Sac rate= PSI per minute

[kenbert61]

Hi everyone first I would like to thank all the wonderful divers on these forums.I quite often read thru the subjects and learn what I can.
Kinda of a lazy question here...I have been diving mostly off the wonderful coast of NC and plan my second tank off my suunto computer on the boat, but i don't like to log my dives till that nite when I can plug my info into my laptop and carefully go over the days dives.
My computer gives me PSI per minute example the last 2 dives were 71 psi per minute and 59 psi per minute is there a quick formula to know my sac rate from these numbers or should I just stay old school with what I was taught...thanks in advance.

 

[Blackwood]

My computer gives me PSI per minute example the last 2 dives were 71 psi per minute and 59 psi per minute is there a quick formula to know my sac rate from these numbers or should I just stay old school with what I was taught...thanks in advance

Yes, there is. PSI/minute * 1.

:mooner:

In all seriousness, do you often switch cylinders? If not, PSI/min is perfectly fine to track. Otherwise, convert to volume. What cylinder do you use?


[(Pressure consumed)/(dive time*average ambient pressure)] * (rated volume/rated pressure).

What's in the square brackets can be replaced by the numbers your suunto is giving you (I assume it's compensating for depth).

Examples could be:

[2000 PSI / (30 minutes * 3 ATA)] * (100 CF / 3442 PSI) = .65 CF/min/ATA

or

[140 BAR / 30 minutes * 3 BARA)] * (15 L / 1 BAR) = 23 L/min/BARA

 

[iztok]

Hmmm.. PSI/min depends on the depth. Deeper you go, higher the number.

 

[DA Aquamaster]

He's referring to an average gas consumption for the dive of 71 psi per minute and 59 psi per minute.

Of course, it is pretty meaningless without comparing average gas use to average depth.

 

[kenbert61]

Yes I was referring to avg gas consumption as a new diver I am working to bring down my SAC rate I didn't know about the depth, because my Air consumption is much lower at deeper depth in the Ocean then lesser depth in the quarry...I am sure because I feel much more comfortable in the warm blue ocean then the cold quarry.

 

[TSandM]

It's important to know if your computer is normalizing the consumption rate to the surface. If it is, you can compare values from dive to dive. If it isn't, you need the average depth of your dive to get a number that you can compare from one dive to the next.

If you are always diving the same cylinders, and simply want to know what the trend in your gas consumption is, using psi/min is fine. If you use different cylinders, or want to compare with other people, then converting to cubic feet/min is a more useful number. To do that, you not only need to normalize to the surface, but you need to know the working pressure and volume capacity of your tank.

 

[kenbert61]

so far I have just been diving alum 8o's

 

[Blackwood]

so far I have just been diving alum 8o's

What are you asking in this thread?

How to calculate SAC rate from scratch?
How to use the number your computer gives you to plan dives?
How to relate the number your computer gives you to volume?

 

[fdog]

Just a quick look at the numbers, since the OP indicates it's an Al 80:

2.5 x 0.71 = 1.8 cu. ft. per minute

This looks like the computer is reporting psi/minute from depth, so, as Lynne indicates, you'll need to find the average depth to convert this to something comparable from dive to dive.


All the best, James

 

[MXGratefulDiver]

Hi everyone first I would like to thank all the wonderful divers on these forums.I quite often read thru the subjects and learn what I can.
Kinda of a lazy question here...I have been diving mostly off the wonderful coast of NC and plan my second tank off my suunto computer on the boat, but i don't like to log my dives till that nite when I can plug my info into my laptop and carefully go over the days dives.
My computer gives me PSI per minute example the last 2 dives were 71 psi per minute and 59 psi per minute is there a quick formula to know my sac rate from these numbers or should I just stay old school with what I was taught...thanks in advance.

The simple answer to the above bolded question is yes ... but you need to know the average depth calculated over the course of your dive. Check to see if your computer provides that information ... many do.

The formula converts average depth to pressure, in atmospheres absolute (ATA) ...

For ocean diving, the formula is

P = depth/33 + 1

For your quarry dive it's

P = depth/34 + 1

Once you've determined P (the pressure at your average depth), you divide the gas consumption numbers you provided in your original post by P ... that gives you your surface consumption rate ... which many refer to as SAC.

To more fully understand how it all works, there's an article on my website that you may find informative ...

... Bob (Grateful Diver)