RMV math…

[60plus]

SAC stands for surface air consumption, ie the number of litres per minute or cubic feet per hour that passes through the divers lungs at when breathing at the surface. As the diver goes deeper more air passes through his lungs but it is no longer the SAC because the diver is not at the surface. The volume of air passing through his lungs is a multiple of SAC, that may also vary due to other factors.
RMV stands for respiratory minute volume, ie the volume of gas passing through a divers lungs in a minute. For a given diver at the surface it will be the same as SAC. The actual volume of passing through the divers lungs as he goes deeper does not change but the quantity of gas he breaths and the rate of depletion from the cylinder does. For diving calculation purposes RMV calculations regard volume as quantity (quantity actually being volume multiplied by density).
I hope the above statement is sufficient for those who have been criticising me to realise that I do know what SAC and RMV is.
I state once again that this thread was originally in basic scuba where I posted that it was not necessary for recreational divers to calculate their dives based on SAC because actual consumption varies based on many other factors (or words to that effect). All they need was fair idea of how long a given cylinder size and pressure would last and would it be sufficient for the expected dive profile. It is far better they check their SPG regularly and know what the should and should not do if it varies from expectation.
I dive at Capernwray, a quarry with a diving centre. Both my cylinders are 15L and normally at Capernwray they fill to just over 200 bar with the cylinder quite warm. This is more than enough for any of the dives at Capernwray and I often have about 100 bar left after 45+ minutes dive. If Capernwray are filling my cylinders and I intend to go to Wastwater or Coniston I ask then to top up the cylinder to 220 bar when cold. I do not have to do a calculation to as them to do this, I simply want to maximise my dive time at what is a deeper dive site. Similar approach on holiday, normally I dive with 15L but they had run out of 15L tanks because there were a lot of divers at a CMAS photography event. So I just asked them to fill a 12L as high as they could, about 225 bar, and dived to the limit of that. Another example, if I am going over 20m or into a wreck, cave or circumstances that may make accessing te buddies air difficult I carry a 3L pony. I don't bother with a calculation, I know 3L should get me safe if my primary supply fails. If SAC was so vital them PADI and SSI would be teaching it for recreational depths, they are not, they simply teach that you use you cylinder contents faster as you go deeper so watch your SPG.
I accept that if you are decompression diving (more that a few minutes) and optimising the size of cylinders to carry then a calculation would be appropriate. If you are using a rebreather then you need to calculate volume of CO2 that has been absorbed to determine scrubber life.

 

[Wibble]

3 litre PONY…

If 200 bar, then you’ve 600 litres available.

Have you done the gas calculations for 30m/100ft? It’s really scary.

At 30m you have 4ATA. If your normal SAC is 20 litres per minute, you’d be breathing 80 litres per minute at 30m. 600/80 = 7.5 minutes before it is empty.

Of course you won’t be breathing 20 litres per minute, you’d be stressed and frightened and will easily double your normal breathing rate to a SAC of 40 litres/min, or 160 litres/min at 30m depth, or empty it in 3 minutes.


You’re doing recreational dives which are No-Decompression-Limits. BUT, all deeper dives have a "mandatory" safety stop which you should do if you’ve had a problem.

A 3 litre PONY is too small in reality.

I do recommend you put this kind of dive into a spreadsheet and run the numbers as it reveals how small the safety margins are with small PONY cylinders.

Scenario: main tank free flows and scares the bejesus out of you. It takes you at least a minute to get sorted, is your buddy around to give you gas? Then you switch to the PONY. You may well attempt to close the valve on the main tank, etc. you switch to the pony and get your thoughts together, 2 mins on the bottom at least. Will you put up your SMB? Then you ascend, slowly. Do you do a safety stop? Can you see how much gas is left in the pony? Are you stressed and breathing like hell? Are you afraid of running out of gas?


Four weeks ago this happened to me. Diving sidemount and solo, my second regulator on the bungee necklace freeflowed. Gas spewing out of the regulator and buffeting my head around. WTF thought I and tried to stuff my fingers in the reg and bash it around, nope. Thought I’d stick it in my mouth and try breathing from it. Nope. Then started to shutdown that side, remembering I’m breathing from it and swapped to the other reg. shut it down. Needed a minute to compose myself. Then tried turning that side on again, but no, still freeflowed. Checked my gas and had lost 150 bar! Aborted the dive and swam for the exit ramp.

Fear, prevarication, diagnosis, the masses of bubbles, stress. It is a killer. That would have been terrifying on a 15 with PONY in the sea.

 

[tursiops]

3 litre PONY…

If 200 bar, then you’ve 600 litres available.

Have you done the gas calculations for 30m/100ft? It’s really scary.

At 30m you have 4ATA. If your normal SAC is 20 litres per minute, you’d be breathing 80 litres per minute at 30m. 600/80 = 7.5 minutes before it is empty.

Of course you won’t be breathing 20 litres per minute, you’d be stressed and frightened and will easily double your normal breathing rate to a SAC of 40 litres/min, or 160 litres/min at 30m depth, or empty it in 3 minutes.


You’re doing recreational dives which are No-Decompression-Limits. BUT, all deeper dives have a "mandatory" safety stop which you should do if you’ve had a problem.

A 3 litre PONY is too small in reality.

I do recommend you put this kind of dive into a spreadsheet and run the numbers as it reveals how small the safety margins are with small PONY cylinders.

Scenario: main tank free flows and scares the bejesus out of you. It takes you at least a minute to get sorted, is your buddy around to give you gas? Then you switch to the PONY. You may well attempt to close the valve on the main tank, etc. you switch to the pony and get your thoughts together, 2 mins on the bottom at least. Will you put up your SMB? Then you ascend, slowly. Do you do a safety stop? Can you see how much gas is left in the pony? Are you stressed and breathing like hell? Are you afraid of running out of gas?


Four weeks ago this happened to me. Diving sidemount and solo, my second regulator on the bungee necklace freeflowed. Gas spewing out of the regulator and buffeting my head around. WTF thought I and tried to stuff my fingers in the reg and bash it around, nope. Thought I’d stick it in my mouth and try breathing from it. Nope. Then started to shutdown that side, remembering I’m breathing from it and swapped to the other reg. shut it down. Needed a minute to compose myself. Then tried turning that side on again, but no, still freeflowed. Checked my gas and had lost 150 bar! Aborted the dive and swam for the exit ramp.

Fear, prevarication, diagnosis, the masses of bubbles, stress. It is a killer. That would have been terrifying on a 15 with PONY in the sea.

Nice reply, but a waste of your time. He does not need to plan a dive, because he is doing the same dives with the same tanks, over and over. What is bad is he does not plan for something going wrong or for a buddy needing gas. And he is completely wrong about PADI.

 

[L13]

I hope the above statement is sufficient for those who have been criticising me to realise that I do know what SAC and RMV is.

Nope, it indicated you do not know what RMV is.

 

[Jcp2]

What might be interesting is breathing off a regulator for some time while sitting and reading a book or watching a movie to get a sense of what one’s basal inactive SAC is. Then whatever it is, one can go no lower, so that realistic judgements about what is possible while diving can be made. For instance, if my basal SAC is 0.5 cu ft/min, then it’s not possible for me to match somebody who has a diving SAC of 0.4.

 

[Wibble]

What might be interesting is breathing off a regulator for some time while sitting and reading a book or watching a movie to get a sense of what one’s basal inactive SAC is. Then whatever it is, one can go no lower, so that realistic judgements about what is possible while diving can be made. For instance, if my basal SAC is 0.5 cu ft/min, then it’s not possible for me to match somebody who has a diving SAC of 0.4.

There are always two SAC rates to measure; the bottom phase SAC and the Decompression phase SAC. I always erred high so used 15 litres/min for the bottom and 12 lpm for decompression even though deco was frequently 9 lpm.

Honestly @60plus you really should know your SAC especially as you use a 3 litre PONY

 

[Gareth J]

As @Wibble says, creating a spreadsheet with cylinder capacity and is a good way of getting a visual view of how long a cylinder with a specified amount of gas will last at differing depths and differing breathing rates.

The pdf is laid out similar to the BSAC88 tables, a format I am familiar with so an easy visual reference for me. The cylinder volume and breathing rate can be changed (in the original file).

This particular version is to aid dive planing when we are managing student divers. As such it has cylinder pressures from 240 to 150 bar and breathing rates from 25l/min to 15l/min for a standard 12l cylinder. It is a quick reference

 

[60plus]

And he is completely wrong about PADI.

Tell me where in the PADI OW manual it discusses SAC calculation.

 

[60plus]

RMV does not vary with depth. It is a scientific term (not a diving term) and is the volume change of the lungs in a minute and has nothing to do with depth (volume per breath X number of breaths per minute).

L13 What prompted you to post this was my statement "That RMV varies with depth" If you look at my earlier long post I stated that RMV does not vary with depth, it is the density and hence quantity that varies. When I wrote "That RMV varies with depth" it was a mistake due to rushed typing.

 

[scubadada]

3 litre PONY…

If 200 bar, then you’ve 600 litres available.

Have you done the gas calculations for 30m/100ft? It’s really scary...

If you know your RMV, this calculation is very easy. I usually dive solo. I calculate my pony requirement for a minute at depth to figure things out, a normal ascent, and a 3 min safety stop, all at twice my average RMV.

A 3 liter pony is close to 19 cu ft. I can do my calculated ascent, as described above, from 130 ft with about 16 cu ft, leaving me with just under 500 psi and 3 cu ft left in my pony. I'm very rarely diving at 130 ft.

Anyone can do the same calculations.

I have my RMV for my last 1792 dives, 0.36 +/- 0.04 cu ft/min (mean +/- std dev) under a whole variety of diving conditions. 95% of my dives are between 0.28 and 0.44 cu ft/min. 0.44 cu ft/min is a very strenuous and/or cold dive.