SAC Rate Changes with Depth

[kelemvor]

SAC is psi/min
rmv is cf/min

Think you have that backwards.

 

[pjhansman]

I'm gonna go with RMV being Respiratory Minute Volume which by default means it's cubic ft or litres consumed per minute....yes? The two acronyms seem to be used interchangeably, but I'm thinking we're talking about RMV here .

 

[Bob DBF]

SAC is psi/min
rmv is cf/min

Think you have that backwards.

I believe @hammet is correct, however they are used interchangeably so much it's quite confusing. As long as one can plan their gas requirements and keep track of gas on the dive, it obviously makes no difference.

A rose by any other name...


Bob

 

[Kevrumbo]

Just to be clear - SAC can not be measured in psi. The psi depends absolutely on the volume of the tank and it's working pressure. SAC can only be measured in gas volume.

(Tank) Rated Vol / Working Pressure = cf/psi

cf/psi x psi used = cf used

Your SPG reads in pressure units, so how can SAC not be measured and expressed in psi/min or bar/min? As long as you state "pressure SAC" along with the volume rating of the cylinder you're using, this is within convention and there is no ambiguity.

Manually performing the measurement with an AL80 (11 liter/bar) Tank with full service pressure of 207 bar:

I consume 30 bar of air in 10min at depth 3 ATA (20msw:66fsw), for a depth consumption rate ("DCR") of 3 bar/min.

My surface consumption rate ("SCR") is therefore: 3 bar/min ÷ 3 ATA = 1 bar/min per ATA.

Hence my "pressure SAC" with an AL80 (11 liter/bar) Cylinder is 1 bar/min per ATA;
In US Imperial Units, this is 14.5 psi/min per ATA with an AL80 (0.025 cf/psi) Tank.

To calculate "volume SAC" or same as RMV, multiply by the cylinder rating factor of the AL80:

(1 bar/min per ATA) x (11 liters/bar) = 11 liters/min per ATA;
Or in US Imperial Units: (14.5 psi/min per ATA) x (0.025 cf/psi) = 0.36 cf/min per ATA.

Surface air consumption rate to be meaningful need be expressed in gas volume/ time. Let's not forget what the acronym stands for when we try to define it. . .
Cameron

Yes, as a standard measurement across all cylinders, RMV or "volume SAC" must be stated in volume units per time. Here's where potential confusion lies:

Given the RMV above of 11 liters/min per ATA and a single AL80 tank (11 liters/bar):

11 liters/min per ATA ÷ 11 liters/bar = 1 bar/min per ATA pressure SAC.

If you instead use a manifolded AL80 twinset of 11 liters/bar cylinders (or 22 liters/bar total):

11 liters/min per ATA ÷ 22 liters/bar = 0.5 bar/min per ATA pressure SAC.

Therefore you have the same standard RMV or volume SAC rate across all cylinder sizes as well as twinsets, but not necessarily the same pressure SAC rate.

Clearly, it makes more sense that Double AL80's last twice as long and have a pressure that moves at a slower rate per minute, which is half of a single tank AL80 pressure rate per minute, because the Doubles have twice the volume capacity for a given RMV of 11 liters/min per ATA.

Anyway, the lesson is: if you state a "pressure SAC rate", you must quote the tank(s) size or total cylinder volume rating associated with it. . .

(Assuming OP, @Charred: are you using an AL80?)

 

[stepfen]

My surface consumption rate ("SCR") is therefore: 3 bar/min ÷ 3 ATA = 1 bar/min per ATA.

How come something called "Surface consumption rate" has a unit of whatever per ATA??? Surface is surface (i.e. 1 ATA) no matter what. Hence SAC (i.e. Surface Air Consumption) to me makes perfect sense to be expressed in litters (or cubic feet) per minutes.

I don't want to open a debate. I just use this to show that to me we are talking about the same thing with different terms/definitions. Some people understand it and prefer it one way while others understand and prefer it the other way.

As long as one understands the concept and can do correct planning in any way that would be enough. Problems arouse when somebody don't understand any way good enough or when people try to mix them and still expect to make sense or even worst try to impose one way or the other.

Just my 2cents

 

[hammet]

How come something called "Surface consumption rate" has a unit of whatever per ATA??? Surface is surface (i.e. 1 ATA) no matter what. Hence SAC (i.e. Surface Air Consumption) to me makes perfect sense to be expressed in litters (or cubic feet) per minutes.

I don't want to open a debate. I just use this to show that to me we are talking about the same thing with different terms/definitions. Some people understand it and prefer it one way while others understand and prefer it the other way.

As long as one understands the concept and can do correct planning in any way that would be enough. Problems arouse when somebody don't understand any way good enough or when people try to mix them and still expect to make sense or even worst try to impose one way or the other.

Just my 2cents

It's called SURFACE air consumption because it your breathing rate in psi normalized to surface pressure to take depth out of the equation.

RMV goes one more step by taking cylinder size out of the equation as well as depth.

 

[stepfen]

It's called SURFACE air consumption because it your breathing rate in psi normalized to surface pressure to take depth out of the equation.

RMV goes one more step by taking cylinder size out of the equation as well as depth.

That's exactly my point. To you it makes sense. To me it does not (and no matter what you tell me it won't so plz don't try).
I can still plan a safe dive (to my level) though...

 

[agilis]

What am i missing? SAC is Surface Air Consumption. Depth consumption rates will obviously be different as air compresses.

 

[Kevrumbo]

It's called SURFACE air consumption because it your breathing rate in psi EXTRAPOLATED to surface pressure to take depth out of the equation not to take the cylinder size out of it.

Actually, the proper term is "normalized reference" to the surface.

What am i missing? SAC is Surface Air Consumption. Depth consumption rates will obviously be different as air compresses.

How come something called "Surface consumption rate" has a unit of whatever per ATA??? Surface is surface (i.e. 1 ATA) no matter what. Hence SAC (i.e. Surface Air Consumption) to me makes perfect sense to be expressed in litters (or cubic feet) per minutes.

I don't want to open a debate. I just use this to show that to me we are talking about the same thing with different terms/definitions. Some people understand it and prefer it one way while others understand and prefer it the other way.

As long as one understands the concept and can do correct planning in any way that would be enough. Problems arouse when somebody don't understand any way good enough or when people try to mix them and still expect to make sense or even worst try to impose one way or the other.

Just my 2cents

That's exactly my point. To you it makes sense. To me it does not (and no matter what you tell me it won't so plz don't try).
I can still plan a safe dive (to my level) though...

Because to have the proper units consistent with the algebraic operation to calculate a depth consumption rate ("DCR"), you must retain the "per ATA" of the particular SAC rating.

Example:
At a depth of 4 ATA (30msw) and a pressure SAC of 1 bar/min per ATA with a 11L/bar Aluminium cylinder:

Your depth consumption rate ("DCR") will be (4 ATA) x (1 bar/min per ATA) = 4 bar/min.

Another Example:
At a depth of 10 ATA (90msw!) and a pressure SAC of 1 bar/min per ATA with a 11L/bar Aluminium cylinder:

Your DCR will be (10 ATA) x (1 bar/min per ATA) = 10 bar/min at 90msw.

For 200 bar in your tank, you will have 200 bar ÷ 10 bar/min = 20min of Air at 90msw
(Assuming you maintain the same DCR as you lose consciousness due to the extreme narcosis).

 

[Altamira]

I thought I understood SAC, but now my head is about ready to explode. Time for a margarita! Maybe after a few of those, everything in this discussion will become clear.