Anyone not measure their SAC?

[lowviz]

I don't doubt or refute anything that you are saying, we both know you live this stuff. But maybe you live too close to the trees to see the forest as a newbie to gas planning would.

A diver just beginning to appreciate SAC/DAC/RMV (pick your fav and stick with it) has little appreciation of how variable such a single number representation of one's gas usage is. Over time, that number will become more meaningful to them. However, it is a simple exercise to determine one's resting rate and one's working rate. After which, that person has gained a much more immediate and valuable insight into themselves. Those single point determinations now fall on a range of one's expected values.

 

[TrimixToo]

I carefully measured my gas consumption over several dives, both the main part of the dive and deco, during one of my technical diving classes (I can't recall which...probably Deco Procedures). Then I plugged the average plus .1 CF/min into my dive planning software. Then I dove while using my computers, and using turn pressure as a guide to surfacing with a third left. After a couple of years, I removed a half-tenth, then after a while longer, another half-tenth. I now plan at .7 CF/min at 1 ATA for back gas, but I always turn on pressure.

All that said, to me the measurements represent a starting point to be modified as operational experience dictates. My consumption rate at a specific depth and workload is a single data point on a spectrum. While it can inform my planning, it cannot accurately predict what I will use or when I should turn.

 

[boulderjohn]

In my first few years of diving, I had no idea what SAC or RMV or SRC (or whatever) meant. I did a lot of dives, and I never had a problem with it. During those years I never saw a scuba cylinder that was not an AL 80. It did not take me long to have a pretty good sense before a dive of how long it would last at the depth I was planning to go. In other words, I had a pretty good idea of my SAC without knowing there was such a thing. I still do dives of that nature today, and although I do know my SAC in a more formal sense, I really don't do anything differently. For most divers, I think that is fine.

With technical diving, things are different, you really need to be sure you have enough gas for the planned dive and any reasonable complications that might occur. I therefore make sure students have a pretty good idea of their SAC rates. By "pretty good idea," I am referring to the fact that you really cannot predict it as nicely as some people seem to think you can. Thus, for gas planning, I use numbers that are a tad higher than I actually expect, and I make sure each member of my team has a very comfortable reserve. Consequently, I don't have to pay a lot of attention to it these days, because I know I am not cutting it at all close. I know that on the dives I am doing, I am coming back with more gas than I would predict, so my know my actual SAC is better than the one I am using.

 

[Kevrumbo]

Let's see . . .I have a 11L Aluminium tank with a 200bar starting pressure and nominally consume 30 bar in five minutes at 18m depth in temperate 15 deg C waters diving the Kelp Forest off Catalina Island:

Five minutes elapsed time and my SPG should read 170bar -unclip & check it! It does. . .

Another five minutes for total Ten minutes elapsed time and my SPG should read 140bar -check! It does.

Fifteen minutes elapsed time and my SPG should read 110bar -check! It does.

Twenty minutes elapsed time and my SPG should read 80bar -check! It does,
and then start a multi-level ascent to the shallows at 9m & eventual safety stop. . . surface with 32 bar remaining and Thirty-Five minutes Total Dive Time.

@boulderjohn -It's not that hard to perform the simple iterative subtraction above during the dive and "predict" within 5 bar what the SPG will show remaining, all easily derived and applied from RMV and pressure SAC rate.

 

[_Ralph]

I record in/out pressures, the electronic log takes care of calculating an average "SAC"

I use it to gauge how demanding the dive was, and for future, i.e. how much gas I need to do the same dive (I dive several different sizes of tanks, so knowing that I need xyz of size yzx is handy)

_R

 

[MXGratefulDiver]

sac rate is measured in volume/time/ata. It has nothing to do with psi until you calculate for that.

Not really ...

NWGratefulDiver.com

... Bob (Grateful Diver)

 

[jlcnuke]

I've got 8 dives in 82F water, 2 of those drift diving and hardly moving my legs, 2 of them in about no current moving comfortably, 4 (short) dives fighting current while doing skills (UW nav, S&R stuff) with a leaking hose to my SPG. Then I've got 2 dives in 68F water relaxed swimming in a quarry.

I somehow don't think the information from that small, varied, group of dives is going to provide me with "good" numbers to use for planning purposes. Taking that into account, and the fact that I have no plans for anything but "simple" rec dives for the near future, I really don't see what good calculating SAC or RMV would do me right now.

For the record, SAC and RMV were never mentioned to my recollection in either of my OW courses (did two, more than a decade apart). It was mentioned, but definitely no emphasis/detail discussed about it, during my AOW course.

 

[lowviz]

...//... I have no plans for anything but "simple" rec dives for the near future, I really don't see what good calculating SAC or RMV would do me right now. ...

Depends on your mindset. If you are only planning on being back on the boat with 500, then forget the calculations.

On the other hand, assume a simple buddy pair out-and-back exterior exploration on a tied-in boat. One diver is a gas sipper and the other is a gas hog with a big tank. Dive plan is to be back on board with 500. If the gas hog loses his/her gas at the extreme of the dive then hard decisions have to be made. However, consider the option where both set their turn pressures to ensure that they are carrying sufficient gas for an emergency. For that, you need a reasonable estimate of each other's consumption rate and turn when the first diver reaches his/her limit.

 

[MXGratefulDiver]

For the record, SAC and RMV were never mentioned to my recollection in either of my OW courses (did two, more than a decade apart). It was mentioned, but definitely no emphasis/detail discussed about it, during my AOW course.

It was not part of the curriculum I was provided by my agency to teach those classes either ... which is why I wrote the article. In the AOW class I taught (technically ASD for my agency) I required my students to take gas consumption measurements and calculate how much gas they would need for their deep dive, based on the dive plan. The purpose of the exercise was less about them learning how to do the calculations than it was to give them an opportunity to understand why going to 100 feet on an AL80 probably wasn't a good idea. Most of them concluded without my ever having to tell them that it wasn't an adequate gas supply for that dive ...

... Bob (Grateful Diver)

 

[Kevrumbo]

. . .A diver just beginning to appreciate SAC/DAC/RMV (pick your fav and stick with it) has little appreciation of how variable such a single number representation of one's gas usage is. Over time, that number will become more meaningful to them. However, it is a simple exercise to determine one's resting rate and one's working rate. After which, that person has gained a much more immediate and valuable insight into themselves. Those single point determinations now fall on a range of one's expected values.

Ok, here's a representative gas consumption and tank(s) table range: you will find that most nominal pressure SAC rates fall between 1 to 2 bar/min per ATA with a particular cylinder for single tank diving.

Below are some example pressure Surface Consumption Rate (SCR) values for a variety of common cylinders with an arbitrary volume SCR (also known as volume SAC rate or RMV):

Given a arbitrary nominal volume SCR of 22 liters/min per ATA (that's 0.78 cuft/min per ATA in US Imperial Units, a reasonable & achievable volume SCR for most novice divers, and exercise level for fit advanced divers with sustained active finning):

Cylinder Size | Pressure SCR
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.

Given a arbitrary nominal volume SCR of 15 liters/min per ATA (0.53 cuft/min per ATA in US Imperial Units, relaxed with minimal finning for advanced divers):

11L/bar tank (AL80): 1.4bar/min per ATA;
12L/bar tank (Steel HP100): 1.3bar/min per ATA;
13L/bar tank (AL100): 1.2bar/min per ATA;
15L/bar tank (Steel HP119): 1bar/min per ATA;
16L/bar tank (Steel HP130): 0.9bar/min per ATA;
11L Twins (Double AL80's): 0.7bar/min per ATA;
12L Twins (Double HP100's): 0.6bar/min per ATA;
16L Twins (Double HP130's): 0.5bar/min per ATA.

Given a arbitrary nominal volume SCR of 11 liters/min per ATA (0.39 cuft/min per ATA in US Imperial Units, drift diving floating neutrally buoyant & going with the current):

11L/bar tank (AL80): 1bar/min per ATA;
12L/bar tank (Steel HP100): 0.9bar/min per ATA;
13L/bar tank (AL100): 0.8bar/min per ATA;
15L/bar tank (Steel HP119): 0.73bar/min per ATA;
16L/bar tank (Steel HP130): 0.68bar/min per ATA;
11L Twins (Double AL80's): 0.5bar/min per ATA;
12L Twins (Double HP100's): 0.45bar/min per ATA;
16L Twins (Double HP130's): 0.3bar/min per ATA.

The point is that SAC rate for most nominal activity on single tank is going to fall in between 1 bar/min per ATA and 2 bar/min per ATA. And how easy is it to figure multiplying factors of numbers like 1 and 2? --All you need now is a convenient time interval like 10 minutes and your metric depth converted to ATA as multiplying factors, and you will then know what your Depth Consumption in bar will be over that time interval, with a particular tank, and level of physical activity. . .