Methods of measuring basic gas consumption for recreational divers (1) SAC Rate.

[Packhorse]

I always thought it way pretty average too untill I started seeing people post SACs of .4 and even .3 or less.
Of course I d like to get mine lower but not to low if you know what I mean.

 

[TheHobster]

if anyone is interested I have built a spreadsheet for this; also includes a time at depth calculation - email me at hphobbie@ameritech.net and I can reply and I attach it - all suggestions gladly accepted

it is based on taking average depth of a dive; time and air consumption - then you can calculate SAC based on the tank being used, then can calcualate trime at depth on whatever tank you are using

 

[cummings66]

I always thought it way pretty average too untill I started seeing people post SACs of .4 and even .3 or less.

I've wondered if some of those really low rates are their best and not the normal rate? IE I can do a .35 during the warmer season in our local lakes, but during the Winter when our temps outside are cold enough to spit snow and ice and the water bottom temps are around the 40's my rates are higher. Typically my best in the Winter so far is .45 with an average of .7. Summer my best as said before is .35 with an average of .6.

My best rates are solo diving, the higher rates reflect diving with buddies. Most divers tend to move quickly through the water which uses more air, and of course I keep up since I'm the buddy which means I use more air. I love my predive brief, I'll say I like to move slowly so stick by my side. We hop in and away they go like a torpedo, darting from one site to the next.

Trust me, even with a .6 or .7 you'll be better then most of the divers I've seen. That's not air-hog rates in my book.

 

[grazie42]

Originally posted by Bill51
My predive rule of thumb is also 1.0 CFM for general dives, but I bump it up to 1.2 CFM if I’m going to be diving a current or with a camera, and 1.5 if I’m going to be combining multiple heavy air consuming activities. That has always been very conservative for me, and I haven’t run out of air for years. I wondered if anyone else adjusted their SAC.

If I have the conversions right (please correct me if I´m not) 1,5 cfm is a consumtion of 43 L/min @ surface(!). I guess we all have different SACs and that´s all well and good but regardless of what your consumtion is, if conditions on a dive are bad enough that it´s 50% higher, I know I won´t be enjoying myself. I´d rather abort or change some parameter of the dive than keep diving because I know I "have enough gas"...

I add a padding of 15-30% (depending on the dive) to my actual SAC for planning dives. To me, it´s a warning signal if I notice that my actual consumtion is close to planned consumtion during a dive, and I´ll do a "mental check" to determine the cause of the elevation (unless it´s something obvious like current). A greater change than that would indicate to me that it´s time to change the plan because you´re doing a dive that you´re obviously not comfortable with...

ymmv

 

[Rick Murchison]

If I have the conversions right (please correct me if I´m not) thats a consumtion of 43 L/min @ surface(!).

1 CF = 28.3 L (approximately)
Here's a pretty good site for your back pocket
Rick

 

[grazie42]

I was pretty darn close then, I was using the 1,5cf figure posted by Bill51 (edited original post to reflect this)

P.S Thanks for the link...imperial measurements are like greek to me...I´ve managed to learn that a 80cft tank is a 11 L tank (a size that is only used for stages here) and thats about it...

 

[Packhorse]

P.S Thanks for the link...imperial measurements are like greek to me...I´ve managed to learn that a 80cft tank is a 11 L tank (a size that is only used for stages here) and thats about it...

That all depends at what pressure rating the 80 cf.

a 10l at 230bar = 81cf



http://www.caralarms.co.nz/Air consumtion at depth.xls

Here is a excell spreadsheet that I made that calculated bottom time for 1 or 2 divers with adjustable tank sizes and SAC.
Its in Metric.