SAC vs RMV

[SC_Hoaty]

Morning Gary. Check this out - Calculating air consumption SAC / SCR / RMV [Archive] - ScubaBoard

---------- Post Merged at 07:42 AM ---------- Previous Post was at 07:21 AM ----------

And from checking Google, it appears that various folks define SCR as a rate either pressure based (and cylinder specific) or volume based (and cylinder independent). The common theme is that in order to be depth independent, all are normalized to sea level (the Surface reference).

When the surface consumption rate is discussed, if units are included, it is clear if the discussion is using a pressure or volume based measurement.

Arguing about the definition of the terms (SCR is ALWAYS pressure, RMV is ALWAYS volume, ...) is rather pointless. Just define what the terms are for purposes of the discussion, and get on to important things like who will run out of breathing gas first. Otherwise it is just semantics.

 

[Insta-Gator]

Morning Gary. Check this out - Calculating air consumption SAC / SCR / RMV [Archive] - ScubaBoard

Good morning, Senor Hoaty.

I like Walter's explaination, though I'm not sure I'd go sit on the bottom for 10 minutes just to figure out my RMV. But his math matches my understanding of the math and the definition of terms.

wish we'd gone metric in the 70's.

---------- Post Merged at 07:49 AM ---------- Previous Post was at 07:44 AM ----------

Question for the folks on the other side of the pond:

When you calculate your l/minute (a volume measurement) do you refer to it as your surface consumption rate (SCR or SAC)?

Perhaps that's where the volume vs pressure confusion is originated, somewhere over the mid-Atlantic. You know how meaning can be lost in translation.

reference note: Diving Log 5.0 is a product from Germany

 

[DA Aquamaster]

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.

If their calculation gives an output in PSI per minute, then it is tank dependent. You don't have to go any farther than the unit analysis to see it, regardless of what they might say about it.

If I use 50 psi per minute from an AL 80, I am using 1.28 cu ft per minute:

77 cu ft / 3000 psi = .0257 cu ft per psi, .0257 cu ft per psi x 50 psi = 1.28 cu ft.

If I am using 1.28 cu ft at a depth of 30 feet:

1) I am at 1.9 ATA;

30/33+1 = 1.9

2) and my SAC/RMV/SCR or what ever else you want to call my rate of gas use at depth corrected to 1 ATA at the surface will be .67 cu ft per minute.

However, if I was diving a steel 72, my SAC is now

71 cu ft / 2475 psi = .0286 cu ft per psi, .0286 cu ft per psi x 50 psi = 1.43 cu ft., 1.43 / 1.9 = .75 cu ft per minute.

It's not a huge shift, but it's a change. However, if I have a set of double 100s and I use 50 psi per minute at 30 ft, I'm an incredible gas hog:

200 cu ft /3442 psi = .0581 cu ft per psi, .0581 cu ft per psi x50 psi = 2.90 cu ft., 2.90 cu ft / 1.9 = 1.53 cu ft per minute.

Thus any unit of gas consumption, if defined by psi per minute, is extremely tank dependent.

Actually RMV is calculated based on SAC for a specific cylinder constant.

Taking that back to cave diving practice, if I have a SAC rate (whether you call it SAC, RMV, SCR or even just plain Bob) of .6 cu ft per minute, and I am doing a cave dive with a diver using an pair of AL80s, another diver using a pair of steel 72s while I am still using a pair of LP 95s I'll know that:

The pair of steel 72s has a tank factor of 5.8 (0.286 x 100 = 2.86, rounded to 2.9), the pair of AL 80s have a tank factor of 5.2, and the LP 95s have a tank factor of 7.2 (95/2640x2=.0719, .0719x100 = 7.2). This means that 100 psi is equal to 5.8 cu ft, 5.2 cu ft and 7.2 cu ft resepctively.

If we are diving into a cave with strong outflow where I'm comfortable using a fill third of gas, then I'd plan on using 1200 psi of my 3600 psi filled LP 95 tanks, and that would be 86.4 cu ft (7.2x12=86.4). However, if I lose all my gas I'll need to exit on another diver's reserve gas, so the smallest volume tanks come into play here. in this case, 1/3rd of the double 72s needs to be considered since they hold 154 cu feet at 2475 psi, providing a smaller third of only 47 cu ft. (The AL80s hold 154 cu ft at 3000 psi and have a 51 cu ft "third", however if the steel 72s were overfilled to 3000 psi, then they'd hold 174 cu ft (5.8 x 30=174) with a 58 cu ft "third" rand the AL 80s would now be the smaller set.)

So I'd want to limit my larger tanks to a smaller third of only 47 cu ft to match the steel 72s. 47 / 7.2 = 6.53, or 650 psi, meaning that if I started with 3600 psi, I'll now turn the dive at 2950 psi rather than 2400 psi. to ensure I use no more than 1/3rd the volume of the smallest set of tanks on the team.

That's where a correction based on tank volume comes into play, but whether you do it all in psi, work with cubic feet or use a tank factor that creates a volume of gas per 100 psi unit. all the tanks involved have to be normalized so that each diver knows how much gas in psi can be used on the dive.

In planning terms, if the average depth of the cave is 90 ft and I am swimming 50 feet per minute, with a SAC of .60 with 47 cu ft available for the penetration I know that I will be at 3.7 ATA (90/33+1=3.7) using 2.24 cu ft per minute (3.7x.60=2.24), and can swim for about 21 minutes (47/2.24=20.98) and 1050 ft (21 minutes x 50 feet per minute) before I have to turn the dive.

Of course, I could work it in psi as well:

0.60 cu ft per minute in double LP 95s is 8.3 psi/minute (7.2/100 = .60/X, [.60x100]/7.2 = 8.3),
8.3 x 3.7ATA = 30.71 (30 psi/minute)

650 psi "third" / 30 psi per minute = 21 minutes, x 50 feet per minute = 1050 ft.

So it really does not matter what you use or what you call it, as long as you understand the basic concepts involved, and I am pretty sure that if I dive another 28 years, the argument about SAC versus RMV will still be raging and will still be just as meaningless - and psi or BAR will still be tank dependent if divers are trying to equate them with a specific volume.

 

[Insta-Gator]

If their calculation gives an output in PSI per minute, then it is tank dependent (really???). You don't have to go any farther than the unit analysis to see it, regardless of what they might say about it.

If I use 50 psi per minute from an AL 80, I am using 1.28 cu ft per minute:

77 cu ft / 3000 psi = .0257 cu ft per psi, .0257 cu ft per psi x 50 psi = 1.28 cu ft.1.28 cu ft is a volume, not a pressure. Volume IS tank dependent (RMV) pressure (PSI) is NOT tank dependent.

If I am using 1.28 cu ft at a depth of 30 feet:

1) I am at 1.9 ATA;

30/33+1 = 1.9

2) and my SAC/RMV/SCR or what ever else you want to call my rate of gas use at depth corrected to 1 ATA at the surface will be .67 cu ft per minute. Volume

However, if I was diving a steel 72, my SAC is now

71 cu ft / 2475 psi = .0286 cu ft per psi, .0286 cu ft per psi x 50 psi = 1.43 cu ft., 1.43 / 1.9 = .75 cu ft per minute.

It's not a huge shift, but it's a change. However, if I have a set of double 100s and I use 50 psi per minute at 30 ft, I'm an incredible gas hog:

200 cu ft /3442 psi = .0581 cu ft per psi, .0581 cu ft per psi x50 psi = 2.90 cu ft., 2.90 cu ft / 1.9 = 1.53 cu ft per minute.

Thus any unit of gas consumption, if defined by psi per minute, is extremely tank dependent (STRONGLY DISAGREE).

Don't mix volumes and pressure in the same sentence for a definition. PSI (pounds per square inch) can only be a pressure, not a volume. 3000 PSI in a 30 cu ft tank is 3000 PSI. 3000 PSI in a 130 cu ft tank is still 3000 PSI. When you mentally or through habit convert from 50 PSI to 1.28 cu ft, you can not do this without knowing the size of the tank, as you illustrated, but at that point you have stopped talking about pressure and started talking about volume (RMV .. Respiratory Minute Volume)

Please refer to the NOAA formula for SAC. It is in PSI (pressure) not any volume. Where in their equation do they refer to size of tank???

 

[DA Aquamaster]

Read the last half of the post I added while you were typing yours to show how it's applied in the real world. Oddly enough you might realize we agree, but that you just don't see it as NOAA uses tank volume, they just don't note it up front - but it's an essential factor in the PSI figure they use.

If you look at the NOAA calculation you posted, they are clearly mixing psi and volume - something you suggest should not be done.

SAC = (PSI/time)/ATA.

Where do you think the PSI in the calculation came from? It came from the SPG readings on the reg attached to a specific tank for the gas used over that time at that depth.

I gave you thrree examples above where the SPG reading and PSI per minute would reflect three different volumes based on 3 different tanks - and we agree on that. Just apply that back to the NOAA calculation, but consider where the SPG readings came from and how tank volume effects them. You'll have to conclude that their SAC calculation is tank dependent as the output is in PSI and will only be accurate when used with that type of tank (or a tank with the same tank factor / internal volume - but we can address that later if you are interested).

 

[SC_Hoaty]

I believe DA Aquamaster did his math just fine. His point (and I agree with it) is that your consumption rate expressed in a standard-volume-per-unit-time is not tank dependent. If you express consumption in pressure-per-unit-time, you are baiting a trap - because it it so tank dependent. For the extreme, compare the time it would take you to empty a Spare Air or small pony vs double 120s. The calc that yields your SAC in psi/min is going to depend on which tank you are breathing from.

Why is this important? For dive planning. Will you run out of gas or bottom time.

If their calculation gives an output in PSI per minute, then it is tank dependent. You don't have to go any farther than the unit analysis to see it, regardless of what they might say about it.

No problems with this statement - if you think this is incorrect, go back to the Spare Air vs double 120s.

 

[Insta-Gator]

Read the last half of the post I added while you were typing yours to show how it's applied in the real world. Oddly enough you might realize we agree, but that you just don't see it as NOAA uses tank volume, they just don't note it up front - but it's an essential factor in the PSI figure they use.

If you look at the NOAA calculation you posted, they are clearly mixing psi and volume - something you suggest should not be done.

SAC = (PSI/time)/ATA. (true)

Where do you think the PSI in the calculation came from? It came from the SPG readings (true) on the reg attached to a specific tank (NOT True if you are referring to tank size) for the gas used over that time at that depth.

I gave you thrree examples above where the SPG reading and PSI per minute would reflect three different volumes based on 3 different tanks - and we agree on that. Just apply that back to the NOAA calculation, but consider where the SPG readings came from and how tank volume effects them. You'll have to conclude that their SAC calculation is tank dependent as the output is in PSI and will only be accurate when used with that tank.

3000 PSI is 3000 PSI, regardless of tanks size.

 

[fire_diver]

3000 PSI is 3000 PSI, regardless of tanks size.

3000 psi is meaningless for a consumption calculation unless you are measuring it against a tank volume. That like asking what the MPG of car is, but ignoring the number of gallons that started in the tank.

 

[DA Aquamaster]

3000 PSI is 3000 PSI, regardless of tanks size.

There's no room for disagreement there and I'm happy to throw you that bone, but as noted in the post above it is totally meaningless for dive planning or SAC calculation purposes.

And more to the point you brought up in the first place, in the NOAA calculation you drug in here, the PSI output of the calculation is totally meaningless without knowing tank's capacity.

 

[Insta-Gator]

DA Aquamaster, if you can show me where and how to set the tank size on any SPG, I will confess to having lost what little sense I was born with.

I don't know about you, but I was trained to monitor my "pressure" using an SPG (submersible Pressure gauge) but, hey, if you think pressure is meaningless, then have at it.

We will have to just agree to disagree.