First dive at 40 meters - Newbies recreational

[Storker]

The fact that you had such a short dive time of 40 mins and used 140 bar shows your air consumption was quite high. Much higher than you assumed before the dive.

Let's do the math, based on the limited data we have.

Even if it is not the first time for the DM, he played along). As we are congratulating each other (40,5 m might not be much for some but it was an achievement for my wife and I), computers start beeping: 1 minutes to NDL. WTF! We must have descended too slow and spend too much time between 30 and 40.
Damned! We can’t stay any longer. And we forgot the camera ( no picture to immortalize the moment). SPG at 150/ 160 and going down fast.

210 to 160 bar is 50 bar. Times 12 L is 600 surface liters.

The PADI RDP says that NDL @40m is 9 min. 1 min left, that's 8 min. 600 by 8 is 75, divide by 5 bar is 15 surface liters per minute. Perhaps on the slightly high side for a wetsuit dive in warm water, but not bad at all for a drysuit dive in cold water. Particularly since the OP has <50 dives under his weight belt; most n00bs that I've dived with use quite a bit more (generally closer to 20 than to 15 SLM. Or even >20)

Now, what would a proper reserve be here? 1 min @40m, that's 75 L. Ascent at 10 m/min, average depth 20m, that's 4 (min) x 3 (bar) x 15 (SLM) = 180 L. 3 minutes safety stop @5m, that's 3 (min) x 1.5 (bar) x 15 (SLM) = 67.5 L. Total 322.5 L. Times two for increased consumption due to stress, times another two to account for donating to your buddy and we get a minimum reserve of 1290 L. In a 12 L tank, that's close to 110 bar. Plus some 20 or so bar to account for SPG error and to be certain to be able to inflate their BCD at the surface.

So the only two things the OP did right was to leave before they went into deco, and to have a fairly decent reserve. Me, I'd be more conservative than that, especially considering that everyone is narked at 40m, no matter whether or not they notice it. I definitely would allow for a more time than one minute at 40m, on air, to sort things out and start a controlled ascent. And 10 m/min is pretty fast. Lot of n00bs would probably struggle with their buoyancy control at that ascent rate, and corking is a definite risk.

 

[UCFKnightDiver]

Others have touched briefly on this, but if you had a catastrophic loss of gas at depth and needed to share air, you likely wouldn't have enough gas to make it to the surface/complete deco. You said there was a safety bottle hanging at 20ft, but you were drifting with the current? So that bottle was useless to you then.

12L tank is HP100cuft I assume, better than an AL80, but not great.

Sorry, going to do these calcs in imperial because that's what I'm most familiar with.

Doing some math, you said it took you likely - - 12+ minutes to get to the bottom with 3ish minutes on the bottom, at this point you had 150 bar or 2100 psi so you have about 58cuft of gas left
- Let's assume you have a normal SAC rate of .7 cuft/min, then problem and catastrophic loss of gas at depth. Breathing rate goes to 1cuft/min (it's probably way more) and you're sharing air so double that, plus you're at depth so you're really breathing 10cuft per minute. It takes you 2 minutes to solve the problem and start heading back up. So you've breathed 20cuft and are down to 38cuft.
- Sounds like your ascent took 25ish minutes due to current and deco. With 9 minutes of deco and travel time between stops let's call it 15 minutes for deco and 10 minutes to get to 65 ft (first deco stop). Average depth for this portion of dive is 99ft for 10 minutes, so we're using 80cuft.

- First stop is 65ft for 2 minutes. You use 6 cuft here.
- Second stop 40ft for 2 minutes, you use about 5 cuft here
- 3rd stop 20ft for 5 minutes, you use 16 cuft here.

See the problem here? You had 58cuft of gas when you started your ascent, you needed 107cuft. Obviously if there was an issue you would've likely considerably shortened the time to the first deco stop, but even if you reduced that to 6 minutes or 10ft/minute, you still would have used 48ish cuft on your ascent to first deco stop for a total of 95cuft needed and you had 58cuft available when your buddy catastrophically lost her gas in this scenario. We could further simplify things and cut out the first two deco stops, but you would still need 84cuft, which you didn't have. Sure, your DM is there, but I think this math illustrated the dangers here. Had it just been the two of you and no safety bottle at 20ft, you likely would have been out of luck.

 

[Dody]

@Dody while you were drifting with your wife, did any of you shoot a DSMB?

Regarding the seemingless equalization between 30m and 40m, it is expected as it becomes easier the deeper you go due to a lower relative increase in pressure.

No, we did not. The plan was to shoot the DSMB at safety stop or in a case of emergency if we were drifting too much. We were not in such an emergency situation yet as we could still see the rift ( close to the anchor line) and the DM. My priority was to ascend to at least 20 meters. Tunnel thinking?

 

[Blackcrusader]

Yes, I will keep that in mind. When I (rightly or wrongly) assumed that we were not ascending fast enough, I should have inflated our BCDs. I thought about it but I actually fought against doing it for fear of a runaway ascent. Narcosis???

I may have made a mistake this was a training dive as you kept writing about your instructor who you now write did not attend the dive on the the DM.

You wrote "It’s been a wonderful 50 days journey from the day we visited a dive shop for the first time to now AOW with 37 dives" Enjoy.

With inflating a BCD you have 3 places to purge air normally both shoulders and back right rear. So you won't inflate BCD to max and have a rapid ascent. Also dive computers can show you your ascent rate so you can manage an ascent watching your DC.
You can see here where I practice an ascent using BCD and of course lungs don't forget to continually breath out and you can slow your ascent doing that. Never fully breath in when ascending.

I took this video as my son wanted to do his OW. So I could explain he could see the ascent range on the right and also the NDL change as you ascend. You can practice this a lot on your dives with your wife so you can get a feeling of what a nice steady ascent is. When you are finning against a current and trying to ascend you burn a lot of air and you can get tired if prolonged, or suffer leg cramp as I have seen many divers get. Some of my friends who are instructors have this video to show their students.

I wear my dive computer on my right hand. I can see it if using the left inflator hose or using my right shoulder purge valve. I am giving another diver an assisted ascent I use my left hand to hold that diver and my right hand where I can see my computer while holding my purge handle in case I need to prevent rapid ascent as you can also get up currents as well.

Narcosis? Not necessarily. Inexperienced in this situation, yes. Both Narcosis and inexperience possible. The two can be deadly. Your certifications mean nothing underwater.

 

[Dody]

He mentioned not being able to ascend from 40m to 37m easily due to current: Later horizontal current which he drifted with.

The 45 degree ascent was not intentional at first. We wanted to ascend vertically. But if you look at trajectories, you go up vertically with an horizontal current and your ascent is a curve.

 

[Dody]

@Dody while you were drifting with your wife, did any of you shoot a DSMB?

Regarding the seemingless equalization between 30m and 40m, it is expected as it becomes easier the deeper you go due to a lower relative increase in pressure.

So my sonar is not effective below 20 meters

 

[Dody]

Let's do the math

The PADI RDP says that NDL @40m is 9 min.

So the only two things the OP did right was to leave before they went into deco, and to have a fairly decent reserve. Me, I'd be more conservative than that, especially considering that everyone is narked at 40m, no matter whether or not they notice it. I definitely would allow for a more time than one minute at 40m, on air, to sort things out and start a controlled ascent. And 10 m/min is pretty fast. Lot of n00bs would probably struggle with their buoyancy control at that ascent rate, and corking is a definite risk.

I love the math. SSI states 5 minutes @40m.
But I am interested in what you think I did wrong for continuous improvement and I am not sure that I understand. Can you please explain further?

 

[Blackcrusader]

The 45 degree ascent was not intentional at first. We wanted to ascend vertically. But if you look at trajectories, you go up vertically with an horizontal current and your ascent is a curve.

Do yourself a favour. Go to www.divinglong.com and download the software. Scan in your dives and then go to your 40m dive. Put in your AL80 tank with 200 bar and end 40 bar. Then save the dive log. Then you can go to the profile save your data and you will see your sac rate. You can then see your air consumption on the right. My 45m dive not much different to your 40m dive except I did not exceed NDL and I did a 70 minute dive time and had 60 bar at end of the dive.

OK so on this profile, you can see at left my current depth at 11.33m time 30.03 mins average depth for dive 14.2m and my sac rate for the total dive time. You can see even at 30 mins I still have 120 bar left. Second half of the dive I was swimming against a slight current to get to the boat pickup area.

What you think your sac rate of 11l/min is not going to be what you get on your dive. It may well shock you how much you miscalculated.

 

[Dody]

Others have touched briefly on this, but if you had a catastrophic loss of gas at depth and needed to share air, you likely wouldn't have enough gas to make it to the surface/complete deco. You said there was a safety bottle hanging at 20ft, but you were drifting with the current? So that bottle was useless to you then.

12L tank is HP100cuft I assume, better than an AL80, but not great.
Doing some math, you said it took you likely - - 12+ minutes to get to the bottom with 3ish minutes on the bottom, at this point you had 150 bar or 2100 psi so you have about 58cuft of gas left
- Let's assume you have a normal SAC rate of .7 cuft/min, then problem and catastrophic loss of gas at depth. Breathing rate goes to 1cuft (it's probably way more) per minute and you're sharing are so double that, plus your at depth so you're really breathing 8cuft per minute. It takes you 2 minutes to solve the problem and start heading back up. So you've breathed 16cuft and are down to 42cuft.
- Sounds like your ascent took 35ish minutes due to current and deco. We can gas math out your deco stops first and then can figure out the length of time from 130ft to first deco stop and estimate gas usage there.
- First stop is 65ft for 2 minutes. You use 4 cuft here.

Well. I am not sure I agree on that one. From my calculation at 40 meters and with our SAC in the last 10 dives, we still had 30 minutes of air. Let's say 15 minutes in case of buddy breathing. We were not in a deco dive. So all we had to do was to ascent 40 meters which is 5 minutes at 9 m/ min plus 5 minutes safety stop at 5 meters and 2 minutes at 20 meters. Without panicking, it is enough. And the DM was around. But I agree, not so much safety margin. Pony with a regulator will be a must in my next dives.

 

[Blackcrusader]

This was not a deco dive. But I agree that it could have been if everything had gone wrong. What do you mean by BASICS?

if you have a deco stop the it is a deco dive. You wrote your computer beeped to get you to do deco stops yes? You had to do a 2 min stop at 20m so that sir makes this a deco dive even if you say you only planned a dive not to exceed NDL limits. You say you had 1 min to NLD at 40m and struggled to ascend, while struggling you are finning and could have been in deco already but been back in NDL clock later as you ascended.