Air Consumption

[CD_in_Chitown]

Avg Depth - (18FSW divide by 33) + 1 = ATA of 1.54
PSI Used / dive time (2300 / 55mins) / ATA (1.54) = 27 PSI/Minute This is your SAC (Surface Air Consumption)
SAC 27 divided by (Tank PSI/ capacity) 27/ (3000 / 80) = RMV .72 cubic ft per min

Not bad for the eighth dive, not bad for any dive chasing bugs. I'm usually pushing 7 when I go crabbing, nice relaxing dives get into the .4s and warm water relaxing dives drop me into the .35 range. I still use .6 as a planning RMV though, you never know what might happen.

 

[BlueWaterMadness]

Three keepers. Quite a few escapees. Tons of shorties.

 

[dumpsterDiver]

I think that is a good breathing rate.. I used to do similar dives and to extend my bottom time, I would ascend to the surface and breath off the snorkel anytime I started to get a little winded from chasing a lobster or being excited. Rising to the surface from 15-20 feet and floating there for 2 minutes for 2 or 3 times per dive used to extend my bottom time. If you can see the bottom, going to the surface will not mess up your dive plan usually. I used to dive alone mostly so buddy communication was not a problem.

 

[Diver0001]

I was Lobstering that dive. Using an AL80. What exactly is the XALER method? I will have to try the weighted breathing routine. Thanks for the advive. STAY WET.

Well, given what you were doing it's not that bad. It's much more important to develop a feeling for how fast your tank is empty and/or to be able to plan for it than it is to achieve a lower SAC rate.

That said, I calculate your SAC at 26 litres per min. Assuming you're basically normal with some practice you'll be able to reduce that to about 12, perhaps a little less. The key is to learn how to relax your muscles. The more tension you keep in your body, and the faster you try to do everything, the faster you'll go through your air.

Take a couple of dives to make sure that your weighting and trim are dailed right in and then dive very slowly and concentrate on letting go of the tension in your muscles. That's a start

Also, breathe from your belly, like a musician. That will help and it will give you better ventilation.

BTW, "XALER" is just "RELAX" spelled backwards.

R..

 

[Fionab]

I don't understand PSI but a 55min dive so early on in your diving experience doesn't sound too bad to me.

My advise would be don't worry about it, it all comes with experience. I have found that it doesn't matter which forum I log on to lots of people seem to be preoccupied with breathing rates which seem to turn in to a competition. I don't know what mine is and I don't care, I know that on every dive I have enough carry out the plan and to enjoy myself then return to the surface with some to spare. :lol2:

 

[rmannix]

for your 8th dive that's not a bad SAC rate at all, especially with the long transit swim.
Just keep diving and it will get better.

 

[CD_in_Chitown]

I calculate your SAC at 26 litres per min.

How does that compare to .72 cu ft/min? (And isn't that RMV, I thought SAC was the surface air consumption or PSI/MIN measure?)

When are all the rest of you blokes gonna get onboard with the Imperial system? :14:

And for one last math lesson on air consumption

If BlueWaterMadness wanted to drain that tank 80 * .72 = 111.111111 mins total possible dive time (no reserve, I mean empty tank here)

 

[Diver0001]

How does that compare to .72 cu ft/min? (And isn't that RMV, I thought SAC was the surface air consumption or PSI/MIN measure?)

When are all the rest of you blokes gonna get onboard with the Imperial system? :14:

And for one last math lesson on air consumption

If BlueWaterMadness wanted to drain that tank 80 * .72 = 111.111111 mins total possible dive time (no reserve, I mean empty tank here)

Ok, let's make sure I got this right.

18ft is about 5.5 metres

that makes about 1.55 ata

an 80cf tank is about 2200 litres

He had 600psi left, which is about 20% of 3000. 20% of 2200 is 440 litres

which means he made the dive with 1760 liters.

At 1.55 ata that's like breathing about 1136 liters on the surface in 55 minutes, which is .... 20.65.

Ok. I made a mistake somewhere. 20.65 litres when converted to cubic feet is about .73. Good enough for our purposes.

In fact, I think the only way to calculate for diving is in metric. You can pretty much do it in your head if you don't have to keep converting from that archaic system which is all based on how big a particular person's thumb happened to be.....

R..

 

[CD_in_Chitown]

At 1.55 ata that's like breathing about 1136 liters on the surface in 55 minutes, which is .... 20.65.

I don't follow the math used to determine this surface equivalent, can you provide the equation. (Forgive the stupid imperialistic pseudo-mathmetician )

Ok. I made a mistake somewhere. 20.65 litres when converted to cubic feet is about .73. Good enough for our purposes.

Not wildly inconsistent with my math so I'm not sure on the mistake, I'm just not clear how you did it.

In fact, I think the only way to calculate for diving is in metric. You can pretty much do it in your head if you don't have to keep converting from that archaic system which is all based on how big a particular person's thumb happened to be.....

Blasphemer!

 

[Diver0001]

I don't follow the math used to determine this surface equivalent, can you provide the equation. (Forgive the stupid imperialistic pseudo-mathmetician )

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..