Question on Rock Bottom

[WhiteSands]

If one were to calculate Rock Bottom (RB) by the book, diving at any depth 18m or less will yield a RB of 50 bar (about 725 PSI), which I was told was the absolute minimum for RB. I.e., say if my average depth was 10m, I still start surfacing at 50bar.

However, I feel that starting to surface at 50bar at 18m is cutting it kinda close in my personal experience. There were times when I surfaced with 10 bar or even less. If I were sharing air I don't think I would make it.

It could also be due to the inaccuracy of my SPG, I noticed that it drops off much faster from 50bar to 0bar then from 200bar to 150bar.

In "The Fundamentals of Better Diving" by JJ, it states that one of the objectives of a dive is to return to the boat/surface with 50bar still in the tank.

If that is the case, how can one start surfacing at 50bar?

Appreciate your input, thanks.

 

[TSandM]

If you are surfacing from 10 meters and using 40 bar to do it, either you are not doing a direct ascent (in other words, your ascent rate is much too slow) or your SAC rate is significantly above the 1 cubic foot per minute consumption that is assumed as a STRESSED consumption rate.

Unfortunately, I have never gone through rock bottom calculations in metric -- but if you are taking 3 minutes to get from 30 feet to the surface (min deco), you have an average depth for that period of 15 feet, or 1.45 ATA. So you have roughly 4.5 ATA-min of gas consumption. You have used 40 bar, or 600 psi, and I'm assuming this is in a 12l or Al80 tank. That's 15 cf of gas, divided by 4.5 ATA-min . . . you're running almost 3 cubic feet per minute of gas consumption, which is EXTREMELY high. You're right; none of the rock bottom calculations are going to be valid if your baseline consumption is that high. (But I have only seen consumption like that in highly stressed OW class students. You really have to be panting to go through that much gas.)

Regarding the "surfacing with 50 bar" idea -- Rock Bottom is just what the phrase implies. It is the absolute MINIMUM amount of gas you can have in your tank at a given depth, and still be reasonably sure of being able to surface yourself and a buddy while sharing gas. It's not necessarily a pressure you should plan on reaching. But for most people, it should only take about 6 bar to surface from 10m (in a 12l tank), so going up at just a tad over 50 bar should still get you on the surface with a reasonable amount of gas in the tank.

 

[WhiteSands]

Thanks for the response Lynne.

I think there were two issues that probably contributed to my low gas on surfacing, and it wasn't the consumption.

One was that I took time to shoot an SMB.

Second was me misunderstanding the ascent rules on previous dives. I had initially thought that you needed to stop for 1 min at every 3m stop, but later realized on re-reading the manual that the 1min was inclusive of the time it took to ascent to the next stop, after the first stop.

However, even after I realized my second error, I still noticed surfacing from 18m (about 60ft) with only 20bar left in my SPG. If I were sharing air, I don't know if it would be sufficient for two. However, to be fair I did not exactly monitor this very closely. I will write this down for my next dives just to be sure.

 

[robertarak]

Thanks for the response Lynne.

I think there were two issues that probably contributed to my low gas on surfacing, and it wasn't the consumption.

One was that I took time to shoot an SMB.

Second was me misunderstanding the ascent rules on previous dives. I had initially thought that you needed to stop for 1 min at every 3m stop, but later realized on re-reading the manual that the 1min was inclusive of the time it took to ascent to the next stop, after the first stop.

However, even after I realized my second error, I still noticed surfacing from 18m (about 60ft) with only 20bar left in my SPG. If I were sharing air, I don't know if it would be sufficient for two. However, to be fair I did not exactly monitor this very closely. I will write this down for my next dives just to be sure.

It is really less of a manual, and more of a text to be used in conjunction with a qualified instructor. For experienced divers it is a good reference, but nothing takes the place of quality instruction. Your desire to learn these skills and techniques is admirable. Look for a instructor that can help you.

 

[WhiteSands]

It is really less of a manual, and more of a text to be used in conjunction with a qualified instructor. For experienced divers it is a good reference, but nothing takes the place of quality instruction. Your desire to learn these skills and techniques is admirable. Look for a instructor that can help you.

Hi Robert, I am taking this course under a qualified instructor.

 

[TSandM]

The consumption you listed would be about five times the amount I calculated would normally be used in that short ascent, which means that either you took a VERY long time to shoot the bag, or you lost a lot of gas in the process (were you inflating with a purged regulator?), or your gas consumption is much higher than our normal estimates.

 

[WhiteSands]

The consumption you listed would be about five times the amount I calculated would normally be used in that short ascent, which means that either you took a VERY long time to shoot the bag, or you lost a lot of gas in the process (were you inflating with a purged regulator?), or your gas consumption is much higher than our normal estimates.

Noted. I will try to do a more careful observation on my next dive.

On the SMB, I was using oral inflation.

 

[J-Vo]

However, even after I realized my second error, I still noticed surfacing from 18m (about 60ft) with only 20bar left in my SPG. If I were sharing air, I don't know if it would be sufficient for two. However, to be fair I did not exactly monitor this very closely. I will write this down for my next dives just to be sure.

If you want your Rock Bottom (RB) pressure to work for you and your buddy, you must add your and your buddies RB volumes before calculating your RB Pressure. Also, if your SAC is a lot higher than the average diver you should be calculating your RB rather than using any trick shortcuts you may have been given.

 

[WhiteSands]

If you want your Rock Bottom (RB) pressure to work for you and your buddy, you must add your and your buddies RB volumes before calculating your RB Pressure. Also, if your SAC is a lot higher than the average diver you should be calculating your RB rather than using any trick shortcuts you may have been given.

I believe the standard RB formula already accounts for 2 persons.

(Breathing rate x 2 divers x ave_depth_ATA x time_to_surface_including_stops) / tank_volume_rating

PS: This is for metric calculations. There is a lot more math wizardry involved when calculating for imperial....