Question Definitions for SAC and RMV?

[Divin'Papaw]

I learned SAC & RMV early on in my wreck diving course and then again in my TDI tech courses. The definitions I learned & use are #1 & 4.

 

[tursiops]

@scubadada I'm not saying they made it up, just said they cemented it when they started using it in their manuals. This is not discounting AUP's use but the prevalence of transmitters in recreational diving is considerably higher now than it ever has been and IMO Shearwaters use of the terms when compounded with TDI's use of the terms have certainly cemented them.

Hi @tbone1004

AUP/PPS/Oceanic... has marketed the MH8A transmitter since 1997, there are many more out there than Shearwater is responsible for. Now, current brand importance/recognition, you got it.

Yes, Oceanic has used SAC as pressure/min and RMV as volume/min for a long time...long predating Shearwater's use of AI and their definitions....which are the same. it seems to be the Europeans who are the holdouts....

 

[L13]

RMV, Respiratory Minute Volume, is the amount of gas (cf3) consumed per unit time (minute), with no adjustment made for depth. So, all other things being equal, your RMV at 33-feet deep is double your RMV at the surface.

So SAC is RMV adjusted for ambient pressure (as due to depth), and a more useful metric in making comparisons topside. RMV is good to have in mind when you're at 100-feet with an 80-cf tank on your back, and need to be mindful it won't last long at that depth.

Pretty sure this is wrong and RMV is constant regardless of depth. As @tbone1004 said, RMV is well defined medical/scientific term.

 

[wetb4igetinthewater]

RMV is not a term I learned from any diving course or reading materials; I do see SB posts using the term. I use the terms Surface Consumption Rate (SCR) and Depth Consumption Rate (DCR), which are in units of volume per minute. I believe SCR and SAC have the same meaning.

I really wish GUE did not invent these new terms. It wasn't necessary.

 

[tursiops]

By the way, NOAA defines RMV as volume/minute and SAC as pressure/min. See attachment.

 

[drrich2]

Pretty sure this is wrong and RMV is constant regardless of depth. As @tbone1004 said, RMV is well defined medical/scientific term.

In that context I doubt pressure was considered; I'd imagine in the medical field it would be assumed to be one atmosphere pressure, if it was thought of. If that's the case, it wouldn't be an issue of whether it is or isn't a constant regardless of depth, because depth wouldn't have been considered.

I wonder to what extent the scuba community directly referenced the medical field for the term, but it's worth a look. WikiDoc.org (which I didn't know was a 'thing') has a page on it; from that:

"Respiratory minute volume (or minute ventilation, or flow of gas) is the volume of air which can be inhaled (inhaled minute volume) or exhaled (exhaled minute volume) from a person's lungs in one minute. This is normally registered when a person has a ventilator supporting the person's breathing due to a sickness or injury.

Minute volume is calculated by taking the tidal volume and multiplying the respiratory rate (the number of breaths per minute a person is taking). The higher the minute volume the more carbon dioxide (CO2) the person is releasing, the converse is the lower the minute volume the lower the amount of carbon dioxide the person is releasing. For example a person who is hyperventilating would have an increased minute volume.

A normal minute volume would be 5-8 liters per minute."

There's no mention of pressure being taken into consideration.

From the Free Dictionary by Farlex we get:

"res·pi·ra·to·ry min·ute vol·ume (RMV),
the minute volume of breathing; the product of tidal volume times the respiratory frequency. See: pulmonary ventilation."

And their definition of tidal volume:

"ti·dal vol·ume (VT),
the volume of air that is inspired or expired in a single breath during regular breathing."

The volume of air used at depth from the scuba tank (which isn't compressed by the external pressure until it leaves the tank) would be greater at depth than at the surface. Put another way, at 33 feet we go through a tank at twice the speed. Our RMV at 33 feet would then seem to be double that at the surface, if we judge by the volume leaving the tank, rather than the volume entering and leaving our lungs.

 

[tursiops]

In that context I doubt pressure was considered; I'd imagine in the medical field it would be assumed to be one atmosphere pressure, if it was thought of. If that's the case, it wouldn't be an issue of whether it is or isn't a constant regardless of depth, because depth wouldn't have been considered.

I wonder to what extent the scuba community directly referenced the medical field for the term, but it's worth a look. WikiDoc.org (which I didn't know was a 'thing') has a page on it; from that:

"Respiratory minute volume (or minute ventilation, or flow of gas) is the volume of air which can be inhaled (inhaled minute volume) or exhaled (exhaled minute volume) from a person's lungs in one minute. This is normally registered when a person has a ventilator supporting the person's breathing due to a sickness or injury.

Minute volume is calculated by taking the tidal volume and multiplying the respiratory rate (the number of breaths per minute a person is taking). The higher the minute volume the more carbon dioxide (CO2) the person is releasing, the converse is the lower the minute volume the lower the amount of carbon dioxide the person is releasing. For example a person who is hyperventilating would have an increased minute volume.

A normal minute volume would be 5-8 liters per minute."

There's no mention of pressure being taken into consideration.

From the Free Dictionary by Farlex we get:

"res·pi·ra·to·ry min·ute vol·ume (RMV),
the minute volume of breathing; the product of tidal volume times the respiratory frequency. See: pulmonary ventilation."

And their definition of tidal volume:

"ti·dal vol·ume (VT),
the volume of air that is inspired or expired in a single breath during regular breathing."

The volume of air used at depth from the scuba tank (which isn't compressed by the external pressure until it leaves the tank) would be greater at depth than at the surface. Put another way, at 33 feet we go through a tank at twice the speed. Our RMV at 33 feet would then seem to be double that at the surface, if we judge by the volume leaving the tank, rather than the volume entering and leaving our lungs.

Your lungs don't get twice as big at 33 ft. How can the RMV double?
I think you are conflating the volume of gas with the number of molecules in that gas.

 

[drrich2]

Your lungs don't get twice as big at 33 ft. How can the RMV double?

The volume of gas leaving the tank per unit time doubles.

The volume of gas (even at double the density) entering and leaving your lungs does not, true.

But it's the first concept that is practically useful.

The patients who are the focus of treatment in the medical field for the most part aren't breathing their full gas supply off pressurized tanks alone. Even people with nasal cannula or an oxygen mask supplying O2 are getting supplemental oxygen enriching their air; they don't breathe exclusively off the tank's volume.

 

[L13]

The volume of gas leaving the tank per unit time doubles.

The volume of gas (even at double the density) entering and leaving your lungs does not, true.

But it's the first concept that is practically useful.

The patients who are the focus of treatment in the medical field for the most part aren't breathing their full gas supply off pressurized tanks alone. Even people with nasal cannula or an oxygen mask supplying O2 are getting supplemental oxygen enriching their air; they don't breathe exclusively off the tank's volume.

The volume of gas leaving the tank does not change(actually, the tank's volume stays constant,only the pressure in the tank changes). Your lungs have the same volume at 33 ft as at the surface. However, if you take that volume of air from 33 ft to the surface, its volume will double because it contains twice the number of molecules as a breath at the surface.

 

[drrich2]

(actually, the tank's volume stays constant,only the pressure in the tank changes)

A fair point. It'd rather be correct to say that the mass of air used per minute is double at 33 feet vs. the surface. I'm used to thinking of the volume as the amount of something, and that doesn't work when pressure changes drastically.

I doubt that level of nuance is going to catch on in the recreational dive community in a broad way, at least at the lower levels.

Did a little poking around online and it turns out this isn't a new thing. See Chronicle of an Older Diver from way back in 2009! From that article:

"It’s interesting that there is no agreement among agencies as to the exact definition of Respiratory Minute Volume (RMV) and Surface Air Consumption (SAC).

My original certification agency, NASDS, defined SAC as the surface equivalent number of PSI per minute on a dive, which is (PSI Consumed/Total Dive Time)/((33+Depth)/33).

IANTD does the same thing, and defines RMV as the SAC/(Working Pressure/Rated Cylinder Volume).

DSAT defines SAC as the same as IANTD and NASDS define RMV, and defines RMV is the (Tidal Volume minus Respiratory dead air space) times breaths/minute. I added the parenthesis to what I read in the book (p42 Tec Deep Diver Manual) because otherwise it doesn’t make any sense. Oddly the Encyclopedia of Recreational Diving doesn’t seem to mention either term. Wikipedia’s definition of RMV is similar to DSAT but doesn’t factor Respiratory Dead Air Space.

In the SDI solo diving course SAC is defined as the volume per minute consumed at rest on the surface. They recommend breathing from a tank while sitting around watching TV or like activity to measure this. For me, consuming about 325 PSI of a 3000 PSI working pressure tank with a rated volume of 77.4 cubic feet in 30 minutes, this was about .28 cubic feet per minute. They call what IANTD and NASDS call RMV the SRMV, or Surface Respiratory Minute Volume. They then go on to recommend that before each dive the RMV by multiplying by the number of absolute atmospheres of pressure and also by a “Dive Factor”, which takes into account effort and should be at least 1.5 for an easy dive, and perhaps more than 3 for a high effort, cold or stressful dive.

Disconcertingly, their sister agency, TDI, recommends determining SAC using a swim at depth, the very thing that SDI says doesn’t work, and implies that RMV is just another term for SAC."

I don't have my SDI Solo Diver manual handy, but as I recall, SAC was defined in terms of volume (cf3) / minute, and I thought it was adjusted for depth.