Deco dive plan sheet

[Jax]

But I LIKE math. I already know all the formulas. I'm funny like that.

 

[netmage]

But I LIKE math. I already know all the formulas. I'm funny like that.

But I thought you were Army? :mooner:

 

[Jax]

:shocked2:


:chairfight:

 

[Jax]

Y'know, this is funny as all get-out!!! It wasn't just six-eight months ago that I was being filleted for loving my computers!!!!!

:spit:

 

[ucfdiver]

Y'know, this is funny as all get-out!!! It wasn't just six-eight months ago that I was being filleted for loving my computers!!!!!

:spit:

There's a difference between cutting your own tables with modern algorithms, flying a computer on faith alone, and using antiquated deco tables. I would be willing to make a hypothesis that most who criticized flying the computer would suggest using a modern decompression algorithm.

 

[Jax]

There's a difference between cutting your own tables with modern algorithms, flying a computer on faith alone, and using antiquated deco tables. I would be willing to make a hypothesis that most who criticized flying the computer would suggest using a modern decompression algorithm.

Such as? What's your favorite?

 

[ucfdiver]

Such as? What's your favorite?

I use whatever algorithm gets me out of the water the fastest, VPM+2, Buhl 20/85, or ratio deco.

For short dives, ratio deco makes the most sense, I know that full deco bottles will outlast the duration where ratio deco is useful. For longer dives, I cut tables using VPM or Buhl, develop a trend, and (roughly) stick to it. I don't really care about 5min of deco one way or the other on these dives, because % wise, it's very little.

For instance, my last deco dive was at Ginnie Springs, just shy of 170min using back gas which was 28% Nitrox, 2 stages of 32% nitrox, and 1 stage of 21/40 trimix (for buoyancy reasons). For my starting point, I assume everything is 30%. I know that 3 AL80 stages and 2xLP108's cave filled have around 500cu ft of gas, of which we're going to use 1/3 in and about 1/6 out with the flow observing 1/3rds, so let's assume usable gas is (1/3+1/6)*500 = 250. From figuring a rough estimate based off of SAC calculations, previous experience in this cave, and guess work, we assume that this will get us 150-180min of bottom time-- the high flow caves really murder your SAC, and it's super low for the beginning of this dive on a scooter, so it's a huge gas range and we know this.

Now let's see what decoplanner has to say... This is an estimate, but even the algorithms are, so I'm OK rounding.

Now, let's find a trend between bottom time depth and deco time. Yellow is our target range. We'll notice that at 90ft avg depth, every 5min beyond 160min of bottom time results in 2min extra 02. So 20min over, add 20/5 = 4 *2min = 2min deco. 10ft shallower = 50% less deco. 10ft deeper = 50% more deco.

OK, these are crude estimates, but the potential for unknown is in this area, so knowing how long we can stay, what depth and what that does to our deco gas is extremely important. We can't pull tables out every 5min of the dive, so a general trend is a huge convenience when making on the fly decisions involved in cave diving.

Now, assuming a full AL40, and SAC of .5 (kinda high for deco), I know that an AL40 will give 50min of deco time at 20ft. I also realize that 32% nitrox takes (roughly) 2x as long to deco out as o2 in the case of a failure, so if my buddy and I split that bottle, we're each going to get 25min on o2, and 50min on back gas, so knowing "on the fly" how many minutes of deco we have allows ourselves to know what we're committing to.

Because of this deco gas limitation, I also know in the back of my head the entire dive that the following ranges (in red) are off limits.

Once back to deco, I refer to my tables, but I always try to develop an easy to remember formula for the potential range of depth and time for each dive I do. It's very quick to look at, and can make on the fly decisions much easier and more educated.

Admittedly, if you handed me a navy dive table without directions, I wouldn't know what to do with it. Of course, if you gave me a PC running OS/2 I couldn't use it either, and they're equally as useful in my mind, so I'm not too worried.

 

[Jax]

Of course, if you gave me a PC running OS/2 I couldn't use it either, and they're equally as useful in my mind, so I'm not too worried.

:spit:



Good discussion, thank you!

 

[ucfdiver]

:spit:



Good discussion, thank you!

No problem. Did that clear up why I don't think flying a computer blindly is a bad idea, but have no issue using a deco planner with zero experience on navy tables?

IMO tables are a way for open water divers to visualize what depth and time do to their NDL's, which is lost with a computer. They need to know how large of a tank to rent to stay down 20min @ 100ft and ascend safely maintaining minimum decompression (or safety) stops. I don't have a major issue if a well disciplined OW diver chooses to plan a dive with a computer as a backup, in fact I think this is smart until you prove you can accurately estimate average depth.

Once at a technical level, you need to know the following-

• How much gas do I need to complete my objective (20min @ 150ft, 3000ft swim into a 90ft deep cave @ 50fpm, etc)?
• How much deco gas do I need?
• How much gas do I need to get an OOA buddy to his first gas switch?
• How much gas (and what type) do I need to cover a failed deco bottle?
• Where should I place this gas for maximum safety?

Tables really don't teach you this. Vplanner/Decoplanner and similar software is a way to start these conversations with current deco theory as your starting point. Further to the point, these software even show graphs of tissue loading over time to show students all 16 compartments, the falloff of tissue loading as you near saturation, the exponential off gassing at gas switches, etc. This helps you visualize the mathematical model representing your body. Exporting to excel, you can witness how models cross each other (switch which has longer deco) as you add helium or bottom time.

 

[Jax]

Just imagine what will happen when I finally take a trimix class.