No deco and dive computers

[uncfnp]

There's alot of depth to the question though. Do you know your SAC/RMV rate? Do you understand what rock bottom gas planning is?

Let's assume you'll be diving 32% nitrox with a max planned depth of 110' and your tank is a 3442 PSI 130. At 110' you should start your ascent no later then when you have 1100 psi left. That's about 42CF of gas. As a comparison the rule of thirds ( one for me, one to get back and one for my buddy to get back ) would indicate 43cf but that should be close enough. That leaves you 2342psi to use on the bottom or about 88cf.

Now let's assume your air consumption is .75cf/min at the surface. At 110' ( 4.33ATA) you'll be consuming 3.25cf/min. The NDL for 110' on 32% is 25 minutes. Your suunto will probably give you about 22 minutes in plan mode. With 88cf of bottom gas you will lively have enough gas to put your suunto into deco.

So I'd say when you are within 3 minutes of an NDL or at 1100 psi (which ever comes first ) you start your ascent. If you have plenty of gas left then ascend 10 - 20' and see what your computer gives you for NDL. Repeat this process. Dive until you have at least several minutes of NDL left or enough gas. ( 90' 900psi 80' 800 psi ).

When it's time to ascend, go no faster then 30 feet per minute. Consider a 1-2 minute stop at 60' and do your safety stop at 20'. Then ascend at 10 feet per minute to 10', consider taking another 1-2 minute stop and ascend as slow as you can to the surface from there.

If one were planning this dive for maximum bottom time with a smaller tank, say an HP 80, with a pony bottle as the RGS, what would you estimate as rock bottom and possible bottom time on nitrox 32 and a SAC rate (true surface and not including dive factor) of .32? Trying to decide if I will have enough gas with my 80's that I am comfortable with or rent 100's which are difficult for me to manage. By my estimate the 80's should do. Thanks.

 

[wmason11]

windapp, I understand it now but didn't at the time, except in a sort of intuitive sense. Deco For Divers was good reading.


I now see on another thread that the OP is doing this as part of a dive shop group trip. He'll be with an instructor/divemaster whom I presume can answer his questions.

ya we will. Im good now, just clearing up some stuff. But I haven't met him lol. I am friends with the other guys at our shop. apparently hes leading this trip.

 

[yardpro]

it all depends on the computer
last weekend i was diving nitrox with an air suunto computer. I was at the bottom with 1 min left of ndl time, i started to ascend and the computer jumped to 3 min, then 5 min, then 10 min, time increasing as i went up.

When you go from 90 feet to 60 feet your ndl time will about double..as you raise your ndl will increase much faster than your ascent rate, so you will not go into deco.

 

[Mike]

I will be diving 130s tanks on a trip and will be diving at depths over 100 ft. We will be diving nitrox and I know I will run out of no deco time before air time so when im at a certain depth and my no deco time runs out, I need to simply go shallower to have more no deco time right? Prob a silly question but im just makin sure im not making a mistake. You should definitley start ascending slowly and start making your way up to the boat right? Just in need of clarification. I have never dove with large tanks like that

Personally I'd question the nitrox and consider not using it.

Please check out your dive computer's manual and learn how it will be functioning and displaying your deco information.

You basically have it right though. As your NDL time counts down you will eventually be going into Deco. Depending on factors such as depth and time spent at it, you might easily just eliminate your deco obligations by ascending to a shallower depth... or you might not and have a deco obligation at your safety stop... there are many factors and many what ifs, but your basic premise is correct.

However, like I said, I'd probably dump the nitrox as the nitrox introduces other problems in regard to your depths and the benefits especially on your first deco dives are going to be out weighed by the issue of what happens if compounding all of this there comes a depth issue on top of things? What if your buddy F's up and gets narc'd and goes deeper than he should? You now are in deco and on top of it are faced with violating your maximum depth due to nitrox?

Be real careful and dive with a really good buddy. Adding further tasks will make things more dangerous through task loading.. I'd leave the camera on the boat if you were planning on bringing one... and do not do this dive without a complete understanding of your dive computer.

 

[chrpai]

@uncfcp-

The math wouldn't change for me because I wouldn't make that plan. I want to have my rock bottom reserve in both tanks for complete redundancy. You don't mention your pony size but I assume it's <=40CF which was my rock bottom in this example. That said, you could always sling an 80 instead of a pony.

Some people don't like to dive a tank deeper then it's size. So a lot of people would want a 100cf to go to 100fsw. That said, an 80 will do, just maybe not as long as you might like.

 

[dmoore19]

I think you said that you would be using a Cobra 2 as your dive computer. You might want to check divecomputertraining.com and see if there is a class for it. It will give you a representation of what the screens look like and what to expect to see should you accidentally get into an obligation. If you are going to be using the computer for determining you dive time and planning it would help to know what the computer is telling you.

 

[lowviz]

...//...
1. Running out of no decompression time is like running out of air. It should never happen because you are keeping an eye on your computer.
2. Your computer will give you a decompression schedule if you over-stay. Just follow it, and then follow the instructions in the manual for the surface interval afterword.

I'm with you all the way, I just choose to go further. Having a plan in place for being without a computer is important to me. Calms my mind. My SPG and some form of depth gauge are all I need.

Great narcosis test:

Take out your slate and divide your max depth into 222222 (six two's)
Take the result and divide your depth into it again.

It is good to be able to perform this calculation at any depth as the result is very close to when your first mandatory stop is going to appear diving air. Knock of 10' from the max depth for EAN 32, subtract 20' for EAN 36. So if you can get the answer before you 'time out' you are "within your range".

-you won't find this in the books...

---------- Post added June 9th, 2013 at 10:11 PM ----------

...//... Some people don't like to dive a tank deeper then it's size. So a lot of people would want a 100cf to go to 100fsw. That said, an 80 will do, just maybe not as long as you might like.

She is within the realm of reasonability (with her killer SAC).

 

[chrpai]

She is within the realm of reasonability (with her killer SAC).

But what's her buddies SAC?

But ya, she'll be fine.

 

[lowviz]

But what's her buddies SAC?

But ya, she'll be fine.

damn, you know I just logged back on to cover this oversight...

 

[uncfnp]

She is within the realm of reasonability (with her killer SAC).

Very rarely it does pay to be a petite female :blinking:

But what's her buddies SAC?

But ya, she'll be fine.

My "buddy" will not do these dives with me so our agreement is that I hire a DM for at least the first day. I figure the DM can take care of himself.