Do you use your SAC for anything? POLL

[thanksforallthefish]

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[BoltSnap]

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[aquaregia]

I suspect there is some fixation on SAC rate, specifically people trying to lower it as if it is the measure of a good diver (just as there can be with how much weight people carry). That is a bad thing and not particularly helpful.

I whine about this a lot, and I'm gonna do it again It seems very common that people return from a dive trip complaining about some hoover buddy who cut short their dive due to high air consumption. As long as people complain like this, guys like me with high gas consumptions will continue to fixate on lowering their SAC.

 

[markmantei]

Whew, just caught up on the thread. Good posts. Obviously SAC and RMV rates are very important to tech diving, but for a drift dive on a reef @ a carribean resort it is much less important. I am lazy, but since the ai computers and software calc the SAC rates I look at them every dive.

While not fixated on SAC, as a fairly new diver I try to keep breathing rates in the back of my mind when diving. Reviewing my SAC post dive gives my an idea of how I did on a particular dive. Trying to lower it not to prove I'm a better diver, but to increase bottom time and do more diving.

I do find it incredibly useful for deconstructing a dive. It is absolutly interesting to see how much variation there is due to factors such as a stressful situation underwater, working hard in wierd positions (halloween underwater pumpkin carving was murder on the SAC). Also, i'd have to check, but if memory serves, my SAC in warm carribean with much less gear is close to half what it is in the cold lakes in a drysuit. That does make sense though, following the current is less kicking = lower sac, let equipment = less drag = lower sac.

 

[Kevrumbo]

My SAC/RMV in tropical warm waters is typically 30% better than it is in temperate cold SoCal homewaters. However, after a week now diving in Palau 30deg C water temp, I've lowered that to around 50% of my nominal cold water SAC (from 22 litres/min to 11 litres/min).

This is how I'm using this value with a 11 litres/bar tank (i.e. an AL80) here in Palau:
11 [-]litres[/-]/min divided-by- 11 [-]litres[/-]/bar equals 1 bar/min pressure SAC rate.

All my dives are averaging 20 meters depth going with the drift current; 20 meters is 3 ATA (divide 20 by 10 and add 1 gives a depth in atmospheres absolute of 3 ATA).

Therefore 1bar/min multiplied by 3 ATA equals a depth consumption rate of 3 bar/min at 20 meters. Checking my bottom timer every 10 minutes, I expect to consume 30 bar (3 bar/min multiplied by 10min equals 30 bar), and accordingly my SPG should read 30 bar less in that 10 minute time frame.

So by 30 minutes elapsed dive time at 20 meters, I expect to be down 90 bar or at half tank (AL80 full tank is 200 bar). At 40 minutes elapsed time, I'm ascending off the wall into the shallow coral plateau around 9 meters (down 120 bar from 200 bar total, or 80 bar remaining in tank). And finally at the 45 to 50 minute mark, I'm at 6m and my 3-5min safety stop with 60 to 70 bar left. I surface and I know even before looking at my SPG that I have around 50 bar remaining in my tank.

This is how you should actively use your SAC rate with your particular tank, knowing how much breathing gas you have left, not just only pre-planning, but also during the actual dive real-time-on-the-fly . . .additionally, you have a SPG that reads in units of pressure: why not convert your SAC rate to a Depth Consumption Rate (DCR) in pressure units to make use of it???

 

[spoolin01]

Kev, here's the thing I don't get about this - for the type of diving you're describing. Why is it better to do the math, then monitor your watch, so that you can predict what your SPG is going to tell you, when monitoring the SPG is simpler, and ultimately what counts? You're going to act based on the SPG whenever it disagrees with the whole forecasting regime anyway, so why not keep it simple? Neither method works if you're not watching the indicator, so why do two when only the one really matters?

 

[TSandM]

spoolin, it's a cross-check on the SPG. If your predicted reading doesn't match the actual one, you have to worry about a stuck gauge (if it's too high) or a leak somewhere that will mean you'll need to end your dive before you expected to.

 

[billmas]

I whine about this a lot, and I'm gonna do it again It seems very common that people return from a dive trip complaining about some hoover buddy who cut short their dive due to high air consumption. As long as people complain like this, guys like me with high gas consumptions will continue to fixate on lowering their SAC.

Hey Aquaregia (BTW a lot of this post isn't directed at you),

I'm with you on that matey, been there, done that and got the T'shirt.

I'm a big guy, not particularly overweight or unfit or anything, just a big guy. On my last holiday diving, I turned 5 of the 6 dives. I can imagine that many of the divers who had paid the same as me for the same dives, were probably a little p'd off about it. No one said anything, maybe they were all cool about it, but I hate being that guy.

I had told the dive centre before I went and I was very clear.... "I need a 15l tank". Thay said "no you don't". Cue farcical rendition of a panto.... "On no you don't", "oh yes I do", "Oh no you dont"... etc etc ad nauseum.

Yes, knowing my SAC should have been useful in that situation but wasn't (because the dive centre wanted us in and out as quick as possible so they could take our money and go home).. but the SAC(max/average/min) is (IMHO) very useful otherwise.

My typical SAC is useful in dive planning (and I too would love to get it lower), but my peak SAC dictates to me about gas reserves. I'm a newbie at all this, but I'm a computer programmer - numbers are my bread and butter, they run around with ease in my head all the time.

That is what made me sit down and write the code to do my rolling SAC during the dive, and to be honest I'm glad I did. Yeah, I'm doing 9l/min (0.3cu/ft?) during the safety stop but I can be doing 70 l/min (3 cu.ft?) when I'm stressed.

When I sit on a boat and the dive briefing says "turn the dive at x bar" or somesuch, I generally disregard it. I know the numbers, I know the peak SACs, I know that I could have my buddy on my Octo, I run my own numbers in my head to make sure they're safe for my buddy and I. I sincerely hope my numbers are never tested

Typically on a boat over here, it is something like "turn the dive at 100 bar", or on a shot dive, "begin the ascent at 100 bar" etc etc etc. This is regardless of whether the dive is planned to 10m or 30m, or whether I have a 10L or 15L tank ... I've heard the same on every briefing I've had To me, this is quite ridiculous.

One of my buddies dives twin 12's, I dive a single 15+pony, another dives a single 12. Quite simply, how can "turn the dive at 100 bar" be safe and appropriate for all of us? It's not. It suggests to me that for "liability" reasons, there is this set number that everyone sticks to... IIRC it is even in my learning materials for getting certified ... turn the dive at "x" or hit the surface with "y". I personally believe that the set number isn't actually always safe... so I'll keep running my own numbers.

What concerns me, even when recreational diving is that there is this ethos "as long as I turn my dive at x bar we'll be safe" but I believe that is clearly untrue.

I'm not trying to say I'm better or more qualified than the boat skipper or other divers, I'm not, I'm a less than 50 dives newbie.. ie a proper newbie learner. My buddy on the twin 12's (with thousands of dives) hasn't ever given any thought to his SAC. I'm the newb, he's the accomplished diver but he still runs on "turn the dive at x bar" regardless of depth or any other factors. It has been drummed into him that as long as he does that he'll be safe. For his average SAC, it is probably true, but for his peak SAC... I don't know, neither he or I know what his peak is.

What I'm trying to say, it is only by knowing my typical and peak SAC that I can make my own decisions as to what will and will not keep me alive.

For example....

... You're at 30m
... You're on a 12l tank (I'm guessing that's an LP100?)
... Your buddy goes on your Octo
... You have half of your initial air left
... You both have eyes as wide as plates, so triple your SAC for "Oh Sh*t" purposes.

How long have you got? How long before you begin ascent? 1 minute, 2 minutes? Can you safely do the 9m per minute ascent? And a safety stop? What if you can't. Should you risk a bend (alive but bent) instead of drowning (dead but not bent)?

(It's a rhetorical question ... to everyone... please don't answer specifically! But if you haven't answered it to yourself in the next 5 or 10 seconds or so.... you're 30m down remember and you won't get much longer to think about it? This might be your last chance, the difference between (1) dead (2) bent (3) alive and healthy.

Those are the numbers I run in my head all the time, I'm not a tech diver, but to me they're about staying alive. My typical SAC is great for dive planning, my peak SAC is a minimum for getting us out alive. I only know my peak SAC from having that information available from all of my dives.

Anyway, It's getting late... only two weeks before some warm water diving in the Mediteranean for me... and guess what... it's on a 12l tank instead of the 15l I normally dive "Oh no you're not" "oh yes you are" etc etc

(One day I'll start another thread about my last diving in the med... diving on a prayer is the best way I can describe it!).

Dive Safe All,

Kind Regards
Bill the Newbie Diver but Accomplished Counter

 

[BoltSnap]

Bill,

Are the tank volumes you list (15L, 12L, etc.) set for a standard pressure? What is it if so?

 

[billmas]

Bill,

Are the tank volumes you list (15L, 12L, etc.) set for a standard pressure? What is it if so?

Hi Burhaneddin,

Typical tanks here are 232 bar (~ 3350 psi), you can enter that for the 'start' pressure into one of the green boxes, but you can also run it for any other pressure.

I've found a site with tank sizes in imperial and metric...

XS Scuba Worthington Steel Cylinder Specifications

So a
- HP80 is ~ 10 litre, 232 bar
- HP100 is ~ 12 litre, 232 bar
- HP120 is ~ 15 litre, 232 bar

Kind Regards
Bill