Help needed please

[Tonybaker1977]

Morning all,i have an exam for ocean diver very soon and one thing im struggling on is cylinders and air working out:o on the interactive learning under GASES UNDER PRESSURE (part 2).Questions 7 to 9,how do i work them out as i aint to sure to be honest,any help would be great,thanks alot :)

[David G]

As Ben says later on, this is Sports Diver stuff not Ocean Diver.

For your Ocean Diver Quiz you just need to remember to 'turn-around' once you have used 1/3 of your air, and ascend at 2/3 used, 1/3 remaining for the ascent and reserve.

But for future reference:

I'll do the first one for you, but then you try the next!

10 litre cylinder with 232bar pressure.

We want to leave 50Bar as reserve so we only have 232-50 = 182 bar to dive with.

If we multiply the 10 litre cylinder capacity by the pressure 182 bar we get the total amount of air in the cylinder if we were to empty it on the surface.

This is 1820 litres of air.

Now we are going to dive at 20m so we need to know the absolute pressure which is 2Bar of water (1 bar for each 10m) plus 1 bar of atmospheric pressure, so absolute pressure is 3 bar.

We know that we use 25 litres of air every minute at the surface so we will use the Surface rate multiplied by the absolute pressure at the depth we are diving to see how much we use at the depth; so we will use 25 * 3 = 75 litres of our free air every minute during this dive.

If we divide this 75 into the 1820 we will get how long the air will last, so 1820/75 = 24.27 minutes.

So I would choose answer D, 24 minutes as my answer.

Hope this helps,

David
FCD and AI; so will need my coat if it is wrong:D

[Dave Bewick]

Happy to help, but I don't have the OD workbook. If it isn't too long could you type out the question?

Edit: David G got there first so I guess I'm redundent!

[Ben Panter]

Hi Tony,

Morning all,i have an exam for ocean diver very soon and one thing im struggling on is cylinders and air working out:o on the interactive learning under GASES UNDER PRESSURE (part 2).Questions 7 to 9,how do i work them out as i aint to sure to be honest,any help would be great,thanks alot :)

For reference, the questions Tony is talking about can be found here:

http://www.bsac.org/learn/quiz/cdquiz6pt2.htm

I think that the material to answer these questions has moved on to Sports Diver, so don't panic! I've just gone through the overheads and I can't find anything to the contrary, can anyone else confirm?

The interactive learning section on the website is undergoing redesign, you'll notice that it refers to a 'club diver' qualification, which no longer exists. If you could do the rest of the questions you've got a good start.

Ben

7. A 10 litre cylinder, WP 232 bar is to be used for a dive to 20m and a reserve of 50 bar has been decided on. Assuming a surface air consumption of 25 l/min, what approximate dive duration can be expected? See page 63 Study Guide

8. A 12 litre cylinder, WP 232 bar is to be used for a dive to 30m and a reserve of 50 bar has been decided on. Assuming a surface air consumption of 25 l/min, what approximate dive duration can be expected? See page 63 Study Guide

9. A diver wishes to remain at 20m for 20 minutes and to have an adequate reserve of air. Assuming an air consumption rate at the surface of 25 l/min, what would be the air requirement? See page 63 Study guide

[PeteM]

Morning all,i have an exam for ocean diver very soon and one thing im struggling on is cylinders and air working out:o on the interactive learning under GASES UNDER PRESSURE (part 2).Questions 7 to 9,how do i work them out as i aint to sure to be honest,any help would be great,thanks alot :)

Question 7: Ten litre cylinder at 232 bar, you want to have 50 bar as reserve. Therefore you can use 182 bar of air (232-50), this is 1820 litres of free air. You have a surface air consumption of 25lpm, at 20metres the pressure is 3 bar therefore at 20m you air consupmtionis 75lpm. 1820 / 75 gives 24 and a bit so your dive time is 24 minutes

Q8: Same logic as above, you have (232-50) * 12 litres of free air = 2184. Pressure at 30m = 4 bar therefore your consumption at depth is 100 lpm. 2184 / 100 = just under 22 minutes

Q9: Similar logic, you are at 20m therefore pressure of 3bar so consumption at depth of 75lpm. You want to be there for 20 minutes so you will need 20 * 75 litres for the dive = 1500. But you need a reserve to make sure you get back safely. With Ocean diver we used to teach a reserve of 25% so you get to the correct answer by dividing 3 and multiplying by 4 (about 2000). Now however we teach leaving a third for ascent, safety stop and reserve so these days a better figure would be 1500 / 3 * 2 = 2250 (the interactive learning needs updating when we have time). This of course means the answer to question 10 should now be C

HTH

Pete

[PeteM]

P.S. And has been pointed out don't get stressed about this - this has been moved to Sports Diver. Read the replies and by all means ask questions to further your knowledge but don't worry as it will not be tested.

[scotpages]

Question 7: Ten litre cylinder at 232 bar, you want to have 50 bar as reserve. Therefore you can use 182 bar of air (232-50), this is 1820 litres of free air. You have a surface air consumption of 25lpm, at 20metres the pressure is 3 bar therefore at 20m you air consupmtionis 75lpm. 1820 / 75 gives 24 and a bit so your dive time is 24 minutes

Pete

Hi
This is exactly what I'd have thought would be a definition of free air.
However on a recent exam this was not an option.

The question was:
A 12 litre cylinder charged to 200 BAR gives how much free air?
From the working above the answer would be 200-50 = 150 BAR of free air, giving a total of 1800 litres.
This was not one of the options!
Can anyone say what the answer would be, and why?

[Gordon]

I would say that becuase they have not specified a reserve that
free air = 12*200=2400L
But there is a thread on here somewhere about instructors being able to alter the marks if you can explain your reasoning and it is valid