000 pics of nudibranchs - so go figure...

And it's a good thing you had so much air left in your tank. But you do win the long distance swimming award.