Safety First and a tiny bit of scuba math

I’ve decided to post my thoughts on random subjects a bit more readily.

I used to do that, I used to just blab away, without really a care in the world whether or not anyone gave a hoot what I was writing.  It was interesting and meaningful to me and that was ALL that mattered.

Then I got all serious.  A few years back I had this random idea that I was going to intern and become an instructor for a different diving agency.   One that is pretty serious (or at least was) about how you represent.   A modicum of decorum felt necessary, partially because I was old skool bucking the trend and partially because I’m female.   I felt the need to over achieve if you get my drift.  Don’t get me wrong, it was not necessarily the fault of this decision tree I’d followed but if you go back and read my blogs prior, there was definitely less filtering going on. It became less regular, unadulterated and more vanilla me.   The occasional “back in the old days” forum post would escape my keyboard, but for the most part, relatively tame.  Like I said, low fat vanilla.

My instructor card is from NAUI (National Association of Underwater Instructors). I’m updated (they make us re-up on standards and such) but not currently active.  Retired, or Semi-Retired, meaning, if you for some absurd reason decide you really want me to be your instructor, in addition to a wee bit to cover my time, it will take the cost of my fees + insurance + a trip to somewhere warm for your open water dives and then we come back here to the Puget Sound for some nice Emerald Sea diving because you will have made a commitment to do Basic and Advanced.   NAUI was a great option for me, I chose it back in the early 90’s as my agency of choice because back then, they were a not-for-profit and encouraged instructors to teach above the standards, i.e. if you taught in a cold water environment such as the northwest, they fully supported the idea of adding in some more dives or including drysuits in your program or whatnot.   There was not this “customer is always right” attitude that has pervaded the system, they did not bow to the establishment of entitlement that people sometimes have when they’ve paid money for a class that is teaching them to survive in the alien environment of inner space.

But I digress.   I just wanted to give my fine readers a bit of catch up and fair warning before leaping off on an unfiltered tangent.

Tonight my tangent of choice is a response to some decisions that get made by newer or misguided divers which simply blow my mind, and then proceed to get disseminated via forums and the likes.

I was reading a forum post and a diver made a comment that relates to pre-dive check.   For the non-divers in the audience, a pre dive check is that last round of checks that assure your air is on, your kit is functioning correctly and your hoses are not tangled in a way that might prevent you from delivering air to a diver in need.   Its the last chance to make sure you and your buddies are on the same page with regards to what gas you are all breathing, what the dive plan is, max depth, planned duration, what the turn pressure is and if its a new buddy, how you communicate underwater “this is my signal for X or Y”.   We do this because as Jacques Cousteau put it, our wrap around underwater vehicle will soon be taking us into the silent world.  Not really silent because there are all sorts of noises underwater, but silent as in “I don’t have to hear you talking to me” silent world.   I like to say a lot of being a safe diver is garbage in, garbage out.  If you fill your tanks with good breathing gas, maintain your equipment, and follow some very basic rules (turn gas on, come up with X psi in your tank, etc..) and keep a functioning regulator in your mouth when underwater, then there is a REALLY good chance you’ll survive.

This is where my ability to suspend disbelief met its match.

When my air usage was higher and we were diving with a guy who didn’t seem to use any I got in the habit of not using any air I didn’t have to. You can check for yourself, if your air is off or even almost off and you take a breath, the gauge will twitch. If it is off the gauge will drop down and stay. With a full tank and the valve open I can’t even see a twitch when I take a breath. One breath does it for me unless someone else touches my stuff.

No, I’m not gonna change the words to protect the guilty either.  This is misguided.  Period.  There is so much more going on in a pre-dive check than “one breath” can tell you.  Yes, you CAN see if your valve is on or off, but…

First and foremost I am 100% against turning on a valve to ‘check pressure’ and then turning it ‘off’ to save gas.  If any of your gear is leaking enough to make saving gas an issue then you need to get it serviced.   Turning gas off before dives in this fashion kills divers.  Period.  It kills divers in 10 feet of water at sites where we take basic open water students for their certification dives because the dive is known for its benign nature.   I can leave my kit sitting for days with the valve in the on position and if there is not a leak, then, guess what?!?!  It stays full!  It may not be the best for the piston seat, but it doesn’t leak.

I ALWAYS encourage people to take a couple breaths from each regulator (with them underwater) to make sure they are both functional (your primary and your backup).  In doing so you will find any tears in the diaphragm (will fill with water and fail to deliver gas unless you manually press the purge valve), or seaweed stuck in the exhaust valve or diaphragm (it will breath remarkably wet and again, may not be able to draw a vacuum to toggle the demand valve and have to depress the purge valve for a breath) while you are on the surface and not 90′ underwater when everything matters 1000 fold.   In the event that all heck breaks loose and you have to hand off your primary regulator and go on your backup, or hand off your octopus/safe second, the last thing you and your stressed out buddy need is a mouth full of water when hoping for a full, dry, life sustaining breath.

Another part of my pre dive is I put gas in my BCD, often to a point where the overpressure relief burps, that way i know everything is functioning and I also know (if it keeps burping) that I have a slow leak in my inflator valve that needs fixing.   I hook up my drysuit and I give it a couple good puffs of gas into my suit.  Both BC and drysuit tests are a habit i’ve formed that allows for both physical and audio feedback that:  a) my tank is on, b) i remembered to hook it hoses and c) all the valves work the way I expect them to.

If i’m diving with someone new to me, they may even choose to take a couple breaths off the regulator that would be coming to them in the case of an out of gas situation, and I would encourage that behavior.  REMEMBER, we can’t breath water.   To steal a quote from my awesome dive buddy Lamont “By far the easiest trick to being able to dive safely is just to protect your ability to breathe underwater at all costs. Once you lose that ability your options are incredibly limited.”  The bottom line, if you have a regulator in your mouth that is giving you the ability to breathe, then you have time to deal with most anything.

Let’s look a littler more at the logic or lack thereof in the “one breath” for pre dive statement.  Say someone does an abbreviated pre-dive check to ‘save gas’ so that they can stay underwater longer/keep up with the breathing of their buddy.   Reality. Check. Time.

Y’all must have been sleeping through the Surface Air Consumption lesson in your open water one class, because here’s how it shakes out.

Average surface consumption rate we usually use for a comfortable swimming diver is .5 cubic feet per minute.   That’s actually a high rate for what we are looking at.

In a healthy, young adult, tidal volume is approximately 500 mL per inspiration or 7 mL/kg of body mass.  Avg. respirations per minute, lets go with 12.

500ml X 12 = 6000ml
by my calcs 6 liters = .21 cu ft.

So say you hoover on your regulator on the surface for a full minute.  You just removed .21 cubic feet from your tank.

Again, for my non-scuba diving readers that have made it this far, a standard scuba tank is ~80 cubic feet.   When you go to depth, for each 33 feet you increase the pressure by 1 atmosphere.

What does this mean?   Leaving nothing for reserve (which we never do) and leaving aside the fact that the tank is not actually a real 80 cubic feet,  if you were to sit around in your living room watching “into the blue” and “the Cave”, you’d get approximately 380 minutes of time.

Since we are now going to go underwater, and want to have a reserve lets say ‘be on the surface with absolute minimum of 500psi’ (translates into 13.3 cubic feet)  so subtract 13.3 cubic feet from 80.   That leaves us 66.6 cubic feet to do whatever we want with, right?

Back to scuba math for a second…  since you will be swimming and not sitting on the couch drooling over Jessica Alba in a bikini, lets assume you will use a bit more gas.  The number we often tell new divers to use is .5 cubic feet per minute as their surface air consumption (SAC) rate.  We give you the tools to calculate your ‘real’ SAC rate but for now, let us just use .5 cuft/min.

Still not so bad.  66.6 divided by .5 = 133 min.    Okay, so at least that way you don’t have to suffer through “the Cave”

Here’s where scuba maths gets fun.  Remember what I said earlier about 33ft = an atmosphere of pressure?

so at 33 feet your SAC will double which means, when someone asks you ‘how long does your tank last’ you can pretty honestly tell them ‘well, if i’m in ~30′ of water, a bit over an hour’.

At 66 feet you have now increase your absolute pressure to 3 atmospheres absolute (ATA) so you’d multiply your SAC rate by 3.  This equals 1.5 cubic feet per minute.  (I’m leaving out gas needed for descent and ascent but that is just for simplicity sake) so say 66.6 usable cubic feet of gas in your tank, divided by 1.5 = 44 min.

You are getting the pattern here, right?   OK, so back to our original discussion what does this mean in real life.  Say you decide to forego a normal pre-dive safety check and opt to just take one breath on one of your second stages (not checking the backup at all), not testing your BC and drysuit etc…  Keeping in mind the maximum you’d use if you sucked on your tank for a full minute on the surface is likely around .2 cubic feet per minute.   Well, you do the math, how much ‘time at depth’, say at 66 feet did that shortcut save you?   Complicated?   Not so much.

At 66 feet we figured out that you’d be using 1.5 cubic feet per minute, right?   OK, dredging up elementary school math here…  Gotta get a percent of that number and multiply it by the number of seconds in one minute to figure out how many seconds that shortcut saved.  .2 divided by 1.5 = .133333 now multiply that by 60 seconds and you get just shy of 8 seconds.

Is it really worth 8 seconds of dive time to put yourself and your buddy at risk?

  3 comments for “Safety First and a tiny bit of scuba math

Leave a Reply

Your email address will not be published.

%d bloggers like this: