Issues with SAC terminology

[John Van Ostrand]

On Tue, Jul 18, 2017 at 9:54 AM, Willem Ferguson <
willemferguson at zoology.up.ac.za> wrote:

> This is in some ways a sensitive issue and maybe reflect existing
> discussion on this mail list that I am not aware of. The term "Surface Air
> Consumption" (SAC) is probably used wrongly most of the time, including by
> myself. This is because it is conflated with the Respiratory Minute Volume
> (RMV). Since, in our broad way of talking, we measure SAC as cu ft/minute
> or liters/minute, this is the volume that is referenced and should more
> accurately be indicated as RMV. SAC rate is a pressure indication, the
> number of bars of gas used per minute and not a volume. Corrected for
> cylinder size, it allows calculation of RMV. This is also consistent with
> the NAUI and IANTD training materials that I have contact with. Shouldn't
> we at some stage get the terminology accurate and replace SAC in the UI
> throughout with RMV?
>

There doesn't seem to be a consensus in tech diving how SAC and RMV are
defined. At one time I was told SAC is pressure change and RMV is volume
consumed, both representing consumption at 1 ATM. The manual from the Adv
Trimix course I just took from TDI said SAC is the volume of gas consumed
at 1 ATM and RMV is volume of gas used at a certain depth.

The common spoken (informal) usage among pre-tech divers in my experience
is that SAC is volume consumed at 1 ATM. From my limited tech experience
the most common use is of SAC as pressure and RMV as volume both at 1 ATM.

Personally I think think pressure change is pointless as anything but an
intermediate metric since it relies on the tank specs on which it was based
on. SAC as volume is the metric to keep in mind when planning dives and RMV
is the target metric to use when planning real world gas usage.



-- 
John Van Ostrand
At large on sabbatical
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