SAC vs RMV

[DA Aquamaster]

DA Aquamaster, if you can show me where and how to set the tank size on any SPG, I will confess to having lost what little sense I was born with.

I don't know about you, but I was trained to monitor my "pressure" using an SPG (submersible Pressure gauge) but, hey, if you think pressure is meaningless, then have at it.

We will have to just agree to disagree.

That's fine, but why exactly are you monitoring your pressure? At some point in the process, you have to interpret what that pressure means, and when (if?) you do that, you have convert that pressure to a volume.

Come to NC and we'll go diving. I'll hand you a reg and an SPG and you tell me how long you'll want to stay on one of the 110 ft wrecks here. What I won't tell you is how large the tank is - it might be an X8-130 under filled to 3000 psi with 113 cu ft in it.

Of course the reg and SPG might also just be attached to a 6 cu ft pony strapped to an empty tank on your back, but hey, by your standards you're just fine as you are trained to look at your SPG and note the psi, and from your perspective, the SPG will say 3,000 psi when you look at it and start your descent to 110' so you'll be good to go.

Obviously tank size is a critical factor to making any sense at all out of what the SPG tells you and what "3000 psi" actually means to you on the dive.

Personally, I was trained to look at the SPG, and then interpret what the number actually means.

MY SPG tells me an awful lot more than just "PSI". I was trained to note if the needle actually moved since the last observation and to compare what it tells me (the pressure but more importantly the remaining volume of gas in the tank, once I factor in the tank size) to what I expect it to tell me at that point in the dive based on my anticipated gas use at the depth, time and workload involved. And if the reading does not match my estimate, it tells me something may be a miss.

 

[MXGratefulDiver]

SAC and RMV are nothing more than two different ways of expressing the same value. When the relationship between pressure and volume is understood, they are both useful.

For the recreational diver ...

RMV is useful for diving planning ... i.e. "Do I have enough gas to do this dive?" ... because we know the volume of gas our tank holds when it is full.

SAC is useful for dive execution ... i.e. "Do I have as much air at this point in the dive as I expected to have?" ... because it's an indicator of how much air we have left relative to a full tank.

An analogy is planning a trip in your car ... one where you know you won't have access to a gas station until you get back. You know how big your gas tank is, and how many miles per gallon you can expect to get. So determining whether or not you have enough gas is done in volume (gallons). During your trip, you monitor your gas gauge. It doesn't tell you how many gallons you have ... although you can calculate it easily enough if you know how big your gas tank is. But what your gas gauge really tells you is how much gas you've consumed ... and have remaining ... relative to a full tank.

An SPG does exactly the same thing.

Either way, knowing your "miles per gallon" is a critical piece of information. It's also helpful to know how varying conditions will affect your consumption rate ... just as it's useful to understand how varying conditions affect your miles per gallon when you drive your car ...

... Bob (Grateful Diver)

 

[graham_s]

Pressure is meaningless unless you know the volume of the cylinder.
200 bar in a 15 litre cylinder is 3000 litres of air
200 bar in an 8 l pony bottle is 1600 litres of air.
Both bottles are 200 bar.
That's why the pressure is meaningless taken in isolation.

 

[SC_Hoaty]

3000 PSI is 3000 PSI, regardless of tanks size.

Yes, but with very different tank sizes, the SAC in PSI/min will then vary widely. However, the RMV (in scfm/min) could remain almost constant.

 

[Cave Diver]

I've beat my head against the wall trying to clarify the differences between these two terms for years. I've finally given up and accepted the fact that many people seem to use them interchangeably.

 

[TSandM]

Pressure isn't meaningless. Pressure can even be good for planning purposes . . . IF you use the same tank all the time, and know what your gas consumption in psi/min at various depths is. As I think I wrote somewhere above here, my cave instructor would go purple when he asked us what our gas consumption was, and we answered in cf/min -- his point was that you don't monitor cf during a dive, and that's true. If I'm trying to decide whether I have enough gas to search for somebody, all I need to know is how much pressure I took out of the tank to that point, and how much I need to exit; I don't need to go through the calculations to convert everything to cf and back to psi.

If you use the same tank all the time, you can skip that step -- unless you want to figure out which of the divers you're with is likely to gas limit the dive. THEN you really need to know consumption in volume, so you can compare across dissimilar tanks. That's the example DAA wrote up so nicely above. Similarly, if you go somewhere and borrow or rent a tank different from what you normally use, it's nice to have a way to transfer what you know about your own tank to a different one.

Pressure is not meaningless, especially during a given dive; volume is more generally applicable for planning.

 

[lowviz]

I've beat my head against the wall trying to clarify the differences between these two terms for years. I've finally given up and accepted the fact that many people seem to use them interchangeably.

I'm on the same page.

SAC has become an "overloaded" word. Function overloading - Wikipedia, the free encyclopedia

SAC always refers to gas usage normalized to the surface. It doesn't take long to key on whether or not the discussion concerns pressure or volume. Give up Gary, it has become common usage.

 

[thirdcoastdiver]

In a recent class, I observed that the instructor was using SAC with a subscript of P or V to signify pressure or volume. This approach makes it clear what you are discussing, pressure or volume. It may add to the confusion over SAC and RMV definitions.

 

[Insta-Gator]

Thanks thirdcoastdiver.

Since this is in the basic scuba discussions forum, I'd like to make an observation that I prefer not reignite debate.

Note to New Divers:

1. Pay attention to what your instructor teaches you. While you are in training it is very important that everyone in the class uses and understands the same terms and definitions.

2. This thread is NOT intended to be all authoritative. It's a discussion of customary term usage, not reinventing math/science.

3. I personally have issues with some of the comments made in this thread and advise the new diver to ask your instructor, don't rely on this thread.

4. Since my SPG and/or computer measure PSI, it is simply a relative full/empty monitoring device. Some computers (mine included) will actually calculate air time remaining during the dive.

5. For dive planning, especially at depth or overhead environments, it is critical to know and understand your gas volume available and your gas volume usage rate. Again seek further education on how to plan for such dives and how you will monitor these levels.

Safe Diving

 

[DA Aquamaster]

Since it's a basic diving forum I won't debate it but I'd like to make an observation that is valuable for new divers to know.

Fairly early in my diving career, I was at about 100' and I noted my SPG reading, and it indicated 1800 psi - still plenty of gas (46 cu ft left), no problem. 6 or 7 minutes later I checked again as I should have been close to my planned pressure to start the ascent (1000 psi). I looked and noted 1800 psi, great still plenty of gas. Then about 30 seconds later it sunk in that it said 1800 psi before, but it still says 1800 psi now, when it should be several hundred psi lower. First, it occurred to me that the SPG is stuck and the second SPG reading is obviously wrong meaning I have no idea how much gas I really have now. Of course, then it occurred to me that since I had no idea what the first reading should have been at the time, I could not be sure the needle had not already been stuck and that I had less gas than I thought then, meaning I really had no idea how much gas was left now.

It was the last time I ever looked at an SPG and did not compare the reading with what I thought I should have left based on an informed estimate of gas consumption based on depth, time and SAC.

Pay attention to what your instructor teaches you, then realize:

1) no instructor is perfect, some have serious flaws in both how they teach and their diving skills in general and even the best instructor is not infallible; and
2) a scuba certification is nothing more than a statement that you meet certain very low minimum standards and that it is only a license to continue learning, so get on with the continuous learning phase of your diving career.

----

I'll refrain from making any comment about the wisdom of relying on an air integrated computer to make decisions for you on how much bottom time you have left, other than to indicate that the other SPG failure I had was on an air integrated computer that was still happily predicting loads of bottom time based on a flawed reading. In this case, the QD fitting on the first stage had screwed out just enough to close the fitting and trap gas in the line, but not enough to vent the line, resulting in a constant SPG reading that no longer had any relationship to the the tank pressure.

Absent my prior experience and familiarity with predicting SPG readings in advance, I may well have trusted that computer right up until I sucked the tank dry.

2000 psi is simply not 2000 psi when the SPG is wrong.

You, not your air integrated computer, are the "intelligence" that is ultimately responsible for the safety of your dive.