Computing SAC in your living room

[TN-Steve]

Hey Gang,

Have a question, this seem like a good idea to me, which always makes me want to triple check it.

I need to determine my SAC rate so I can start to do effective dive planning. Right now it's more in the "We all jump into the quarry, swim around a while, and when somebody gets low, we surface". Since I'd rather have a better idea of how long, how deep, how much than that, I need to get some good numbers.

I was thinking (and here is where it gets dangerous), I've got a perfectly good cycle trainer at the house, a HRM that I'm very familiar with, and the ability to get my heart rate up to the approximate level I feel when diving (don't wear a HRM during dives, but I've done enough long distance cycling and touring to have a good feel for how much I'm exerting myself. )

It seems like by getting warmed up on the bike, putting my reg in and doing a measured time / psi ride I can very precisely (at least precise enough) determine my basic SAC. Once I've got that, everything else comes easy.

I was also thinking that I could then increase my work load, see how that affects it, and also see what happens when I'm working hard enough to start to over-breath the regulator.

I've read everything I could find on gas management for rec. divers, NW_Diver and the posts at Spherical Chicken were tremendously helpful.

What, if anything, am I missing. When I'm diving with the club, it's a lot of up/down, not holding a steady depth, and I really don't want to ask somebody to watch me swim back and forth at 33 feet for 15 minutes.

Thanks in Advance.

 

[drrich2]

The SDI Solo Diver course manual gets into this issue; calculating a true surface air consumption rate at rest on land, then multiplying by your dive depth pressure factor (1 at the surface, 2 at 33 ft, 3 at 66 ft, etc...) and then multiplying by a difficulty factor based on estimated difficulty of the dive (e.g.: might be 1.5x for an easy reef dive) to get an estimated Respiratory Minute Volume (RMV). That provides some rough numbers to get started, which experience can then hone further. I got my measured SAC by taking a tank with known volume and pressure, breathing off it (with mask on so I didn't use my nose) at rest for a set period of time (20 minutes for me), compared PSI change to tank volume to get the gas I'd used, divided by 20 minutes & got my SAC.

I hope I recounted all the above without any screw ups. Apology in advance if I did not.

It's a bit different than what you're proposing to do.

Are you diving a computer that lets you download dives to your desktop computer, and use start & end tank pressures to calculate your SAC?

Richard.

 

[TN-Steve]

The SDI Solo Diver course manual gets into this issue; calculating a true surface air consumption rate at rest on land, then multiplying by your dive depth pressure factor (1 at the surface, 2 at 33 ft, 3 at 66 ft, etc...) and then multiplying by a difficulty factor based on estimated difficulty of the dive (e.g.: might be 1.5x for an easy reef dive) to get an estimated Respiratory Minute Volume (RMV). That provides some rough numbers to get started, which experience can then hone further. I got my measured SAC by taking a tank with known volume and pressure, breathing off it (with mask on so I didn't use my nose) at rest for a set period of time (20 minutes for me), compared PSI change to tank volume to get the gas I'd used, divided by 20 minutes & got my SAC.

I hope I recounted all the above without any screw ups. Apology in advance if I did not.

It's a bit different than what you're proposing to do.

Are you diving a computer that lets you download dives to your desktop computer, and use start & end tank pressures to calculate your SAC?

Richard.

Yea, I would be factoring the workload in, and I've got a pretty good feel for what range my HR / Exertion level is hanging out in when I'm active. The HRM would just be a confirmation, I know what resistance levels / rpms on this trainer gets me to certain zones pretty well by now.

Nope, my Mares Puck isn't AI, and I'm not planning on spending 1/2 the price of the computer to get their proprietary USB cable to download the data. Would give me a profile, but not the Air data in any case.

Good point about using the mask, hadn't considered that.

Steve

 

[redacted]

I'm not sure what you could do with a SAC computed at the surface under an exercise load. I'm not sure how you can relate that to scuba (unless your DPV is pedal operated). I did mine during the SDI solo course. Living room, easy chair, mask on. This should produce a lower limit that is probably not achievable in water with any reasonable level of exertion but it does give you a lower bound to work with. I then spent a few hours in Scuba Club Cozumel's shore dive and worked out some more useful estimates for very low effort dives. Now I just work on making all my dives as low effort as I can.

Just a few data points: As I recall, my living room SAC was about .25. I'm pretty sure I stayed awake for the over 30 minutes I spent in my easy chair. I even did it twice to check the results.

In the SCC shore dive (avg depth 20 ft), I was running as low as .35 if I took it real easy and .4 with a bit more effort. I usually plan my gas usage based on a SAC of .5. If I get to working hard, like in current, I probably get up into the .6+ area.

Where it really helps a lot is in recognizing and managing those conditions that elevate your SAC. I do pretty good at avoiding conditions and actions that elevate my SAC.

 

[TN-Steve]

My idea is that I'll be able to approximate my actual air consumption when I'm doing my typical "easy kicking, not pushing hard at all" swim mode. That should give me a good number to do planning of off, once I've verified it.

 

[KenGordon]

Nope, my Mares Puck isn't AI, and I'm not planning on spending 1/2 the price of the computer to get their proprietary USB cable to download the data. Would give me a profile, but not the Air data in any case.

You do not need air integration just average depth, time, gas in and gas out. Some dive computers will give average depth when showing the log:

Suppose an average of 16m, gas in of 200bar, out of 65bar and 45 minutes for an 11l cylinder:

200-65 = 135 bar used
135 bar divided by 45 minutes is 3 bar/minute
3 times 11l is 33 litres per minute at depth
Divide by the pressure at 16m, 2.6 bar to get SAC

33/2.6 = 12.7l/min

AI is helpful as it will show how different conditions change your consumption rate, swimming with/against the tide, faffing with an SMB, putting gas in a BC on the way down, getting stressed when you bang your head on unexpected metal plate or being all calm and relaxed on a stop.

 

[waterpirate]

I think the living room excersise is fruitless. This is where filling out a good log entry will help. You need to know:
start pressure
end pressure
dive time
depth
temprature of water
exertion level
mental state/ nervous, anxious, relaxed ect.

Over a number of dives, your real air consumption will present itself. Part of a passing grade for full trimix was to do a given dive and predict what the end pressure would be for all gas when back on the boat to within 250 psi. That is a fun excersise to see if you are on track or not.

YMMV
Eric

 

[jboneng]

SAC and RMV is two values I always note down in my log after a dive, SAC and RMV is dependent on to many variables to be simulated on land, so the best thing to use in dive planing is, in my opinion, to find the newest log entry that matches the dive you are planning and use the SAC rate noted there. Calculating SAC is pretty straight forward.

 

[Basking Ridge Diver]

RMV = ((PSI Used / Working Pressure) x Cylinder Capacity) / (((Avg Depth / 33) + 1) x Minutes)
Example on an AL 80 =((1900/3000)*80)/(((25.023/33)+1)*35) = .823

Relatively easy but you need to know your Start and Ending Pressure, average depth and time.

 

[tbone1004]

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. 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.

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.

Unfortunately the Mares puck, like most cheap recreational computers doesn't spit out average depth which is stupid because it needs avg. depth to calculate decompression limits, so it should spit it out. I guess the pro version does at quick google search. This is reason #1 why I tell people to avoid every dive computer that doesn't spit out average depth. If you don't have that number, you have no way to gauge how your body actually felt on the dive. If you look at a history of your dives, you can see which dives you were comfy on, which you were stressed, which you were over-exerted etc. Can't do that without having your SAC calculated.