Air Consumption Index - Is there a formula ?

[shark_tamer]

String,

If I introduce EANx and water temp in your formula, it would probable read like this :


( TC X ( SP - EP ) / Dive time ) / ((15/10 ) +1) X EANx X To

where TC = Tank Capacity, SP and EP would be Start and End Pressure.

By introducing EANx, minimal variation between EANx32 and EANx36, and anothe minimal variable To = Temperature.

At the end we have a difference .... whatever it is ?

 

[String]

EANx wont affect anything in the slightest so id suggest removing it from the formula as it doesnt make the slightest bit of difference.

To use temperature in it you need to use the P1V1/T1 = P2V2/T2 gas laws and incorporate that. You always need to remember to use kelvin as a unit so (273+25) for 25c and so on. You cant just shove it in the end like you did. Its the first thing that works out not the last.

You also cant just mutliple by "EanX" - its not a unit of measure or anything so has no function,use or purpose in the formula other than to make all the results incorrect.

To get a fair comparison with temperature you'd have to normalise for a specific temperature. If you insist on that id suggest using 25c/298 purely as its recognised as RTP.

Then factor for say 28c by using 298k to 301k using the above formulae.

This simply converts your starting pressure and ending pressure.

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?

Its 4am and i really dont feel like testing that now but im fairly sure about this. May check tomorrow.

 

[shark_tamer]

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

I did use the Search mechanism ... entered "Air Consumption" and nothing remotly ressembled this amongst the 335 threads !!

.... thread titles that is ???

 

[shark_tamer]

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?

Its 4am and i really dont feel like testing that now but im fairly sure about this. May check tomorrow.

Introducing temperature for me is somewhat of a priority ... Half of my diving is done in the Caribbean, and other half done here in Canada ... Therefore a difference of water temperature of about 10oC to 15oC !!

Sleep tight !!!


Thanks

 

[String]

But its meaningless in terms of a formula like above. The other poster was mentioning the actual physics in changing a tank pressure which if it exists is a tiny amount, below the accuracy tolerance of an spg even.

Factors such as you getting cold on a dive, hot, exertion etc will have a noticable affect but they cant be shoved into a formula like that as its not a straight forward ratio.

What you need to do is:

(i) make sure you know AVERAGE depths for all the dives
(ii) Work out the SAC for all the dives (see above for how)
(iii) compare THAT figure and nothing else between the dives

So you may for example have 17l/min on 15c and 14l/min at 25c.

Then you can say for example you use on average 20% more air at home than carribean.

You cant just make a fit-all formula for it, it has to be done the above way.

Going any further is masked by other factors such as change in exposure protection home vs abroad, changing in weight associated with that, lack of currents and so on. There is not going to be a direct mathmatical link for water temperature vs SAC that holds true for you all the time - far too many other factors involved to dwarf any theoretical mathematical difference.

The best you can take from this is "im the carribean wearing XXXX kit in calm conditions my SAC is roughly YYYY" and "at home wearing AAAAA exposure protection in calm conditions my SAC is roughly BBBBB".

Its useful but can't be computed with a forumula like you've tried to above.

 

[shark_tamer]

Its useful but can't be computed with a forumula like you've tried to above.

:shakehead :shakehead :shakehead

Ok I give up ... Yes, that easily.

 

[Kevrumbo]

Here . . .just resurrected this post in another thread:
http://www.scubaboard.com/showpost.php?p=2364861&postcount=13

 

[String]

:shakehead :shakehead :shakehead

Ok I give up ... Yes, that easily.

Its simple, calculate SACs, compare SACs.

The "index" youre after is the SAC.

Yes, its that simple.

 

[shark_tamer]

I'm back for another stab at it !!!!

Yes I did sleep well last nite ... but when I got to bed, I was still making up potential formulas ???

The formula I ( we, us... ) would like to come up with would be something that calculates my own air consumption and compare it to other divers .... Personally, it would be excellent to improve my ACI from year to year, but how do I kow if I'm an AIR HOG or an average air consumption diver ??

Example: After 10 dives, my ACI was 2.1 ( fictional figure ). After 50 dives, my ACI is 1.7 .. I have improved ( read " Wow I'm good !! " ) but if the dive community averages 0.9ACI, well this leaves room for improvement !!!!

Figure crunching time

**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.

Note that I have multiple ACIs ( #1 tru #4 ) ; ACI#1 & #2 numbers were to large … was looking for a ratio near 1.

I like ACI #4 … dive #1 ACI is 0.08 ( good ) compare to dive #25 ACI 0.25 ( worst )

Nota: Attached file was made with Micorsoft's Excel ... You can actually replace variable if you want to play with it !!


View attachment Air consumption index.xls

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

 

[String]

Youre just ignoring the well established perfectly suitable method.

That is SAC.

Calculate SAC for each dive, compare those. Thats it. Thats all you need to do. It really is that simple.