Discussion on gas management

[Mad Scientist]

Two common problems I see with rec divers are

1.) Overweighted - espically true here the NE

2.) They have never been how to manage their gas. One of the reason I can't resist bashing PADI and I feel like any PADI card isn't worth the plastic it's made from is that they never teach you how to really plan/manage your gas. I have a PADI OW, AOW, and deep card from them and we were never taught anything beyhond "come back with 500psi or come back with 100psi" they never taught me a simple way to figure out when I needed to turn around (psi wise) realted to my depth and tank size.

Has anyone (beside Techdiver) been taught about Rock Bottom? It's a simple way to figure out how much gas you and your buddy will need to make it back to the surface while sharing air related to your SACs, the depth, and the tanks you are wearing. Once you know what your Rock Bottom psi is - you never go below that. You then can substract your RB from your total psi and figure how much gas you can use over the course of the dive. There is really no other safe way to plan a dive.

It's a huge failing of the industry that every certified diver doesn't know about Rock Bottom.

[Tech Diver]

I agree with Mad Scientist about the importance of gas management. A key aspect of this is knowing your SAC rate. For those who are unfamiliar with this term, SAC stands for Surface Air Consumption and is a value that indicates the volume of gas that a diver breathes over the period of one minute. Knowing this allows you to easily predict how much gas you require for a given depth and time.

Using some convenient numbers for the following example, let's assume a diver has a SAC rate of 1 cu-ft per minute. The diver plans to spend 20 minutes bottom time at 100 ft and is using an 80 cu-ft tank. Remember that at 100 ft the pressure is 4 atmospheres, so the diver's gas consumption rate is 4 times the SAC rate. Namely, 4 cu-ft per minute. So 20 minutes of bottom would require 80 cu-ft of gas. Since the diver only has an 80 cu-ft tank, he will use up his entire gas supply while still underwater before starting even starting his ascent!

Real SAC rates are usually not as high as in the example. If you are curious, my rates are 0.38 cu-ft/min while resting (used for deco calculations), 0.53 cu-ft/min while swimming normally (used for main dive calculations), and something like 2 cu-ft/min for extreme exertion. By the way, notice how much more gas you consume when you have to exert yourself such as fighting a current or struggling with a piece of equipment.

I apologize if I have bored everyone to death with this topic. For those who are interested in more information on how to determine your SAC rate I can post the equations. The follow-on to this simple example is as Mad Scientist stated: knowing your Rock Bottom PSI.

[luminary]

I have never calculated Rock Bottom formally. I do have a pretty good feel for what my typical consumption is at certain depths but would be very interested in learning how to "quantify" that.

[Mad Scientist]

Originally posted by luminary
I have never calculated Rock Bottom formally. I do have a pretty good feel for what my typical consumption is at certain depths but would be very interested in learning how to "quantify" that.

The whole point of RB is that you figure how much air you need to ascend (and do deco if there is any) for both divers. You then use a pretty simple formula to turn this figure (in cu) into psi - that's your rock bottom that you can't touch, as soon as you hit it, you must go up.

It's a bit much to teach on the internet - but very easy to do onc eyou learn. Plus, I have a list of Rock Bottoms for different tanks at 0-60, and 60-100' - I can just use those numbers and I know my Rock Bottom on any dive.

Both Techdiver and I have had some training with GUE (www.gue.com), although he does alot more advanced stuff than me. In their rec course they teach Rock Bottom gas planning. I now feel like allot of the diving I had been doing before the class was unsafe. I had a little more than 200 dives all here in the chilly Northeast with about 50dives with steel doubles - I had breezed through all my PADI classes. The GUE class (100% recreationa diving) really was a wake up call.


You want to rely on skill and training, not play the odds game as to whether you make it back alive.

[nitroxdiver009]

my padi instructor went over the rule of thirds and stuff

[billsreef]

Originally posted by Tech Diver
For those who are interested in more information on how to determine your SAC rate I can post the equations.

Please do post the equations. I think we all could benifit from it

This has been excellent thread with many good points and personal stories. Worthy of being the first sticky thread in this forum. Diving safety is something we should all be thinking about, especially emergency planning.

[Mad Scientist]

Originally posted by nitroxdiver009
my padi instructor went over the rule of thirds and stuff

Rule fo thirds is different than Rock bottom - sometimes using a 1/3 of you air supply is way to conservative, other times it may not be enough. "Real" rule of 1/3s is 1/3 of you air supply not counting Rock bottom.

Diving overweighted is very, very dangerous - if you have a problem with your buddy bolting to the surface stop diving with him or her untill they get better training and/or a safer attitude.

[gregt]

IMO if you need to spend time calculating rock bottom you aren't recreational diving. Recreational diving should be using methods that are much more conservative than rock bottom. You should never be in a position for rock bottom to matter. JMO, albeit an extremely conservative one.

[Tech Diver]

HOW TO CALCULATE YOUR SAC RATE:
The premise behind the equation is that if you measure the change in your tank pressure over a known period of time at a known depth, you can extrapolate a value indicating the rate at which you consume your gas at the surface. Then you can use this value to predict how much gas you will use at any depth.

EQUATION:
(SAC rate) =
(pressure_change) / (rated_tank_pressure) x (rated_tank_volume) / (ATA_at_depth) / (minutes)

EXAMPLE:
A diver descends to a depth of 20 feet
The pressure gauge indicates 2300 psi
The diver begins to swim normally - neither fast nor slow
After 17 minutes the pressure is 1750 psi
Tank used is rated for 3000 psi and 80 cu-ft

pressure_change = 2300 - 1750 = 550 psi
ATA_at_depth = (20 + 33) / 33 = 1.6 ATA

Let's plug in the numbers:
(550 psi) / (3000 psi) x (80 cu-ft) / (1.6 ATA) / (17 minutes) = 0.54 cu-ft/min = SAC rate
A SAC rate of 0.54 cu-ft/min is a very realistic value

APPLICATION:
Now that we know that the diver's SAC rate is 0.54 cu-ft/min, we can use this to determine how much gas will be used for a dive to 90 feet for 25 minutes bottom time (this is the NDL from the PADI tables), plus 3 minutes ascent time (assuming an ascent rate of 30 ft/min), plus 5 minutes safety stop at 15 feet.

Bottom phase
pressure at 90 feet = (90 + 33) / 33 = 3.7 ATA
consumption rate = (3.7 ATA) x (0.54 SAC rate) = 2 cu-ft/min
gas used at 90 feet = (2 cu-ft/min) x (25 minutes) = 50 cu-ft

Ascent phase
Use an average depth. Namely, 45 feet
pressure at 45 feet = (45 + 33) / 33 = 2.4 ATA
consumption rate = (2.4 ATA) x (0.54 SAC rate) = 1.3 cu-ft/min
gas used for ascent = (1.3 cu-ft/min) x (3 minutes) = 4 cu-ft

Safety stop phase
pressure at 15 feet = (15 + 33) / 33 = 1.5 ATA
consumption rate = (3.7 ATA) x (0.54 SAC rate) = 0.8 cu-ft/min
gas used at safety stop = (0.8 cu-ft/min) x (5 minutes) = 4 cu-ft
Total amount of gas used
(bottom phase) + (ascent phase) + (safety stop phase) =
(50 cu-ft) + (4 cu-ft) + (4 cu-ft) = 58 cu-ft

CONCLUSION
If you are on vacation and using an 80 cu-ft cylinder, chances are the tank is not completely filled because it was filled fast (making it hot) and it since cooled down. Using the above example, if you are thinking spending every last minute of the PADI NDL of 25 minutes doing a wall dive, you better think twice. This dive requires at least 58 cu-ft of gas, and a partially-filled 80 cu-ft cylinder is not enough to provide an acceptable margin of safety. If you assume that your tank is only 90% full (72 cu-ft), and play with the number of minutes in bottom phase calculations so you use no more than 2/3 of your tank (the ascent and safety stop remain unchanged), you will find that the rule-of-thirds limits this dive to only 20 minutes. Here are the calculations:
Let's limit our gas consumption to
(72 cu-ft) x (2/3) = 48 cu-ft
Subtract the non-changing ascent and safety stop volumes to get the limit on the bottom phase volume
(48 cu-ft) - (4 cu-ft) - (4 cu-ft) = 40 cu-ft
Divide the bottom phase volume by the consumption rate at 90 ft
(40 cu-ft) / (2 cu-ft/min) = 20 minute limit for bottom time

TIPS
1) When swimming around to calculate your SAC rate I find it easy to just push my hands against a large rock/boulder and swim in place. That way your depth does not change.

2) Make yourself a table so you don't have to calculate any of these values while on the boat, etc. Or if you have a handheld computer you can download some planning software for free.

3) If you are overwhelmed by these equations try a conservative estimate (e.g. always assume it takes 8 or 10 cu-ft for a combined ascent and safety stop) and only calculate the bottom phase. I hate to say it because I have a "precise" personality, but some margin of safety is better than none at all.

[Tech Diver]

Errata:

Please note that in a previous posting where I describe the calculations of the SAC rate, I wrote:

Safety stop phase
pressure at 15 feet = (15 + 33) / 33 = 1.5 ATA
consumption rate = (3.7 ATA) x (0.54 SAC rate) = 0.8 cu-ft/min

The last line should say:

consumption rate = (1.5 ATA) x (0.54 SAC rate) = 0.8 cu-ft/min

The calculations should all still be correct. However, please let me know if I made any arithmetic errors.

Peter

[Mad Scientist]

Originally posted by gregt
IMO if you need to spend time calculating rock bottom you aren't recreational diving. Recreational diving should be using methods that are much more conservative than rock bottom. You should never be in a position for rock bottom to matter. JMO, albeit an extremely conservative one.

Greg,

Nothing could be farther than the truth. RB is ideal for rec diving. There is no reason to be more conservative. It's made for rec diving. Plus, although it's good to know how RBs are calculated, you don't need to figure them out for every dive. I have list of RBs, I just use them .

In terms of your average rec diver not needing them - how about this fact: the Rock Bottom for an al80 at 0-60' is 1,000psi. Think about that. Are you telling me that you are using a more conservative method? If so, how much air would you come up with on a 60-100' dive with an al80?

Or how about this example: you are diving an al80 is the Caymans, there is current, you need to get back to the anchor at the end of the dive. Your RB is 1,000pi. 3000psi (full al80)-1000psi (RB) = 2,000psi usable psi. Since you need enough air to swim out, get back, and have air for your buddy to get back, you diving this by 3 (rule of 1/3s) - now you have 600psi. 3000psi (starting pressure)- 600 = 2400 is your turn pressure. This isn't conservative enough????

[gregt]

Ok, so conservative isn't exactly the word that works best. Perhaps foolproof is more appropriate.

[Mad Scientist]

Originally posted by gregt
Ok, so conservative isn't exactly the word that works best. Perhaps foolproof is more appropriate.

Greg, again, I don't want to sound like a jerk or be argumentative, but Rock Bottom is really the only safe was to plan gas managment for recreational dives. Also, the Rock Bottoms that are listed are based on "normal" conditions - once you learn how to calculate Rock Bottom you can play around for them. For example, the "test book" Rock Bottoms gives you and your buddy 1 minute at depth to deal with the problem before you begin to ascend sharing air, some people might want to 2 minutes - this would increase you end Rock Bottom psi.

[gregt]

We will need to just agree to disagree on this. As I said, I am a conservative diver, and I believe that if you are in a situation where you can't tell if you have enough air just by looking at your gauge and using common sense then you aren't recreational diving, IMO.

[Mad Scientist]

Originally posted by gregt
We will need to just agree to disagree on this. As I said, I am a conservative diver, and I believe that if you are in a situation where you can't tell if you have enough air just by looking at your gauge and using common sense then you aren't recreational diving, IMO.

Hey Greg, I don't mean mean to press you too hard on the issue. But, what do you consider a rec dive, are we talking dives to only 20 or 30 feet?

Just to be clear (and again I'm not trying to be disrespectful) if you were doing a dive to 80' in the Key off a dive boat and you were diving a high pressure 100 and yoru buddy was diving a steel 95 how would you know at what point to turn back to the boat? Are you diving in a way that you would only get 5 or 10 minutes of bottom time?

[gregt]

Hey Greg, I don't mean mean to press you too hard on the issue. But, what do you consider a rec dive, are we talking dives to only 20 or 30 feet?

Just to be clear (and again I'm not trying to be disrespectful) if you were doing a dive to 80' in the Key off a dive boat and you were diving a high pressure 100 and yoru buddy was diving a steel 95 how would you know at what point to turn back to the boat?

We both agree that if you can't answer your last question you shouldn't be making the dive. I simply don't think it is realistic to think that a recreational diver is going take the proper time and attention to safely use the method you are describing. I'll leave it at that.

[Mad Scientist]

Originally posted by gregt
I simply don't think it is realistic to think that a recreational diver is going take the proper time and attention to safely use the method you are describing. I'll leave it at that.

Maybe that has something to do with the rash of recreational deaths in the last few years - I'll leave it at that.

[Bomber]

Originally posted by Mad Scientist
Maybe that has something to do with the rash of recreational deaths in the last few years - I'll leave it at that.

Keeping up with those numbers is one small part of our job.

When you factor in the correct number of actual rec divers, number of rec dives. Factor out things like the windjammer accident.

Deal with the real numbers, rec accidents and rec deaths have shown a steady drop in the past 11 years.

I'll leave it at that.

[Mad Scientist]

There's been a spike in the last few years, including this year were deaths are way out in front on the average - check out the newest DAN report.

While you're hear though, may I ask you a NOAA question? In another thread in this forum it was posted that NOAA divers used 1.6Po2 during the working portion of their dives, is this correct?

And again, to get back to the thread, I would invite anyone to post how they would decide how much air to use in the example I posted earlier (80' reef dive in the keys with a current off a charter boat - you are diving a hp100 tanks, your buddy is diving a steel 95 , at what psi do you turn back to the boat?). I'd like to compare other methods to Rock Bottom. Or is everyone just winging it and playing the odds.

[Bomber]

Originally posted by Mad Scientist
There's been a spike in the last few years, including this year were deaths are way out in front on the average - check out the newest DAN report.

My department is one of the sources that DAN uses for their numbers. How they want to twist the numbers to sell more insurance is up to them.
There has been no spike in real numbers thus the relaxing of requirements.

Originally posted by Mad Scientist
While you're hear though, may I ask you a NOAA question? In another thread in this forum it was posted that NOAA divers used 1.6Po2 during the working portion of their dives, is this correct?

That makes as much sense as saying "NOAA divers use air". Depending on what they are doing, how long and how often they are doing it, depth or lack of depth, etc etc. We are free to chose what fits the situation best.

[Mad Scientist]

Originally posted by Bomber
Depending on what they are doing, how long and how often they are doing it, depth or lack of depth, etc etc. We are free to chose what fits the situation best.

So, no standards huh??? Again, this stems from another thread where a RC member was justifying diving NITROX to depths were had a working PO2 of 1.6 - he said he went by NOAAs guidlines.

[Bomber]

How can you turn "what fits the situation best" into "no standards"?

[Mad Scientist]

Originally posted by Bomber
How can you turn "what fits the situation best" into "no standards"?

The fact that you did answer my question made me think that NOAA left gas choice issues up to the individual diver on a given dive. A standard would be (for example) - 1.4 max Po2 during the working portion of the dive, not "well, whatever seems best."


Again, I wanted to know what the standard max PO2 is for NOAA divers - you gave me a "what fits best" of course I'm going to think that means that there is not a standard??????

Since I asked an Rock Bottom question as well, can I also NOAA does not have standard method of gas managment? Furthermore, as I know several NOAA divers, I know you guys don't have standard diving rig (gear matching etc.).

[Bomber]

A standard would be (for example) - 1.4 max Po2 during the working portion of the dive, not "well, whatever seems best."

Oh OK, We're working at 90ft and of course we use "1.4 max Po2".

Does that fit your standard or whatever FITS THE SITUATION BEST. not seems best.

Obviously, different dives require different standards. You're trying to make one dive, one shoe fits all.

[Mad Scientist]

Originally posted by Bomber
Oh OK, We're working at 90ft and of course we use "1.4 max Po2".

Does that fit your standard or whatever FITS THE SITUATION BEST. not seems best.

Obviously, different dives require different standards. You're trying to make one dive, one shoe fits all.

But a different depth you would allow for a higher Po2? I'm still unclear if you guys have standard?