Calculating SAC?

[dumpsterDiver]

I always expressed SAC as cu-ft per min, not psi per minute.

If you are changing tanks, then knowing your SAC rate in cu-ft would seem more useful (in any case it is easier to remember). My instructor (40 yrs ago) taught us to use 1 (one) for the SAC. Not too hard to remember.

His advice was if you plan your dive with 1, it is pretty unusual to get into trouble unless you start working really hard. Planning for a SAC of 1 cu-ft per minute does have some significant value due to the simplicity of it and if you want to cut things tighter and assume that you will not get cold or stressed or work hard or get too excited, then you can start making plans to use a lower SAC. Learning to do the estimation with 1 does make it easier to learn to use fractions of this number later.


However, someone with an applied math degree should not have to really ask any questions about this stuff.

 

[tursiops]

What Is a Surface Air Consumption (SAC) Rate?

No really.

Do you believe everything you read on the internet?

 

[CptTightPants21]

Do you believe everything you read on the internet?

I am confused. What do you think is incorrect?

 

[redacted]

I am confused. What do you think is incorrect?

Generally speaking, when "sac" is discussed here on scubaboard, it is in units of cu ft per minute. It seems to be much more useful and understood.

 

[CptTightPants21]

Generally speaking, when "sac" is discussed here on scubaboard, it is in units of cu ft per minute. It seems to be much more useful and understood.

I get that and I have stated as much in my post on the first page. Regardless of what the common convention is ( I too specify SAC in cu ft per minute), the dictionary definition of SAC rate is in PSI per minute

 

[tomfcrist]

^^^^Fact

 

[bradells]

I'll second most posts here.

I personally can either dl my dive from the computer and give it my in/out pressure, and it spits out a number of my SAC. But this is after the day of diving, over a beverage, back at home/hotel/boat.

I also do this during my SI, but calculated manually by using a guesstimated average depth, amount of gas I used, and dive time. I'm slowly getting better at guesstimating my average depth.

I'll also second Dopplers book, the Six Skills, as it goes into further using your SAC to get accurate predictions of gas consumption before you dive.

For example, in nice warm water (say Roatan), with minimal suit exposure (swim trunks and a rash guard), drifting around (not finning) looking for mermaids, my SAC is easily lower than 10L/min.

Add some current, actually finding a mermaid, some sharks, etc show up, my SAC goes up by a percentage. It could be 12L, 15L or even spike to over 30L!!

Same with cold water in the PNW. I've had some dives where I've been close to 10L, in my shell drysuit, in doubles, carrying a stage (ie, not as streamlined as swim trunks and a rash guard in the Caribbean), but usually around the 14-16L mark.

Knowing that I may be dropping into a wreck in some current, diving with a new team, different wildlife, new dive site , etc can add easily double my SAC. Knowing this, makes it easier to plan a dive with the gas available. A very handy tool for when you know your averaged 'normal' SAC rate is.

I'm slowly adding in current, and other SAC 'factors' into my logging, to gauge how much it does affect my breathing rate. But it isn't a quick process ...

BRad

 

[CptTightPants21]

OK I will bite (and probably will be wrong): An HP 80 (I think) runs a pressure of about 3450 PSIG to hold 82 Cu/Ft of air (@ 1 ATA) and a LP80 holds 78 Cu/FT @ 2400 PSI so it is not an apple to apple comparison due to the 5 % greater capacity of the HP. I still think the actual internal physical volume of the HP is smaller so I am going to guess that given equal pressures the LP80 will have a greater volume of air in it.

The LP will have more gas, although you need to be careful with your reasons.

For any tank: Remaining Air is = (Remaining PSI/Working Pressure for full tank)*tank size

LP tanks will either have 2640 PSI, 3000, or 3100 PSI working pressure, HP tanks will mainly have 3442 or 3500. The smaller working pressures in the denominator means that for the same PSI and tank size, the LP will always have more air.

 

[tursiops]

I get that and I have stated as much in my post on the first page. Regardless of what the common convention is ( I too specify SAC in cu ft per minute), the dictionary definition of SAC rate is in PSI per minute

What dictionary?

SAC has taken on several meanings over the years, and has evolved. Many references/blogs/articles are not keeping up with the evolution.
Back when, PSI/minute was common. Today, that is NEVER used in the technical community, and probably only used in the recreational community by those who only use AL80s.
But SAC is NEVER just PSI, which is what your post said. It is ALWAYS per time, usually a minute. PSI is not a rate; PSI/minute is.
The problem with using psi/min for SAC is that people think they are breathing psi....when in fact they are breathing cuft (or liters, or course). That could be a fatal mistake. if they switch tanks.

RMV, happily, is actually an agreed definition....and is always volume/minute.
Another confusion is whether SAC is what you are actually using, including your work or swimming, or whether it is a resting rate....the lowest consumption rate you might have.
With RMV it is the actual consumption rate, no confusion. Usually.
So it is important to be clear what you mean, and to use the right units.
What if I tell you my SAC is 15, or 0.4, or 11? Do you have any idea what I mean? In fact, those are all the same....15 psi/min for an AL80, or 0.4 cuft/min or11 liters/min for ANY size tank. But that is a resting rate...what if I am swimming hard, against a current? Is my SAC still 0.4 cuft/min, or is it now 2 or 3 times that? In fact, TDI would argue my SAC has not changed, but my RMV has.
Most divers seem to use SAC to mean RMV....the actual consumption rate. So please be clear.
And it varies from TDI to SDI to PADI to NOAA.

Just be sure to use the units, and to say what you mean.

My resting SAC is 0.4 cuft/min. It only approaches that on deco or during some photo dives in recreational gear in warm, no current water. On most dives my Surface RMV (that is, actual consumption, adjusted to the surface) is 0.45-0.5 cuft/min if not working, and typically 0.6-0.7 cuft/min if swimming around a lot. But it can escalate to 1.5-2 cuft/min if seriously stressed and working very hard. I avoid that!

 

[EFX]

But that is a resting rate...what if I am swimming hard, against a current? Is my SAC still 0.4 cuft/min, or is it now 2 or 3 times that? In fact, TDI would argue my SAC has not changed, but my RMV has.

Good explanation until I read the above. Can you clarify? My understanding is that if RMV changes based on dive activity so will the corresponding SAC. As you say it is important to clarify what you mean by SAC especially in the new divers forum. SAC (surface air consumption) per se, does not specify a volume/time or a pressure/time rate. It is unfortunate that different terms have cropped up. SSI's Science of Diving book calls RMV SRV (Surface Respitory Volume), a term I prefer over the more medically sounding RMV. We should use the terms SACP and SACV to distinguish between the two terms.

For new divers SACP can change with different tank sizes (gas volume at 1 ATA) and even tanks with the same volume with different working pressures as already mentioned. This is why SACV is preferred. The reason SACP is necessary is that unless you have a flow meter attached to your reg your SAC as measured during an actual dive will be in psi/min. Once you've determined this value the SACV can be calculated.