How do I calculate my real SAC rate?

[Colliam7]

Ok so I am bored and stuck at home! . . . I thought I should attach a reg and figure out my SAC rate. Any suggestions on how I can do that? I can time myself for 10 minutes breathing from a reg but if I am sitting down with an empty relaxed mind, that may not give a fairly accurate SAC. I can moderately exert and then try to breath the same reg after the same level of exertion but would that be considered accurate SAC rate?

I tend to agree with Bob, and others, that it is best to measure it a) in-water, and b) under both relaxed conditions and under exertional conditions. But, if you are sitting around the house (with nothing better to do), assemble your rig, put it on, and walk around the house at a steady but relaxed pace for 10 minutes (I would suggest 20) while breathing from the reg. Then, do it again while exerting - this could be walking more rapidly, running in place (harder to do), or simply moving your arms vigorously for 10 minutes (tiring on the arms).

This might give you a ROUGH idea of your SAC rate, under both relaxed and exertional conditions (you really want to know both). But, as several have noted, there are other variables that affect SAC, and it is not necessarily a constant, to 2 decimal places, it, so don't look at whatever you calculate as THE (one and only) SAC rate that defines your air consumption. Still, possibly a useful, snowy day exercise.

 

[rhwestfall]

put on your gear, vacuum the house, record it all for a you-tube video..... PLEASE!!!!

 

[101recon]

... wear heavier undies ...

... Bob (Grateful Diver)

This definitely helps. When I first started diving dry I just wore my MK2 undergarment. This is OK for 45 min. or so in cold water. For colder/longer dives I started adding Under Armour, and if needed, a pair of regular thermal underwear under the MK2. The change in SAC was far less dramatic. It's amazing how much difference being comfortable will make.

 

[scubadada]

SAC is almost useless and must be corrected for a number of dive related factors. I follow my SRMV as calculated by my AI PDC. I find these values very useful for my dive gas requirements.

 

[Foxfish]

Ok so I am bored and stuck at home! I was scheduled to go for an ice-dive but the roads are blocked and I am sitting in my home with 3 filled tanks. Two HP 100s and one HP 80. I thought I should attach a reg and figure out my SAC rate. Any suggestions on how I can do that? I can time myself for 10 minutes breathing from a reg but if I am sitting down with an empty relaxed mind, that may not give a fairly accurate SAC. I can moderately exert and then try to breath the same reg after the same level of exertion but would that be considered accurate SAC rate?

Any and all tips and suggestions welcome.

Cheers -

CS

You don't sound sufficiently confused so I will try to remedy the situation. SAC stands for 'surface air consumption'. It is quoted as the volume of air you breathe per minute or the air pressure reduction in your tank per minute. The two are related by the ideal gas laws. Where I live it is normally quoted as a volume with units L/minute and the following discussion relates to this use of the term.

When you are diving, the volume of air you breathe in a given time interval varies depending on your depth. Well actually it doesn't, in fact it remains essentially constant assuming your level of exertion remains constant and a few other factors normally neglected.

But if you take this air consumption and convert the volume of air breathed during the time interval to the free air volume, ie. the volume at the surface pressure of one atmosphere, you will find it varies substantially with depth according to the ideal gas law P1V1=P2V2. When talking about 'surface air consumption' you'd be forgiven for thinking this was the meaning of the term, ie. your free air consumption derived from the what you breathe at a given depth. You would be wrong. The surface air consumption quoted by most people has nothing to do with this surface air consumption. It is a conspiracy intended to confuse all but the select few.

I suspect the term was initially used to refer to someone sitting on a couch on the surface, breathing from a regulator and then calculating their air consumption but was then adopted to refer to your air consumption underwater. While the way in which these two rates are determined is unrelated, they are inextricably linked by the fact that when you breath on the surface and under water, the volume change in your lungs and diaphragm as they expand and contract remains essentially constant even though the free air volume of the air breathed varies. Think of it like a bike pump. The swept volume of a bike pump when fully extended and full of air is the same above and below the water (once you've worked out how to get air into it underwater).

The formula used for calculating what most divers call SAC is

SAC = (Change in Pressure).(Tank Volume)/(Time Interval)/(Average Depth).

The SAC calculated in this way refers to the volume of the compressed air breathed at the average depth within a given time interval.

If you think about it hard enough, you will find this is derived from the ideal gas law P1V1=P2V2. Conditions 1 and 2 conditions refer to inside the tank and conditions at the average depth of the dive. The time term converts the volume into an air flow rate or air consumption rate.

In my experience, when a SAC is quoted by most people, is not referring to the air consumption calculated on the surface while sitting in a couch but their actual air consumption during a dive. So the term respiratory mean volume RMV at least avoids some of the confusion caused by just calling it SAC.

The term surface respiratory volume SRMV is used by SDI/TDI. Respiratory minute volume RMV is used in other contexts apart from diving (probably a medical context):

http://en.wikipedia.org/wiki/Respiratory_minute_volume

so I presume that surface is tacked onto the front primarily to add to the confusion. I've seen it in one of their course books. It refers to the compressed volume you breathe at the average depth for the given time interval, not the the volume you breathe converted to a free air pressure. May be the word surface is added to indicate it was derived from a SAC.

On the recent SDI/TDI course I did, they gave an example of how to determine the SRMV. It was done by factoring the SAC. My preferred method, in the absence of a convincing argument to the contrary, is to work out my RMV based on measurements from an actual dive. For a stressed RMV I'd again just take the measurements while stressed. Again it is what most people call their SAC.

Having said that I have calculated the SRMV the SDI/TDI way and found if anything it is a little conservative. My SAC is about 12 L/min (0.42 cfm) and my actual RMV on a relaxed dive is typically 15 L/min (0.53 L/min) which is a factor of 1.25 times the SAC. SDI recommends using a factor of 1.5 for an easy dive which would mean an SRMV of 18 L/min (0.63 cfm) used for planning. In recent times my RMV varies little on an easy dive so that is the number I use for dive planning.

I trust you are now as confused as I was when first introduced to SAC.

 

[Peter Guy]

Why does one want to know their SAV (RMV) rate? The primary reason, AFAIK, is to be able to calculate (estimate) how much gas (air) you need to do the dive you plan to do. IF that is true, then it is my belief the most important SAC calculation is for a whole dive, not just resting, working, stressed, etc.

Towards that end, I encourage my students to do just one SAC calculation -- for their dive -- but to do it for every dive they do. After a while they'll end up with a pretty good idea of their consumption rate for the type of dive they are planning and thus they can do a decent job of estimating their planned consumption.

YMMV

 

[scubadada]

I agree with Foxfish, most divers probably use SAC and SRMV to mean the same thing. Nomenclature is most important to ensure accurate communication. There is certainly an opportunity for miscommunication on this topic.

I choose to use SMRV and my AI PDC calculates it for every dive I make. As Peter Guy points out, it is actually quite constant, even covering a reasonable spectrum of diving conditions. For my last 358 dives, my SRMV has been 0.37 +/- 0.04 cu ft/min (mean +/- std dev). This information is extremely useful to me for my gas planning. Though not a perfectly normal distribution, this means that about 95% of all my SRMVs should be between 0.29 and 0.45 cu ft per minute, and that is borne out by my experience. The actual range of all 358 dives has been from 0.29 to 0.63 cu ft per minute. I had 4 reasonalbly strenuous dives in a wetsuit in very cold water in Massachusetts that account for the high end of the range.

I've learned a lot from following my SRMV, you can too.

Good diving, Craig

 

[mala]

any tips or suggestions welcome.???

Well whatever your sac rate (or whatever you want to call it)calcs are on the surface what do you do if the plan changes underwater?

can you do cubic feet/psi/feet of seawater in your head?

metric is the answer.

a short story-

So shortly after I started diving I was doing a wreck in the channel and I needed a bit of extra gas.
Take this ali 80 as a stage my mate said.
ok I said that sound s good.-how much gas does it hold?
It holds 80 cubic feet at operating pressure he said.
Ok-I said -whats the operating pressure?
207 bar he said
Its got 190 bar in it I said
Ok he said so its not full.
so how much gas does it have?
Dunno he says.
I called it a 10ltr at 190 so that's 1900lts

the tank was a 11.1 ltr tank at 190 bar

so 2109 lts


doing a sac calc is ok on the surface if you want to estimate your gas usage on a dive but its real hard to make adjustments during the dive if you cant do the math.

metric is your friend and psi is your enemy.

 

[MXGratefulDiver]

Why does one want to know their SAV (RMV) rate? The primary reason, AFAIK, is to be able to calculate (estimate) how much gas (air) you need to do the dive you plan to do. IF that is true, then it is my belief the most important SAC calculation is for a whole dive, not just resting, working, stressed, etc.

Towards that end, I encourage my students to do just one SAC calculation -- for their dive -- but to do it for every dive they do. After a while they'll end up with a pretty good idea of their consumption rate for the type of dive they are planning and thus they can do a decent job of estimating their planned consumption.

YMMV

Which works quite well ... but it's not the whole picture. The other question that needs to be asked is "how much gas do I need to reserve for emergencies". And for that, you should assume that you'll be stressed out dealing with the emergency, which will lead to a higher consumption rate. The purpose of the "city miles" calculation is to help you determine a reasonable number, based on your consumption rate and planned depth ... with equal consideration for how much gas your buddy may need if the emergency is an OOA.

As we all should know by now, the 500 psi rule of thumb may or may not be sufficient ... and rock bottom calculations do rely on a "working SAC" ...

... Bob (Grateful Diver)