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What's DIR really about?
DIR - An essay
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Calculations on the fly
Dive planning on the fly
How much lead?
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Gas switching procedure
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Average depth for deco?
Using the min deco table
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Nitrox Class Part 2

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Dive Planning On The Fly

Read Calculations On The Fly first if you haven't. This article will assume that you can calculate this stuff in your head.

When we dive we need to keep track of a couple of things that relates to our dive plan. Depth, time, gas and decompression obligation are the most important ones. When doing non-decompression dives we need to check that we don't extend our dive time.

Turn depth into ATAs

Depth, time and gas is closely related.To be able to use them we need to convert depth into ATA. Easy enough in metric (ATA=msw/10 + 1) but a bit more challenging using the imperial system.

The formula is: ATA = ft / 33 + 1
That is (almost) the same as ATA = ft * 3 / 100 + 1
Let's try that one!

140 ft is in ATA?
140 * 3 = 100 * 3 + 40 * 3 = 420
420 / 100 = 4.2
4.2 + 1 = 5.2 ATA

45ft is in ATA?
45 * 3 = 45 + 45 + 45 = 135 (another way to multiply with 3)
135 / 100 = 1.35
1.35 + 1 = 2.35 ATA

Another approach is to memorize a few depths and just guesstimate for any depths in between. Remember that +/- 10 ft = +/- 0.3 ATA

  • 0ft = 1 ATA
  • 33ft = 2 ATA
  • 66ft = 3 ATA
  • 99ft = 4 ATA
  • 132ft = 5 ATA
  • 165ft = 6 ATA
  • 198ft = 7 ATA
Gas planning

Doing a fast dive plan you need to know how long your gas is going to last at the depth you intend to dive. I prefer to convert gas into time because time is what is interresting during the dive. To do in our head we first need a base line to start from.

For simplicity I use a 12L/200bar as the starting point. A 80cuft bottle is about the same size, a set of double twelves or double 80s is twice that, a set of double 18s or 104s (double17s) are tripple the amount of gas.

Using 20 liters per minute or 0.7 cuft/min you get about 120 minutes out of a 12 liter/200bar or an 80cuft/3000psi bottle at the surface. So 120 minutes is our base line. If you divide that with the depth in ATAs you know how long you can stay.

For example at 30m/100ft/4ATA we can stay 1/4 of 120 minutes until we run out of gas in a single tank. That's about 30 minutes.

Using all your gas at depth is a bad idea since we need some reserve gas if we have a problem. Assume that you wanted to reserve 1000psi/70bar you can still use 2/3thrd of your gas supply. In the example above 30m/100ft that would be 20 minutes in real life (2/3rds of 30min).

Let's say your are doing a dive to 150ft/45m/5.5ATA and you intend to use half of your gas supply in your double 80s/double 12s. How long is the dive going to be?

Well, half the doubles is one full bottle so 120 min at the surface. 120/5.5 is a bit tricky but 120/5 = 24 and 120/6 = 20 so a little more than 20 minutes (22 min) is what you can expect.

Adjusting the base line

But what if your gas consumption rate is lower (or higher)? You could use the same basline and adjust for higher or lower consumption or you could make your own baseline to start from.

If you use 15 lpm (liters per minute) or 0.5cuft/min instead of our base line, it's 75% of the gas consumption meaning your will get 33% more time at depth. So instead of 22 minutes at 45msw/150ft we can stay about 29 minutes (22+22/3).

Using stages

Usually in cave diving and also ocean diving we use stages to extend our bottom time. The nice thing is that a stage normally a 80cuft bottle so we can use the same calculations as above. In the cave however it is dropped at half the pressure going in and you will breath the other half going out. Let's try some examples.

How long will it take before you need to drop your stage, diving to an average depth of 25m or about 80ft?
Converting it to ATA we get about 3.5 ATA. 120/3=40 and 120/4=30 so about 35 minutes using a full stage and half of that is about 18 minutes.

Cave training

During GUE Cave 1 training you are allowed to use 1/6 or 35 bar/500 psi for penetration. Common tanks used with a dry suit are 104s which are basically the same size as three 80cuft stages. 1/6 of three stages is the same as half a stage, meaning we can use the same calcs as above to figure out how long it will take before we have to turn. 500 psi/35 bar is also 1/6 of a set of full doubles so the same applies here.

If we are diving a site where we have 18m/60ft average depth how long before we have to turn the dive?

18m/60ft is about 2.8ATA (3ATA - 10%) giving us 40 minutes + 10%, about 45 minutes total. Half of that is 22 minutes. So you are likely going to turn the dive after about 22 minutes.

Calculating swimming distance

A common swimming pace in cave diving is 50ft or 15m per minute. Poking around on your average wreck you are going to swim slower but could speed up if you had to. Likewise if you are running or retrieveing a guideline you also likely to be going slower.

Taking our Cave 1 example above where we would have to turn after 22 minutes, how much penetration would we get?
Well, 50ft per minute * 22 minutes = 1100 ft (100*22/2). In meters it's 15 m/min * 22 min = 330m (15*2+10%).

Using scooters

Using a scooter you will basically tripple your swimming pace, running at 150ft/min or 45m/min. If you are up against a lot of flow you will be slower also carrying a lot of gear will slow you down.

Let's say we are going to do a scooter cave dive to 200ft/60m using one stage for penetration and saving all our backgas. How far are we likely going to go in the cave?

200ft/60m is 7ATA. 120/7 is tricky, let's do 14% instead. 10% is 12 minutes and 5% is 6 minutes so 14% is about 17 minutes. Since we are scootering and likely more experienced we are probably going to use 15lpm or 0.5cuft/min, maybe less. That's 33% more and 17/3 = about 6 minutes longer so 17+6=23 minutes. Since we have to turn when we reach 1/2 our pressure we can do a about 11-12 minutes. Converting that to scooter speed we will do 150ft * 11 min = 1650ft (150*10+10%) or 45m * 11 min = 495m. So about 1650ft or 500m penetration is what we can expect with a bottom time of about 23 minutes. Pretty neat isn't it?

Planning non-decompression dives

When you do non-decompression you need to stay with in the dive limits of your choosen table. Minimum deco means that we always do some decompression stops to minimize bubbles even if most tables would not call for it. Anyway, what we need to know is how long we can stay at a certain depth.

Using my minimum deco tables we get the following times using 32% nitrox:

100 ft = 35 min, 80 ft = 50 min, 60 ft = 85 min, 50 ft = 160 min

30 m = 35 min, 25 m = 50 min, 20 m = 75 min, 15 m = 165 min

The times between the imperial and the metric version differ because it is not the same depths. The metric one uses 5 meter increments while the imperial one uses 20 or 10 feet increments. If you need a depth like 90ft you can interpolate and take a value in between, in this case about 40 minutes (42.5 actually).

If you memorize these times (or times from another table) you just need to start ascending before your time is up. Doing multi-level dives you need to keep your weighted average depth below the limits of your table.

Let's say we want to dive to 80ft/24m on a single tank. Gas planning say we can stay an absolute maximum of 120/3.5=35 minutes but looking at the table we can do 50 minutes before we start getting into real decompression. So this is an easy dive since our gas will be the limit, not our decompression.

However, doing the same dive using doubles we can do 70 minutes at depth before running out of gas. If we want to reserve some gas for minimum deco and problem solving we could save 50bar / 750 psi for that. That gives us about 75% to use of our gas. 75% of 70 minutes is about 52 minutes (70/2/2=17.5, 70-17.5=52). That's 50 minutes at 80ft/24m. A nice dive plan with a lot of dive time, don't you agree?

Summary

These are a couple of calculations you can use to plan your dive - in the water or on land. It will give you an appreciation of how long you can stay, how far you will go and if you have to do any deco before you do it. This flexibility is something a dive computer can't do since there is no way to enter all the parameters into it. And for advanced dives you know what to expect without breaking out a laptop.

In the next part I will delve into decompression on the fly.

Have fun!

Peter





This page was last modified 6 September 2006
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