Calculating SAC Rate and RMV

[coldsmoke]

I sent over an e-mail to a friend of mine that was an attempt to address entry level gas planning. I thought I'd post it here. It's basic and takes a few assumptions, but hopefully some will find it useful.

About SAC rates - Since we talked about it a little yesterday and I was doing mine, I thought I’d calculate yours. It’s a great tool for gas planning but it is also a great way to track your comfort in the water. As you become more comfortable in the water, more methodical in your movements, develop better buoyancy control, and adopt a more streamlined position you will notice a significant decrease in your consumption rate.

You’ll need a little background for it all to make sense – so here goes:

Data Needed:
Dive Time – 46.4 minutes
Average Depth – 21.21 ffw
Start Pressure – 3000 psi
End Pressure – 400 psi
Tank – AL 80

• ATA = Atmospheres Absolute – Surface pressure at sea level is 1 atmosphere or 14.7 psi. Every 33 feet or 10 meters you descend underwater you increase pressure by another atmosphere or 14.7 psi. So at 33 feet deep we are at 2 ATA’s (one for the normal pressure at the surface and another for the 33 feet of water above us. At 66 feet we are at 3 ATA’s, 99 feet 4 ATA’s, etc.
  • The formula is Depth/33+1 = ATA

• SAC = Surface Air Consumption – expressed in psi/min. (people generally confuse this with RMV –see below). SAC is dependent on tank size so it is really only useful on tanks of the same size or during your dive to monitor your breathing rate. SAC is calculated by dividing the psi you used by your bottom time and then adjusting for depth in ATA’s.
  • So you started with 3000 psi and ended with 400 psi. Our run time was 46.4 minutes so you used 56.0 psi per minute. (3000-400=2600/46.4=56.0)
  • Now adjust for depth. Our average depth today was 21.21 ffw or 1.64 ATA’s. (21.21/33+1) Now divide your psi/min by ATA’s and you get your SAC. 56.0/1.6 = 35 psi/min SAC.

So how is this useful? Well it’s mainly used to track your gas consumption while you are diving so that you know if you are going to have adequate gas to finish what you set out to do. I make it simple. I would multiply my SAC by 5. So every 5 minutes you know you will reduce your psi by 170 psi times your ATA’s. So at 33 feet you should use 340 psi every 5 minutes, at 66 feet 510 psi, etc. Note this calculated SAC will only be good for you when you are diving your Al 80 tanks. What if you change to a high pressure 100 or a set of double LP95’s

• RMV = Respiratory Minute Volume – expressed in cubic feet/min. RMV is used to express your consumption regardless of tank volume. This is so you can change out tanks or dive with other divers with different tanks and still be able to plan your gas needs. It’s just a little more involved that SAC. First you need to relate cubic feet to psi with regards to the tank(s) in question. This is done by multiplying your tank volume by its rated pressure.
  • In the case of an Al 80 – it holds 77.4 cu ft of gas at 3000 psi. So each psi is 0.0258 cuft in that tank. (77.4/3000) There’s an easier way but I’ll get to that later.
  • So if you used 2600 psi then you used 67 cuft. (2600*.0258) Divide cuft used by run time. 67/46.4 minutes = 1.44 cuft/ min.
  • Now just adjust for depth just like calculating your SAC. 1.44/1.6 ATA’s = 0.9 RMV.

So what is this used for? Well say you want to do a 100 foot dive. How long will you be able to stay there with an Al 80? Well you consume 0.9 cuft per minute at the surface so at 100 feet you will consume 3.6 cuft per minute. (0.9*4 ATA’s). So you will be able to stay there for 21.5 minutes. **Note this excludes many other dive planning considerations like rock bottom (gas needed for you and your buddy to ascend properly), and mdl’s.** You obviously wouldn’t dive to 100 feet for 21 minutes because then you’d be out of gas and over your mdl limits if you are diving air. I was just simplifying the explanation.

• Tank Factors – Here’s a quick way to deal with tank volumes at rated pressures for calculating your RMV. There’s really only a hand full of tanks out there that you would dive as primary tanks. Tank factors are just close estimates that make calculating this stuff in your head way easier. I have tank factors for the tanks I use memorized and the rest written in my wet notes.
  • AL 80’s have a tank factor of 2.5. So to figure how many cubic feet you used on a dive using tank factors you take your psi used, in this case 2600 psi and divide by 100. 2600/100 = 26 and now multiply that by your tank factor. 26*2.5= 65. Not quite as accurate as the method above but certainly acceptable for any gas planning you should be doing. Double 80’s have a tank factor of 5.
  • The tanks that I was diving with yesterday were double HP 100’s. A single HP100 has a tank factor of 3 and doubles have a tank factor of 6. My start pressure was 1900 and my end pressure was 1300. I used 600 psi. 600/100 = 6. 6*6 = 36. I used 36 cuft on that dive. My SAC was 600psi/46.4minutes/1.6ATA = 8.1psi per min SAC. My RMV was 36cuft/46.4 minutes/1.6ATA = 0.485 RMV

Hopefully that all makes sense.
NWGratefulDiver on the board also has a great article. Much better than mine! Can't seem to find his web site right now. I'll look for it and edit if I find it. Maybe he'll chime in.

Hunter

 

[scubajcf]

NWGratefulDiver.com

 

[Blackwood]

Hopefully that all makes sense.

It does, though I have a couple of comments.

One is that you are applying salt water density to fresh water. However, since it's a fairly negligible difference, it isn't a cause for concern.

Another, and this is REALLY nit picky, but
"First you need to relate cubic feet to psi with regards to the tank(s) in question. This is done by multiplying your tank volume by its rated pressure."
That ain't right, but since you go on to explain the math, it doesn't matter.

Another...
"SAC = Surface Air Consumption – expressed in psi/min. (people generally confuse this with RMV –see below)."
I don't think people confuse SAC Rate with RMV. It's just terminology. Personally, I express SAC Rate as either pressure/time or volume/time (don't use the term RMV for anything). Partially because that's how I learned, and partially because RMV seems a nonsensical term so I refuse to adopt it. That they couldn't come up with anything better than Respiratory Minute Volume baffles me. On the other hand, SAC Rate (surface air consumption rate, i.e. the Rate at which you Consume Air at the Surface) is perfectly descriptive. Truth be told, I would support different terminology for pressure and volume figures if they seemed reasonable, like maybe SVR and SPR (surface volume rate and surface pressure rate). But RMV just annoys me. Too intentionally jargonized.
[/diatribe]


Finally, if I didn't already know what a tank factors are, i.e. round(100*V_rated/P_rated), I'd like to. Everything else in your email you spelled out the what/why. Here you just gave the how.


Anyway, good work overall.

 

[coldsmoke]

It does, though I have a couple of comments.

One is that you are applying salt water density to fresh water. However, since it's a fairly negligible difference, it isn't a cause for concern.

Another, and this is REALLY nit picky, but
"First you need to relate cubic feet to psi with regards to the tank(s) in question. This is done by multiplying your tank volume by its rated pressure."
That ain't right, but since you go on to explain the math, it doesn't matter.

Oops! Thanks for the heads up. I will fix it.

Another...
"SAC = Surface Air Consumption – expressed in psi/min. (people generally confuse this with RMV –see below)."
I don't think people confuse SAC Rate with RMV. It's just terminology. Personally, I express SAC Rate as either pressure/time or volume/time (don't use the term RMV for anything). Partially because that's how I learned, and partially because RMV seems a nonsensical term so I refuse to adopt it. That they couldn't come up with anything better than Respiratory Minute Volume baffles me. On the other hand, SAC Rate (surface air consumption rate, i.e. the Rate at which you Consume Air at the Surface) is perfectly descriptive. Truth be told, I would support different terminology for pressure and volume figures if they seemed reasonable, like maybe SVR and SPR (surface volume rate and surface pressure rate). But RMV just annoys me. Too intentionally jargonized.
[/diatribe]

I agree completely, and I use the term SAC when I am actually expressing my RMV as most people do, but I was trying to be as correct as possible. RMV annoys me too.

Finally, if I didn't already know what a tank factors are, i.e. round(100*V_rated/P_rated), I'd like to. Everything else in your email you spelled out the what/why. Here you just gave the how.

Thanks for adding this. I thought about it but I was already thinking it was too long to hold most peoples interest and my target was mostly the rec diver using AL80's. I figured if someone wanted more info they could seek it out.

Hunter

 

[Deefstes]

Good post, thanks coldsmoke. Two things I'd like to raise:

1. I've seen different articles dealing with air consumption and the way you strike a difference between SAC and RMV is not used likewise in all. I agree with your arguments but I also agree with Blackwood.

Basically, what I'm saying is that SAC is not neccessarily expressed in psi/min. As you know SAC stand for Surface Air Consumption, in other words what is your consumption rate of surface air (as opposed to compressed air). I see no reason why the mount of air should be measured in pressure (give a certain cyllinder) rather than volume.

But yes, this is all nit picking, forgive me.

2. I live in the "rest of the world" where expressions such as "cubic feet" and "pounds per square inch" are usually only understood by the old and wise (who don't SCUBA dive). While the math stays the same for SI units there is one or two principles that need to be understood in order to make this work for metric units.

Most importantly, the total volume of air in a cyllinder, using imperial units, is given by the cyllinder's rating (assuming it is filled to working pressure). In other word an 80cuft 3000psi cyllinder contains 80cuft of air if it is filled to working pressure. As the pressure decreases the volume can be obtained by multiplying the starting volume of 80cuft with the fraction of pressure remaining. In other words the same cyllinder, filled only to 2700psi, will contain a volume of air equal to 80x2700/3000 = 72cuft.

The total volume of air in a cyllinder, using metric units, is calculated differently. The rating of the cyllinder is given in the cyllinder's physical size. In other words a 12l cyllinder contains 12 liters of air if uncompressed. This value is then multiplied by the pressure to which the cyllinder is filled. If the cyllinder is filled to 220 bar, it contains 12x220 = 2640 liters of air.

The rest of the math stays the same. So using the same values in your example above (but the approximate metric equivalents of course), here is a metric example.

Dive Time – 46.4 minutes
Average Depth – 6.5 m
Start Pressure – 206.8 bar
End Pressure – 27.6 bar
Tank – 11 liters (not that you get such a cyllinder but this is to approximate your values)

Calculated:
SAC = 3.86 bar/min (using your definition of SAC)
RMV = 25.75 l/min

A while ago I put together a little Excel spreadsheet with which to calculate SAC. It has two sheets, one for imperial units and one for metric units. I've attached it for what it's worth.

 

[coldsmoke]

Basically, what I'm saying is that SAC is not neccessarily expressed in psi/min. As you know SAC stand for Surface Air Consumption, in other words what is your consumption rate of surface air (as opposed to compressed air). I see no reason why the mount of air should be measured in pressure (give a certain cyllinder) rather than volume.

Technically speaking SAC is expressed in pressure/min and RMV is expressed as volume/min. Just like BTU is expressed in degree F/pound of pure water and calorie is an expression of degree C/gram of pure water. I didn't make it up, and it's certainly nit picking, but to express SAC in volume/min isn't correct.

BTW, metric is way easier!! Thankfully we're moving that way. It's only taken about 30 years for us to understand what a 2L soda bottle is. Maybe the next 30 years we should work on meters. But wait - what would that do to football? Crap, I guess we're doomed to imperial units forever.

Hunter

 

[DA Aquamaster]

I agree SAC is by definition expressed in PSI per minute while RMV is expressed in cubic feet per minute (yes, I know I am ignoring the 90% of the world that uses liters and bars - but they have a much easier task anyway given that they use liters and bars)

That, said RMV is a term that does not exactly roll off the tongue like SAC does nor does "Respritory Minute Volume" have the same intuitive meaning as "Surface Air Consumption". Consequently SAC has become the common term regardless of whether you are working in psi or cu ft.

 

[DA Aquamaster]

Maybe the next 30 years we should work on meters. But wait - what would that do to football? Crap, I guess we're doomed to imperial units forever.

Hunter

yep, I am not sure I could adjust to seats on the 54 and a half meter line.

 

[TSandM]

Nice thread, guys! I'm always delighted to see someone recapping this information for newer divers. Whether you call it SAC rate or RMV, the bottom line is that here are the tools to KNOW whether you have enough gas to do the dive you're proposing to do, BEFORE you're at 100 feet and looking at 500 psi on your gauge.

 

[Blackwood]

I agree SAC is by definition expressed in PSI per minute while RMV is expressed in cubic feet per minute

It's not universal; it depends entirely upon who is doing the defining.

I have a number of dive texts which refer to SAC Rate in CFM.

But really, it's a non-issue. As TSM noted, the bottom line is what matters.