Do you Plan your dive or Dive your plan within NDLs?

[stuartv]

No, SAC rate is expressed in liters/cuft per minute. So SAC is a volume and has nothing to do with the size of the tank.

Edit: Sorry, but this is stuff that is supposed to be taught in an Open Water Diver Course.

It was taught... differently than what you are saying. Maybe the course materials have changed since you took it?

 

[Patoux01]

And then claiming a scientific background

 

[stuartv]

so the point still stands, what does SAC expressed in pressure per unit time do for you? the answer is absolutely nothing, it is just used as a stepping stone to get from DAC to RMV by those that think it is helpful to break the math up.

My SAC is 27psi-min on an AL80. If I'm diving an AL80 and I'm at 100', I know I'm at 4ATA. 4 * 27 is easy enough. 108 psi/min. In 10 minutes, I'll drop 1080 psi on my SPG. Look at my SPG and see where I'll be in 10 minutes. Handy and easy - even in Imperial units.

I generally work in RMV (cu-ft/min), but SAC seems like it can be handy.

 

[Bennno]

What materials are those? If you don't mind my asking.

 

[Storker]

if I'm using a 100, it's around 1.4? Still not simple arithmetic, like the poster who keeps going on and about multiplying and dividing by 1 keeps saying. And when I'm on a 120, it's 1.2? Doubles? 0.7 or 0.6?

This is - IMO - a pretty darned good argument for losing your surface cuFts and tank factors and converting to the Euro way of doing it: Water volume and service pressure.

12L 232 bar = 2800 surface liters.
15L 200 bar = 3000 surface liters.
15L 232 bar = 3500 surface liters.
If your SAC - or RMV, I'm not going to argue that semantic point - is 15 SLM, the 12x232 will give you an hour at 20m/3 ATA, the 15x200 will give you some 5 minutes more, and the 15x232 will give you about an hour and a quarter. Easy peasy, no need to memorize - or look up - those tank factors. All you need is simple, middle school level arithmetic. Which at least I can do even if I'm narked out of my gourd.

 

[tbone1004]

Storker, that's why I converted to bar gauges....

SAC is normalized for the surface in pressure per what you guys are saying. DAC is what you actually measure, which is pressure used over time, not accounting for depth. I.e. your depth air consumption vs your Surface air consumption when the DAC is normalized to surface pressures.

 

[lowviz]

... converting to the Euro way of doing it: Water volume and service pressure. ...

That would make it way too easy.

Don't forget that most (but not all) of our ratings are at 10% over. Now your assumed volumes are off. No rhyme or reason as to which gets rated that way either. Layers upon layers of confusion for a brain-dead simple measurement.

 

[Bennno]

My SAC is 27psi-min on an AL80.

That is a volume though. It's roughly 18 liters/.65 cuft. You are always measuring in volume, psi per min in a specific tank size is just a weird way of giving the volume.

 

[Kevrumbo]

Easy Scuba Maths In Metric
[Or how an American Diver figured out the logical use of a Bar SPG]:

How much easier & intuitive is it to work with "1 bar/min" Metric vs "14.5 psi/min equivalent" in US Imperial Units???

My Surface Consumption Rate (SCR) after drift diving so many years on holiday in Palau's 28deg C tropical water temp is a personal best 11 litres/min per ATA.

Using this SCR value with a 11 litres/bar tank (i.e. an AL80 Cylinder):

Divide 11 litres/min per ATA by 11 litres/bar equals 1 bar/min per ATA .

So again --How much easier, intuitive & advantageous is it to work with "1 bar/min" and the metric system in general for Scuba? Well, the arithmetic can all be figured easily & quickly in your head and on-the-fly:

All my dives were on Nitrox32, averaging 20 meters depth always 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 at 20 meters, my 1bar/min per ATA gas pressure consumption rate will increase threefold --that is 1bar/min per ATA multiplied by 3 ATA equals a depth consumption rate of 3 bar/min at 20 meters. Hence checking my elapsed bottom time every 10 minutes, I expect to consume 30 bar (3 bar/min multiplied by 10min equals 30 bar), and accordingly I already know my SPG will read 30 bar less in that 10 minute time frame. (If however the actual SPG reading indicates 30% or more consumption than expected, then there is a leak problem or I am physically exerting/breathing harder than normal and probably would consider aborting the dive).

So by the first 10 minutes delta time at 20 meters, I expect to be down 30 bar from a full AL80 tank at 200bar, or 170bar remaining actual SPG reading (3bar/min multiplied by 10min is 30bar consumed; and 30bar consumed from 200bar total full tank is the SPG showing 170bar remaining pressure). At the end of another 10 more minutes delta time drifting along at 20 meters, I've consumed 30bar from 170bar, or 140 bar remaining in tank. And finally after another 10 minute period at the elapsed dive time mark of 30 minutes total, I've consumed 30bar delta from 140bar, or 110bar remaining and nearing half tank.

At 40 minutes elapsed time, I'm ascending off the deep wall into the shallow coral plateau around 9 meters (down 30bar from 110bar, 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 SCR with your particular tank, knowing how much breathing gas you have left not only on pre-planning, but also during the actual dive at depth, real-time-on-the-fly --all with easier to use metric units . . .additionally, you have a SPG that reads in units of pressure: why not convert your SCR to a Depth Consumption Rate (DCR) in pressure units to make use of it???

In summary & recap: divide your volume SCR (or SAC/RMV rate) by your particular tank's cylinder rating factor to get a figure in pressure units per minute since your SPG reads in pressure units -not volume units. Multiply this SCR in pressure units by your planned depth in ATA, and you'll know what your Depth Consumption Rate (DCR) per minute in pressure units at that depth will be. And the Metric System for Scuba diving makes the arithmetic much easier especially if your pressure Surface Consumption Rate (SCR) turns out to be roundable up to convenient integer like 1 or 2bar/min per ATA.

----
Some example pressure SCR values for a variety of common cylinders, given a arbitrary nominal constant volume SCR of 22 litres/min per ATA (a reasonable & achievable breathing RMV for most novice divers):

11L/bar tank (AL80): 2bar/min per ATA;
12L/bar tank (Steel HP100): 1.8bar/min per ATA;
13L/bar tank (AL100): 1.7bar/min per ATA;
15L/bar tank (Steel HP119): 1.5bar/min per ATA;
16L/bar tank (Steel HP130): 1.4bar/min per ATA;
11L Twins (Double AL80's): 1bar/min per ATA;
12L Twins (Double HP100's): 0.9bar/min per ATA;
16L Twins (Double HP130's): 0.7bar/min per ATA.

 

[LeadTurn_SD]

How do you pre-plan a multilevel dive or do you? Do you plan the dive using a manual approach (table or other method) or do you use a paid subscription software application?

Does your pre-plan include your expected RMV, gas volume and time requirements to stay within the NDLs? Or do you jump in and monitor on the fly your dive based on your SPG (AI) and PDC?

It can be a boat or shore dive - I am interested in your approach to a pre-plan or lack of pre-plan.
Thanks in advance.

For me it is as simple as deciding on a turn pressure and maximum depth / time for the dive.

Most of my dives are reef dives, from shore, using AL80's and usually not exceeding 70 feet. Make the initial descent, continue out to the reef's drop off, decide whether to explore to the left or right along the reef, reach (or more often, stay above) my maximum planned depth, cruise along until turn pressure is reached, turn the dive, and to add a little extra "cushion" I often ascend about 15- 20 feet or so and return along the reef wall at a shallower depth, then turn for shore at my landmark and burn any remaining gas at 15-20 feet looking for little critters in what is in effect a VERY long safety stop.

With an Al 80 on single dives not exceeding 70-80 feet, gas is always the limiting factor for, not NDL's. My plan therefor is mainly to make sure I return to the entry/exit point with enough gas for safety, and that is about it.

I no longer do deep air or go into deco, so my planning is dirt simple.

Best wishes.