RMV math…

[Doc]

Take into account the gas volume represented by charged LP & HP hoses.

There’s that.

 

[lowwall]

Not as I understand it. Your lung volume is what it is, and thus RMV is not depth dependent. What varies is the number of gas molecules a given breath will pull in as depth varies.

In the big picture, Volume (and thus RMV) is applicable across cylinders. SAC will be specific to a particular cylinder (or cylinder). The ratio of gas capacity and cylinder pressure is the conversion between RMV and SAC and is true for both imperial and metric. (Capacity at 1 bar equals the water volume for metric folks. The "tank factor" I referred to above is one form of that ratio.)

If I'm concerned with the rate my cylinder pressure drops, I scale my SAC by the ambient pressure at whatever depth. Scaling my RMV by ambient pressure gives the rate of change of the volume that gas occupies at surface pressure.

My understanding is different. I did a quick Google search of "rmv sac scuba" before posting and thought I had verified it via this page: RMV vs. SAC: What’s the Difference? - CaveDiving.com

The thing about RMV is that it varies with depth. For example, at a depth of four atmospheres — that’s 30 meters or just under 100 feet — our RMV will be twice what it would be at a depth of two atmospheres or ten meters/33 feet. This is just Boyle’s law in action.

In order to make RMV usable, we need to convert it to something else. This is where Surface Air Consumption or SAC rate comes in.

SAC rate is nothing more than RMV adjusted for ambient pressure at sea level. In other words, if your RMV at a depth of four atmospheres is eight liters per minute, it means your SAC rate is one-fourth of that or two liters per minute.

But after reading your post, I looked further and your interpretation does appear to be the prevailing view today. I like how it is stated on Your RMV Rate, How To Calculate It To Be A Better DIver!

SAC stands for Surface Air Consumption (rate). Many divers use the terms RMV and SAC interchangeably. Strictly speaking, however, SAC is supposed to be measured in PSI per minute or BAR per minute. Because it is a measurement of pressure, SAC is specific to the tank that it was measured on. This means your SAC rate will be different when you use a different-sized tank.

As I final check, I went back to see what the US Navy Dive Manual has to say. Interestingly, they do not use the term SAC. They only use RMV which is measured as a volume (either liters or cubic feet) feet per minute.

They do have a formula for measuring "consumption rate" or C:

C = ((depth in feet + 33)/33) x RMV in cubic feet per minute

So the consumption rate at the surface (which we could call "surface consumption rate" ) = ((0+33)/33) x RMV = 1xRMV = RMV. Which is what we would expect except it's in cubic feet per minute not psi or bar per minute.

 

[lowwall]

I was working at NASA when two engineering teams decided to use different units of measure and didn't tell each other. So when the control unit sent "10" until touchdown to the landing unit...the control unit meant meters and the landing unit thought it was feet and all hell broke loose and destroyed the landing craft.

Are you somewhat mis-remembering the Mars Climate Orbiter loss or did they do this twice?

Mars Climate Orbiter - NASA Science

Key Facts Launch Dec. 11, 1998 Launch Site Cape Canaveral Air Force Station, Florida Launch Vehicle Delta II End of Mission Sept. 23, 1999, lost on arrival

solarsystem.nasa.gov

An investigation indicated that the failure resulted from a navigational error due to commands from Earth being sent in English units (in this case, pound-seconds) without being converted into the metric standard (Newton-seconds).

The error caused the orbiter to miss its intended orbit (87 to 93 miles or 140 to 50 kilometers) and to fall into the Martian atmosphere at approximately 35 miles (57 kilometers) in altitude and to disintegrate due to atmospheric stresses.

 

[tursiops]

Since this is the Basic Scuba forum, I want to note that we have not always been using the terms RMV and SAC (surface air consumption) rate correctly in this thread. RMV varies with pressure. SAC is RMV at 1ata and is thus a fixed number.

It can get confusing because you tend to use RMV to figure out your SAC rate. And then when you are planning a dive, you use SAC rate to figure out RMV.

For example, let's say you completed a 40 minute dive at an average depth of 20m/66’ and used 50 cubic feet of gas.

Your RMV for the dive is 50/40 or 1.25 cubic feet per minute. To figure out the SAC you divide RMV by the average ata which is 3 in this example, so SAC is .417.

Now that you have the SAC rate, you can calculate RMV at any depth. It's simply SAC x ata.

You know, this might be easier to grasp if we used weight rather than volume. I'll save that thought for another post.

As a newb...this had me confused as hell for the last hour. The documentation I was using online (see above) was calculating SAC using PSI. So the SAC would be shown as 16.15 not 0.42 and I was racking my brain with how the heck can his SAC be a number lower than 1 when my calculations are always a number greater than 10.

I may be more special than others....but it took me that long to figure out people are not saying what unit they are using when they quote SAC

SAC in cuft/min or SAC in psi/min

Not as I understand it. Your lung volume is what it is, and thus RMV is not depth dependent. What varies is the number of gas molecules a given breath will pull in as depth varies.

In the big picture, Volume (and thus RMV) is applicable across cylinders. SAC will be specific to a particular cylinder (or cylinder). The ratio of gas capacity and cylinder pressure is the conversion between RMV and SAC and is true for both imperial and metric. (Capacity at 1 bar equals the water volume for metric folks. The "tank factor" I referred to above is one form of that ratio.)

If I'm concerned with the rate my cylinder pressure drops, I scale my SAC by the ambient pressure at whatever depth. Scaling my RMV by ambient pressure gives the rate of change of the volume that gas occupies at surface pressure.

Not quite. Conventionally, SAC is in units of pressure per minute (per atm, as well, if we're being absolutely correct about units).

Lol, I stopped fighting that battle long ago. Overwhelming usage is RMV for the cuft/min rate and SAC for psi/min (or the metric liter/min and bar/min rates).

Yes, they are both consumption rates, so Surface Air Consumption seems applicable, but that's definitely the minority view.

Bottom line is the fewest number of people will be confused (or try to correct you) if you give units and choose "SAC" vs "RMV" based on those units. It's an imperfect world...

If you think you have the "correct" usage for SAC and RMV, you may be deluded. There is a whole thread on this.
Many use SAC tp mean pressure/minutes at 1 ATA, and RMV to mean volume/min per ATA. But that is not agreed, and there is no authority to refer to.
The only way out of the dilemma is to say what units you are using; if you overtly say "psi/min at the surface" then I don't care what you call it, I can understand what you mean. If you tell me your SAC is 0.7, then I have to guess that you mean cuft/min and hope that you mean "at the surface." If you tell me your SAC is 20, then I have to guess that either (1) you mean liters/min and hope that you mean "at the surface," or that (2) you mean psi/min but I don't know how to interpret that unless I know what size cylinder you are breathing from, and either way I have to hope that you mean "at the surface."

So if we are discussing breathing rates and are going to dive together, please don't make me guess and assume what you mean. Tell me your units, do you mean at the surface, and what size is your cylinder. Then and only then can we successfully communicate.

 

[quietlife4me]

Are you somewhat mis-remembering the Mars Climate Orbiter loss or did they do this twice?

Mars Climate Orbiter - NASA Science

Key Facts Launch Dec. 11, 1998 Launch Site Cape Canaveral Air Force Station, Florida Launch Vehicle Delta II End of Mission Sept. 23, 1999, lost on arrival

solarsystem.nasa.gov

An investigation indicated that the failure resulted from a navigational error due to commands from Earth being sent in English units (in this case, pound-seconds) without being converted into the metric standard (Newton-seconds).

The error caused the orbiter to miss its intended orbit (87 to 93 miles or 140 to 50 kilometers) and to fall into the Martian atmosphere at approximately 35 miles (57 kilometers) in altitude and to disintegrate due to atmospheric stresses.

that's the one - 24 years ago my brain was better (maybe)

 

[quietlife4me]

So if we are discussing breathing rates and are going to dive together, please don't make me guess and assume what you mean. Tell me your units, do you mean at the surface, and what size is your cylinder. Then and only then can we successfully communicate.

Where's the "preach it brother" emoji?

 

[L13]

People who use SAC say RMV changes with pressure.
People who use RMV say SAC changes with pressure.
Everybody(almost) uses their preferred term at 1 ATA and multiplies by depth in ATA for consumption.

People who use RMV use liter/min or cuft/min.
people who use SAC use 100psi/min or cuft/min (or occasionally liter/min).

There are people who do not conform to any of the above statements .

 

[tursiops]

It doesn't help that Shearwater displays SAC (pressure/min) and Subsurface calls its calculation of volume/minute (after you've put in the requisite info) SAC, instead of the more usual RMV.

 

[scubadada]

Using standard definitions is most important when communicating with others. This topic has been beaten to death on SB

SAC is pressure/time/ata, psi/min/ata or bar/min/ata and is cylinder dependent
RMV is volume/time/ata cu ft/min/ata or l/min/ata and is cylinder independent

RMV is used to make gas requirement calculations regardless of the cylinder(s) used

 

[L13]

You do you, amigo. My doubles TF is about 6 cuft/HPsi rather than metric 26 liters. I know which is easier for me to calc with in the water.

For your own cylinders if you measure pressure in PSI, TF is easy to use since you already know it.

But, if you walk up to an arbitrary tank labeled in cuft you will have to look up or calculate TF if you are using PSI (or lookup/calculate liters if you are using BAR).

If your tanks are labeled in liters (most of the world) and your pressure is in BAR (most of the world), then everything is easy, with no lookup/calculating of TF.

Unfortunately, I live in the USA, and all my buddies are imperial unit types. So I end up using PSI, cuft, and TF .