Gas Management Questions?

[guyharrisonphoto]

I have been reading up on gas management and calculation procedures and have learned a lot from various websites (thanks, DevonDiver, for all of your resources)!

I have a question regarding "rock bottom" gas management. I understand the concept, and have even done a few calculations for some different dive scenarios. The amount of gas required is surprisingly more than I expected, and I am glad I have taken the time to begin learning this concept.

The "rock bottom" concept is based on the worst case scenario of having to breathe two divers all the way up from depth if there is an OOG situation, through all necessary stops and still having 300 psi at the surface. We dive single tanks (HP 130s), and some of my scenarios show that this would cut the dive short of the planned dive time (including ascent and deco stops).

I see, however, that you can also calculate what I will call your "individual rock bottom" based on what it would take to get yourself to the surface (and still have 500 psi reserve). Same for your buddy.

I think that the rock bottom idea is great for dive planning in anticipation of an emergency. However, what if you and your buddy are a close team, and routinely check your own and each other's air supply? If all is progressing normally and both of us have sufficient air to complete our dive plan as we hit the "rock bottom" pressure, should we still thumb the dive , or can we continue our dive until we hit our deco limits (or until one of us hits their "personal" rock bottom pressure)?

Thanks,

Guy

 

[Reef_Haven]

You thinking is clear to me. "I understand it" and "I'm willing to disregard it".
You'll both be fine as long as everything goes as planned.

 

[boulderjohn]

The complete Rock Bottom concept is indeed very conservative, but if you think it through, it is not as conservative as it appears. This is especially true if you are doing a normal recreational multi-level dive.

First of all, it on a normal multi-level dive, your rock bottom calculations for your deepest depth change when you get to a shallower depth. You need to have a certain amount of gas in reserve at 100 feet for sure. Ascend to 60 feet and things change dramatically. If you work your way slowly to the surface on a multi-level dive, you may find that they don't impact your dive planning nearly as much as you think.

Next, it assumes you will want to do all stops (etc) on the way to the surface. For technical diving, that is a must, because those stops are absolutely necessary. For NDL diving, you really don't have to make stops on the way to the surface. You may decide that the stops are not critical and not to include them in you calculations. That would be your choice.

Finally, the "leave 500 PSI in your tank" rule is not part of the rock bottom concept. That is a different animal altogether. If you are trying to do rock bottom AND leave 500 PSI in your tank, you are mixing two different systems. One should be used instead of another. They should not be added together.

 

[ae3753]

Great to hear that you're researching gas management and rock bottom. Many good resources on the Internet and ScubaBoard.

To echo boulderjohn, rock bottom is for a specific depth, not for the dive. Rock bottom at 100' is different than rock bottom at 60'. If you haven't had a chance yet, work out the rock bottoms for 100', 60', and 30'.

 

[DevonDiver]

...you can also calculate what I will call your "individual rock bottom" based on what it would take to get yourself to the surface.... Same for your buddy....I think that the rock bottom idea is great for dive planning in anticipation of an emergency.....should we still thumb the dive , or can we continue our dive until we hit our deco limits (or until one of us hits their "personal" rock bottom pressure)?

In addition to John's excellent advice, I would add:

1) Calculating 'individual' rock bottom is just a means to the same end. That end being a minimum gas reserve to bring both divers to the surface. It's useful if two divers have very disparate air consumption - which would mean that calculating the rock bottom based on one diver's SAC and doubling it would be inaccurate. In this instance, both divers calculate an 'individual' rock bottom and add them together.

2) Rock Bottom is meant to cater for all OOG contingencies. Running out of air through neglect/inattention is just one contingency. Other contingencies might include: equipment failure or stressful event leading to elevated SAC/accelerated gas consumption. I once had a cold-water freeflow (at 125ft) that drained my tank in under 2 minutes - no warning and not sufficient to reach the surface. I've also seen a (very large) German trainee-divemaster empty an AL80 in under 3 minutes when he was chased by a tenacious Titan Triggerfish. On both occasions the divers were vigilant of their gas and otherwise diving responsibly. OOG emergencies can happen, which is why prudent divers ensure they maintain enough gas to share for a safe ascent from whatever depth they are at.

 

[Mike Boswell]

I have been reading up on gas management and calculation procedures and have learned a lot from various websites (thanks, DevonDiver, for all of your resources)!

I have a question regarding "rock bottom" gas management. I understand the concept, and have even done a few calculations for some different dive scenarios. The amount of gas required is surprisingly more than I expected, and I am glad I have taken the time to begin learning this concept.

The "rock bottom" concept is based on the worst case scenario of having to breathe two divers all the way up from depth if there is an OOG situation, through all necessary stops and still having 300 psi at the surface. We dive single tanks (HP 130s), and some of my scenarios show that this would cut the dive short of the planned dive time (including ascent and deco stops).

I see, however, that you can also calculate what I will call your "individual rock bottom" based on what it would take to get yourself to the surface (and still have 500 psi reserve). Same for your buddy.

I think that the rock bottom idea is great for dive planning in anticipation of an emergency. However, what if you and your buddy are a close team, and routinely check your own and each other's air supply? If all is progressing normally and both of us have sufficient air to complete our dive plan as we hit the "rock bottom" pressure, should we still thumb the dive , or can we continue our dive until we hit our deco limits (or until one of us hits their "personal" rock bottom pressure)?

Thanks,

Guy

Bob (NWgratefuldiver) has a terrific writeup on this and its helped me a lot. I would definitely print out his stuff and read it and tuck it into your log book.

But as to your question, I think the point of rock bottom is to get BOTH divers to the surface in case one of them goes OOG.

 

[ScubaSam]

Bob (NWgratefuldiver) has a terrific writeup on this and its helped me a lot. I would definitely print out his stuff and read it and tuck it into your log book.

But as to your question, I think the point of rock bottom is to get BOTH divers to the surface in case one of them goes OOG.

Echo Mike's recommendation. Here is the link to Bob's Understanding Gas Management article.

NWGratefulDiver.com

 

[Hatul]

For those who dive with an AI computer I'll throw out this quick and dirty way of gas management for recreational single tank diving.

Set the tank reserve at 650 psi. That allows 150 psi for 3 min safety stop with 500 psi at surface. Now, at any depth divide the depth in feet by 30 to get the number of minutes to surface at recommended rate of 30 ft/min. Make sure the Remaining Air Time is greater than the time to surface. If you follow this you'll always have some reserve because the Air Time assumes you stay at depth.

The equivalent for Rock Bottom Time to save your buddy: you simply require the Air Time to exceed twice the time to surface. In this case the 3 minute safety stop will take both of you from 650 psi to 350 psi at surface (650-2*150). As an example say you're at 90 ft. It takes 90/30= 3 minutes to surface safely so simply start your ascent when Air Time hits 6 minutes. This allows the flexibility of always staying on top of the Rock Bottom Time in a multilevel dive. This method gives a reserve for hyperventilation and to exchange regs because the Air Time assumes you stay at depth, so it underestimates the amount of air left as you make your ascent.

Adam

 

[lamont]

I have been reading up on gas management and calculation procedures and have learned a lot from various websites (thanks, DevonDiver, for all of your resources)!

I have a question regarding "rock bottom" gas management. I understand the concept, and have even done a few calculations for some different dive scenarios. The amount of gas required is surprisingly more than I expected, and I am glad I have taken the time to begin learning this concept.

The "rock bottom" concept is based on the worst case scenario of having to breathe two divers all the way up from depth if there is an OOG situation, through all necessary stops and still having 300 psi at the surface. We dive single tanks (HP 130s), and some of my scenarios show that this would cut the dive short of the planned dive time (including ascent and deco stops).

You should be able to run out a dive at 100 feet for 30-ish minutes on 32% to the limits of the NDLs and then multilevel that into a 60-80 minute dive with 130 cu ft tanks and still have plenty of rock bottom left. If you can't do that, then you've got an issue with comfort in the water and/or fitness driving up your SAC rate.

However, yes, the point of rock bottom is to keep you conservative, if it didn't impact your dive planning and cut dives short there would be little point to it.

I see, however, that you can also calculate what I will call your "individual rock bottom" based on what it would take to get yourself to the surface (and still have 500 psi reserve). Same for your buddy.

I think that the rock bottom idea is great for dive planning in anticipation of an emergency. However, what if you and your buddy are a close team, and routinely check your own and each other's air supply? If all is progressing normally and both of us have sufficient air to complete our dive plan as we hit the "rock bottom" pressure, should we still thumb the dive , or can we continue our dive until we hit our deco limits (or until one of us hits their "personal" rock bottom pressure)?

Nope. Because that's when you're going to find out that gauge error in the SPG means that your buddy didn't have as much gas as they expected. Or they have a free flow issue that drains their tank dry at depth. Now they're going to want to go onto your gas and you're not going to have enough for both of you, which could get very exciting...

 

[ianr33]

The "rock bottom" concept is based on the worst case scenario of having to breathe two divers all the way up from depth if there is an OOG situation, through all necessary stops and still having 300 psi at the surface. We dive single tanks (HP 130s), and some of my scenarios show that this would cut the dive short of the planned dive time (including ascent and deco stops).

You really doing deco dives on a single tank? No H/Y valve?