SAC Amusement

[scubadada]

Yep, that's the spot...similar time span to what I'm seeing. I appear to be a bit longer before it settles down...lack of experience maybe leading to less smoothness establishing neutrality?

I've lost track about the purpose of this thread, is it about your RMV in the middle of the pack, or something else?

 

[Slow]

Has to do with the sampling rate, mine is 30 seconds, don't know what yours is.

Hmmm...Interesing point. My sampling frequency is 10 seconds and doesn't report SAC until about 2 minutes into the dive.

 

[Slow]

I've lost track about the purpose of this thread, is it about your RMV in the middle of the pack, or something else?

Purpose? Nothing important, just some thoughts to share with my fellow newbs who might be trying to figure out some of the same things regarding their own gas usage. I hadn't seen any specifics or details discussed in anything I could find online, so thought maybe this would be a point of info that others might find useful.

 

[Streydog]

Above is a dive where I was fighting current to get to a wall without getting pushed out to open water, then stay on the reef. Below is a much easier light drift dive not going against current much. My SAC has improved quite a bit, I was a huge air hog. I think the more you dive the better it gets, dropping weight and having to use less air in in your BC helps. I used to be thinking about my next breath in while I was exhaling, now I am more comfortable slowly exhaling. Both graphs do show a much higher SAC at the beginning of the dive though.

 

[Caveeagle]

1) Yes, buddy was AL80 as was I. Next trip I'm going to be asking the dive op for 100 cf tanks.

Good call. HP100s are really nice tanks. You should be able to ditch some weight off your belt too.

2) As a newb I don't entirely trust my own opinion on my trim etc. That said, after seeing me dive, everyone else on the dive boat, including the DMs, thought I was joking when I told them how new I am to diving. They said that under the water I looked like I'd been diving a long time. Apparently my trim and streamlining are good. I've spent a lot of time in the pool working on that, so it was gratifying to hear it has paid off. I do have one piece of gear most people don't carry, a 6cf pony bottle, strapped to my chest. My standard position is to hold my arms across my chest, which keeps the bottle and reg hugged close to me. Extra drag for sure, but I don't think it's a ton...fwiw, even when sitting still I have higher gas usage than other people do who are moving around. I'll keep working on streamlining for sure, but I don't think I'm a long way off of ideal.

Don't take their word for it.. not trying to be a jerk, but many OW divers, DMs and Instructors have a pretty WIDE definition of what "good" looks like. You may very well be a natural.. Just double check. Get somebody to GoPro some video of you swimming around in full gear. Often your buddy will really resist giving you honest criticism. And DMs that are hoping you will plant that tip on them at the end of the day.

..And move that pony over to your side. I can almost guarantee its creating drag clipped across your chest.

3) Controlling breathing is something everyone says not to do, and I understand why. I think if they saw the way I habitually breathe when riding a bike, they'd immediately say that it's not beneficial when diving. I think that the standard instruction of "breathe like you normally do" is somewhat incomplete when you don't know how someone normally breathes.

If your "bike" breathing is what you call normal, then we may have found your problem. I am no cardio expert, but I do ride a road bike from time to time. You just cannot compare the two activities. Sure if your body is in really good cardio condition, then you might be more efficient than me when we are both fighting current, and breathing like freight trains, climbing hill.. But the object in diving is to try and keep your heart rate and breathing rate as close to your resting rate as possible. So even if you are in better shape than me.. but I am just chilling and floating around.. your gonna breathe more air if your thinking about your breathing a ton.. In/Out/In/Out/In/Out...ect.. Its just too deliberate, and your consumption is going to show it. Once you are comfortable in the water, and you start to think about other things.. you should start to improve.

5) Good point on accuracy of the measurement...the initial spikiness could just be due to pressure sensor inaccuracies. What is MSA?

For DATA / Analysis junkies only: Short version: don't try to analyze measurements made in inches, when your tools are only accurate to within yards...

Measurement system analysis (MSA) is an experimental and mathematical method of determining how much the variation within the measurement process contributes to overall process variability. There are five parameters to investigate in an MSA: bias, linearity, stability, repeatability and reproducibility.

 

[cneal]

New divers, and I was very bad at this, will not trust the equipment and inhale quickly instead of a nice slow and relaxed breath. This results in a greater volume in each breath and faster rate. Once you relax not only will your air consumption drop you won't need as much weight.

The best result will come with bottom time. Get out from behind the desk and dive.

 

[bowlofpetunias]

You are right Scuba is not supposed to be competitive but people ARE still competitive. The three areas I see people being competitive tend to be SAC rates, number of dives and depth of dives. I've seen people do 4 dives by surfacing multiple times while the rest of the group did one dive Too many people have gone to break their depth records only to turn up in A&I threads.

IMHO the thing abut SAC rates is somewhat understandable. Seems like very few people mind shortening the dive because their buddy is low on air but everyone hates BEING the buddy that is shortening the dive. There really is an easy solution and you have already touched on it. Get a bigger tank! Knowing your SAC should be about being able to put together a safe dive plan based on realistic gas needs.

Sure dealing with things like trim, bouyancy and stress can make a difference. Some people have bigger lung volumes some people just need more air period! It just shouldn't be a big deal beyond planning!

I admit I do things like fill my BCD with exhaled air rather than the power inflator especially at the beginning and end of the dive. I think it is good practice to inflate orally at the surface to establish positive bouyancy so if anything happened that you had to it would be no big deal at all. I tend to try to fill my wetsuit with some water on the surface if I can. That way I don't have to fight the trapped air to descend. Proper weighting means you have to work less as well.

For clarification I have never been a scuba instructor but have taught (thousands of) first aiders and Paramedics for years before retirement. IMHO some of the issues new divers can have is that having told never to hold their breath and to "breathe normally" they get so focused on breathing they wind up breathing more than they need to. Normal breathing does involve a pause.

A simple analogy on breathing.

Homer Simpson is sitting in a control centre monitoring a gauge:

take a breath and O2 sats go up Co2 levels go down... Homer is happy....no activity needed
pause... Co2 sats start going up.... Homer is lazy and no alarms have gone off
Co2 reaches a "trigger" level Homer's alarm goes off and he pushes the Exhale (vent) button
breathe out Co2 Homer pushes the Inhale button
breath in....

Here is one of the best explanations of breathing for new divers ever posted...Air consumption tips?

 

[Saniflush]

I admit I do things like fill my BCD with exhaled air rather than the power inflator especially at the beginning and end of the dive. I think it is good practice to inflate orally at the surface to establish positive bouyancy so if anything happened that you had to it would be no big deal at all.

I do this as well. Never thought about it being good practice, just always figured my sac rate was so bad there was no sense in me using some of that new, fresh outta that tank air when I already had some "used air" ready to go.