Do you use your SAC for anything? POLL

[BoltSnap]

Thanks Bill.

Saying that a tank is a 12L (or anything else), doesn't really specify its true gas capacity (when filled to its specified working pressure) unless one knows the working pressure (BAR's) for that Tank in the metric system. In the imperial/US system, the actual tank capacity is specified (capacity of gas when the tank is filled to its working pressure). One would need to know the working pressure for the tank in the Imperial/US system to know the physical size of the tank and to figure out the ratio of the PSI/ft3 for gas consumption/SAC Rate calculations.

 

[aquaregia]

Kev, here's the thing I don't get about this - for the type of diving you're describing. Why is it better to do the math, then monitor your watch, so that you can predict what your SPG is going to tell you, when monitoring the SPG is simpler, and ultimately what counts? You're going to act based on the SPG whenever it disagrees with the whole forecasting regime anyway, so why not keep it simple? Neither method works if you're not watching the indicator, so why do two when only the one really matters?

I have an idea of time passing without looking at a clock. It's not perfect, but way better than my idea of the pressure remaining in a tank.

 

[billmas]

Thanks Bill.

Saying that a tank is a 12L (or anything else), doesn't really specify its true gas capacity (when filled to its specified working pressure) unless one knows the working pressure (BAR's) for that thank in the metric system. In the imperial/US system, the actual tank capacity is specified (capacity of gas when the tank is filled to its working pressure). One would need to know the working pressure for the tank in the Imperial/US system to know the physical size of the tank and to figure out the ratio of the PSI/ft3 for gas consumption/SAC Rate calculations.

Hi Burhaneddin,

Yes the working pressure will usually be 232 or 300 bar (mostly 232). One of the nice things about l/bar is that you can calculate volume of air in the tank regardless of the working pressure by...

TankSizeLitres x BarInTank = Air in Litres Available

So you only need to know the current pressure, not the working pressure. Same for monitoring or calculating SAC's, you don't need to know the working pressure... it is (generally) irrelevant in making any calculations.

So if I have a 12l tank at 120 bar, I have 1440 litres of air available. It is the same formula regardless of tank size or working pressure.

The working pressure specifies maximum, so again to work out the maximum capacity of any tank is TankSizeLitres x MaxWorkingPressure.

Hth
Kind Regards
Bill

 

[Kevrumbo]

My SAC/RMV in tropical warm waters is typically 30% better than it is in temperate cold SoCal homewaters. However, after a week now diving in Palau 30deg C water temp, I've lowered that to around 50% of my nominal cold water SAC (from 22 litres/min to 11 litres/min).

This is how I'm using this value with a 11 litres/bar tank (i.e. an AL80) here in Palau:
11 [-]litres[/-]/min divided-by- 11 [-]litres[/-]/bar equals 1 bar/min pressure SAC rate.

All my dives are averaging 20 meters depth going with the drift current; 20 meters is 3 ATA (divide 20 by 10 and add 1 gives a depth in atmospheres absolute of 3 ATA).

Therefore 1bar/min multiplied by 3 ATA equals a depth consumption rate of 3 bar/min at 20 meters. Checking my bottom timer every 10 minutes, I expect to consume 30 bar (3 bar/min multiplied by 10min equals 30 bar), and accordingly my SPG should read 30 bar less in that 10 minute time frame.

So by 30 minutes elapsed dive time at 20 meters, I expect to be down 90 bar or at half tank (AL80 full tank is 200 bar). At 40 minutes elapsed time, I'm ascending off the wall into the shallow coral plateau around 9 meters (down 120 bar from 200 bar total, or 80 bar remaining in tank). And finally at the 45 to 50 minute mark, I'm at 6m and my 3-5min safety stop with 60 to 70 bar left. I surface and I know even before looking at my SPG that I have around 50 bar remaining in my tank.

This is how you should actively use your SAC rate with your particular tank, knowing how much breathing gas you have left, not just only pre-planning, but also during the actual dive real-time-on-the-fly . . .additionally, you have a SPG that reads in units of pressure: why not convert your SAC rate to a Depth Consumption Rate (DCR) in pressure units to make use of it???

Kev, here's the thing I don't get about this - for the type of diving you're describing. Why is it better to do the math, then monitor your watch, so that you can predict what your SPG is going to tell you, when monitoring the SPG is simpler, and ultimately what counts? You're going to act based on the SPG whenever it disagrees with the whole forecasting regime anyway, so why not keep it simple? Neither method works if you're not watching the indicator, so why do two when only the one really matters?

I have an idea of time passing without looking at a clock. It's not perfect, but way better than my idea of the pressure remaining in a tank.

Okay . . .all the heavy math derivation above aside.

It simply means in rote application --I'm nominally using 30 bar of gas in a ten minute interval at 20 meters, and just subtracting that out from my total starting pressure of 200 bar (or subsequent remaining pressure during the dive). The simple motivation and gist of it all is that this easy arithmetic result should jibe with the actual value shown on my SPG for every succeeding 10 minute interval at 20 meters.

As Lynne pointed out:

spoolin, it's a cross-check on the SPG. If your predicted reading doesn't match the actual one, you have to worry about a stuck gauge (if it's too high) or a leak somewhere that will mean you'll need to end your dive before you expected to.

That's how you should strategically be using your SAC rate and SPG together. . .

 

[4sak3n]

When I sit on a boat and the dive briefing says "turn the dive at x bar" or somesuch, I generally disregard it. I know the numbers, I know the peak SACs, I know that I could have my buddy on my Octo, I run my own numbers in my head to make sure they're safe for my buddy and I. I sincerely hope my numbers are never tested

Typically on a boat over here, it is something like "turn the dive at 100 bar", or on a shot dive, "begin the ascent at 100 bar" etc etc etc. This is regardless of whether the dive is planned to 10m or 30m, or whether I have a 10L or 15L tank ... I've heard the same on every briefing I've had To me, this is quite ridiculous.

One of my buddies dives twin 12's, I dive a single 15+pony, another dives a single 12. Quite simply, how can "turn the dive at 100 bar" be safe and appropriate for all of us? It's not. It suggests to me that for "liability" reasons, there is this set number that everyone sticks to... IIRC it is even in my learning materials for getting certified ... turn the dive at "x" or hit the surface with "y". I personally believe that the set number isn't actually always safe... so I'll keep running my own numbers.

What concerns me, even when recreational diving is that there is this ethos "as long as I turn my dive at x bar we'll be safe" but I believe that is clearly untrue.

I've wondered this too on occasion after running my own numbers and plotting out rock bottom pressures for a multitude of cylinder sizes and depths.

The conclusion that I came to was that the skipper/DM/operater/guide/whoever probably (correctly imho) believes that most divers don't know how to or haven't taken the effort to run the numbers. Thus they set a number that will probably, for most folk, get them and their buddy out of the water safely. This obviously doesn't take the outliers like yourself into account but they can't plan for every eventuality or otherwise it will erode their client's enjoyment. Should they set the ascent pressure at 150 bar in case 1/50 people has a stressed SAC high enough to warrant that? 1/30? 1/20? Where does one draw the line?

In short, they make an educated guess and expect everyone who knows better (like you and myself) to ignore their rule and go by what we have calculated.

I'm afraid that it is rather a sad indictment of the diving teaching industry that operators have to assume that most of their clients don't understand gas management planning.

 

[Kern]

Thanks Bill.

Saying that a tank is a 12L (or anything else), doesn't really specify its true gas capacity (when filled to its specified working pressure) unless one knows the working pressure (BAR's) for that Tank in the metric system. In the imperial/US system, the actual tank capacity is specified (capacity of gas when the tank is filled to its working pressure). One would need to know the working pressure for the tank in the Imperial/US system to know the physical size of the tank and to figure out the ratio of the PSI/ft3 for gas consumption/SAC Rate calculations.

With the WC (Water Capacity) of the tank you know the pressure per volume of the tank without any calculations. Using your example of a tank with a WC of 12lt, you know instantly that for every bar used, you go through 12lts of gas.

The WC & WP (Working Pressure) are stamped on every tank.

 

[BoltSnap]

Kern,

I understand that fully. My point is that if I am simply told a "15L" tank without knowing the working pressure of the tank, I wouldn't know the capacity of the tank in total. The working pressure can be 232bar, 200 bar, etc. The tanks I bought here in Libya are 15L tanks with a working pressure of 200bars. If they were 232 bar tanks, then they would hold more air and thus just saying 15L isn't enough.

Do you see my point?

 

[Kern]

Kern,

I understand that fully. My point is that if I am simply told a "15L" tank without knowing the working pressure of the tank, I wouldn't know the capacity of the tank in total. The working pressure can be 232bar, 200 bar, etc. The tanks I bought here in Libya are 15L tanks with a working pressure of 200bars. If they were 232 bar tanks, then they would hold more air and thus just saying 15L isn't enough.

Do you see my point?

Sure, all metric tanks need both numbers to define their maximum capacity. The problem really, is that we all tend to think everywhere is the same. For instance, just yesterday I posted almost exactly what Bill said earlier, thinking that because I'd never encountered a metric steel tank that wasn't 232bar or 300bar, & less than 1% of those being 300bar, I'll often just say 12lt. To date, you're the 1st person, (in the metric world), that didn't understand that to mean 232bar, knowing that I'd have specified 300bar otherwise.

PS. I'v just done it again actually. Because I always use steel tanks for BG, that's what I usually think about, when obviously, I should also think people may be talking about aluminium tanks. In Oz they can be 207bar, 240bar, or if they're Catalinas, they can have DOT markings & be 3000psi.

Oddly, when Australians talk about aluminium tanks, they almost never talk in terms of WC & WP, but usually talk in cft.

 

[BoltSnap]

Are 300 Bar tanks readily available?

 

[Kern]

Are 300 Bar tanks readily available?

Sure, you can order a steel 300bar Faber any time.

300 Bar Cylinders : Sterling Leisure Pty. Ltd.