Computing SAC in your living room

[KenGordon]

it needs avg. depth to calculate decompression limits,.

You need the whole profile if doing what a dive computer does. 10 minutes at 50m followed by 10 minutes at 10m is not the same deco as 20 minutes at 30.

Different tissue groups become saturated at different times, the limiting group changes, it is a fairly complicated and not entirely linear set of relationships.

 

[MXGratefulDiver]

One thing I absolutely don't like about NW's post on the subject is that he uses CFM for RMV, and PSIM on SAC. That makes SAC a completely useless unit unless 100% of the time you dive the same tanks, and it also means nothing if you're talking to people about it.

Actually that's not so. Later in the article (I'm assuming your talking about this article on my website) I explain how to convert your measurements from one cylinder size to another, using the "baseline" of the cylinder. I also explain why it matters to understand your gas consumption in both volume (cubic feet per minute) and pressure (psi per minute) and how to make the conversion from one to the other.

Basically, it's important because when you're planning for your dive you know how much gas your tank holds ... in cubic feet ... at it's working pressure. So you can get an idea whether or not you're carrying adequate supply for the dive you're planning to do. When you're diving, you only have your pressure gauge to tell you how much gas is in your tank. So volume is good for planning, and pressure is good for executing.

Most of us express our SAC in CFM because it makes it meaningful and if the US wasn't so arrogant and idiotic about units, we'd switch our gauges over to bar where calculating SAC takes about, oh half a second. Much more useful way to discuss things.

No argument there ... I'd love to have seen us convert to the metric system 55 years ago ... when it was first attempted. But we Americans tend to want to do things "our" way ... and since we live in that world it's best to learn how to survive in it.

Anyway, yes you can estimate your SAC by cycling, but generally if you sit on the couch and hang out that will give your true RMV at rest which is important because this is the absolute lowest your air consumption will be. Multiply for 1.5x for deco, 2x for dive planning and you're pretty close.

I prefer in-water measurements. The way I do it in my AOW class is to have the students do a hard swim at a constant depth for 5 minutes, taking before and after consumption measurements ... then rest until their breathing rate comes back down to normal and do a 10-minute leisurely swim, taking before and after measurements. Use those two sets of numbers to establish your high (working) and low (resting) consumption rates, and consider that your actual consumption during the dive is going to be somewhere in between.

In most of these discussions there seems to be the notion that SAC is a hard number. It's not ... it's actually a range. Consider it much like the "highway" and "city" miles per gallon you get in your car ... your actual use depends on a number of factors.

Think about how you breathe in real life. If you're laying on the couch, you're breathing at a certain rate. If you're taking a leisurely stroll, it's a higher rate. If you're jogging, it's yet higher. And if you're running from that big dog that just came charging out of the driveway you just jogged past, it's right out through the roof.

Diving's like that ... how quickly you go through your gas will depend on a number of variables. Multiplying by 2X to get your working SAC from your resting SAC will put you in the ballpark ... but we're all different. In the measurements I've done as described above, I've seen that number go up more than 3X ... it really depends on the individual.

Now, granted, most people don't really want to do that much arithmetic ... and in most cases they don't need to. I explain to my students that we're not going through this exercise because I want them to spend their dive planning time doing arithmetic, so much as I want them to grasp the concepts of what happens when you dive. Knowing how much your consumption rate increases when you're working hard helps you reinforce the idea that when you run into those conditions you need to either shallow up or shorten your dive ... and a pretty good ballpark of by how much. Monitoring your gauge will do the rest, but you first need to know why those steps are necessary. It's part of being proactive, rather than reactive, to the circumstances of the dive.

I have my AOW students take their consumption measurements and apply them toward their deep dive plan ... to predict how much gas they're going to need to do that dive. It's not uncommon for those with smaller tanks ... like AL80's ... to come to the conclusion that they can't do that dive because their tank's too small. That's really the point ... I want them to understand that before they go deep ... not once they're down there.

... Bob (Grateful Diver)

 

[tbone1004]

Ken, I know how the algorithms work, but they still all have the ability to calculate it since they are live tracking depth and time

Bob,
I know you did the tank factors in there, but I still think it's better to run everything in CFM and then convert that to PSI rather than vice versa, same math, but gives you the initial output in something usable. As an engineer and a diver who was trained by one of the original cave divers from day 1, I always think of my gas supply in cubic feet and treat PSI as a fraction of my full tank volume. Unfortunately most tanks don't have easy math so you have to round, but it gets you close enough. Thinking in cubic feet also makes it easier to convert to DAC since the numbers rounded to safety get you close. Also why I wish we did our depth in meters.... Oh wait, my petrel is in metric and my gauges have bar and psi on them

What you can do to get a good baseline is the following.

Sit on couch with as small of a tank as you have. AL30's are particularly good for this if you can get a hold of one, smaller the better though. Breathe while watching TV and have one of those clicker things they use to count people. Count the number of breaths you take while breathing off of the regulator, do it for at least 10 minutes. You can do shorter time intervals if you have smaller tanks, but you need a gauge in 100psi increments to get the best results and you need to breathe at least 300 psi out of it, so on a 30 for most people that's about 10 minutes.
The SAC you get gives you your resting SAC rate under ideal circumstances. The more important thing is by counting the number of breaths, you get your approximate resting tidal volume.

Better yet, if you can, you can make yourself breathe as if you were breathing on scuba taking "normal" breaths and that gives you your tidal volume while you're walking around. You can also do this on a treadmill or exercise bike or something so you are actually having to use the normal amount of volume instead of the top 15-20% we use when resting. Just do it to where you are breathing like if you're walking through the grocery store. The more clicks you get and the more you can breathe off of the tank, the better your numbers are going to be.

Once you get your approximate tidal volume calculated, you can then get your exertion levels up to "diving" circumstances, and over a 30 second or 60 second period, count the number of breaths you take and this will give you a pretty good baseline of your SAC under different circumstances.

This is all way more involved than it needs to be, and the best thing is really to have a computer or bottom timer with avg. depth on it and just calculate it over the course of a dive. Every one of my dives has SAC marked on it and it is a good indicator to how I was feeling on that dive. They can bounce between 0.35 and 0.65 depending on how I'm feeling and what I'm doing.

 

[scubadada]

I agree with several others, your SRMV is the most valuable information to have. It takes into account exertion, water temperature, and other factors that contribute to your gas consumption.

My primary computer is an AI Oceanic VT3. The download software automatically captures air consumption, average depth, and dive time. It uses a default cylinder of your choice or you designate an alternative cylinder to automatically calculate SRMV for each dive.

I have a SMRV for my last 425 dives over the last 4 1/4 years. Though my SRMV is affected by exertion and, in a minor way by water temperature, the values are actually quite uniform. The mean +/- standard deviation is 0.37 +/- 0.04 cu ft/min. That means 95% of my SRMVs fall within the range of 0.29-0.45 cu ft/min. The real range is slightly greater, 0.28-0.63 cu ft/min. The highest SMRV was a deeper, dark wreck in moderate current at 47 degrees in a wetsuit. I use my own results to successfully and accurately plan my gas requirements. I use twice my average SMRV, a value I've never actually reached, to plan emergency gas needs such as pony volume needed for a normal ascent from recreational depths.

Good diving, Craig