34 replies to this topic

[shadragon]

Regardless of the tank size on my back, the % mix in it, the depth I am at or the buddy I am with I monitor my depth and pressure gauge and amend my dive profile as needed so I can surface safely with my planned reserve intact.

- On one dive in very heavy current I burned through 2,000 psi in 3 minutes of heavy kicking at 14 feet.

- I have used an entire 3000 psi AL 80 in 11 minutes.

Don't let your "dive plan" override "dive reality". What is, is and you need to continually adapt your dive profile to the conditions you find yourself in.

[WreckWench]

... 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.

Kate you can find multiple people who will agree with just about anything but that don't make it so!

Your own common sense and diving experiences and knowledge have prevailed and you asked a great question and you have gotten good information which you can use to draw your own conclusions.

[drbill]

As mentioned, it all depends on your SAC rate. I've repeatedly dived to 160' on an 80 and 200' on a 120 with plenty of gas for the profiles I dived. I do carry a pony, but the only time I've used it on those dives was when I had regulator problems while doing my deco.

[peterbj7]

Very much my experiences too, Bill.

[JimG]

Kate, I too have heard "don't dive deeper than the cu ft of your tank" - basically a poor man's gas planning "rule of thumb". It's an artifact of what I call the "sound byte" philosophy of training, wherein people are taught some catchy sayings that are easy to remember, but may or may not have any bearing on the reality of their diving. Other examples of this include:

• Never ascend faster than your smallest bubbles
• Be back on the boat with 500 PSI
• You can't get bent diving with a steel 72 (yes I have actually heard this one! )

As several people have mentioned, these are no real substitute for proper dive and gas planning based on depth, time, and knowing your true air consumption rate (which can easily be measured and/or calculated).

One thing I have started doing to help my OW students with this during confined water training is to periodically ask them to tell me their remaining air pressure without checking their SPGs. This gets them in the habit of mentally estimating remaining pressure based on depth and time. In most cases the students are able to consistently get within 100-200 PSI of their actual pressure by the end of the last pool session, which helps them a lot when we go to open water (FYI, I did not come up with this idea - stole it from DiveGeek aka Mark ). In the AOW class, I have the students actually measure their air consumption in the pool, and then they can use that for gas planning on their deep dive, in addition to depth, time and Nitrogen loading.

Having a good handle on your actual air consumption rate is one of the keys to worry-free diving, IMO. It's something to really start paying attention to, particularly as you get into more advanced forms of diving.

-JimG

[JimG]

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

I use the 1.6 setting.

Except that those are the MOD limits for 32% @ 1.55 ata, not 1.6. Looks like maybe your computer is forcing you to be a little more conservative than you thought you were being.

-JimG

[Landlocked Dive Nut]

There are 3 separate personal decompression model settings on my Suunto, each more conservative than the last. To select the best model for you personally, you're supposed to take into consideration the following:

cold exposure
diver below average physical fitness level
diver fatigue
diver age
diver dehydration
stress
obesity
and previous history of DCI

You can select the P0, P1 or P2 model depending upon how many (if any) of the above you want to factor into your personal conservative comfort level. I happen to use the most conservative setting because of my age, weight & stress levels. Nothing wrong with diving conservatively, especially as we get older! Well, it has worked well for me -- so far, anyway.

IMPORTANT NOTE: If you dive with two computers, find out if they both have this type of setting and make sure you set them to the same model setting.

[secretsea18]

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

I use the 1.6 setting.

Except that those are the MOD limits for 32% @ 1.55 ata, not 1.6. Looks like maybe your computer is forcing you to be a little more conservative than you thought you were being.

-JimG

Close enough for me! Especially since I believe that ALL the air in my tank is there to be used! \


Like I said, the Suunto Vytec is the "never get bent, wet or deep" computer.

[Hipshot]

• Be back on the boat with 500 PSI
• You can't get bent diving with a steel 72 (yes I have actually heard this one! )

As several people have mentioned, these are no real substitute for proper dive and gas planning based on depth, time, and knowing your true air consumption rate (which can easily be measured and/or calculated).

The rationale for returning to the boat with 500 (or 400, as I've sometimes heard) psi is that if a diver surfaces a distance away from the boat, they'll have enough air to avoid a long surface swim. It's probably not a bad idea.

When I was first learning how to dive (back in 1969) we were taught that you normally couldn't get bent on a 72. That was assuming that you didn't have any residual nitrogen from a previous dive and went to a depth where there'd be a realistic bottom time for a 72; you'd almost have to try to get bent using a 72.

Rick



The world is a stage, but the play is badly cast - Oscar Wilde

[georoc01]

• Be back on the boat with 500 PSI
• You can't get bent diving with a steel 72 (yes I have actually heard this one! )

As several people have mentioned, these are no real substitute for proper dive and gas planning based on depth, time, and knowing your true air consumption rate (which can easily be measured and/or calculated).

The rationale for returning to the boat with 500 (or 400, as I've sometimes heard) psi is that if a diver surfaces a distance away from the boat, they'll have enough air to avoid a long surface swim. It's probably not a bad idea.

When I was first learning how to dive (back in 1969) we were taught that you normally couldn't get bent on a 72. That was assuming that you didn't have any residual nitrogen from a previous dive and went to a depth where there'd be a realistic bottom time for a 72; you'd almost have to try to get bent using a 72.

Rick

I also heard that the 500 reserve is not yours, its for your buddy in case of a failure.

The world is a stage, but the play is badly cast - Oscar Wilde

[secretsea18]

• Be back on the boat with 500 PSI
• You can't get bent diving with a steel 72 (yes I have actually heard this one! )

As several people have mentioned, these are no real substitute for proper dive and gas planning based on depth, time, and knowing your true air consumption rate (which can easily be measured and/or calculated).

The rationale for returning to the boat with 500 (or 400, as I've sometimes heard) psi is that if a diver surfaces a distance away from the boat, they'll have enough air to avoid a long surface swim. It's probably not a bad idea.

When I was first learning how to dive (back in 1969) we were taught that you normally couldn't get bent on a 72. That was assuming that you didn't have any residual nitrogen from a previous dive and went to a depth where there'd be a realistic bottom time for a 72; you'd almost have to try to get bent using a 72.

Rick

I also heard that the 500 reserve is not yours, its for your buddy in case of a failure.

While I had been taught that it was to ensure that there was air pressure still in the tank upon surfacing, thus preventing water from getting into the tank.

[peterbj7]

You only need around 20 bar/300psi to ensure that.

[WreckWench]

Many schools of *reserve air* thought advise the "rule of thirds"...

So if your tank has 3000 psi (or roughly that) then you use 1000 psi out and 1000 psi back and 1000 psi back on the boat aka reserve for either:

a. long surface swims
b. buddy needing air issues
c. other problems

If you are diving a low pressure tank then a full fill is 2400psi and it would be 800 out, 800 back and 800 reserve.

VERY SERIOUS DIVERS such as cave divers, cavern dives, many wreck divers, many tech divers, etc. use this rule and in more serious diving conditions I strictly follow it too. Although I rarely get back on any boat with less than 1000 psi after any dive UNLESS I've done two dives on one tank! LOL!


I guess the real message is that you should not habitually be draining your tank low...you need to leave some contingency air in it RELIGIOUSLY so that you'll have it when you need it!!!

[shadragon]

Couple of personal observations:

- The 500 psi is supposed to be there when you get OUT of the water. It is not designed for a surface swim or keeping water out of your tank. Seals keep water out of your tank. Nor is it designed for your buddy. Your buddy is supposed to come out of the water with 500 psi as well!!! The 500 psi is a contingency reserve (a very small reserve) that will hopefully never be needed, but is there if it is. As an aside, if you ever use that reserve then something went wrong during that dive and you need to examine what happened and take corrective action in the future.

- There is a reason why a 4' 2" diver diving on an AL63 signs the same liability waivers at the dive shop as everyone else. Tank size has nothing to do with whether you get bent or not. You can get bent on a 3 Cu. Ft Spare Air if the conditions are against you. Dehydrated, tired, post surgery, age, PVO or whatever. All computer algorithms and all dive tables are based on theoretical models. There are no cut and dried rules for DCS. Anyone diving to any depth on compressed air is always at risk of getting bent.

- Students should be taught to constantly check their gauges and / or computers. They are there to give you accurate information and remove subjectivity. Narcosis can affect judgement or reason and "feeling fine" is illusionary and a false sense of security. There is a reason cave divers do not go on gut instinct.

- Every dive is different. Even doing the same dive site 2 hours apart. Conditions, temperature, current, visibility, obstacles, surge and you are never exactly the same twice. SAC rate is theoretical and subject to change on a moments notice. Amend your dive plan as required as often as needed so that you emerge from the water safe and sound.

Talk to everyone, listen to everyone then make up your own mind as to what works best for you.

Dive safe.

[secretsea18]

Couple of personal observations:

- The 500 psi is supposed to be there when you get OUT of the water. It is not designed for a surface swim or keeping water out of your tank. Seals keep water out of your tank. Nor is it designed for your buddy. Your buddy is supposed to come out of the water with 500 psi as well!!! The 500 psi is a contingency reserve (a very small reserve) that will hopefully never be needed, but is there if it is. As an aside, if you ever use that reserve then something went wrong during that dive and you need to examine what happened and take corrective action in the future.


Dive safe.

Heck and I thought that you are supposed to start going to the safety stop at @ 500 psi.