Do you Plan your dive or Dive your plan within NDLs?

[tbone1004]

even with NOAA they admit that it is to reduce confusion because in order to follow the indirect measurement of tank paradigm that they use here, you have to have some sort of a tank factor, which is the root of the problem

 

[Charles2]

My brain hurts

 

[scubadada]

depends on the dive.

If I'm going in a quarry or some other mostly confined area, I just fly by the computer because I'll never get close. If I'm doing ocean diving I'll preplan somewhat. I know my SAC rate *don't use RMV, it's the wrong term, SAC is the right term*, and I know the tank size, and I know the bottom. So I do rock bottom calculations and then work back from what is left and that is my bottom time limit. That combined with the mix will give me a nitrogen intake level and allow me to plan decompression. I use Pastodeco because it's easy and cheap and allows me to input gradient factors, but if you are diving with a computer it is crucial to use plan based around their algorithm.

I don't bother with NDL's because I am deco certified, so if I rack up a 5-10 minute mandatory deco on backgas, it isn't the end of the world, I'd rather use the gas at the bottom then limit myself with an NDL

My impression is that surface respiratory minute volume, SRMV, is the correct nomenclature

 

[lowviz]

As I understand it (or don't), RMV is invariant with respect to depth. It is a measure of the volume of gas you move through your lungs in a minute. It is referenced to your lungs.

SAC is a measure of depletion of your gas supply, it varies greatly with depth. To avoid a bit of confusion they coined the term DAC.

 

[lowviz]

SAC can be calculated at any depth and is then normalized to the surface.

 

[Bob DBF]

My impression is that surface respiratory minute volume, SRMV, is the correct nomenclature

RMV cuft/min @ 1atm
SAC psi/min using RMV and adjusting for tank size and depth for dive planning

Both are used interchangeably all the time. I personally use SAC, which is actually RMV in my case, to calculate usage on the fly to know how many cuft are left in my tank. Not real scientific.


Bob

 

[EFX]

Thanks Bob for posting the units with the terms. As the terms tend to be used interchangeably there can be some confusion as to which term is actually meant. PADI and SSI use both terms and emphasize their intended use for gas planning (first measure SAC in psi/min and then calculate RMV in ft3/min). According to one tech diver on SB TDI uses SAC with units of ft3/min. This diver emphasized that RMV is unnecessary; why add another term. Always use SAC. My understanding talking with tech divers is that SAC is understood to mean ft3/min. Interestingly, my Shearwater Perdix PDC (a technical dive computer) in its dive planning menu asks for RMV which is ft3/min to calculate gas usage for the planned depth and time.

 

[dberry]

As I understand it (or don't), RMV is invariant with respect to depth. It is a measure of the volume of gas you move through your lungs in a minute. It is referenced to your lungs.

SAC is a measure of depletion of your gas supply, it varies greatly with depth. To avoid a bit of confusion they coined the term DAC.

OK, now this is counter to much I've read (and been taught). SAC doesn't vary with depth because SAC is calculated for only one depth - 0 fsw. The "S" stands for Surface, which makes sense because it has been adjusted to 1 ATA (0 fsw). SAC can be calculated after a dive from your AC (air consumption in psi / min) and your average depth (to get the average pressure on the dive). Units are psi/min (AT ONE ATA). True SAC (as defined by NOAA and PADI, at least) sort of sucks because it depends completely on the volume of the cylinder in use. And the system in North America makes you work to find the actual volume of your cylinders.

Of course your AC (air consumption) DOES depend on depth. If you always use the SAME cylinder you can just plan pretty well using the SAC and planned depth to figure out how many psi you'll use per minute.

RMV is the volume of air you use in a minute (cuft / min), again corrected to the surface (1 ATA). This number does NOT depend on the size of your cylinder. It is basically the SAC corrected for the "cylinder constant" for whatever cylinder used when you originally measured your SAC. Now you have a number you can use with any cylinder, but you'll still need to know the volume of the new cylinder (or the cylinder constant) and the planned depth before you can figure out how many minutes you'll have before the tank runs dry.

So, both SAC and RMV are values that are true on the surface ONLY. Both need to be corrected for pressure at depth.

Now MY head hurts.

 

[lowviz]

You are correct. Bitten by the good old "it" again. Allow me to indicate what I was thinking when I wrote that rather ambiguous sentence:

... SAC is a measure of depletion of your gas supply, it (rate of depetion) varies greatly with depth. To avoid a bit of confusion they coined the term DAC.

The following post is equally ambiguous, let's just say that one's SAC can be calculated from measurements taken at any depth.

 

[Kevrumbo]

RMV cuft/min @ 1atm
SAC psi/min using RMV and adjusting for tank size and depth for dive planning

Both are used interchangeably all the time. I personally use SAC, which is actually RMV in my case, to calculate usage on the fly to know how many cuft are left in my tank. Not real scientific.

Bob

so the point still stands, what does SAC expressed in pressure per unit time do for you? the answer is absolutely nothing, it is just used as a stepping stone to get from DAC to RMV by those that think it is helpful to break the math up.

Why do you derive this using volume units to check & confirm against your SPG --which indicates remaining gas at depth in pressure units? In other words, how about tracking gas consumption for your particular tank from the start in psi (or bar) per minute, per depth factor in ATA ?

Example: 22 liters/min per ATA volume Surface Consumption Rate divided-by a 11 liter/bar tank (an AL80) equals 2 bar/min per ATA pressure Surface Consumption Rate.

So at 21 meters depth which is 3.1 ATA (21 divided-by 10 plus 1 is 3.1 ATA), you stay for 10 minutes. 10 minutes multiplied-by 3.1 ATA multiplied-by 2 bar/min per ATA equals 62 bar consumed.

Confirm with SPG that it shows a delta of 62 bar less in that 10 minute interval, so if you started with a full 200 bar cylinder, the SPG indicates "138 bar" remaining (i.e. 200 minus 62 equals 138 bar).

Doesn't that make better algebraic/arithmetic sense and unit consistency?