Explanation of *on the fly computing* - LONG

[ Follow Ups ] [ Post Followup ] [ California Scuba Diving BBS ] [ FAQ ]

Posted by MHK on July 19, 2001 at 15:38:49:

In Reply to: MHK's dive profile posted by Chris on July 19, 2001 at 14:56:02:

OK, here we go.. I've e-mail the above graph to Chris prior to Seahunt posting his profile but, as you will see, it's COMPLETELY irrelevant what profile is used.. I want you to understand the theory, not necessarily the specific profile..

I also caution that this should not be used as a substitute for QUALITY training, and I recommend John Walker in that regard.. I'm going to provide the end result [ ie; the shortcuts] that have been developed, honed over years and many dives but I want to encourage everyone that you can't learn how to dive on the internet.. These methods work, despite what some would have you believe and you become a safer and more efficient diver as a result of the training and education..

You all know that we strongly recommend standardization, some call us zealots because of it.. When we say holostic it's because each part compliments the other.. Accordingly, we use 32% for all dives to 100' and trimix thereafter. Since I'm doing this profile for Seahunt I'll assume he's using 32% and I'll address air or 36% if anyone has follow ups..

Accordingly 32% provides a 20% EAD advantage.. That's trick #1.. If you are at 100' on 32% your NDL calculations can be done at 80'.. So you'll need to remember 20%. [ for two reasons, I'll give you the other one in a minute].. BTW, 36% EANx is a 25% advantage..

#2, all you need to do is observe your AVERAGE depth. The reason that I asked to use the graph was to make it simple for this illustration. If you look at the graph you can very easily see that your mean depth is 70'.. So 70' on a 32% is really 55' [ rounded]. So I'd use 60' for conservation...

Trick three, what's the NDL at 60'????? 60 minutes.. I told you that I wouldn't ask you to memorize tables so how do you know it's 60 minutes????

Easy CalAbDiver gave it to you yesterday [ thanks Karl you almost gave it away :-)]

The Navy understood a long time ago that you need to keep things simple underwater and you shouldn't do complex calculations..

So what does the follwoing have in common:

40' equals 80 minutes NDL
50' equals 70 minutes NDL
60' equals 60 minutes NDL
70' equals 50 minutes NDL?????

You get the point.. Those are the Navy tables..

They have in common that the time and the NDL equals 120.. Add the depth and the time together and get 120.. It's slightly different at 120 & 130 but we'll get to that..

So therefore if you know you're average is 60', well guess what??? You've got 60 minutes..

It's not magic and it's not hard...

You build in conservation if you want.. Similiarly to what computers do.. When a computer computes the M-value, the conservation factor is simply adjusting the depth or time.. In other words, if your average is 70' you could use 80' if you want to be more conservative, or if your time is 40 minutes you could use 45 minutes...

Now I said that 20% is important for 2 reasons and I gave you the first reason.. The second reason is because many believe that the Navy tables are too aggressive. And by in large that is true. The Navy has chambers available, the Navy diver's are all in good shape and they have a 20% DCS acceptance rate.. In other words the Navy thinks it's ok for 20% of diver's to get bent when they created the tables..

Acordingly, you adjust... In other words, the reason why we use 32% is because it is 20% more advantageous.. So rather than compute an EAD, you could use your actual depth...

These are all the reasons why I say it's important to understand not just HOW to do it, but WHY... I can't give you all the WHY's on the internet so I encourage training and education..

If all else fails and for some reason we [ my buddy and I] had a brain fart underwater and both forgot how to add to 120, we keep copies in our Wetnotes, but you can see it isn't very difficult and can be adjusted *on the fly* when you have the fundamentals understood...

Walker is great at explaining this stuff and I encourage anyone to take a GUE Fundamentals class from him because these kinds of things are just the tip of the iceberg...

This stuff isn't rocket science and as I've said very often, please do NOT abandon your computers immediately.. Try this method while using your computer and see what you come up with.. You'll be surprised that it will match your computer..

Furthermore, we always throw in deep stops and safety stops... It's all about understanding how the deco curve [ or NDL limit curve] works and to keep it moving...

Let me know if it wasn't clear and I'll follow up publicly or privately...


Follow Ups:

Post a Followup




[ Follow Ups ] [ Post Followup ] [ California Scuba Diving BBS ] [ FAQ ]