A simple poll. What term do you use to describe the amount of gas consumed per minute at the surface, and what do you measure that in?
Gas management terminology
- Thread starter AndyNZ
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OP
I've tended to use RMV as a volumetric term (litres per minute) and SAC as a pressure term (bar per minute).
We've just gone through all our teaching materials and found a mix of RMV, SAC and SCR used in different places in different ways. Looking to standardise on a single term, so curious as to what other people use!
We've just gone through all our teaching materials and found a mix of RMV, SAC and SCR used in different places in different ways. Looking to standardise on a single term, so curious as to what other people use!
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I do the same as Andy ... SAC is expessed as units of pressure per minute, RMV as units of volume per minute.
Both mean the same thing ... just expressed in different ways. And I've seen both terms used differently than I use them. Frankly, it doesn't make much difference as long as you comprehend when it's appropriate to use which, and why ... for imperial measurements, I tell my students to use volume per minute for gas planning, and pressure per minute for calculating consumption rates during the dive.
... Bob (Grateful Diver)
Both mean the same thing ... just expressed in different ways. And I've seen both terms used differently than I use them. Frankly, it doesn't make much difference as long as you comprehend when it's appropriate to use which, and why ... for imperial measurements, I tell my students to use volume per minute for gas planning, and pressure per minute for calculating consumption rates during the dive.
... Bob (Grateful Diver)
I use SAC/cubic feet even though I know (or at least believe) it is "technically" incorrect (or so says my Dr. wife). RMV is "technically" about volume while SAC is "technically" about pressure. But I don't breathe pressure, I breathe volume!
To me it is just easier to comprehend SAC as volume, so I do!
To me it is just easier to comprehend SAC as volume, so I do!
When I say my air consumption is .35-.40 that is RMV. Because there is such a difference in how people use the different terminology, I usually just call it air consumption.
Kevrumbo
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SCR or Surface Consumption Rate, expressed in either pressure or volume per minute at always a 1 ATA reference (a volumetric unit expression is understood to be normalized across all tank ratings).
Example: My cold water SCR is 22 litres/min*ATA
DCR or Depth Consumption Rate, expressed again in either pressure or volume per minute, but multiplied by the particular depth of interest in ATA.
Example: (22 litres/min*ATA)(4 ATA) = 88 litres/min DCR at 30m depth.
Since your SPG is in pressure units, the more useful metric during the acutal dive is the transformed SCR and DCR in pressure units based on your tank(s) rating:
Example: My SCR is 22 litres/min*ATA;
I use double manifolded AL80's with a total tank rating of 22 litres/bar;
Therefore 22 divided-by 22 equals One bar/min*ATA (this is my SCR now in pressure units for the double AL80's).
So then at 30m depth or 4 ATA, I expect my SPG to decrease by a DCR of 4 bar/min. . . [i.e. (One bar/min*ATA) multiplied-by (4 ATA) equals 4 bar/min Depth Consumption Rate in pressure units]
Example: My cold water SCR is 22 litres/min*ATA
DCR or Depth Consumption Rate, expressed again in either pressure or volume per minute, but multiplied by the particular depth of interest in ATA.
Example: (22 litres/min*ATA)(4 ATA) = 88 litres/min DCR at 30m depth.
Since your SPG is in pressure units, the more useful metric during the acutal dive is the transformed SCR and DCR in pressure units based on your tank(s) rating:
Example: My SCR is 22 litres/min*ATA;
I use double manifolded AL80's with a total tank rating of 22 litres/bar;
Therefore 22 divided-by 22 equals One bar/min*ATA (this is my SCR now in pressure units for the double AL80's).
So then at 30m depth or 4 ATA, I expect my SPG to decrease by a DCR of 4 bar/min. . . [i.e. (One bar/min*ATA) multiplied-by (4 ATA) equals 4 bar/min Depth Consumption Rate in pressure units]
RMV ..... in cubic feet per minute at the surface then adjusted for depth and appropriate workload factor ....
SAC is ok so long as you specify that it is in cuft / min.
This needs to be converted to a turn pressure or rock bottom (see i know the term now) pressure after tank factors are established.
Needle up good -- needle far left bad !
Cheers
JDS
SAC is ok so long as you specify that it is in cuft / min.
This needs to be converted to a turn pressure or rock bottom (see i know the term now) pressure after tank factors are established.
Needle up good -- needle far left bad !
Cheers
JDS
I have always used SAC (based in cubic feet / minute) just because that is what came out of the TDI manuals.
I suspect RMV is technically more accurate, but the only place I have heard the term used really extensively is in the Antipodes.
I suspect RMV is technically more accurate, but the only place I have heard the term used really extensively is in the Antipodes.
I generally refer to my gas consumption rate as my SAC and reckon it in the units SCF/min because SAC is what most divers call it and I work in imperial units. I realize that there is confusion when referring to SAC or "surface air consumption" because of the differing definitions of the term. The term "respiratory minute volume" or RMV expressed in SL/min or SCF/min is more precisely defined in the literature and, in fact, when in a teaching situation is what I will use. Any measure of gas consumption rate in terms of units of pressure over time I consider useless because it introduces unnecessary ambiguity.
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