Question Definitions for SAC and RMV?

[Cheizz]

So when you calculate SAC, you apply pressure and tank volume (so in effect, you calculate a volume/amount of air).

Well, since volume = pressure * tank size, speaking of volume per minute seems more efficient to me (less variables to communicate).

I made some tables for myself (to put in my wetnotes) as a reference for my expected air consumption. Made them for different SAC rates (l/min) and tank sizes that I own...

 

[Divin'Papaw]

I assume Shearwater need a tank size as well in order to calculate SAC rate? all the other information they get from your dive profile and transmitter info.

That is Shearwater's calculated SAC rate. It is NOT tank independent. They leave that up to you to calculate either on your own (I have it built into my dive log spreadsheet) or using another dive log software.

In the parlance I use, SAC is NOT tank independent. I call that RMV.

 

[drrich2]

But I think it is stupid to use not just a medical term, but a medical term that use the same units (when in metric) and has a close to similar meaning.

'Stupid' is a value judgment; if the term is not causing substantial confusion in the recreational diving community, serves a legitimate and useful purpose, and is already widely established and impractical to change without causing considerable disruption/confusion solely on a semantic principle, then the attitude 'When in Rome, do like the Romans' may work better than tilting at the semantic windmill.

From this thread it seems to me the understanding and use by divers of the term SAC is quite inconsistent. RMV sounds like the road to a more universal term, and already in use.

Well, since volume = pressure * tank size, speaking of volume per minute seems more efficient to me (less variables to communicate).

I agree, strongly prefer amount (volume?) per minute rather than pressure/ per minute, which in U.S. use seems to mean use RMV, not SAC.

 

[Cheizz]

I agree, strongly prefer amount (volume?) per minute rather than pressure/ per minute, which in U.S. use seems to mean use RMV, not SAC.

Before today, I had never heard of RMV.

 

[johndiver999]

If you understand the math there is really no need to be so concerned with the terminology.

If you are planning a dive by yourself, you can call it fruit loops per smile, for all it matters.

If you are planning a dive with other people (some of which may have different sized tanks than you), it seems that it makes sense to deal with a consumption rate that is based on volume per minute. After that is established, then the rate can be converted to a form which is more useful as in pressure units per minute (tank dependent), which can then be communicated within the dive team.

In other words, I don't see a whole lot of value in talking about (or immediately communicating) psi (or bar) per minute, unless everyone has the same tanks. So we are always going to start with volume per time and then once that is defined move on to (i.e., convert to) pressure per time, right?

So it is essential to have available a term which pertains directly to volume per time.

 

[Manatee Diver]

'Stupid' is a value judgment; if the term is not causing substantial confusion in the recreational diving community, serves a legitimate and useful purpose, and is already widely established and impractical to change without causing considerable disruption/confusion solely on a semantic principle, then the attitude 'When in Rome, do like the Romans' may work better than tilting at the semantic windmill.

We are allowed to have our own judgements.

And I will continue to give my SAC with units. And lets be honest no one cares except people on the internet. A small fraction of divers even know their SAC rate, and even in smaller fraction of that do anything of value with it like planning dives.

So if we are sitting at a picnic table at Ginnie with a map in front of us and I go "My planning SAC rate is 0.7cuft/min." No one is going to bat an eye, and if they do I roll up my map and say "I say good day to you sir!" like Willie Wonka (the good movie).

 

[inquis]

My planning SAC rate is 0.7cuft/min."

I would do the same. I tend to guard my words when talking to internet divers, because the likelihood of someone insisting I'm confusing SAC and RMV is high.

 

[Divin'Papaw]

And I will continue to give my SAC with units. And lets be honest no one cares except people on the internet. A small fraction of divers even know their SAC rate, and even in smaller fraction of that do anything of value with it like planning dives.

I would do the same. I tend to guard my words when talking to internet divers, because the likelihood of someone insisting I'm confusing SAC and RMV is high.

Same here. If I'm ever discussing air consumption with someone, I always give units. I can clearly grasp from the units they provide what they are talking about. I don't care if they call it SAC, RMV, SCR or something else. The units tell me whether we are talking apples to apples. If someone mentions their SAC to me without units, I always ask what units. If I get a blank stare, I move onto another topic because clearly the conversation is pointless.

 

[Manatee Diver]

I would do the same. I tend to guard my words when talking to internet divers, because the likelihood of someone insisting I'm confusing SAC and RMV is high.

I would just tell them that is what I was trained to use, my TDI books are from before the change over. And if they still want to be annoying that is what the ignore/block button is for.

 

[scubadada]

I assume Shearwater need a tank size as well in order to calculate SAC rate? all the other information they get from your dive profile and transmitter info.

My Shearwater Teric will display real time SAC if you want to see it. I almost always dive rental AL80s and know that my average RMV of 0.36 cu ft/min translates to a SAC of 14 psi/min. I don't glance at my SAC often, most frequently when I am significantly exerting myself. I have seen SACs as high as around twice my average. It's interesting.

The Teric on board log gives you your average SAC for the entire dive, as does the downloaded log. Shearwater currently does not have a mechanism for entering your tank characteristics so that RMV can be automatically calculated. You must calculate the RMV manually by multiplying by the tank factor, 0.258 cu ft/psi for an AL80. I, and others, have suggested to Shearwater that they fix this.

On the other hand, my nearly 13 year old Oceanic VT3 does not give any real time or on board information about gas consumption. However, in the download, both SAC in pressure/time and RMV in volume/time are displayed. The tank characteristics can be chosen from a dropdown list and there is a default, AL80 for me.

I posted this in detail on another thread RMV math…