Air Consumption

[CD_in_Chitown]

Still with me?

Next you need to move the 1760 litres to the surface. You do that with 1760/1.55. Tht's the amount of gas used divided by the ata's. That gives 1135.48-something so I just said 1136.

Bells ringing, lights coming on....

personally, my problems in math are monumental...

Now I understand how you did it. I can't do it in my head in imperial or metric though.

 

[cfsdiver18]

Oh......hmmmm.....ok. I'll try.

Well, you know how to work out that 18ft is 1.55ATA, right.

In metric, 18 feet is 5.5 meters. You get 1 ata for being on Earth, you get 1 more ata for every 10 metres so 5.5 meters is .55 of an ata, which makes 1.55 ata.

So, what is 27 metres? That's 1 ata for Earth plus 2 for 20 metres plus .7 for the extra 7 metres.... or 3.7 ata.

That's what makes working these things out so much nicer in metric. In Imperial you get 1ata for every 33ft. 27 metres is 89 ft. By the time you've worked out the ata for 89ft I could have done 10 more metric examples....especially if you had to do it in your head like I just did....

So back to our example. an 80 cf tank has a liquid volume of 11 litres (I just happened to knwo this without looking it up or I would have had to look it up). ie. you could pour 11 litres of water into the tank before it was full. The working pressure is usually about 200 bar so the volume of air in the tank is 200*11 or 2200. That's the total number of litres of air in the tank at 200 bar (ie. full). This is how most of the world works out the amount of gas in a tank. Most everywhere tanks are also called 10 litre, 12 litres, 15 litres etc etc. In other words, they're measured by the volume of liquid they would hold and not the gas volume at it's working pressure.....

So now we konw that the ata was 1.55 and the tank had 2200 litres of air in it when it was full.

The next step is to take off what he didn't use. That was 600psi. In this case I used 3000psi/600psi to get the 20% and then used 0.8*2200 (= 1760) to find out how much air he used for the dive. 100% - 20% = 80% ergo 0.8*2200....

Still with me?

Next you need to move the 1760 litres to the surface. You do that with 1760/1.55. Tht's the amount of gas used divided by the ata's. That gives 1135.48-something so I just said 1136.

then you divide 1136/55 to get the liters per minute, which was 20.65

so assuming we did the whole thing in metric then you know this:

volume (v)- 11 litre tank
pressure (p)- 200 bar full pressure
reserve (r)- 20% (or 40 bar) left over
depth (d)- 5.5 metres average depth
time (t)- 55 minutes

so the equation is (((1-0.2)*(11*200))/(1+0.55))/55

or to make it look official (((1-r)*(v*p))/(1+(d/10))/t

personally, my problems in math are monumental so I just do it like this:

work out average ata
work out litres used

SAC (actually RMV) is litresused/ata/time

R..

Man I thought when I left school I wouldn't have to do math like that any more. My head hurts just reading that stuff let alone having to figure it out for someone.