When does Wreck=Tech?

[tombiowami]

At what point would you all recommend using the thirds rule on an ocean/wreck dive? The ones I am looking at would be around 100', little current, good vis, warm water. If not using thirds, is there a specific pressure in which you start the climb up?

I am also looking for general guidelines also. Strictly no deco, single tank.



Tommy

 

[Uncle Pug]

Originally posted by tombiowami
I am also looking for general guidelines also. Strictly no deco, single tank.

Are you planning penetration?
Do you need to come up and upline or can you drift on ascent?
What do you mean by "start the climb up"?

Do a search on Rock Bottom for a general guidline.

 

[tombiowami]

Thanks, will do. No penetration, no drifting. Start the climb up just means ascent in verbose.

Tommy

 

[Rick Murchison]

OK... no penetration, no staged deco obligation... let's plug some numbers.
I'm going to assume you'd like to be back aboard with 500 psi in your tank.
Let's assume a Surface Consumption Rate of 25 psi/minute on your AL80 (.65 CFM), and a 100' dive.
We're going to allow five minutes at our safety stop.. 3 stopped and two to get back aboard, but I'll figure my consumption at 16.5 feet (1.5ATM) for the whole time - gives me a little gravy. That means I'll use 1.5X25X5, or 187.5 psi for the safety stop and the trip to the ladder from the safety stop. I'll round that up to 200, and plan to arrive at my safety stop with no less than 700 psi.
I'm going to figure my ascent at 30 fpm, so it'll take me about three minutes to get from the site to the safety stop, at an average depth of 57.5', which I'm going to round up to 66', or three ATM, and figure I'll need 25X3X3 or about 225 psi for the ascent - and since I like to deal in nice round numbers I'm going to round that up to 300, so I need to start my ascent up the anchor line at 1000 psi.
Now I need to know how much gas I'll need to get back to the anchor line - that's going to be dependent on the site and how far from the line I am, but if I've left the line with 2800 psi, for example, and have been working up-current, then I'd plan my turn at 1/2 the available gas + 200psi, or, in this case 2800-(2800-1000)/2+200=2100 psi.
This is admittedly conservative, but very comfortable.
Rick

 

[Hal Iotis]

Thanks, Rick! That was a very informative post. I'm not planning to do that dive, but the calculation sure comes in handy, and is certainly better than an educated guess. This wasn't covered in AOW...

 

[Jeff M. English]

Excuse me everyone,

UP, I can't find the thread on the Rock Bottom gas management theory that you mentioned. I've heard of this before, a gas management plan from GUE for open water exercises but it's never been explained to me.

I'd appreciate a good link or a rehash if it's not a bother, if it is I'll go off and find it myself, I was just hoping to save the time.

Thanks
Jeff

 

[Uncle Pug]

I am in a rush right now so will give some details later.

Depending on the dive halves might be excessive and thirds might not even be enough...

On all scenarios assume no penetration and no deco...


1) Descent and ascent with no need to make an unline or anchor line (dift for pick up)...
No thirds or halves... just figure your Rock Bottom and dive until you reach Rock Bottom or the NDL.

2) Descent with optional use of up line for ascent means you have access to direct assent if needed so halves can be used... should a failure occur then you would just do a direct ascent. In this case you need to back Rock Bottom out of your supply then figure halves then subtract that from total supply to get turn pressure... you must still make sure not to go past NDL.

3) Descent with manditory ascent using up line means you need to back Rock Bottom out of your supply then figure thirds then subtract that from total supply to get turn pressure... you must still make sure not to go past NDL but that is unlikely.

For instance... if you only figure thirds on a dive where you absolutely need to get back to the up line and you have a failure at the furthest point.... and your buddy needs to use his third of your gas to get back to the line you are in trouble... you have used one third going out and the other two thirds coming back... but you still must make the ascent! Straight thirds does not work for OW... you need to keep a Rock Bottom in reserve.

Rock Bottom is the amount you and your buddy need to make it to the surface (or first gas switch for deco diving) making all stops and ascending at no more than 30fpm. This reserve gas is not what you need to get back onto the boat with... this is the gas you need to use for ascent times two (your buddy.)

I explain how to figure this Rock Bottom in another thread (I think about snorkels ) and you can do a search to find it. Having an abitrary reserve of whatever psi that you really have no plan for is unwise IMO.... better to figure out ahead of time what your Rock Bottom reserve is for various max depths.

Shoot this wasn't brief at all.... well... I still gotta rush but if there are any questions put 'em here:

 

[Jeff M. English]

Mr. Pug sir,

Thank you for the snorkel reference, made finding the post a lot easier.

Got the Rock Bottom thing, pretty much what I figured, makes sense.

Thanks