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How much gas should I come up with


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#1 Capn Jack

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Posted 27 March 2009 - 12:24 AM

So the way to help yourself is to keep diving, but more importantly, to work hard to improve your skills, and your knowledge about diving. Coming up with 500-700 pounds will put you in good stead with people who don't plan their dives. But it will get your excluded from dives with the "dive Gods" who know that doing that would likely not leave enough air to breathe to get you and your buddy to the surface on many dives. So learn WHY that's not the best idea, and begin to plan your dives a bit better.



I'm still new myself. Everything that I have been taught says that 500-700 pounds is a good dive. What am I missing here??
If 700 lbs is not enough then what is considered enough.

Maurice

not to scare anyone - but I've gone through 700 pounds in 3-4 minutes... so, the question is - how much do you need to make sure you AND your buddy get to the surface safely (i.e. controlled ascent).

Most dive boats throw a general number to hit the ladder with - like 500# - I guess the first question I'd ask is how big is your tank? AL80 @ 500# is a lot different than a steel 104 @ 500#.

Who's diving? My SAC is about 3-4x Wreck Wenches (she actually generates air sometimes) - so you should learn your consumption and adjust accordingly - If WW and I both hit 15' with 500#, she could hang there 4 times longer than I could - 500# is not equal to 500#.

How deep are you going? If you're coming up from 100' you need to start sooner than you would from 50' since you're fighting physics that's geometric with depth. It'll only take you about 1 minute longer from 100, than 50, but you'll be pulling much more gas out with every breath at 100 vs 50

Picture this - you've had a great dive - and along comes your buddy - totally OOA, and their OOA signal is to grab your reg out of your mouth from behind, taking your mask off with it (ask me how I know about this scenario) - your buddy is going to have lots of issues, and really poor SAC at this point - and now you're maskless, and wondering WTF, and finally find your Octo - and assuming you don't put it in upside down and get a mouthful of water to boot - you're going to be kinda excited about now. You had 700# just before your mask came off - now you have two in extremis divers sucking off the same tank.... need I go further?

Like folks have been saying - enjoy, but learn, practice, dive more. If you're on an SD trip, you're likely to have a few fairly experienced divers along, most likely an instructor or two, and always at least one DM (WW). Use the surface intervals to pick their brains - ask - no such thing as a stupid question.
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#2 PerroneFord

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Posted 27 March 2009 - 12:47 AM

We need to have a sticky here that discusses the concept of "Rock Bottom" or minimum gas, or something.


The fundamental principle of gas management goes like this:

1. Determine your SAC rate (Surface Air Consumption Rate). How many cubic feet you breathe at the surface. Generally, for emergency planning we assume this to be 1 Cuft per minute.
2. Determine the depth of your dive. For illustrative purposes let's say 100ft (or approximately 4 atmospheres)
3. The goal is to complete the bottom portion of your dive with sufficient gas (air) to get both you AND your buddy to the surface with all safety stops should one of you suffer a catastrophic failure of breathing gas.

So let's put that through an example.

Let's assume that I plan to do a dive to 100ft, and my SAC rate is 1cuft per minute. We'll do some rounding in the example, but bear with me.

Let's say my buddy has exactly the same breathing rate I do.

At 100ft after 15 minutes or so, we decide to end the dive and begin our ascent. And his LP hose ruptures and empties his tank. We now have to complete the dive using just my tank.

Ok, we are at 4 atmospheres. So my gas is going to go 4 times faster than it would at the surface.

We will assume that we spend 1 minute at depth trying to get the problem sorted out, and get everything squared away. We will assume that we ascend at a safe 30ft per minute. And we assume that we will do the common 3 minute safety stop at 15ft.

So we spend 1 minute at 100ft sorting the problem. That's a 1cuft/minute SAC rate * 4 atmospheres = 4 cuft.
We ascend from 100ft to 70ft in the next minute. Thats 1cuft/min * 3 Atmostpheres = 3 cuft
We ascend from 70ft to 40ft in the next minute. Thats 1cuft/min * 2 Atmospheres = 2 cuft
We ascend to from 40-15 in the next minute. (rounding happens here) That's 1 cuft/min * 2 Atmospheres = 2 cuft (really this would be a bit less, but the others would have truly been a bit more so it works out)
We stop at 15ft for 3 minutes so that's about 3.5 cuft
We ascend slowly to the surface, so we'll add 1 more cuft.

Grand total = 16.5 cuft.

So let's turn that into PSI. And we'll use an Al80.

At 3000psi the AL80 hold 77cuft. (3000psi * 16.5cuft) / 77cuft = 642psi

So it would take 642psi to execute this ascent, for ME alone. If we assume my buddy breathes the same as I do, we'd have to double that figure to get us both to the surface with this plan. Now I need 1285 psi in the Al80 to do this plan safely.

So, at 100ft, I would need to end the dive around 1300psi if I wanted to ensure I could get both myself AND my buddy safely to the surface completing the safety stop, and adhering to common ascent rates.

So let's look at some factors

1. We assumed the buddy breathed at the same rate as us. For you ladies, if you dive with men, or larger people in general, you may want to add a bit of a fudge factor
2. We assumed that coming up with NO gas left was acceptable. If you are in the ocean, you may want to have some reserve in case we surface in heavy seas or away from the boat
3. We assumed that the ascent was PERFECT and that there were no other problems. No one paniced, no one got off the line, no one lost a mask, etc. You might want to reserve 100 psi to accomodate for this
4. We assumed that we had a perfectly accurate guage. If you haven't checked yours, you may want to.

On the positive side, we also assumed a SAC rate of 1.0. This is pretty heavy. My emergency SAC rate is about .8. However, as a newer diver, like when I took Cave, it was 1.2 or so. Understanding how a buddy breathes can be important.

We also assumed that doing the safety stops was necessary. Safety stops are not required. It's good to do them though. And for planning purposes, we can make plans to do them. If you're running short on gas, you can bypass them, or do an abbreviated stop.


The primary lesson here is to actually sit down and do some REAL planning and think through these kinds of issues. Many people pride themselves on milking a tank and coming up with 500psi just as the captain or DM said to. However, they never realize that in a bad situation, they are not leaving enough reserve to rescue themselves or their buddy if things go sideways. Recreational diving operates under the assumption that everything will always go perfectly. Thus there is no need to plan. Technical diving operates under the assumption that something will ALWAYS fail, and perhaps more than one thing, so careful planning and adequate reserves are always used.

Now that we know that it would take about 1300psi in an AL80 to get the average diver and their buddy to the surface, from 100ft, you can simply remove 1300psi at the start of your dive from your plan. So your 3000psi is now 1700 psi. And you can do a plan that splits that in half. We can swim away for 850 psi, and then swim back with the remaining 850 psi, and then safely do an ascent. If nothing goes wrong, you'll end up with (3000-1700- 642) about 650 psi on the surface. Right in the middle of the usually acceptable range. But now you will now HOW you arrived there because it was planned, instead of guessed at.

#3 Diverbrian

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Posted 27 March 2009 - 09:33 AM

Actually, one also needs to take into account things like currents and that divers tend to breathe more when stressed (that darned adrenaline) . So, you may want to pad those numbers even more. This is one reason that beginning dive books will tell you to do your best to plan dives into the current if it exists first. That way, you lowest gas consumption will be on the way back to boat.

Planning can be fun and will help keep your dives less stressful while you are executing them.
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#4 PerroneFord

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Posted 27 March 2009 - 09:44 AM

Actually, one also needs to take into account things like currents and that divers tend to breathe more when stressed (that darned adrenaline) . So, you may want to pad those numbers even more.


This is why I calculated with a SAC of 1.0 instead of something more normal. A large man, who is also a new diver, may want to change that to 1.5 though. A smaller diver, or female, or experienced male, may be just fine with that 1.0 calculation.

I had a near-death experience on a dive in 2007, and calculated my SAC rate afterwards. It was less than 1.0 so I feel comfortable with using that rate for myself.

#5 Diverbrian

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Posted 27 March 2009 - 09:49 AM

Actually, one also needs to take into account things like currents and that divers tend to breathe more when stressed (that darned adrenaline) . So, you may want to pad those numbers even more.


This is why I calculated with a SAC of 1.0 instead of something more normal. A large man, who is also a new diver, may want to change that to 1.5 though. A smaller diver, or female, or experienced male, may be just fine with that 1.0 calculation.

I had a near-death experience on a dive in 2007, and calculated my SAC rate afterwards. It was less than 1.0 so I feel comfortable with using that rate for myself.


I do the same thing when planning my dives. I normally figure for a breathing rate a bit higher (for myself) than I normally breathe. I would rather come up with too much gas than not have any.
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#6 Capn Jack

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Posted 27 March 2009 - 10:41 AM

I am going to go out on a limb here - and expect some heat - but I have to say it.

For many years, 60 FPM ascent rate was routine. It has since been - and I agree with the wisdom - changed to 30 FPM.

HOWEVER, we can fix bent. We can't fix dead. I'm going to choose a more rapid than normal ascent rate if I think we're going to be OOA before we hit the surface.

I'm also going to skip the safety stop unless I have plenty of gas. Same with deco btw (but this is in the NEWER DIVERS area)

Let's not talk about how I got in this horrible situation and my lack of planning, not diving gauge, stop watch and tables.... I f****d up - and I'm here - trying to ascend without enough gas....

You may fire when ready, Gridley.
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#7 PerroneFord

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Posted 27 March 2009 - 11:03 AM

Nothing to fire at there! Of COURSE you jet to the surface if you are going to run out of gas.

However, in a discussion about gas PLANNING, it would be prudent to PLAN for having enough gas at the end of the dive so we don't get caught HAVING to jet to the surface because we are short on gas. Again, I apologize for my mindset as I cannot jet to the surface in my diving even if I was willing to take the hit and get bent. I just can't get there. So my gas plan has to be solid, and I have to adhere to it. So naturally, I take those same principles with me to open water. If the plan is solid, there just isn't any need to jet to the surface. It is also AMAZINGLY comforting knowing that your gas plan will allow you to escape from nearly any bad situation in a calm and controlled manner.

This: Oh dang, Timmy's tank just blew up, let's ascend. I wonder who'll win Tic-Tac-Toe at the safety stop!

Not This: Oh dang, Timmy's Tank just blew up, dear God where's the closest chamber! Someone call DAN!!

:teeth:

#8 secretsea18

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Posted 01 April 2009 - 07:06 PM

So the way to help yourself is to keep diving, but more importantly, to work hard to improve your skills, and your knowledge about diving. Coming up with 500-700 pounds will put you in good stead with people who don't plan their dives. But it will get your excluded from dives with the "dive Gods" who know that doing that would likely not leave enough air to breathe to get you and your buddy to the surface on many dives. So learn WHY that's not the best idea, and begin to plan your dives a bit better.



I'm still new myself. Everything that I have been taught says that 500-700 pounds is a good dive. What am I missing here??
If 700 lbs is not enough then what is considered enough.

Maurice



Most dive boats throw a general number to hit the ladder with - like 500# - I guess the first question I'd ask is how big is your tank? AL80 @ 500# is a lot different than a steel 104 @ 500#.

Who's diving?



Hey Mark, you know my answer to this question....

How much air should you come up with? None! At least unless the dive boat gives you a refund for returned unused air in your tank! :respect:

#9 Diverbrian

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Posted 01 April 2009 - 10:18 PM

So the way to help yourself is to keep diving, but more importantly, to work hard to improve your skills, and your knowledge about diving. Coming up with 500-700 pounds will put you in good stead with people who don't plan their dives. But it will get your excluded from dives with the "dive Gods" who know that doing that would likely not leave enough air to breathe to get you and your buddy to the surface on many dives. So learn WHY that's not the best idea, and begin to plan your dives a bit better.



I'm still new myself. Everything that I have been taught says that 500-700 pounds is a good dive. What am I missing here??
If 700 lbs is not enough then what is considered enough.

Maurice



Most dive boats throw a general number to hit the ladder with - like 500# - I guess the first question I'd ask is how big is your tank? AL80 @ 500# is a lot different than a steel 104 @ 500#.

Who's diving?



Hey Mark, you know my answer to this question....

How much air should you come up with? None! At least unless the dive boat gives you a refund for returned unused air in your tank! :birthday:


I do know of divers who attempt to burn that gas off at the safety stop looking at the fish. That is perfectly safe in my mind :birthday: .
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#10 JimG

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Posted 02 April 2009 - 04:17 AM

This is an issue that (in my opinion at least) is not that well covered in most OW courses, and it really needs to be. The general rule of "back on the boat with at least 500 PSI" is fine as far as it goes, but does nothing to help a new diver actually plan their gas usage. New divers have no idea how fast they will consume their gas on a dive, or how much to reserve for the ascent and safety stop.

For many years now, I have been teaching the Rule of Thirds to my OW students as a general guideline for gas planning on an anchor dive - one third of the starting gas supply to swim away from the boat, one third to swim back, and then the final third is held in reserve. Once safely back at the boat, the students have a couple hundred PSI available to burn off just puttering around near the anchor line, and then the final ascent should start at about 600-700 PSI. This generally provides enough gas for a nice slow ascent and a safety stop, and with adequate reserves to share air with a buddy if the need arises.

I start drilling down on "true" gas consumption calculations in the Advanced class. I have the students measure and calculate their SAC rate in a swimming pool, and then use that as a consideration for planning their deep dive. This helps them determine how much gas they will use, how much gas to reserve and (possibly) what size cylinder they will need.

I have found this to be a pretty good way of introducing the concept of gas planning, without bogging the students down with a lot of calculations (at least until they get some experience and a basic idea of how much gas they personally use). It's also not that difficult to teach, and works with almost any cylinder type or size. So far it seems to be working pretty well, but I'd certainly be interested to hear how other instructors approach this same topic.

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#11 peterbj7

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Posted 02 April 2009 - 06:36 AM

Brian referred to stress increasing gas consumption. Most people have no idea just how extreme this can be. It's common for gas consumption to go up by a factor of 7 or more when the diver is stressed and/or working hard. Hence so many deaths from divers who think PSD is just diving and treat it as recreational diving.

#12 billsoccer

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Posted 18 May 2009 - 06:29 PM

:birthday: BTW - This was an excellent example. Reminded me of things I forgot about as soon as my Open Water certification was over. Thanks. Will learn my SAC and do these calcs in the future!

We need to have a sticky here that discusses the concept of "Rock Bottom" or minimum gas, or something.


The fundamental principle of gas management goes like this:

1. Determine your SAC rate (Surface Air Consumption Rate). How many cubic feet you breathe at the surface. Generally, for emergency planning we assume this to be 1 Cuft per minute.
2. Determine the depth of your dive. For illustrative purposes let's say 100ft (or approximately 4 atmospheres)
3. The goal is to complete the bottom portion of your dive with sufficient gas (air) to get both you AND your buddy to the surface with all safety stops should one of you suffer a catastrophic failure of breathing gas.

So let's put that through an example.

Let's assume that I plan to do a dive to 100ft, and my SAC rate is 1cuft per minute. We'll do some rounding in the example, but bear with me.

Let's say my buddy has exactly the same breathing rate I do.

At 100ft after 15 minutes or so, we decide to end the dive and begin our ascent. And his LP hose ruptures and empties his tank. We now have to complete the dive using just my tank.

Ok, we are at 4 atmospheres. So my gas is going to go 4 times faster than it would at the surface.

We will assume that we spend 1 minute at depth trying to get the problem sorted out, and get everything squared away. We will assume that we ascend at a safe 30ft per minute. And we assume that we will do the common 3 minute safety stop at 15ft.

So we spend 1 minute at 100ft sorting the problem. That's a 1cuft/minute SAC rate * 4 atmospheres = 4 cuft.
We ascend from 100ft to 70ft in the next minute. Thats 1cuft/min * 3 Atmostpheres = 3 cuft
We ascend from 70ft to 40ft in the next minute. Thats 1cuft/min * 2 Atmospheres = 2 cuft
We ascend to from 40-15 in the next minute. (rounding happens here) That's 1 cuft/min * 2 Atmospheres = 2 cuft (really this would be a bit less, but the others would have truly been a bit more so it works out)
We stop at 15ft for 3 minutes so that's about 3.5 cuft
We ascend slowly to the surface, so we'll add 1 more cuft.

Grand total = 16.5 cuft.

So let's turn that into PSI. And we'll use an Al80.

At 3000psi the AL80 hold 77cuft. (3000psi * 16.5cuft) / 77cuft = 642psi

So it would take 642psi to execute this ascent, for ME alone. If we assume my buddy breathes the same as I do, we'd have to double that figure to get us both to the surface with this plan. Now I need 1285 psi in the Al80 to do this plan safely.

So, at 100ft, I would need to end the dive around 1300psi if I wanted to ensure I could get both myself AND my buddy safely to the surface completing the safety stop, and adhering to common ascent rates.

So let's look at some factors

1. We assumed the buddy breathed at the same rate as us. For you ladies, if you dive with men, or larger people in general, you may want to add a bit of a fudge factor
2. We assumed that coming up with NO gas left was acceptable. If you are in the ocean, you may want to have some reserve in case we surface in heavy seas or away from the boat
3. We assumed that the ascent was PERFECT and that there were no other problems. No one paniced, no one got off the line, no one lost a mask, etc. You might want to reserve 100 psi to accomodate for this
4. We assumed that we had a perfectly accurate guage. If you haven't checked yours, you may want to.

On the positive side, we also assumed a SAC rate of 1.0. This is pretty heavy. My emergency SAC rate is about .8. However, as a newer diver, like when I took Cave, it was 1.2 or so. Understanding how a buddy breathes can be important.

We also assumed that doing the safety stops was necessary. Safety stops are not required. It's good to do them though. And for planning purposes, we can make plans to do them. If you're running short on gas, you can bypass them, or do an abbreviated stop.


The primary lesson here is to actually sit down and do some REAL planning and think through these kinds of issues. Many people pride themselves on milking a tank and coming up with 500psi just as the captain or DM said to. However, they never realize that in a bad situation, they are not leaving enough reserve to rescue themselves or their buddy if things go sideways. Recreational diving operates under the assumption that everything will always go perfectly. Thus there is no need to plan. Technical diving operates under the assumption that something will ALWAYS fail, and perhaps more than one thing, so careful planning and adequate reserves are always used.

Now that we know that it would take about 1300psi in an AL80 to get the average diver and their buddy to the surface, from 100ft, you can simply remove 1300psi at the start of your dive from your plan. So your 3000psi is now 1700 psi. And you can do a plan that splits that in half. We can swim away for 850 psi, and then swim back with the remaining 850 psi, and then safely do an ascent. If nothing goes wrong, you'll end up with (3000-1700- 642) about 650 psi on the surface. Right in the middle of the usually acceptable range. But now you will now HOW you arrived there because it was planned, instead of guessed at.



#13 Racer184

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Posted 18 May 2009 - 07:03 PM

what is PSD ?

#14 JimG

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Posted 18 May 2009 - 08:11 PM

what is PSD ?

PSD == Public Safety Diver (usually)

-JimG
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#15 shadragon

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Posted 19 May 2009 - 05:29 AM

I found this article many moons ago when I was looking to construct a spreadsheet that calculated turn around pressures at the various depths for various size air tanks and ponies for personal reference. Not only did this Gent beat me to it, he also included the under-laying theory and calculations as background. His observation on the difference between 'regular' SAC rate and 'Emergency' SAC rate is interesting. I present the article "as is". I thought it was a good read, but I leave it to the reader to decide for themselves. I do not like his comment when he said, "You now don't need to be checking your SPG, but only need to check your BT/computer for your dive time." That seems silly to me. You should always check your SPG. Stuff happens. I do like his disclaimer though.
Remember, email is an inefficient communications forum. You may not read things the way it was intended. Give people the benefit of the doubt before firing back... Especially if it is ME...! ;)

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