Do you Plan your dive or Dive your plan within NDLs?
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What particular tank (or twinset) would that be? 10L? 12L? 15L? D7? D8.5? D12? Your 1 bar/min could be anything from 10 to 24 SLM (i.e. from mouse to hoover), depending on the tank/twinset size. I use 10L tanks, and I've never recorded a SAC as low as 10 SLM. And I log my SAC on every dive since my PDC is AI.
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ok, here goes. It is actually much more simple if you ignore RMV.
Respiratory minute volume is measured, not calculated, as a function of tidal volume and number of respirations per unit time. This is a medical term that is almost always expressed as liters per minute. As we do not actually have a way to measure this, and no way to accurately calculate it, it is not the appropriate term. Somehow a few years ago people started getting "smart" and wanted to sound "smart" and used this term taken from the medical professions. Somehow SDI/TDI has decided this is useful and I believe is the only agency currently using it. I could be wrong, don't keep track of everyones scuba courses, but this term has started creeping in over the last decade.
SAC and DAC are unitless, meaning they are expressed in units of consumption. This can be PSI/Min, this can be L/min, CF/min, doesn't matter, there is no unit assigned to it, it is simply the consumption of some unit per unit time.
DAC is the only thing that we measure, and it is ALWAYS measured in unit pressure per unit time, and it is either bar/min or psi/min for our application.
DAC doesn't really do us very good, so you have to normalize it to something useful for dive planning and that is when you divide the DAC by your pressure in ATA. In the same calculation this should be normalized to a unit volume to make it actually useful.
If you want to bind any of them to a unit, then leave DAC as psi/min or lpm because that is what you are going to read at depth, so you can use that for planning how much time you have remaining, and then when you convert that to SAC for pre-dive planning, then it goes from pressure/time to volume/time. No reason to have three terms and use SAC as a stepping stone.
In the US because we are stupid, we measure out tanks indirectly which means we have to use tank factors to equate pressure to useful volume. Dive stats, I used an AL80, burned 2900psi, and stayed at 33fsw for 100 minutes.
American way. 2900psi*77.4cf/3000psi=75cf. 75cf/100mins=.75cfm/2ata=.38cfm
Euro way. That AL80 11l/bar. I used 200bar over 100minutes at 33ft, so I consumed 2200l total, over surface equivalent of 200 minutes, so my SAC rate was 11lpm or 0.38cfm.
Euro way is infinitely easier, so I have gone to bar gauges and measure my cylinder volume directly so I don't have to deal with tank factors and having to do extra math if the bottle isn't filled to working pressure.
Respiratory minute volume is measured, not calculated, as a function of tidal volume and number of respirations per unit time. This is a medical term that is almost always expressed as liters per minute. As we do not actually have a way to measure this, and no way to accurately calculate it, it is not the appropriate term. Somehow a few years ago people started getting "smart" and wanted to sound "smart" and used this term taken from the medical professions. Somehow SDI/TDI has decided this is useful and I believe is the only agency currently using it. I could be wrong, don't keep track of everyones scuba courses, but this term has started creeping in over the last decade.
SAC and DAC are unitless, meaning they are expressed in units of consumption. This can be PSI/Min, this can be L/min, CF/min, doesn't matter, there is no unit assigned to it, it is simply the consumption of some unit per unit time.
DAC is the only thing that we measure, and it is ALWAYS measured in unit pressure per unit time, and it is either bar/min or psi/min for our application.
DAC doesn't really do us very good, so you have to normalize it to something useful for dive planning and that is when you divide the DAC by your pressure in ATA. In the same calculation this should be normalized to a unit volume to make it actually useful.
If you want to bind any of them to a unit, then leave DAC as psi/min or lpm because that is what you are going to read at depth, so you can use that for planning how much time you have remaining, and then when you convert that to SAC for pre-dive planning, then it goes from pressure/time to volume/time. No reason to have three terms and use SAC as a stepping stone.
In the US because we are stupid, we measure out tanks indirectly which means we have to use tank factors to equate pressure to useful volume. Dive stats, I used an AL80, burned 2900psi, and stayed at 33fsw for 100 minutes.
American way. 2900psi*77.4cf/3000psi=75cf. 75cf/100mins=.75cfm/2ata=.38cfm
Euro way. That AL80 11l/bar. I used 200bar over 100minutes at 33ft, so I consumed 2200l total, over surface equivalent of 200 minutes, so my SAC rate was 11lpm or 0.38cfm.
Euro way is infinitely easier, so I have gone to bar gauges and measure my cylinder volume directly so I don't have to deal with tank factors and having to do extra math if the bottle isn't filled to working pressure.
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I do not get it
. My SAC is 12/15L/Min. This is INDEPENDANT of the tank of course.
So my consumption is 1 bar ( or a bit less ) from a 15 L tank.at the surface ( 1 bar ).
Why does it need to be so complicated in Imperials? I wonder.
Sorry Tbone, I wrote may answer just at the time when you were writting your post. thank you for clarifying
. My SAC is 12/15L/Min. This is INDEPENDANT of the tank of course.So my consumption is 1 bar ( or a bit less ) from a 15 L tank.at the surface ( 1 bar ).
Why does it need to be so complicated in Imperials? I wonder.
Sorry Tbone, I wrote may answer just at the time when you were writting your post. thank you for clarifying
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"RMV and Consumption Rate
Respiratory Minute Volume (RMV) is the volume of gas at ambient pressure and temperature being breathed by the diver per minute. This value is expressed in cu ft / min and the value does not vary with the depth and is a measure of how fast the diver is breathing. This is commonly (and incorrectly) referred to as the “SAC rate”.
Surface Air Consumption Rate (SAC) is the pressure of gas in the tank at ambient pressure and temperature being consumed. It is equivalent to the RMV but is expressed in psi / min, and is dependent upon the size of the tank.
Consumption rate at depth (Cd) is a measure of the amount of gas which is actually being used at depth. This is measured in cu ft / min similarly to RMV, but measures how fast the tank is actually being drained. It is the volume of gas being breathed adjusted for a pressure of 1 atmosphere.
These definitions agree with both the US Navy and NOAA definitions."
Source:
http://blog.scriptkiddie.org/2010/0...for-recreational-divers/#sthash.ttGekPAa.dpuf
Respiratory Minute Volume (RMV) is the volume of gas at ambient pressure and temperature being breathed by the diver per minute. This value is expressed in cu ft / min and the value does not vary with the depth and is a measure of how fast the diver is breathing. This is commonly (and incorrectly) referred to as the “SAC rate”.
Surface Air Consumption Rate (SAC) is the pressure of gas in the tank at ambient pressure and temperature being consumed. It is equivalent to the RMV but is expressed in psi / min, and is dependent upon the size of the tank.
Consumption rate at depth (Cd) is a measure of the amount of gas which is actually being used at depth. This is measured in cu ft / min similarly to RMV, but measures how fast the tank is actually being drained. It is the volume of gas being breathed adjusted for a pressure of 1 atmosphere.
These definitions agree with both the US Navy and NOAA definitions."
Source:
http://blog.scriptkiddie.org/2010/0...for-recreational-divers/#sthash.ttGekPAa.dpuf
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incorrect according to whom? The Navy and NOAA actually measured the RMV's of their divers when they created the charts for dive planning, we don't have the ability to measure that on scuba, we only have the ability to measure consumption in unit pressure and convert it to something more meaningful, because it is a calculation instead of a measurement, RMV is still not the appropriate term for what we are doing. Doesn't matter either way as long as you are consistent calculating it, but when you use RMV, SAC, and DAC it gets confusing because SAC is just a stepping stone in the process instead of going straight from a DAC in pressure/time to SAC in volume/time
SAC expressed in pressure (per minute) is not a volume. PSI and bar are not units of volume. So SAC is not the volume used in one minute. It's the pressure drop (in a given bottle) per minute.
The V in RMV stands for volume. That's why it is expressed in cu-ft or liters per minute. Because cu-ft and liters are a unit of volume.
RMV is independent of any cylinder. Thus why I normally only do my own calculations using RMV. SAC is completely dependent on the specific cylinder.
At least, that's my understanding.
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I gave the source.
Fine.
However, if someone uses RMV for whatever reason, it is one simple step to get back to more familiar consumption rates.
How is it that our calculations are not accurate? If I know that my tank is 77.4 cu-ft surface air volume capacity and has a working pressure of 3000psi, and I know that my pressure changed from 2800 psi to 1300 psi in 30 minutes, and I know that my average depth over that 30 minute period was 33 feet, I can calculate an RMV. I get 0.65 cu-ft/min for this example.
What would make the resulting number inaccurate? Whether my arithmetic is accurate or not is not my question. The question is, if you have those pieces of data, which I think we can collect accurately, then what would make the subsequent calculation of RMV be inaccurate?
I recognize that the definition of RMV as used in scuba may be different than the definition of RMV in other fields. But, as it is defined and used in scuba, what makes it inaccurate?
For scuba, what I was taught has always seemed pretty straightforward and simple to me. SAC is psi (or bar) per minute from a given cylinder. RMV is surface air volume (cu-ft or liters) per minute, period.
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the source was some guy who doesn't like it, fine, he never cited anything that said it was incorrect. If you read the citations you would see where NWGratefulDiver says the following
"Your SAC rate is defined as the amount of gas you breathe in one minute at the surface. It can be expressed as pressure (PSI) or volume (cubic feet). For the purpose of this discussion, and to avoid confusion, we will refer to your SAC rate in terms of pressure. When expressing your air consumption rate as volume, we will refer to it as Respiratory Minute Volume (RMV)."
Again, the reason RMV was used is to avoid confusion, not because it is the right term. RMV has a specific formula per the medical textbooks, and the original definition is volume per respiration, times respirations per minute. We can't measure either of those on scuba.
SAC in pressure is pretty much useless. You need to know your unbounded consumption rate in terms of volume per unit time, and by converting it to pressure for a specific cylinder is one extra math problem that you don't actually have to perform. Why waste the time? My sac is 0.5cfm, I have determined that I have 66cf or 2000psi available to me after rock bottom calculations, I am planning a dive to 99fsw, how much time do I have? 0.5*4=2cfm at depth, you have 66cf available to you which is 33 minutes. You don't ever need to do any calculations in PSI, it is all volume and time. You have to equate volume to pressure because that is what the gauges read out, but that is simply. Same dive plan but you have an NDL of 33 minutes and you need to know how much gas you need. 33 mins *4ata *0.5cfm=66cf. I'm diving a HP120, 66/120*3500=1900psi that I can use. Why create a third term that has no use?
edit: We can't accurately calculate it because the gauges aren't accurate enough to tell you the pressure drop for each breath and you aren't counting how many breaths you are taking per minute. Therefor you are averaging the total consumption of a total time, and then using that average to approximate your RMV. If your instructor taught you the way you just explained, what did he say SAC was useful for?
"Your SAC rate is defined as the amount of gas you breathe in one minute at the surface. It can be expressed as pressure (PSI) or volume (cubic feet). For the purpose of this discussion, and to avoid confusion, we will refer to your SAC rate in terms of pressure. When expressing your air consumption rate as volume, we will refer to it as Respiratory Minute Volume (RMV)."
Again, the reason RMV was used is to avoid confusion, not because it is the right term. RMV has a specific formula per the medical textbooks, and the original definition is volume per respiration, times respirations per minute. We can't measure either of those on scuba.
SAC in pressure is pretty much useless. You need to know your unbounded consumption rate in terms of volume per unit time, and by converting it to pressure for a specific cylinder is one extra math problem that you don't actually have to perform. Why waste the time? My sac is 0.5cfm, I have determined that I have 66cf or 2000psi available to me after rock bottom calculations, I am planning a dive to 99fsw, how much time do I have? 0.5*4=2cfm at depth, you have 66cf available to you which is 33 minutes. You don't ever need to do any calculations in PSI, it is all volume and time. You have to equate volume to pressure because that is what the gauges read out, but that is simply. Same dive plan but you have an NDL of 33 minutes and you need to know how much gas you need. 33 mins *4ata *0.5cfm=66cf. I'm diving a HP120, 66/120*3500=1900psi that I can use. Why create a third term that has no use?
edit: We can't accurately calculate it because the gauges aren't accurate enough to tell you the pressure drop for each breath and you aren't counting how many breaths you are taking per minute. Therefor you are averaging the total consumption of a total time, and then using that average to approximate your RMV. If your instructor taught you the way you just explained, what did he say SAC was useful for?
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