SAC vs RMV

[Insta-Gator]

Other than 'local custom' why do people refer to RMV (respiratory minute volume, measured in cubic feet) as SAC or SCR (surface air consumption or surface consumption rate, which is measured in PSI)? NOAA clearly defines their equations for SAC (pressure) and RMV (volume) with RMV being a cylinder specific calculation based on max volume and working pressure.

http://www.ndc.noaa.gov/pdfs/AirRqmtFormulas.pdf

I understand that planning for a more technical dive will focus on gas volumes rather than pressures, but most of us are recreational divers and our gauges/computers reflect PSI not RMV. At depth what good does it do me to know my surface volume consumption rate?

I'm old and lazy so my SAC/RMV are not what they were 20 years ago, but as a rec diver, does this chart really do me any good? (from DivingLog 5.0)

 

[DA Aquamaster]

SAC, if you define it as being related to pressure is cylinder specific, not RMV.

It's an old argument but there has been some flip flop over the last 5-6 decades on the pressure versus volume definition of SAC. In the 1970s and early 80s a SAC rate calculator was sold. It was a circular slide rule with entries for depth, time and pressure that produced a "SAC" in psi per minute, and as such it was specific to the tank used to obtain the data. That pretty much cemented the association of SAC and psi, when prior to that it was considered a volumetric concept.

It really doesn't matter.

Cave divers for example use tank factors to bridge the gap between gas consumption at the surface in cfm and SPGs that deliver data in PSI. Double LP 95s have a tank factor of 7.2. (95/2640= .0359, x 2 = .072, x 100 = 7.2) which basically gives the diver an easy method to equate PSI and cubic feet by taking the tank factor times the 100's of psi used or remaining to figure the volume. For example, 1200 psi would be 86.4 cu ft. (12x7.2=86.4).

The math isn't hard to do in your head either. 10x7.2 = 72 and 2x7.2 = 14 and change, which added together give you a ballpark 86 cu ft. Or just take 7x12 to get a slightly more conservative 84 cu ft.

And you can work it in reverse For example, if you don't want to use a full "third" of cave filled LP 95s (3600 psi) and instead want to limit the penetration gas to 65 cu ft, you can take 65 and divide it by 7.2 to get 9.03, which if you take it by 100 gives you the PSI used before you turn the dive (903 psi, or just plain 900 psi).

Since you round to 100 psi increments anyway you can again round everything down and do it in your head - 7x8=56 (to small), 7x9=63 (pretty close), 7x10=70 (too big), so you go with 7x9 and 900 psi.

Where RMV or SAC is important is in knowing how much gas you'll need for the dive, the reserve and an deco given the depths and times involved.

 

[fire_diver]

Why do people chose one term over another? Ignorance? RMV speaks to the gas used for a specific depth. SAC is converted to and equivilent gas usage on the surface so that you can make easy plans for a specific tank on a future dive.

What does your dive tell you? It says you have a normal SAC range and you should probably use .60 SAC for your planning calculations.

 

[Insta-Gator]

SAC, if you define it as being related to pressure is cylinder specific, not RMV. ( )

It's an old argument but there has been some flip flop over the last 5-6 decades on the pressure versus volume definition of SAC. In the 1970s and early 80s a SAC rate calculator was sold. It was a circular slide rule with entries for depth, time and pressure that produced a "SAC" in psi per minute, and as such it was specific to the tank used to obtain the data. That pretty much cemented the association of SAC and psi, when prior to that it was considered a volumetric concept.

It really doesn't matter.

Cave divers for example use tank factors to bridge the gap between gas consumption at the surface in cfm and SPGs that deliver data in PSI. Double LP 95s have a tank factor of 7.2. (95/2640= .0359, x 2 = .072, x 100 = 7.2) which basically gives the diver an easy method to equate PSI and cubic feet by taking the tank factor times the 100's of psi used or remaining to figure the volume. For example, 1200 psi would be 86.4 cu ft. (12x7.2=86.4).

The math isn't hard to do in your head either. 10x7.2 = 72 and 2x7.2 = 14 and change, which added together give you a ballpark 86 cu ft. Or just take 7x12 to get a slightly more conservative 84 cu ft.

And you can work it in reverse For example, if you don't want to use a full "third" of cave filled LP 95s (3600 psi) and instead want to limit the penetration gas to 65 cu ft, you can take 65 and divide it by 7.2 to get 9.03, which if you take it by 100 gives you the PSI used before you turn the dive (903 psi, or just plain 900 psi).

Since you round to 100 psi increments anyway you can again round everything down and do it in your head - 7x8=56 (to small), 7x9=63 (pretty close), 7x10=70 (too big), so you go with 7x9 and 900 psi.

Where RMV or SAC is important is in knowing how much gas you'll need for the dive, the reserve and an deco given the depths and times involved.

Why do people chose one term over another? Ignorance? RMV speaks to the gas used for a specific depth. SAC is converted to and equivilent gas usage on the surface so that you can make easy plans for a specific tank on a future dive.

What does your dive tell you? It says you have a normal SAC range and you should probably use .60 SAC for your planning calculations.

Sorry. Are y'all using different calculations from these? Based on your definitions, there must be some difference. According to the NOAA Air Requirement Formula, SAC is not cylinder specific. Actually RMV is calculated based on SAC for a specific cylinder constant. Also, RMV is a volume measurement of consumption at the surface, not at depth. So if you have other equations/definitions, I'd love to see them.

 

[fire_diver]

RMV = Respiratory minute volume. The volume of air used in one minute. Where do you measure this? At depth? That number is your RMV. Nothing about RMV indicates it means at 1ATM.

SAC = SURFACE Air Consumption. The volume of air used in one minute, normalized to 1ATM. The measure is taken at depth, then converted to a surface volume.

All diving gas volume used measurements are taken in PSI, which has to be calculated by the tank volume to obtain a gas volume. Anyone expressing a "volume" by giving a psi change doesn't know what they are doing.

 

[SC_Hoaty]

Looking at those formulae, it appears that one uses SAC (calculated as shown in the figure) as a stepping stone to calculate RMV. For a given individual, the RMV should be fairly constant as they change from a small cylinder to a large one, while the SAC would vary quite a bit.

HOWEVER, if you always dive the same tank type (Al-80 for lots of rec divers renting tanks) then RMV and SAC would both be fairly constant. Since SAC is easier to compute, it is useful to track if your air consumption rate is improving or not, with the caveat that if you use a different tank, you should use RMV as your basis to compare.

Make sense?

---------- Post Merged at 09:16 PM ---------- Previous Post was at 09:14 PM ----------

RMV = Respiratory minute volume. The volume of air used in one minute. Where do you measure this? At depth? That number is your RMV. Nothing about RMV indicates it means at 1ATM.

SAC = SURFACE Air Consumption. The volume of air used in one minute, normalized to 1ATM. The measure is taken at depth, then converted to a surface volume.

Nothing measurement is taken in PSI, which has to be calculated to obtain a volume. Anyone expressing a "volume" by giving a psi change doesn't know what they are doing.

Note that your definitions are NOT the same as the ones in the formulae. Not that one is right and the other is wrong - just that they are different.

Without defining terms, agreement may look like arguement.

---------- Post Merged at 09:24 PM ---------- Previous Post was at 09:14 PM ----------

And speaking of defining terms, Gary, since your DivingLog 5.0 printout has no units, you should check the instructions about what it thinks SAC is. Those numbers are nowhere near what I calc for SAC in psi/min for my dives.

Also, the trick in determining the cylinder K is to get the right data. An AL-80 does not hold 80.0 SCF of gas, IIRC - that is a nominal designation.

 

[TSandM]

SAC in psi/min is useful if you and your buddies typically dive the same tank. If everybody dives the tropics on Al80s, then you all know what to expect in terms of gas duration and requirements.

If you dive with people who use different size or pressure tanks, SAC gets pretty confusing. It's easier to normalize everything to volumes and figure out who is the gas-limiting diver that way, and what everyone's rock bottom needs to be.

My cave instructor was driven batty by our insistence on converting everything to volumes. He would shriek, "Cubic feet is no use to me! I want to know how many psi per minute you are going to use!" But everybody down there dives the same tanks.

At home, I'm likely to get in the water with one person using an HP80, me on an HP100, and my third buddy on double 130s . . .

 

[graham_s]

We use l/min and measure the tank size in Litres and Pressure in bar. It makes for easy and intuitive calculation.

For example, I have a 12l cylinder filled to 200 bar.
This means I have 2400l at 1atm.
If I consume air at 25l/min, I will consume that air in 96 mins.
If I take into account my 50 bar reserve, that drops to 72 mins.
As pressure increases at a linear rate, at 10m, I'm consuming air twice as fast (2bar ambient) so the same fill lasts 36 mins.
Easy.
SI units are the way forward.

 

[Kevrumbo]

It's easier to work with pressure units for gas planning & management in the Metric System, especially if your pressure Surface Consumption Rate (SCR) turns out to be a convenient integer.

I have a cold water reference Surface Consumption Rate (SCR) of 22 litres/min(roughly 0.75 cuft/min in US Imperial units).

The common AL80 Tank holds 11 litres volume at the surface standard of 1ATA(or 1.01 bar), for a metric rating of 11 litres/bar.

22 litres/min divided-by 11 litres/bar equals 2 bar/min SCR (Surface Consumption Rate) in pressure units --a more useful quantity to utilize during the dive since your SPG reads in bar pressure units. [And 2 bar/min is much easier & quicker to arithmetically manipulate in your head than 29 psi/min equivalent in US Imperial Units.]

Your depth in meters, which converts easily to ATA (simply divide-by-10 and add 1) becomes your multiplier depth factor for your 2bar/min pressure SCR.

Example: 30 meters depth is 4 ATA (divide 30 by 10 and add 1 equals 4); your 2bar/min SCR at depth -or Depth Consumption Rate (DCR)- now becomes 8bar/min. [4 times 2bar/min equals 8bar/min]. So 10 minutes at depth 30m on an AL80 (11L/bar) tank in nominal conditions, you would expect to consume 80bar of gas and your SPG reading to be down or show a delta of 80bar. . .

What if your SCR, or Depth Consumption Rate (DCR) is lower or better than the example above? --Just scale it as a percentage result:

My SCR/RMV in tropical warm waters is typically 30% better than it is in temperate cold SoCal homewaters. However, after a week drift diving in Palau 30deg C water temp, I've lowered that to around 50% of my nominal cold water SCR (from 22 litres/min to 11 litres/min).

This is how I used this value with a 11 litres/bar tank (i.e. an AL80) in Palau:
11 litres/min divided-by- 11 litres/bar equals 1 bar/min pressure SCR.[Compare: how much easier & intuitive is it to work with "1 bar/min" vs "14.5 psi/min equivalent" in US Imperial Units???]

All my dives are averaging 20 meters depth going with the drift current; 20 meters is 3 ATA (divide 20 by 10 and add 1 gives a depth in atmospheres absolute of 3 ATA).

Therefore 1bar/min multiplied by 3 ATA equals a depth consumption rate of 3 bar/min at 20 meters. Checking my bottom timer every 10 minutes, I expect to consume 30 bar (3 bar/min multiplied by 10min equals 30 bar), and accordingly my SPG should read 30 bar less in that 10 minute time frame.

So by 30 minutes elapsed dive time at 20 meters, I expect to be down 90 bar or at half tank (AL80 full tank is 200 bar). At 40 minutes elapsed time, I'm ascending off the wall into the shallow coral plateau around 9 meters (down 120 bar from 200 bar total, or 80 bar remaining in tank). And finally at the 45 to 50 minute mark, I'm at 6m and my 3-5min safety stop with 60 to 70 bar left. I surface and I know even before looking at my SPG that I have around 50 bar remaining in my tank.

This is how you should actively use your SCR with your particular tank, knowing how much breathing gas you have left not only on pre-planning, but also during the actual dive at depth, real-time-on-the-fly --all with easier to use metric units . . .additionally, you have a SPG that reads in units of pressure: why not convert your SCR to a Depth Consumption Rate (DCR) in pressure units to make use of it???

 

[Insta-Gator]

---------- Post Merged at 09:24 PM ---------- Previous Post was at 09:14 PM ----------

And speaking of defining terms, Gary, since your DivingLog 5.0 printout has no units, you should check the instructions about what it thinks SAC is. Those numbers are nowhere near what I calc for SAC in psi/min for my dives.

Also, the trick in determining the cylinder K is to get the right data. An AL-80 does not hold 80.0 SCF of gas, IIRC - that is a nominal designation.

Ah, ha!!!! Ding, ding, ding, we have a winner.

DiveLog 5.0 defines gas usage as SAC, but displays RMV numbers (as defined by the NOAA formulas) Hence my initial question. I've see people on SB mix and match SAC/RMV in posts for a long time and I just figured it was a local custom or error. But for a software distributor to label it as such leads me to believe the practice may run deeper. Why? The NOAA formulas are very clear and straight forward. If there are other formulas out there with alternative definitions, I'd love to see them. Really, I would.

For those who insist on posting their personal definition of what SAC or RMV are and where it is measured, please go back at find an authoritative source to back up your comments. Otherwise you are only adding to the argument.

I'm not trolling here. I really do want to know where the competing views come from and what mathematical or scientific method/agency/what ever supports your position.

I've found the NOAA formulas and definitions to be consistent with what I was taught. If you were taught differently, share your supporting documentation. Thanks.

---------- Post Merged at 06:46 AM ---------- Previous Post was at 06:35 AM ----------

Kevrumbo, I love your examples. Why the US did not follow through with the cut-over to the metric system some 20+ years ago, I'll never know. Metric is so much easier, being based on 10's. With the metric system and your example I can see how easy it would be to monitor gas at depth by volume and volume consumption rather than in psi. Doing the math in imperial units with a SPG is just not practical (for me.) However if I know my SAC (in pressure units) I can 'guesstimate' my pressure consumption at depth or actually do the math on shore ahead of time.