SAC Calculation

[RTee]

Question: If your gauge (SPG - P=pressure) only tells you your pressure, what good is knowing your RMV (volume) while underwater? Do you smart folk do the conversion while at depth to determine if you have enough volume remaining? If I know my SCR and multiply that by ATA I know my PSI consumption at depth and can clearly use the SPG for valuable information.
Maybe y'all are just smarter than I am.

Valuable information such as if I may ask??? If I have already established my turn pressure or my ascent trigger (NDL, time, PSI) then once I hit the water I can still use my computer/bottom timer to confirm NDL/time comparaison and my SPG to see where I am compare to where I should be. I go up or turn around (whatever we wanted to do or as part of contingency planning) once I hit the first of those criterias.

In my case, I dive 40 cft @ 3000psi, 80cft @ 3000psi, 80 cft @ 3300psi and 2 X 117 @3442. I also dive occasionally 100 cft @ 3000 psi as well as 100 cft @ 3442 psi and dive with friends who own various size of singles and doubles (standard, LP and HP). each having its distinct psi/min based on my gas consumption. It is much easier for me to stick to cft/min and we all understand each other using that common denominator.

If, on the other hand you prefer using psi/min because it is easier for you...ie always using same tank, etc than by all means go ahead.

 

[Insta-Gator]

Sounds like we must be.

Got an 80, SPG he say 3000psi. At 1500psi must have 40cft. Must use 8cft every 300psi. Oh shyt, only got 1000psi left & rock bottom was 27cft, gota go.

Yep, must be

Let's see; my SCR is 15 PSI/Min, I'm at 70' (roughly 3 ATA, plus a little) so my consumption at depth is approx. 45 psi/min. Rock Bottom is about 1,000 PSI. SPG reads 1200 PSI, I've got roughly 4 minutes left at depth ((1200-1000)/45). Volume will increase as I ascend, which is a plus.

I really do get the whole volume method of gas management for planning purposes, but what I'm saying is that when I'm in the water, I prefer to keep the math simple and not have to do a bunch of conversions if I don't have to. To each their own.

(btw, depending on the cylinder, an AL80 usually only 77.4 Cu/Ft, not 80. Don't forget to calculate your cylinder constant before doing all those PSI to CU/FT conversions. Think those lost 2.6 CU/FT might matter at depth?)

 

[fisheater]

Yep, must be

Let's see; my SCR is 15 PSI/Min, I'm at 70' (roughly 3 ATA, plus a little) so my consumption at depth is approx. 45 psi/min. Rock Bottom is about 1,000 PSI. SPG reads 1200 PSI, I've got roughly 4 minutes left at depth ((1200-1000)/45). Volume will increase as I ascend, which is a plus.

I really do get the whole volume method of gas management for planning purposes, but what I'm saying is that when I'm in the water, I prefer to keep the math simple and not have to do a bunch of conversions if I don't have to. To each their own.

Right.

However, you first have to determine what your psi/min consumption is for THAT size tank.


Sent from my iPhone using Tapatalk

 

[rhwestfall]

Ah, back to the need for that great little NASDS Dive Time Calculator again.... shame they are no longer in business. What a great little product it was. Been told there is another Certification Agency selling them....

 

[TSandM]

It's absolutely true that gas consumption in cf/min is useless during any particular dive. (And, in fact, my Cave 1 instructor would get apoplectic when he asked us what our SAC rate was, and we answered in cf/min. He would go off on a tirade about how useless that information was ) But if you dive multiple different cylinders, you are simply going to have to know multiple pieces of information. You can know your SAC in psi/min for every tank, or you can know the "tank factor" for each tank, and do the conversion. "Tank factor" is pretty easy to work with -- it's the number of cubic feet contained in a hundred psi in THAT tank. For example, the TF of an AL80 is 2.5; for an HP100, it's 3 (these are obviously rounded off).

So, if I know that my SAC rate is .5 cf/min, then at 66 feet, it's 1.5 cf/min. That means that, at 66 feet in an HP100, it will take me two minutes to suck down 100 psi, or I'll go through 250 psi in five minutes. That's scuba math, and I can do it in my head on the surface, and even at 66 feet.

 

[Insta-Gator]

Right.

However, you first have to determine what your psi/min consumption is for THAT size tank.


Sent from my iPhone using Tapatalk

Really? Here's the equation form NOAA for SCR (surface air consumption rate)
{image didn't work, please use the link to view the NOAA equations}
www.ndc.noaa.gov/pdfs/AirRqmtFormulas.pdf
Where does it ask for cylinder information? I think there are some folks out there that confuse SCR or SAC (expressed in PSI/Min) with RMV (respritory minute volume) which IS expressed in cu/ft per min. and is dependent on the cylinder constant "k". As far as I can see for SCR, it only wants to know how much air you used for a specific amount of time and divided by the number of atmospheres (ATA) Again, please show me on the SCR equation where the cylinder specific information in entered?>>!!!
Perhaps NOAA has it all wrong. Please call them with your input.

 

[Kern]

I do it completely different to NOAA. I use this formula-

Surface consumption rates = (bar used/time/ambient pressure) x WC

It gives me my SCR in bar/minute & litres/minute.

I just can't get excited over semantics.

 

[Insta-Gator]

I do it completely different to NOAA. I use this formula-

Surface consumption rates = (bar used/time/ambient pressure) x WC

It gives me my SCR in bar/minute & litres/minute.

I just can't get excited over semantics.

Maybe. I tend to get a little OCD when it comes to numbers.

 

[Kevrumbo]

Your primary "gas gauge" during the dive is your SPG in pressure units --so I use the SAC rate converted to bar/min for the particular tank I'm using (my reference RMV is 22 litres/min).

For example, a single AL80 tank (11 litre/bar tank), my pressure SAC rate is 2bar/min. So in 10min breathing on the surface, I'll use 20bar of breathing gas.

But at 30m of depth (4 ATA), I'll consume instead 4-times-20bar equals 80bar in 10min nominal workload finning . . .I then check the SPG to confirm the delta change in pressure.

This is the way you should be actively utilizing your SAC rate, realtime as well as in pre-dive gas consumption planning . . .

See also http://www.scubaboard.com/forums/5714367-post15.html

 

[Kern]

Apoplectic dive instructors, OCD divers, others that think there's only one way things should be done. Wonder if they can drive a car without running out of juice.

I don't seem to have any trouble using an SPG during the course of a dive as a pressure gauge, or a % of volume gauge. Oddly enough, I seem to get to the same place, with the same amount of gas, with a minimum of bother, no matter what information I use, or what way I use it.

By the way Gator, theres no maybe, that equation is the same as those in your NOAA pdf.