Stage Dive Planning

[Scared Silly]

Just so that all of the assumptions are known. What are the underlying SAC rates being used? E.g. Are these their average SAC rates or have they added a buffer into them? If so what is buffer? Depending on the dive I might add in a buffer of 50% to account for current or other extra work loads.

 

[ianr33]

The figures in my original post were using a SAC of 0.6. My actual SAC rate on low exertion dives using doubles is normally 0.5something,so very little,if any "buffer" My Buddy has a SAC rate a little higher,maybe 0.65

Another point I did not mention (as I was asking about stage gas management,not if I needed to use trimix etc etc ) was that this is a shore dive,something like a 20 minute swim out to the dropoff. The posted profile has a direct ascent from 80 feet to 20 feet. The actual dive will have a direct ascent from 80 to around 50 and then maybe 15 minutes slowly profiling back to 20 feet. This will use a bit more gas

I have never come across any currents before in this area but there is a first time for everything.The last thing I want to do is to be swimming into a current to hold position wondering if I have enough gas to complete the stops. I agree that carrying a slung 80 may be a bit more than necessary,but 80's will be readily available and once in the water is no more trouble than a 40,so why not carry a little extra gas ?

The location for this dive is the North East Coast of Grand Cayman. Going to put in just east of the No Dive Zone in Old Man Bay and see if we can get as far as Babylon. This will take us past the named site of Julies Wall and lots of dramatic scenery.

Only 2 1/2 weeks until I am actually diving rather than argueing about it on Scubaboard !!

 

[Thalassamania]

I have no problem with going out in shallow water to drill and test, I’ve been out in a little cove (moderately popular dive spot, shallow water, nice boat ramp entry, sandy bottom) here with rebreathers, band masks, helmets, double 130s with two 40s, you name it … but always with a specific reason.

But on operational dives my goal is to always dive the minimum configuration that is commensurate with getting the task done and with predictable risks. Lately, I see a lot of divers giving themselves more trouble by trying to impress everyone on the boat (or in the parking lot) with all the gear that they “have” to carry to make the dive, its just plain macho stupid.

I plan for my dive, generally at a SAC of 0.75.

I first figure the gas that I'll need for the planned depth (I assume a square profile in most cases).

I figure the gas that I'll need for my ascent and double it (more if my buddy's a big Hoover).

I figure the gas that I'll need for an appropriate deep stop and double it (more if my buddy's a big Hoover).

I figure the gas that I'll need for a 15' safety stop and double it (more if my buddy's a big Hoover).

I figure the gas that I'll need for any required stops and double it (more if my buddy's a big Hoover).

I figure the gas that I'll need if I'm one depth or one time over my planned dive and double it (more if my buddy's a big Hoover).

I sum it all up and decide on the minimum configuration that I can dive for that quantity of gas.

I usually carry 40 cubes of oxygen. I may or may not subtract any gas planned for 20 foot or up from that, depending on my gut feel and how that would effect my rig choice. Beyond that I carry no additional reserve, no 10%, no 1.78 times the phase of the moon, no back on the boat with 500 pounds.

I usually find, since there’s not problem, that I’m back on the boat with a lot of spare gas. And when there are problems, they’re usually early in the dive, so I wind up with even more spare gas. But I always have enough to get me an my buddy back to the surface.