Thinking of Roatan for newish divers

[OMyMyOHellYes]

Can you explain what EAN and MOD are? ETA: I just looked it up!

I know you weren't asking me, but ....

Taking the on-shore, no-dive, hour-long Nitrox cert class can be a really valuable thing to do.

That is what you are asking about.

 

[Allen-1]

I'm about to buy a dive watch and that looks so complicated, my brain kind of blanks. So if your SAC is .57 on the surface and you did a really shallow dive, that would give you 140 minutes on the bottom? I think I understand the rest!
Can you explain what EAN and MOD are? ETA: I just looked it up!

Sorry, didn't answer your question. In short "yes". But it would have to be a very shallow dive, because 10 meters down adds an atmosphere, which doubles your air consumption because it's compressed. I have a hard time wrapping my mind around this - and it's important to understand if you're going to figure bottom time.

 

[NothingClever]

Now, I understand that is because I'm new and don't have my own experiences to fall back upon. I'm sure if I had hundreds of dives in all sorts of conditions, I would have seen enough to handle most situations.

I have hundreds of ocean dives on reefs, walls and wrecks and I still listen to the DMs. Honestly, that’s one important metric I use to gauge their tip.

I don’t really need the DMs (except on technical dives when they clip on my deco bottles, hand me my DPV and receive them from me before exiting the water) but I still listen to them.

The clowns on the boat who go on chatting over the DM are the ones whose skills are most likely to fall short of expectations.

Seasoned divers who know the sites and waters will be respectful of junior divers and stay quiet so they can hear.

Be brilliant at the basics.

 

[therookie]

So if your SAC is .57 on the surface and you did a really shallow dive, that would give you 140 minutes on the bottom?

I think you have the basic idea. Looks like you used 80 cubic feet for the tank and divided that by the SAC rate in cubic feet per minute to get that number, right? In theory, that's correct, but for me when calculating bottom time for a dive there are a few caveats I take into account.

First is that an AL80 doesn't actually have 80 cu feet of air in it when it is filled to 3000 PSI - it's a little less due to the thickness of the tank wall and coatings, which means it can even vary a bit by tank manufacturer just to make things even more confusing. 77.6 cu ft is a common capacity, some are 77.4, and there are probably other variations still. Let's use 77.6 - so dividing that by a SAC of .57 would give you 136 minutes.

However, even that is only a theoretical maximum. From what I have read about how our scuba regulators work, they need a certain amount of pressure before they will even work at all. I think it might depend on the regulator but even when a tank is filled to 3000 psi we would only have maybe around 2800 available for actual breathing use. So that brings us down to 72.4 cubic feet of available air - 127 minutes at that SAC rate.

That's with no reserve - planning on breathing your tank down as far as you can. I've read enough of your posts on ScubaBoard to guess that you would never be comfortable doing that and would prefer to plan for the "standard" reserve of 500 psi. That's not in any way a criticism: I would never be comfortable with it either and would plan for the same. So now using 2500 psi available for use we are down to 64.5 cubic feet of air - 113 minutes at that SAC rate.

But that is at the surface. As soon as you go to any depth that all that number of minutes drops quickly. Let's take a practical example. Since this thread started about Roatan a good use case would be a shore dive when staying at CocoView resort, diving what they call their Front Yard. Here's a portion of a map from their website:

You can see it is very shallow. It's easy to spend a lot of time in water that is around 25-ish feet, and while I have never done it from what I have read there is a ton of life - little stuff like nudibranches etc - in the Front Yard. You can do an enjoyable dive and probably average somewhere around 15 feet over the entire dive time. (For a dive that shallow whether you really need a 500 psi reserve is probably open to debate - personally I would do it anyway just because I am so conservative.)

15 feet is 1.45 atmospheres (1 + 15/33). So our SAC of .57 now becomes .83 cubic feet per minute of "actual" consumption due to the depth. So now taking our 64.5 cubic feet of available air (due to the reserve of 500 psi ) and dividing that by .83 cubic feet per minute, I get a total time of 77 minutes.

Which is a LOT less than 140 minutes, and why I wanted to post this. Possibly far more detail than you wanted, but just to show that there is a lot of "sticker shock" when calculating how long you can actually dive for compared to the theoretical maximums for a given tank and SAC rate.

As always, if someone more (or less!) knowledgeable than me reads this and spots an error please point it out.

 

[scubadada]

...First is that an AL80 doesn't actually have 80 cu feet of air in it when it is filled to 3000 PSI - it's a little less due to the thickness of the tank wall and coatings...

An AL80 does not contain 80 cu ft of gas because of deviation from the ideal gas law at the fill pressure of 3000 psi

Calculating SCUBA Cylinder Capacity | Dive Gear Express®

Or why does my Aluminum 80 SCUBA tank only hold 77 cubic feet of air? ©2018-2020 by Mark Derrick, Gas Blending Instructor at Dive Gear Express ...

www.divegearexpress.com

 

[NothingClever]

I’m not thinking the coating on my cylinder nor a 00.2 cu ft variance in cylinder capacity is going to make that much difference in the duration of my dive. In contrast, my breathing, buoyancy control and propulsion will make a huge difference.

 

[dmaziuk]

I’m not thinking the coating on my cylinder nor a 00.2 cu ft variance is going to make that much difference in the duration of my dive. In contrast, my breathing, buoyancy control and propulsion will make a huge difference.

Of those 3 only breathing actually matters. The other two just make you breathe harder. As can e.g. being cold. Or any kind of stress, real or imaginary -- stress response starts with faster harder breathing to get more oxygen into the blood in case we'll need the extra energy to fight or flee.

Which of course is completely silly on SCUBA as you are already hyperoxic above and beyond what can be achieved by breathing harder on the surface

 

[scubadada]

I’m not thinking the coating on my cylinder nor a 00.2 cu ft variance in cylinder capacity is going to make that much difference in the duration of my dive. In contrast, my breathing, buoyancy control and propulsion will make a huge difference.

The difference in cylinder capacity is not great at 3000 psi, just 3.2 %, take a look at the difference at 300 bar, 4350 psi, 10.2%.