Air Consumption Index - Is there a formula ?

[shark_tamer]

I am in the process of making my own log book on my computer.

I am planning on adding a graph showing my "average" air consumption, showing where I'm improving or not.

I would like something more precise that just a simple formula

Air consumed / Dive time = BARS per minutes.​

A formula that would takes into consideration ( if possible !! ) depth, water temperature, EANx, tank size, etc....

Something that would show that I consume more air in cold water, on deeper diver, EANx21 ( ? ) vs EANx32, even the dives on wich I shared air with a OOA diver . ( That would show as a spike on the curve !!! )

Is there such a formula ?

My little brain is not able to conceive such a formula with so many variables .... or is it not feasable ?

 

[H2Andy]

yes

on every dive, calculate your actual SAC rate, and note the dive conditions

then compare your SAC rates from one dive to another under similar conditions

am i missing your question?

 

[String]

IF metric which it seems you are its easy.

You need a few things from the dive, tank size (litres), starting pressure (bar), ending pressure (bar), dive time (mins), average depth (m).

Breaking this into easy steps (you can just bypass and put into one forumla after understanding)

First, work out how many litres of gas the tank holds.

So Tank capacity x starting pressure.

Now work out how many at end of dive.

Tank capacity x ending pressure

Tank the first from second and thats how many litres of gas were used on that dive.

Now you can work out a per/minute rate

Simply divide this figure by the dive time.

The only thing left to do is convert this to a surface rate.
This is done by converting the depth to atmospheres which in metric is easy as 1atmosphere = 10m to all intents and purposes.

so pressure per minute / (depth in meters / 10) + 1)

That should give you your Surface Air Consumption average for the dive.


Lets try an example.

You have a 12l tank with 200 bar starting. You exit with 50 bar.

Dive time was 45 minutes, average depth, 15m

Starting gas = 12 x 200 = 2400l
Ending gas = 12 x 50 = 600l

Therefore you used (2400-600) litres of gas == 1800l

Now to calculate per minute:

1800/45mins = 40 litres per minute

Convert to surface usage using the averge depth:-

40 / (15/10)+1 = 16 litres/minute

That is the figure you need to use for all the dives normalised for surface consumption.

You can then compare your dives in warm/cold/more weight/less weight and so on to see what effect they have.

Generally you'd expect to see consumption increase if cold (note, this isnt "cold water" as exposure protection comes into this), you shouldnt see any difference with air vs ean, it may go up with a bigger tank due to extra weight, it may not and various other things.

For a ball park figures, "Most" divers use between 10 and 20l a minute. Beginners are told to plan for 25 in case of strenuous conditions on the dive.

 

[shark_tamer]

Something more specific ...

Examples:

Dive # 1

Dive time :50 min
Depth : 25 meters
Water temp :25oC
EANx : 21%

ACI: 1.2


Dive # 2

Dive time :35 min
Depth : 35 meters
Water temp :28oC
EANx : 32%

ACI: 1.3


Dive # 3

Dive time :85 min
Depth : 20 metres
Water temp :26oC
EANx : 32%

ACI: 1.1

Why is my ACI ( Air Consumption Index ) better in dive #3 compare to the 2 others.

Then after 100-200 dives, compare where I do better and where I do bad ..

 

[String]

Without average depth all of those figures are meaningless and utterly worthless -you could have been at 35m for 1 minute or 20 mins for example which would have a massive massive difference in consumption. Water temperature as such doesnt count for anything either as its if you feel cold more than anything else.

Without an average depth you really cant work anything at all with any of the figures or logs.

The only proper way is to calculate SAC as above and then compare it from one dive to another. I guess after 100-200 or so sample points you could normalise the scale to make it a ratio like you've done above but it wouldnt be very useful as there are so many variables that affect it and multiple things on one dive.

 

[Blackwood]

To the OP: There is no reason to involve the gas mix.

IF metric which it seems you are its easy..

It's easy regardless of the unit system.

 

[bperrybap]

Yes there is.
The first thing is to read up on SAC (Surface Air Consumption) rate
and how to calculate it.

To be able to compare your air consumptions you need to compare
against something that will not vary when things like depth or tank
sizes change. SAC rates are computed in volume rather than
pressure to ensure that tanksize is not a factor. SAC rate
also converts the volume of air consumed to an equivalent volume
at the surface based on an average depth of the dive. This
ensures that the depth of the dive is no longer a factor for
comparison.

For metric you will get a SAC rate in Liters/Min or in
Imperial you will get rates in CubicFeet/Min.

As far a temperature goes you can correct for that but
most people don't. The correction is based on a ratio
between the two temperatures as measured from absolute zero.

One thing you will need to have in order to calculate
a SAC rate is your average depth.

Unfortunately, it takes a dive computer to be able to accurately
be able to calculate an average depth for all but a very square profile.
Also, it is usually not the dive computer that calculates the
average depth. Most require a PC/MAC to download the data
and then it is calculated by the PC/MAC s/w.

Hope that helps.
--- bill

 

[H2Andy]

you will find that your air consumption fluctuates based on your fitness that day, how much effort you had to excert during the dive, etc.

what you call your air consumption index is just your SAC rate. the other variables are just not quantifiable in a meaningful way

first, figure your tank's capacity in cubic feet and its working (rated) pressure

let's take an 85cf tank rated at 2640 psi. divide the cf by the rated psi and you get the cf/psi contents of the tank, in this case .00322 cubic feet per psi. that means that each psi in your tank will contain .0322 cubic feet of air

now you can calculate how much actual air you use in a dive. for example, if you used 2000 psi in a dive, you multiply 2000 * .0322 and you will see you consumed 64.4 cubic feet of air that dive

then, by knowing your total dive time and average depth, you can find your consumption rate in cubic feet.

say you did a dive for 20 minutes and your average depth was 40 feet (my computer tells me the average depth for each dive). you can find your actual gas consumtption rate by doing the following:

(cubic feet used / dive time) / (Average Depth/33) +1

so... plug in the numbers:

(64.4 / 20) / (40/33) + 1

=

3.22 / 2.21

=

1.46 cubic feet per minute

if you calculate that for every dive, you can get a sense of how much "real gas" you are using from dive to dive and see if this goes up or down

generally, anything under 1.0 is tolerable, anything under .6 is ok, anything under .4 is really fragging good. of course, this varies from diver to diver, and it's not a competition.

 

[rakkis]

dude.. PLEASE use the search function. this has been discussed to death. the last time was the month of dec when we had about 10 threads on it at least

 

[String]

As far a temperature goes you can correct for that but
most people don't. The correction is based on a ratio
between the two temperatures as measured from absolute zero.

But the change in pressure due to the temperature difference is masked by orders of magnitude the effect of a divers exposure protection and whether he's cold or not.

Working it out the long way on a full tank 25c - 28c caused a change in tank pressure of about 2 bar, approx 1%.

Thats less than a minutes gas even when rounding.

Diver feeling cold or hot or exerted can affect by at least 10x this rate.
Theres also the fact that often tanks are still slightly warm after a fill so cooling plus ambient AIR temperature will affect the starting pressure before the tank gets wet too. Ie 35c air into 10c water will cause a large drop, 10c air into 10c water wont etc. Again this is likely to be far more than the water temperature only so mask any real effect.

Unfortunately, it takes a dive computer to be able to accurately
be able to calculate an average depth for all but a very square profile.
Also, it is usually not the dive computer that calculates the
average depth. Most require a PC/MAC to download the data
and then it is calculated by the PC/MAC s/w.

Certainly all the suunto range calculates on the computer itself and doesnt require download.



(i answered in metric as the original poster asked using metric figures - if he'd have been imperial id have left for someone else as i dont want to learn imperial )