Air Consumption Index - Is there a formula ?

[ECR]

shark_tamer --

Sorry to be harsh, but you aren't listening. String has told you about five times that you have to calculate surface air consumption (SAC) rate, and once very clearly how to do it. This ACI thing you've been hung up on since your original post is -- one last time -- useless because it completely ignores average depth.

You go back and forth between ignoring that crucial variable, and wanting to include every number under the sun -- EAN mix, temperature, and what-all that you've been told have nothing to do with it.

SAC, man, SAC. You can use it to measure your own progress and compare your air consumption to others. Maybe you can even "win" a dive some day.

--ECR

 

[Packhorse]

Once you have worked out your SAC you can put it to use planning your dives.
Bar used X WC (tank capacity) / depth in ata / minutes gives you SAC
turn that around to get air needed for a dive
SAC X depth in ATA X minutes = litres required. (plus reserve)
I have made a speadsheet that allows you to calculate gas required for diffrent depths and SACs for 1 or 2 divers. It also has a temperature vs pressure converter on it.
It can be found here,
http://www.caralarms.co.nz/Air consumtion at depth.xls

As mentioned above SAC is all you need (or SCR RVM as somepeole call it)

 

[bperrybap]

Do a search on SB or Google for "SAC rate" or "SAC
rate calculation", you will find plenty of good reading.

Edit:- just had a thought. Although a change in temperature will affect the pressure of a tank assuming it remains constant throughout a dive then surely it can be factored out as a constant therefore will have NO effect on a dive?

Temperature change can affect the SAC calculation for a given dive.

The change isn't really the actual water temperature so much
as the difference between the initial temperature of the tank
and the water.

The reason that I stated that most folks don't account for it
(I don't) is that normally it is a non issue since its impact is so
small. There are conditions that it can create some
error in your SAC calculation. However, even under those
extreme cases I would not expect to see more than 5-6%.
Normally it is simply not worth worrying about because there
are so many other factors that affect your SAC rate in a much
larger way.

The worst case scenario for creating a SAC calculation error
due to temperature is when there is a big temperature differential
between your tank and the water prior to getting into the water.
This could happen if you had your tanks sitting in the hot sun
or just had them filled on a liveaboard and were diving into very
cold water. The hot tank would cause the air temperature inside
the tank to rise which would cause the SPG or computer to indicate
a higher pressure in the tank which will be used as the
starting pressure if you started to descend immediately
upon entry into the water. As soon as the tank is in the
water for a few minutes it will cool back down and there
will be pressure reduction due to the temperature drop.
This pressure drop due to temperature change,
if not accounted for, will look like a drop due to air breathed
and cause the SAC to be higher than it really was.
While all this is related to physics and can be accounted for,
its not really worth worry about as the error induced is
normally so small.
Just like nobody really worries about the gas used
to inflate your BC during the dive.

For calculation of SAC rates and comparison of SAC rates
on different dives, the actual temperature of the
water you are diving in does not matter. Water temperature
can affect how your body uses air if it makes you feel hot
or cold but it will not affect how you calculate the SAC rate.


--- bill

 

[bperrybap]

**Copied from my log book, dives 15 to 25 done in Cayo Coco, and dives 1 to 9 done in Cayo Largo, both Caribbean dive trips ... roughly same water temp, for basic starting point.

Am I making sense or crunching numbers for absolutly nothing !!!

Making sense, but not calculating anything all that useful.

In order to have a meaningful number than can always be
compared to all your other dives and even dives of other divers,
the number should be something that doesn't compute
differently depending on tank size, dive depth, or dive profile.

None of the ways you have calculated your ACI do this.

SAC rate does do this.
It factors out tank size and dive profile.

The SAC rate calculation does this by doing its calculations
based on volumes and converting that volume to an equivalent
volume as if it were breathed on the surface based on an
average depth.


The reason that SAC is calculated in volume rather than
pressure (BAR) is so that you can compare SAC rates
with different tank sizes.
For example, 200 BAR on a 15 liter tank is alot more air
than 200 BAR on a 11 liter tank.
If you calculate BAR/Min then you could only compare rates
between tanks of the exact same size.
3 BAR/Min rate calculated on a 15 liter tank could not be
compared to 3 BAR/min rate on a 11 liter tank.
While the BAR/min value is the same, the amount of air
being used per minute is not.
By using volume rather than pressure, you are comparing
the amount of air molecules breathed rather than a pressure
drop so the value can be compared regardless of tank size
used for the actual dive.

The reason that SAC converts the volume based on an
equivalent volume at the surface based on an average depth
is to remove the effects of pressure/depth from the comparison.
It allows you to compare your air consumption as if you
never left the surface. It allows direct comparison of SAC
rates regardless of maximum depth or dive profile.

For example, when you take a breath at 20 meters it uses
more air from the tank than when you take a breath at 10 meters.
However, if both breaths were exactly the same size, your
air consumption was still the same.
Carrying that further, if half your dive was 10 meters
and half your dive was at 20 meters, then your average depth
was 15 meters. By using this average depth, your air consumption
was equivalent to if you had done that dive the entire dive time
at 15 meters rather half at 10 and half at 20.
You cannot use a maximum depth because you didn't spend
100% of your dive at that maximum depth. You also cannot
use half your maximum depth as an "Average depth" because
it is highly unlikely that your real dive depth profile averaged
half your maximum depth. You must use a real average depth.
Without a dive computer, it is very difficult to come up with
an accurate average depth unless the dive was a very square
profile.


Carrying SAC & average depth 1 step further, the
average depth is used to convert the volume of air used to an
equivalent volume of air used had the entire dive taken place at
the surface.
This is done so that the final SAC rate can be compared to
any other SAC rate from any other dive or diver regardless of the
depth or dive profile for the dive.

By using SAC rates, divers can compare their values from
any dive to any other dive of their own or other divers
because tank sizes and dive profiles have been factored out.

SAC rate is definitely the way to go.
It is the way all the other Scuba people are calculating and
tracking their air consumption.
It is also needed in order to be able to do gas planning
i.e. calculate how long your tank would last at a given depth.


--- bill

 

[MikeFerrara]

If I'm understanding what the OP is looking for...lets look

A cold diver will consume more air. However a diver who is diving in cold water may or may not actually BE cold. So...we can't directly put temperature into a sac or RMV equation.

However, if we wanted to go through the trouble, we could attempt to statistacally correlate water temp with sac/RMV. We could also add as many other variable to our analysis as we would like.

The easiest way to start would probably be to use the linear regression analysis function provided in most spread sheets. The output will be an equation that allows you to predict based on temperature along with the correlation factor (how well it correlates), standard error of the estemate and so on.

If the correlation factor is low, it means that the change in temperature alone isn't explaining the variation.

The relationship may not be linear but there are mathematical ways of handling that also. You can use multiple linear or non-linear annalysis tools to correlate to as many variables as you would like.

The point here for the original poster is that we can't give you the equation...because it is the statistical analysis that determines the equation and tells us how useful it is in making our prediction.

ok, clear as mud right? Sorry, it's a lot easier to do than it is to explain. RMV in one colum, temp in another and press the go button.

What next...Do some reading on the statistical functions in your spread sheet, brush up in an old statistics text book, maybe do some online searches on correlation methods...or buy a good statistics package (big bucks).

 

[miketsp]

..snip..
A cold diver will consume more air. However a diver who is diving in cold water may or may not actually BE cold. So...we can't directly put temperature into a sac or RMV equation.
..snip..

Absolutely true. I just spent 2 weeks diving 29C water with minimal exposure protection and on the 2nd or 3rd dive of each day I felt much colder than when I habitually dive 15C water with my 7mm semi-dry.

Even so my SACs were excellent because this "warm-water" diving was mainly wreck diving as opposed to my habitual coasting or reef diving where I would be finning constantly throughout the dive.