What is your SAC/RMV rate? (and metric questions)

[TexasKaren68]

I have two questions but figured I'd throw them into one thread because they are related.

First, I'm wondering what kind of air consumption rates some of the veterans on ScubaBoard have? I guess I'd like to know when I reach a "good" RMV rate

So far I've only kept track of the info needed for the calculation on my last four dives (since I bought my computer) and my results for my SAC and RMV are
SAC / RMV
33.08 / 0.88
27.50 / 0.73
33.63 / 0.90
20.87 / 0.56

I have no clue how my air consumption dropped so dramatically on my most recent dive. The only thing I can think of is that it was my (failed) attempt at completing my Search and Recovery adventure dive and I was more concerned about not getting lost and finding the "treasure" than I was about concentrating on breathing.

I'm also not too sure the calculations are 100% correct because these dives were in fresh water, not salt, but from reading another thread it appears that dive computers measure the pressure and then convert that to "feet" for the display. So if you were at 34 feet in fresh water, the computer would show it as 33 feet because it thinks you're always in salt water. Anyway, that's what I learned from another thread so please correct me if I'm wrong.

My second question is about metric calculations. I've learned to dive in the U.S. for the most part, and even on my first couple open water dives in Cozumel, the equipment we used was from my cruise ship and was imperial (SPG/depth gauge). So what the heck am I going to do when I go diving somewhere that uses the metric system? Can someone help me convert my info to use in a metric world? I'd like to see the calculations so I can add it into my spreadsheet (yes, I'm an accounting nerd). I can find calculations for metric results but since I'm starting with imperial units how do I convert that? What is a common tank size (comparable to the AL80) in the metric system? I'm not sure I'm asking this correctly. I guess I just want to know, when I have info in feet and psi, how I can convert that to metric results.

I guess if I travel with my own reg and SPG I'll just need to convert the metric tank volume to imperial to do my calculations Oh, this is so confusing.



Math really was my best subject in school, but in all fairness I did have teachers who would show me what to do first, before I had to do the calculations on my own.

I want to be able to easily calculate my air consumption for different depths, in different countries .

I've been reading NWGratefulDiver's article on Rock Bottom calculations and so far I figure that diving the Spiegel Grove is going to be out of reach for me unless I learn to dive doubles.

 

[MXGratefulDiver]

I have two questions but figured I'd throw them into one thread because they are related.

First, I'm wondering what kind of air consumption rates some of the veterans on ScubaBoard have? I guess I'd like to know when I reach a "good" RMV rate

So far I've only kept track of the info needed for the calculation on my last four dives (since I bought my computer) and my results for my SAC and RMV are
SAC / RMV
33.08 / 0.88
27.50 / 0.73
33.63 / 0.90
20.87 / 0.56

I have no clue how my air consumption dropped so dramatically on my most recent dive. The only thing I can think of is that it was my (failed) attempt at completing my Search and Recovery adventure dive and I was more concerned about not getting lost and finding the "treasure" than I was about concentrating on breathing.

My guess is that you (a) slowed down and (b) were so focused on attaining your goal that you forgot to be nervous.

Both of those things will do wonders for your air consumption.

I'm also not too sure the calculations are 100% correct because these dives were in fresh water, not salt, but from reading another thread it appears that dive computers measure the pressure and then convert that to "feet" for the display. So if you were at 34 feet in fresh water, the computer would show it as 33 feet because it thinks you're always in salt water. Anyway, that's what I learned from another thread so please correct me if I'm wrong.

You're right ... your dive computer knows nothing about actual depth ... it provides "depth" readings using a pressure sensor that converts to feet-salt-water (fsw). Some computers do have a setting that allows you to set them for fresh water, but it isn't really necessary.

I've been reading NWGratefulDiver's article on Rock Bottom calculations and so far I figure that diving the Spiegel Grove is going to be out of reach for me unless I learn to dive doubles.

You'll get there ... and when you drop into the water after having assured yourself that you have enough gas to actually do the dive, you'll be able to relax more ... which will have the positive effect of improving your gas consumption even further ...

... Bob (Grateful Diver)

 

[Insta-Gator]

Your SAC will improve as you gain experience and relax. The less you think about breathing the better your breathing will be (just be sure you are always breathing.)

My SAC (or SCR) is usually between 15 and 16. Thats a RMV between 0.39 and 0.41, give or take. The lowest SAC I can remember from my log was just under 14 and the highest was 21. The key is to relax and go with the flow.

I can't help you with the metric conversion but here's the NOAA equations if you want to play with some of the math.

http://www.ndc.noaa.gov/pdfs/AirRqmtFormulas.pdf

 

[Kevrumbo]

Cold Water RMV or SCR (Surface Consumption Rate) for me is 22 litres/min (roughly 0.75 cuft/min); Tropical Waters typically 30% to even 50% better. . .

See attached document for easy conversion factors for imperial to metric & vice versa (i.e. just using a factor of 10/3 or 3/10 for depth conversions; and 3/2 or 2/3 & adding or taking away a decimal place "0" for pressure conversions).

 

[g1138]

Taking your cylinder factor into account (Al 80)
I got these numbers (rounded):
0.89 cu.ft/min
0.74
0.91
0.56

Now off to compare. You're going to hate me for this. My first pool dive, I had a SAC rate of 36 psi/min (or 1.01 cu.ft/min) on a St 72.
On my last checkout dive I hade 14.3 psi/min on a St 72.
The equates to about 0.41 cu.ft/min (I'll credit it to beginner's luck).

Since my checkout dives, I've been taking my cylinder factor into account for my calculations (so all numbers will basically be in cu.ft/min)
When I'm doing a scientific dive and running a transect my SAC rate usually gets up to 0.80 cu.ft/min (I'll call this my working SAC rate). If I'm having a fun dive and just relaxing around, my SAC drops to around 0.48. If I'm shadowing a group of basic students, I'm usually swimming pretty fast to keep up with them, and my SAC hits around 0.56 avg.

So it goes without saying, SAC rate will mainly depend on many things.
I wouldn't shoot for a number though. There is no universal SAC rate for experienced divers. It'll depend on your dive conditions, your current health condition, your stress, your air input (ie. do you have to inflate and deflate constantly). etc.

As a little pickme up, I have a friend who's been diving a bit longer than me. Has plenty more dives than me, experience to boot, near perfect trim and buoyancy (Fundies passed), DIR set up and all (not that that makes a different). Perfectly calm under the water. Yet his SAC is still stuck at 1.0 cu.ft/min. He's told me it gets even higher when he's working.
So bottom line is I wouldn't sweat it. Just enjoy your dive, try not to get into a situation of worry, and your SAC rate will drop however low your body lets it.


Moving on to your other questions.
Yes your computer calculates in salt water so when you're at 34ft freshwater your computer reads 33ft. Doing the math, your SAC calculations won't be off drastically.
When diving freshwater I personally would just use the number on your computer and then calculate your SAC rate using feet fresh water (34' per 1 atm increase). That's just me though. I always use my max depth and bottom time for calculating my SAC rate, even if I only spent a min out of 30mins on a dive at max depth. It's just me, I'm not too picky with my math.

As for your metric question, I would just log from your own gear if you have it. If you have a computer/gauges just keep it in imperials and don't worry about converting.
If that's not an option then just keep in mind that for sea water 10m = 33ft roughly.
Your safety stop is at 4.5m-6m = 15ft-20ft.
1bar = 15psi
so 10bar=150psi and 200bar = 3000psi.
That's about it I think.

I don't know what the usual tank size is overseas, it'll probably depend on the specific destination.

 

[g1138]

Side note: What does RMV stand for? :blush:

 

[pjhansman]

Side note: What does RMV stand for? :blush:

Respiratory Minute Volume

 

[Charlie99]

So what the heck am I going to do when I go diving somewhere that uses the metric system? Can someone help me convert my info to use in a metric world?

Metric conversions for diving ...

DEPTH
3 meters = 10' (within about 1%)
10 meters = 1atm (actually it equals 1 bar, which is 1.325% less, but close enough)

Convert depth in feet to meters by divide by 10, then multiply by 3.
example: 60' = 6 10's = 3x6 meters.

Convert meters back to feet the other way. Divide by 3 to get the 10's of feet. Or multiply meters by 3, then add another 10%.
24 meters = 24/3 10's of feet = 80.
25 meters = 75' + 10% = 82.5'

PRESSURE
About 14.5psi per bar. Using 15psi/bar is an error of only 3.5%.

Convenient things to remember are 3000 psi = 200 bar. So obviously 100 bar = 1500psi and 50 bar is about 750psi.
70 bar = 1000 psi. and 35 bar = 500psi are also nice to remember.

TANKS
US tanks are described by the free gas (1atm) volume of gas held at the service pressure. In metric tanks are described by the internal volume in liters. (also called "water volume", since that is how much nearly incompressible water can be held in a tank).

GAS CONSUMPTION
1 cubic foot = 28.3 liters. (1 liter = 1000 cubic centimeters. There are (12*2.54)^3 cubic centimeters in a cubic foot.) So a consumption rate of 0.75 cfm is about 21 liters per minute. (0.75 is 3/4. 3/4 of 28 is 21).

Calculating gas consumption at depth is an awful lot easier in metric, since 10 meters-saltwater = 1 bar = approx 1atm. So going between surface consumption and at depth consumption rates is much easier than dealing with the 33'/atm conversion of imperial.

The volume in a tank at a given pressure is also very easy. The gas consumed, in liters is simply Tank size * (bars at start - bars at end).

TEMPERATURE
I find it easiest to just memorize a couple key numbers, and then use the conversion of 9F per 5C to get the others. For smaller adjustments of less than 5C, I just round it off to 2F = 1 C.
10C = 50F
20C = 68F
25C = 77F
30C = 86F
37C = 98.6F (notice how the rounded 37C sounds so precise in F)
40F = 104F (gotta check the hot tub temp before jumping in).

So if someone tells you the water temp is 22, then you know that it is pretty close to 68F + 4 = 72F.

Don't bother memorizing the whole table above. Just pick one around the expected water temp, and then use the 9F per 5C or 18F per 10C adjustment from there. 20C = 68F is pretty close to normal room temp, so that's the most key one.

SPEED, DISTANCE
1km is pretty close to 5/8 of a mile.
Going the other way, 1 mile is pretty close to 1.6km.
100km/hr speed limit is 62mph.

For in water stuff, the key conversion is 1 knot is very close to 1/2 meter/second.

Since miles are about 15% smaller than nautical miles, a weather report that says the wind will be 20 meters/second means that the wind will be about 40 knots or 45mph.

 

[Charlie99]

Yes your computer calculates in salt water so when you're at 34ft freshwater your computer reads 33ft. Doing the math, your SAC calculations won't be off drastically.
When diving freshwater I personally would just use the number on your computer and then calculate your SAC rate using feet fresh water (34' per 1 atm increase).

If her computer is calibrated to fsw, then a reading of 33' on the computer is 1atm or 2ata. your recommendation of dividing by 34' to get the atm increase is erroneous. OTOH, if her computer reads in ffw, then 1 atm = 34ffw and the depths should be divided by 34 to get the atm increase.

I always use my max depth and bottom time for calculating my SAC rate, even if I only spent a min out of 30mins on a dive at max depth. It's just me, I'm not too picky with my math.

Your method of calculation would result in some very interesting SAC rates on my typical multilevel dive of 120', 70 minutes on an AL80. ..... less than 1/4 cfm, even assuming that I drained the tank empty rather than having 800 or 1000psi left as normal.

Even the crudest mental estimate of average depth would be better to use than a max depth that you were at for only 1 minute.

 

[TSandM]

Karen, I guess the best thing I can say is that the average man runs somewhere between .5 and .7 cfm as an experienced, efficient diver. Women run a little less. It's not useful to know what MY consumption rate is; I've been rather unusual in that, even in the beginning. Chasing after the consumption rate of someone smaller or much more efficient than you are will only lead to skip breathing, and skip breathing is dangerous. I'm almost certainly a CO2 retainer, and that isn't a good thing in certain diving settings (like deep caves) so it's nothing to emulate.

What's important is to keep following your own numbers, and expect to see them fall -- although it's important to regard the numbers in the light of the dive. Dives where you swim, or work, or are task-loaded with new equipment or procedures, will have higher consumption rates than relaxed floats in familiar conditions.