SAC vs Surface RMV

[PfcAJ]

I call it a SAC rate, but under the above definition I suppose "RMV" is the term used for that.

 

[Cave Diver]

Jax I *think* this is the post/thread I was referring to. I'll do some more digging later.

I was always taught to get SAC rate by swimming at a prescribed depth for a given time, such as 10 minutes @ 30'

SAC = Gas Consumed/(Depth in ATA x Time)

ATA = (FSW/33)+1
ATA = (30/33)+1 = 1.91

Assuming 700 psi used then SAC = 700/(1.91 x 10) = 36.6psig per minute used.

Respitory Minute Volume (RMV) represents the volume of gas consumed rather then the psi.

RMV = SAC/(Cylinder Working Pressure/Cylinder Volume)

A diver using 700 psi on an AL80 (CWP= 3000psi, CV = 80cft) would have an RMV of:

RMV= 36.6/(3000/80)
RMV= 36.6/(37.5)
RMV= .976 cubic feet per minute

Once you know your SAC for a given cylinder and your RMV for a given cylinder, you can use the following formula to calculate your SAC rate for any given cylinder.

SAC = (CWP/CV) x RMV

Example: A diver with an RMV of .976 wishes to use an AL100cft cylinder

SAC = (3000/100) x .976 = 29.28

Since the SAC rate is based on cylinder volume, knowing your RMV is important for gas management planning when using different cylinders!

Comparing SAC rates is useless if divers are using different size cylinders.

 

[Kern]

Sure. What is it, actually?

I don't suppose you can ever say what it will be on any given dive really. But when I figure my SAC, I do it at rest, then again at my normal work load, & again at extreme work load. This is all done while diving, not while sitting on the surface.

Don't focus on my use of the term SAC. Over here, everyone (at least the ones I'v encountered) call it SAC & measure it in litres per minute. RMV is a term that's never heard.

 

[ianr33]

SAC? RMV? Who cares what you call it ? Only thing that matters is you have a reasonable idea how much gas you are going to use.
Knowing the number in cu ft rather than psi makes sense if you ever use different size tanks or dive planning software.

Best SAC I ever had was around 0.4cu ft/min (Single tank,warm water,very slow dive)

With sidemount and drysuit its around 0.6 unless I'm working really hard. If V-planner tells me I'll only use 2/3rds of my gas with a SAC of 0.6 I figure I'm good to go.

 

[Protist]

A good site I found regarding the formulas....Plus some.
http://www.ndc.noaa.gov/pdfs/AirRqmtFormulas.pdf

 

[mikemill]

Honestly, both SAC and RMV are useless terms in of themselves. Neither have units attached to them which would allow for useful calculation. As a group you can give some default units for each but that doesn't help when talking between groups using different default units. For example TSandM used psi/min for SAC where I think of it in cuft/min but someone else might think of it in L/min. RMV (without specifying surface) is useless since it doesn't say at what pressure. So none of them give enough data on their own.

Much better to think and work in volume, time, and pressure and avoid the sillyness.

 

[Kern]

Just for fun I looked this up in my old TDI deco procedures manual. It's a metric manual, & this is the nomenclature & formula used.

SAC = (bar used/time/pressure depth) x WC

WC = water capacity of the cylinder.

PS. If it wasn't already clear, the WC is also the tank factor.

 

[MXGratefulDiver]

First off, I'm not sure what SDI's version of SAC would be useful for ... a baseline is only any good if it applies to something. Taking measurements while sitting perfectly still on the surface have no real applicability to what goes on while you are diving.

My definitions of SAC and RMV are provided in my gas management article ... which is how I teach them. Essentially, they are two representations of the same value ... expressed in different ways ... and they do not represent a baseline, but rather a range of values that will be dependent on the conditions in which they are being measured.

When we breathe on the surface, we don't have a baseline ... our breathing rate will be dependent on what we are doing ... sitting on the sofa reading a book, walking, running, sleeping ... those will all give you different consumption rates. The same applies underwater. The only thing that truly matters is having some understanding of what the high and low end of your consumption range are, and where various activities will put you on that range.

SAC and RMV both represent that range. The difference is that SAC is expressed in terms of pressure per minute, and is dependent on the cylinder you are using ... while RMV is expressed in terms of volume per minute, and is independent of the cylinder.

RMV is used for dive planning ... because we know the volume of our cylinder.

SAC is used for dive execution ... because we know how much gas we have at any point in the dive by looking at our pressure gauge.

We don't develop an understanding of how different conditions affect our consumption rate by establishing a baseline ... we do so by tracking our consumption over the course of multiple dives, in different conditions, and considering why each is higher or lower than the others. In other words, you look for trends.

Gas consumption ... like so many other aspects of diving ... is not an exact science. There are a lot of different factors that will affect your gas consumption on any given day and any given dive. Understanding what those factors are, and how they affect you, will give you a much more accurate "picture" of how to plan your gas needs than sitting in your living room breathing off a scuba cylinder ...

... Bob (Grateful Diver)

 

[Mike Boswell]

I’m trying to figure (internalize) the differences and usages in these two breathing consumption rates.

First, to baseline the discussion:[1]

SAC: Surface Air Consumption. Measure by taking a tank of known volume, wear a mask or nose plugs, note the starting pressure, breath for 10 to 20 minutes, then note the final pressure. Compute the cu ft (L) used. SAC – Cu ft / minutes.

Surface RMV: Respiratory Minute Volume. Take an average from past dives, rework the figures of consumption at depth to extrapolate what the consumption would be at 1 atm (bar). Figure the consumption per minute.

The SRMV supposedly has other factors ‘against it’ such as work during dive, cold, darkness, and other things that affect volume consumption. The SRMV is variable per dive.

The SAC is supposed to be a more-sure baseline as it is generally not affected much over time (except to vary with physical fitness).


Here’s an interesting difference. The TDI Guide to Advanced Nitrox[2] has nothing about the SAC, but does all computations according to a diver’s RMV.

Q1: Why use a “variable” consumption rate?

Q2: What would be the general / average / expected difference in the SAC vs the SRMV?

Q3: What do you use to compute your gas use and why?

[1] SDI Solo Diving Manual 2007
[2] TDI Guide to Advanced Nitrox 2009

Lot of discussion on SAC here: http://www.scubaboard.com/forums/basic-scuba-discussions/344311-sac-rates.html but the definition of “SAC” doesn’t seem to be the same.

Hi Jax,

Kind of interesting - there seem to be a few different definitions. I went to the Suunto site and got this:

SAC calculation method

To calculate Surface Air Consumption (SAC), Suunto Dive Manager first determines the average depth of the dive. It is the average of all depth reading shown on the depth graph in the dive log entry.

It next determines the total amount of air used on the dive. Dive Manager uses information you provide about cylinder size and about pressure gauge readings. Enter this information on the Air consumption tab in the properties window. Dive Manager then calculates the total volume of air used for the dive.

Dive Manager then determines the volume of gas used per minute and adjusts the result to sea level from the dive's average depth. Finally, it adjusts the result from ambient water temperature to zero degrees Celsius.

The result is a volume of gas per minute at Standard Temperature and Pressure (STP). Because Dive Manager uses volumes at STP, you can directly compare SAC numbers from one dive to another, regardless of depth and water temperature. Dive Manager reports SAC in metric or imperial units. Metric units are Standard Liters per Minute (SLM), and imperial units are Standard Cubic Feet per Minute (SCFM).

 

[TSandM]

So SDI takes your resting surface consumption, and multiplies it times two if you go below 60 feet? That puzzles me, because 60 feet is 3 ATA, so unless you are shore diving and bringing your average depth up closer to 30, a multiplier of 2 isn't even enough to compensate for depth, let alone activity.

It is not horrendously difficult to do a bunch of dives, figure out your average RMV, and then compare that to the average depth of the proposed dive for planning purposes. We learned to do this in Fundies, and we could all do it by the end of the three day class. We did use "scuba math", which involves some simplification and some rounding, but it's good enough for government work. I don't see any good purpose in sticking in a bunch of relatively arbitrary multipliers, when the real math isn't that hard.