So far to do the gas planning you consider necessary for a safe dive you need to know your air consumption rate for the given activity, the finning rate, the direction you will travel for each leg of the dive and the depth profile for each leg of the journey. You would also need to know your tank size or at least be able to make adjustment for you tank size. It implies that you must adhere to the plan.
To make it a little more realistic and applicable to the way most recreational dives that I've witnessed are conducted, assume you do not know the time required to travel each leg of the dive.
Let's assume for the moment that you have the information I listed above. With reference to the previous example, outline your gas planning procedure in detail. From what you have explained to date it would go something like this:
1. Calculate the average depth of the leg in question. This can be done by adding the maximum and minimum depth and dividing by two or approximating the average based on a detailed visualisation of the profile.
2. Calculate your air consumption rate. To be accurate it needs to be adjusted for your activity level and be based on a relatively recent assessment of your rate. Mine can vary significantly over a period of some months.
3. Determine your average finning rate. This assumes you know from previous dives how long it takes to cover a fixed distance. Again this is highly dependent on what you're doing. It also implies that you must maintain that finning rate for the dive plan to work. Assume you have calculated and remembered a range of finning rates from a previous dive. Otherwise you'll need to jump in the water and time your finning rate doing various activities.
4. Knowing your finning rate, determine the time it will take to cover a given segment of the dive plan. For example, if the first leg was to be 50 m and your finning rate was estimated to be 10 m per minute then the time to travel that distance is 5 minutes. You'll need to assume you know all the points of interest on the dive and make allowance for the time you will spend at each. Note that on more complicated dives you may get lost and you would also need to factor in an allowance for time needed to backtrack.
5. Determine the air consumed for the individual leg of the dive by multiplying your air consumption rate by the time at that depth. Then correct the volume calculated to a reference depth. I'll let people like Bob and TS&M explain in more detail how they will do this using simple primary school mental arithmetic.
6. Do 4-5 for each leg of the dive. In the case of this example there are three legs with two being the same.
7. Sum the air consumed for each leg of the dive.
8. Calculate the amount of air your cylinder holds at the required reference pressure and compare that with your estimated air requirement.
9. If the available air is less than your required air adjust change your dive plan to minimise the air needed. Iterate until the available and required air is approximately equal.
So who is going first? Bob or TS&M?
It's not that complicated. I thought you said you read my article ... clearly you didn't understand it. Perhaps that's my fault for not making it clear enough ... but then, given the hundreds of other people who have read and understood it over the past decade, perhaps not.
Here's how my AOW students do the exercise. I teach them to use "scuba math" ... which is simple rounding off to simplify the arithmetic.
1. You research the site and make a plan that estimates the profile you're planning to do ... i.e. where you're going, how long you expect it to take to get you there, how long you're planning to be there, and how long it's going to take you to get back. So for the site I described above, 15 minutes to get to a depth of 30 meters, 8 minutes at 30 meters, 15 minutes to get back up from 30 meters. That's 30 minutes at an average depth of 15 meters (2.5 ATA) and 8 minutes at 30 meters (4 ATA).
2. You should know what your typical consumption rate is. Let's say with the tank you're using it's 25 psi per minute SAC. If you're expecting current or some other condition that's likely going to cause you to work harder, kick it up to 30 psi per minute. But on this particular dive you'd only do that if you were diving in the middle of a large tidal exchange. So let's say you planned your dive around a period of small exchange and go with the 25 psi per minute.
Leg 1 - 15 minutes at 25 psi per minute = 375 psi, normalized for pressure is x 2.5 = 940 psi ... since we're rounding, let's say 1000 psi ... that gives you a bit of a cushion for the unexpected.
Leg 2 - 8 minutes at 25 psi per minute = 200 psi, normalized for pressure is x 4 = 800 psi
Leg 3 - same as leg 1 = 1000 psi
You should plan a minimum of 2800 psi in your tank for that dive. Adding in a desired reserve, you'd want a fill of at least 3300. Less than that, you should plan a less aggressive dive.
Pretty simple ... anyone with a fourth grade education can do that much..
How do you know SAC rate? You measure it ... in class we do this on a given dive by tracking starting and ending pressures on timed portions of the dive at specific depths ... and we do so under varying conditions ... one timed segment while kicking hard, one while moving slowly ... to give you the upper and lower ranges for your specific consumption rate. Students use their actual consumption under those conditions to calculate their "city miles and highway miles" ... to continue the car analogy I gave previously. By researching the site, you get some idea of what conditions to expect, and choose a point on that continuum that you'd expect to be applicable for that particular dive. If you expect rougher conditions, choose a higher SAC number.
You don't need to know how much air your cylinder holds at a specific pressure. If you're using a different size tank than the one you used to measure your consumption rate you only need to know now much volume your cylinder holds per psi (or per bar if you're using metric). This is determined by dividing the volume of your cylinder by the rated pressure ... for example, a high pressure 100 would be 100/3500 or 0.028 cubic feet per psi ... or for practical purposes, 2.8 CF per 100 psi. For an AL80 it's 77.4/3000 or 2.6 CF per 100 psi. So if you made your measurements on an AL80 and are going to be using an HP100, you'd want to normalize ... which you can do by simple division ... 2.6/2.8 = 0.93 or 93% ... which I wouldn't even bother with since it's close enough for roundoff error, and leaving it alone just gives you a bit more reserve if all goes right. If it was the other way, going from larger cylinder to smaller, I'd add 10% to your measured SAC which would more than cover the difference ... and adding 10% to any calculation should be something any reasonably intelligent adult should be able to do in their head ... in the example above it would be 2800 + 280 which would be 3080 psi needed when using the smaller tank (which should be sufficient clues to tell you that you can't do that dive on that tank ... WHICH IS REALLY RATHER THE POINT!)
Now, to your example with the two tech divers ... they would not be making these conversions at the dive site ... this is part of the dive plan that's known well in advance, and they can show up at the dive site with this information readily available ... all they have to do is plug the numbers into the dive profile and perform steps 1-2 above. Simple arithmetic ... those people would be doing that in their heads, and taking probably a minute to do it.
On the other hand, if you're too lazy to bother making a plan, and following the simple steps you supposedly were taught in your OW training, then I can't help you. Go jump in, paddle about until it's time to abort your dive, and enjoy yourself. Just please ... for your own sake ... keep your dive profiles simple enough so that you can abort the dive pretty much whenever you want to without getting yourself in trouble and you should be fine. Leave the more challenging dives to people who have enough knowledge and self-discipline to plan them properly ...
... Bob (Grateful Diver)
---------- Post added November 29th, 2013 at 05:31 AM ----------
Why not just look at your pressure gauge?
... and what will you do if you're at 30 meters and your pressure gauge says you don't have enough gas to make it to your exit point? In your case, I suppose you'd just surface ... which will work in most cases. In some, it won't ... for example, if you surface in a boating lane or a kelp bed or a current that will prevent you from surface swimming back to your boat or point of exit.
What do you do then? Looking at your pressure gauge won't help you resolve the problem you just got yourself into.
... Bob (Grateful Diver)
