Calculating SAC?

[CptTightPants21]

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!

Agree with most of this.

But every instructor, manual, blog, website, etc I have ever seen has SAC rate expressed in PSI per minute. My wreck instructor had me calculate it into PSI and then convert it to cu ft. Can you show me an official source that has it expressed in cu ft and only cu ft. Like I said, I use cu ft to express my SAC rate and have used that convention when talking with other divers.

In my mind, if a person says .x, it is assumed to be cu ft. If a diver was to say his SAC rate was 15 or 11, etc he would need to specify PSI or liters because it isn't as intuitive.

When I state my SAC rate I use my working SAC rate, so me swimming around, task loading, etc.(.45-.5 in your example) If someone was to tell me their SAC rate I would assume it was their working rate as well, and I think it would be dishonest/not helpful if someone used their completely resting/stationary SAC rate in terms of talking or planning a dive.

 

[tursiops]

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.

I guess I was not clear. Sorry. For TDI and some others, SAC does not change with effort level. SAC is not your consumption rate except in a resting state. It is your lowest possible consumption rate. All real consumation rates will be higher, by some multiple. You sit in your living room with a reg in your mouth and a clothespin on your nose to measure your SAC. All real in-water situations (except maybe deco or macro photography!) will give a higher rate...you are consuming more gas per minute than you did in your living room (all adjusted to sea level, of course).

My point was that many divers -- and some agencies -- call ALL the consumption rates SAC....which just causes confusion. My previous example was meant to illustrate a constant SAC but a varying RMV as my work load changed.

---------- Post added November 18th, 2014 at 08:48 PM ----------

Agree with most of this.

But every instructor, manual, blog, website, etc I have ever seen has SAC rate expressed in PSI per minute. My wreck instructor had me calculate it into PSI and then convert it to cu ft. Can you show me an official source that has it expressed in cu ft and only cu ft. Like I said, I use cu ft to express my SAC rate and have used that convention when talking with other divers.

In my mind, if a person says .x, it is assumed to be cu ft. If a diver was to say his SAC rate was 15 or 11, etc he would need to specify PSI or liters because it isn't as intuitive.

When I state my SAC rate I use my working SAC rate, so me swimming around, task loading, etc.(.45-.5 in your example) If someone was to tell me their SAC rate I would assume it was their working rate as well, and I think it would be dishonest/not helpful if someone used their completely resting/stationary SAC rate in terms of talking or planning a dive.

Before I posted my response you quoted, I looked in the PADI Tec manual and in the TDI tec manuals, and in the SDI Solo book. All use vol/min, not pressure/min. The SDI book even says, "SAC stands for surface air consumption and is a measurement in litres or cubic feet (never in units of pressure) that describes how much air a diver breathes every minute while on the surface at rest."

And you would never use your SAC (as a resting rate, as defined above) to plan a dive. You would apply a factor of at least 1.5 to use in your dive planning, and even more if it is a hard-working dive. So, for me, with a 0.4 SAC (resting rate!), I use 0.7 to plan gas use on a dive.

 

[CptTightPants21]

Good to know and I stand corrected.

I was going off of old SSI manuals, my previous instructors, and just about every site I could find on the internet.

So if someone asks you your SAC rate what do you tell them?

 

[EFX]

My point was that many divers -- and some agencies -- call ALL the consumption rates SAC....which just causes confusion. My previous example was meant to illustrate a constant SAC but a varying RMV as my work load changed.

Before I posted my response you quoted, I looked in the PADI Tec manual and in the TDI tec manuals, and in the SDI Solo book. All use vol/min, not pressure/min. The SDI book even says, "SAC stands for surface air consumption and is a measurement in litres or cubic feet (never in units of pressure) that describes how much air a diver breathes every minute while on the surface at rest."

And you would never use your SAC (as a resting rate, as defined above) to plan a dive. You would apply a factor of at least 1.5 to use in your dive planning, and even more if it is a hard-working dive. So, for me, with a 0.4 SAC (resting rate!), I use 0.7 to plan gas use on a dive.

Of course, SAC is Surface Air Consumption. Talk about confusion. I guess it depends on POV. Rec divers measure gas consumption at depth and then calculate what the resulting rate would be at the surface. I suppose SDI divers measure actual rate at the surface and then calculate for depth. This begs the question: how do you measure the rate? Do you put on all your gear, lay on the floor and do swimming motions?

 

[tursiops]

Good to know and I stand corrected.

I was going off of old SSI manuals, my previous instructors, and just about every site I could find on the internet.

So if someone asks you your SAC rate what do you tell them?

I tell them my resting rate is 0.4 cuft/min, and I use 0.7 cuft/min for planning a comfortable dive.
My last recreational trip was 17 dives, with consumption rates from 0.414 to 0.535 cuft/min....mostly around 0.45. There were no hard-working dives.

---------- Post added November 18th, 2014 at 09:29 PM ----------

Of course, SAC is Surface Air Consumption. Talk about confusion. I guess it depends on POV. Rec divers measure gas consumption at depth and then calculate what the resulting rate would be at the surface. I suppose SDI divers measure actual rate at the surface and then calculate for depth. This begs the question: how do you measure the rate? Do you put on all your gear, lay on the floor and do swimming motions?

Doesn't matter where you measure it or where you use it; you always adjust it back to sea level....using the ATAs....before you report it.

My computer gives me an average depth. Then it is easy, knowing the starting and ending pressure, and working through the tank size.
And, no, no swimming motions in the living room, you want a RESTING rate!

 

[uncfnp]

Unfortunately you are all right. I have seen the terms used in every way they have been described in this thread.

I do not have an air integrated DC. I have calculated my SAC several different ways.

The first time, I did a true resting calculation sitting in my living room. I actually like this method because it gives a relative constant rate. I then modify this constant with a Dive Factor (I like the SDI Solo manual for this calculation) according to the conditions and work load of the planned dive. I have had remarkably consistent results with this method. This is much the way described by tursiops in post 32. I later used the method of diving to a given depth swim and note gas used and time then use this to calculate a RMV with swimming.

I then purchased a DC that will give me an average depth with my dive log and have calculated my RMV with this. All have given fairly consistent values.

While the terminology is not used consistently, the best bet is to know what the calculations mean so you always know how to use the numbers regardless of the confusing definitions. Then it's a matter of clarifying terminology with buddies, etc as appropriate.

BTW, As a recreational diver, I use the resting SAC adjusted for DF to calculated amount of gas needed for planned dive but dive my actually gas use.

 

[EFX]

Doesn't matter where you measure it or where you use it; you always adjust it back to sea level....using the ATAs....before you report it.

I think it does matter where you measure it because the activity is different. At depth your measurement is actual dive activity and realistically closer to actual gas usage. Measuring at the surface at rest and applying a multiplier (?) is inaccurate. i suppose it's just as accurate if you drop to depth and remain completely motionless (at rest). But nobody dives this profile. Am I missing something here?

 

[dumpsterDiver]

The best way is to go diving. Start your stop watch when you get to a certain depth, record your air pressure, try to stay at that depth for a while, swim or take pictures or do whatever it is you do, and then when you are ready to change your depth (ascend etc.) note the time and psi consumed. sorta like I used 1500 psi, half a tank (40 cu-ft) while swimming for 17 minutes at around 80 feet ( I get 0.68283185407; if I did the math right)

Then when you get home, "do the math" adjust your actual consumption rate at depth in (cu-ft/min) to an equivalent rate at the surface. Call that rate your SAC and you have a good estimate.. probably better than sitting in a chair and trying to apply empirically derived conversion factors.

People are getting caught up in terminology that is variable and ambiguous or terminology that is not universally accepted.

Your SAC rate is variable depending on a lot of internal and external factors. Since these factors are often unknown or uncontrollable, I think it makes little sense to try to be overly precise with calculation of a SAC rate (so we would round to 0.7 cu-ft /min above). It's only value is as a predictive tool and you can't always foresee exactly how you will feel and what will happen on a dive.

Aren't there spreadsheets around that allow you to plug in this basic info and generate a SAC or RMV or whatever you want to call it?

 

[Edward3c]

Calculate the gas likely to be used when planning a dive by whatever method your comfortable with, and go diving. It really doesn't matter.

 

[uncfnp]

I think it does matter where you measure it because the activity is different. At depth your measurement is actual dive activity and realistically closer to actual gas usage. Measuring at the surface at rest and applying a multiplier (?) is inaccurate. i suppose it's just as accurate if you drop to depth and remain completely motionless (at rest). But nobody dives this profile. Am I missing something here?

Actually a surface resting rate is more accurate for what it is measuring. This method removes the variables of depth and activity and gives a truer SAC. It requires minimal equipment and can easily be repeated as a divers breath control and gas usage improves. It can then be adjusted to compensate for a wide range of diving conditions.

Yes, RMV calculated over time and a range of dive conditions is a great way to verify SAC but not all divers have access to accurate information to allow this.

Realistically, for most new divers, this information is interesting but not really necessary to plan a recreational dive. It can be very useful for calculating redundant gas needs.