While we are all stuck at home and we are unable to go Scuba Diving in Pattaya, I thought I would help you out with a little maths for divers.
Maths is an absolute science and there are a few sums that will help you as a diver.
· How much air do you have in your cylinder?
· How long will my air last?
· How much air do I need to have a third left at the end of the dive?
So let’s look at how you do them.
How much air do you have in your cylinder?
Well here at Dive Station Pattaya we use 12litre tanks usually to 200 bar there for you have 2400 litres of air; if you get one of the big 15litre tanks but only 180bar you have 2700 litres of air.
To calculate this it’s a simple sum,
cylinder size in litres, multiplied by the pressure in bar e.g.
12l X 200bar = 2,400 litres
15 X 220 = 3,300 litres (you get one this full please remember to come back for lunch!)
If you’re using twins then it’s just a matter of doubling the tank volume
12 X 2 = 24 X 210 = 5,040 litres
Test yourself, work out the following:
How many litres in of air in a 12 filled to 190 bar?
Your cylinder is filled to 210 bar and it is a 15 litre?
How long will my air last?
This is a bit more complicated, as we have to know how much air we use before we can work out how long it will last!
You have probably heard other divers talking about their SAC rate (usually those strange techie guys). What they are talking about is how much air would they be using at the surface, or their Surface Air Consumption (SAC) rate. We all learnt in our open water training that the deeper we are diving the more air we use, so we need to use something as a control, this why we use Surface Air Comsumption (SAC).
Calculating your SAC rate
First, to find out our SAC rate, the simple way is to spend several hundred pounds on an integrated air dive computer and laptop. This way, the computer does it for you. (Speak to Danny in the shop and he’ll help you buy one!)
If you don’t have an air integrated computer, you can simply take note of your depth (10 metres works great makes the maths simpler) and tank pressure. Then swim around there at 10metres (watch your buoyancy) for ten minutes and check and note your pressure again. You’ll now know how much air in bar that you use at 10 metres, in ten minutes.
For example, if your start pressure was 190 bar and your finish was 170 bar you have used 20 bar. Remember that we need to know the size of your cylinder, as we learnt in the last sum, that size matters!
Litres per minute
To find your SAC rate in litres per minute you need to complete four sums.
First multiply the bar used by the cylinder size.
So, 20 bar multiplied by 12 litres, equals 240 litres.
Second, we work out the pressure Atmosphere at the depth, by taking the depth, adding 10, and dividing that number by 10. (remember from Open water pressure increases by 1atmosphere for every 10metres of water.
So, 10 metres + 10 = 20, divide by 10, equals 2 atmosphere
Thirdly, we then divide the first number by the second.
So, 240 divide by 2 equals 120.
Finally we divide this number (120) by the time in minutes (10).
So, 120 divide by 10, equals 12 litres per minute
Litres per minute = bar used X cylinder capacity litres divide by (depth+10)/10, divided by minutes.
Lets do that again slightly deeper, at 20 metres we use 40 bar, in 10 minutes whilst using in a 15 litre cylinder.
40 X 15, divide by 20 + 10 divide by 10, divided by 10.
40 X 15 = 600
20 + 10 = 30 divide by 10 = 3
600 / 3 = 200
200 / 10 = 20 litres per minute
How long will my cylinder last?
Now you know how to work out your SAC rate you can calculate how long the cylinder will last. If we use 20 litres a minute at the surface, a 15-litre cylinder containing 200 bar would last 150 minutes (15 X 200 / 20). Not a lot of use when diving as we do not want to spend the time on the surface, plus that’s until is completely empty also what we don’t want either.
More realistically we ask; if we want to complete a 45 minute dive at 17 metres, will we have enough air?
Gas needed equals (time in mins X SAC rate) X ((depth+10)/10)
45mins x 20li/m = 900li, multiplied (17metres + 10) 27/10 = 2.7
This means that well will need 2,430 litres of air to complete the dive.
Mmm, looks like a normal 12litre tank isn’t up to the job for that dive.
How much air do I need to have a third left at the end of the dive?
We know from the first sum that a 12 litre cylinder will not be big enough and if you apply the rule of thirds you need to have an extra third of air left at the end of the dive. There is a simple sum to work this out, multiply the air needed by 1.5 so to complete the dive you will need to multiply 2,430 by 1.5, which means you need 3,645 litres. Now unless you dive side mount or twins you’ll need to reduce the dive time or reduce your SAC rate (this option does just happen it’s a combination of practice and fitness).
I hope these sums will come in useful and they can give you something to do while your sitting in the back garden dreaming of heading off to your next exotic dive destination – which I know will be Scuba Diving in Pattaya with us at Dive Station Pattaya.
If these sums have triggered off the techie side of your brain them head over to our technical pages to look at the courses we offer, it’s the ideal time to start the theory while you’re locked down.
