Friday, October 24, 2008

The Truth About Cell Calibration

This is written by Gerhard...for all those rebreather divers out there (and especially those who have been following the recent reported, alleged, near death on an inspo). If you have any questions....mail G at :)

First things first: cells becoming current limited is a huge reality, can happen for sure! Remember they are one of the weakest links in your re breather. So what to do....

Looking at how the unit calibrate:
When you hit the "yes" button, the display you are looking at is the milivolts divided and measured against air in Oxygen. So to give you an example , one for altitude , one for sea level.

Sea Level: assume ambient 1 bar. Air= 0.21 , O2= 100% Calibration starts and the displays read : 1.02 1.00 0.96 What the unit does now is compare the mV reading to air whilst being in O2 , thus ACTUAL mV will be the following:
1.02 will be basically 10.2mv (thus the inspo now uses a set MV for start calibration assuming about 4.7- 021/100% oxygen- as its linearity checking) so the real MV of the cell submerged in O2 will be 10.2 mV * 4.7 = 47.94 MV , and that would ASSUME the start up MV was 10.2 as well ( It assumes Cell linearity is constant) NOW if you kick the lid off and measure cell 1 with a mV meter and do NOT get a reading of 10.2mV in air.... oops then the assumed linearity is incorrect....more about that later!

The Cell should be kicked out if it falls below 8.3mv approx (39.01mV)

The Hammer boots out ANY cell that does not make 40mv actual as it is predicted to become current limited(including at altitude ,which is a

At altitude the same applies EXCEPT that you must take the MB reading into
Thus if your reading is 0.89 0.88. 0.91 at altitude with a 850mb reading then just multiply the resultant by 0.850 to take it to "altitude"
Thus cell on will be 8.9mV * 4.7 / 0.850 = 49.2mV SEA LEVEL , or 41.83 Altitude reading- good stuff.
So even if you do not see a 1.00 when calibrating at altitude it does not mean the end of the world. According to your altitude it should be fine (above 40mv) SO if you work out the actual MV and see it is below 40mV actual for the altitude regard it as suspicious

NOW what I do NOT like is the fact that it works on a constant linearity assumption when you calibrate( 100% / 21%=4.7). So if you look at an actual as in the hammerhead you have the following:

At sea level my cell mV read in air : 10 9 9 After dousing it in pure O2 for calibration they all stabilize at 39 48 43 mV respectively.
That means that the ACTUAL linearity is as follows :
Cell 1 - 3.9
Cell 2 - 4.8
Cell 1 - 4.3
Thus you can see that it is NOT an actual constant and also cell 1 that seems high did not actually make the calibration and is now booted out as a possible limited cell, but the inspo will calibrate this cell as acceptable (borderline but there) esp if you told it the O2 content was 98%......

At altitude I expose the head to air to see the PO2 shown to verify air:
The mV reads 10 9 9
The Po2 I expect to see is .21 * 0.850 mb = 0.178 PO2 actual that the handsets should display.
All cells displayed within that range since previous calibration.
I wrote down my mV in Oxygen from the previous dive which was 39 48 43
Thus at altitude with pure O2 I expect to see a PO2 of 0.85 shown for each cell.
They all get very close to it.
You can then look at the linearity from each cell then vs now.

So cell 1 is 39/10= 3.9
Cell 2 is 48/9= 5.3
Cell 3 is 43 /9= 4.7
If you calibrate now and all values are still within that linearity level the cell is shown to be constant within calibration range. So if you DID fitness test the cell on the previous dive at 6m and it passed and now linearity check and mV seems stable the cell in theory should not be a problem as it's linearity range did not change (you can only hope!)

Also , in theory it also this tells us is that the cell showing highest linearity should be the most accurate in response range as it can read over a finer scale , no it does not mean it might not be limited, it just tells me the response range is the widest. It is like looking at a 300bar gauge vs a 400bar gauge. The 300 bar gauge will show clearer needle movement across the board as it uses a finer scale. You can see 2bar movement more clearly on the 300bar one than on a 400 bar gauge. So I would expect cell 2 to react nicely during flushes and O2 additions with cell one maybe a bit slower.

Use an approach where you can see yourself what the actual mV in air and actual PO2 is in air in regards to the previous calibration, and once the unit or cells are flooded with O2 then you can record that mV and PO2 shown BEFORE calibrating again. If the linearity is within the same limit the cell should perform in the same fashion ,esp if it is a few months old. You will at some point have 3 new cells in your unit ,it is unavoidable sometimes , but they need to be checked vigilantly esp in the first month. The hammer you can also leave flooded with O2 and come back after a while and look at the ACTUAL mV of the cells to see if there is any deterioration in the actual voltage. If there is , flood with O2 again, if the cells do not come back to previous max mV then get worried! The only way to test if it is current limited is to actually pressurize the unit on the surface or dive to 6m then spike it, deeper would be great. Problem is you do not want to start a heavy dive with an O2 spike through the lungs;-) But you can always go O/C , flush at 8 or so meters while manually manipulating the lungs to try get O2 cycled , look at the readings , then flush again and get on the loop and continue the dive.

Now this all sounds very good , but lets get back to the BUT... There are two options, one is single point calibration- so we use high o2 or 100% O2 to calibrate the cell, OR we try 2 point calibration, so we assume x linearity in air and x in O2 and draw a line, sounds good...and here is the BUT. Cells are NOT linear, they have a curve! The further away the maximum or minimum extends the worse the curve slopes, so your most accurate reading is the CLOSEST POINT TO CALIBRATION. In other words, if you calibrated at sea level- Po2 1 - then your most accurate readings should be in the 0.7-1.3 vicinity, if you calibrate at altitude- Mb 850- then all cells read Po2 0f 0.85.... hmmm...then reaching 1.3 is that little bit harder... Remember cells are usually within 2% error margin of calibrated value, so the only solution to check high end readings is to test the cell in a hyperbaric scenario. 2 Point calibration is not helpful either as I showed you with the hammerhead cells in air and O2 as the actual linearity differs, and as explained on a single point calibration you have a higher reference point from which to work, thus a smaller error!

Next option- Have cell verification like what was designed for the new Cis
Lunar- For those who do not know: It has only 2 cells ,one is the master and has a pure O2 and a diluent (air) injector close to the cell face. Approx every minute the handset hands control over to the slave cell and test fires a small amount of O2 at the master cell and a small amount of Air, it then looks at depth and if the cell does not show correct Mv for the depth the dive is aborted, so a constant hyperbaric test while diving, VERY cool...
except.... when testing was done on various other units ,they found that when blowing on the cell face you are faced with a few issues : If there is condensation on the cell face it sometimes helps to decrease it (yay!) , OR it actually blew moisture past the membrane into the cell causing.... yes you guessed it horribly incorrect readings. And guess which one manifested itself the most ? The latter of course.

The second fun problem (had one of those) is when you get a cell stuck. That basically gets a layer of moisture onto the cell face, it traps O2 behind the water ,and the cell only sees that O2 reading ,which is usually
setpoint. Now the computer sees 1.3 1.3 1.3 while you are happily
breathing the loop down and die embarrassed...or finally, when the O2 behind the cell face gets metabolized then it will fire the solenoid... oops, hopefully you are not an idiot and do realize when your solenoid did not work for a while, or your lung volume breathes down etc... The cool thing is if it was close enough to fire the solenoid before it got stuck it might be within range to start firing and of course will not stop firing the O2 in as the cell cannot see it... he he.

So what to do.....

IMHO ,only other possibilities :Nr 1= FLUSH YOUR UNIT OR GO O/C if you think there is a problem!!!!! The ONLY reliable piece of hardware is the thing between your ears we all assume is a brain ;-) Roll cells over (BEST), if you passed a fitness test on the cells within the previous 24hrs they should be good in theory, buy a cell checker from Narked at 90 (not really worth it-can built one way cheaper- 600 Pound Sterling!) or start diving at the setpoint where calibration passed (yeah sure!) Check your cells and record linearity if possible, but as stated that will just help you to identify if a cell seems to be performing in the same fashion as the previous dive, and where to expect a slow or fast reaction. The chances of loosing 3 cells to current limitation at the same time is VERY slim indeed! But be careful of exposing and leaving the cell in excessive heat or freezing it. Use a system that independently calibrates the cells and does not "take "readings from another. Use a 4th cell if possible.

In regards to cells, AP always claimed a better and more stringent protocol when verifying their cells, seems there might be a small hiccup. AI ( Analytical Industries) makes most of the cells. I have had as many AI cells fail as PSR cells as AP cells. Once again. Roll cells over, check linearity yourself for interest. You can clearly see how an assumed linearity can produce interesting readings. Just use bit of common sense and remember :
the job of the re breather is TO KILL YOU! Your job is to make sure IT DOES NOT!
If you weren't confused before , I hope you are now ;-) Cheers for now.

Tuesday, October 14, 2008

The Argument for Rogue Agencies

The chat group has been wild over the last week or so (I did not pick up mail for 4 days and had over 80 mails sitting there). What jumped for me was a reply from one of the agencies about how they had trained all of the divers listed as ‘heroes’ in one of the responses. Now my name was on that list of heroes and I found myself with a sudden resistance to an agency laying claim to ‘glory’ just because I happen to have a certificate from them. The main argument was on creating divers with the right attitude and the agency in question was ( I guess) trying to imply that they create divers with the right attitude who become role models in the community (nothing like a blatant plug on an impartial chat site). Which just stuck in my throat. Is attitude created or is it something that was there all along ? And if it was created, by who ?
I doubt the answer is simple. Good divers start off with the ‘right’ tendencies that a ‘good’ community will re-inforce. Conversely, bad divers will be influenced by the community in which they find themselves. And there is that word again, community. The diving community is a complex animal, made up of divers, equipment suppliers, dive sites, operators and of course, the agencies. They are all interlinked, but some have more influence than others. A dive site/ operator that does not enforce standards by banning rogue divers creates an environment where there are no consequences and so allows other divers to start breaking rules. Their influence depends on the number of options out there, so dive sites have more influence than say a dive operator who has to compete with a number of similar operators and so is probably less inclined to be concerned and drive away paying customers. Then come the divers themselves, who get to choose to stand up for standards and make it known when they are broken and that it is not OK to do so. Their effect is also variable and often creates cliques of like minded divers who are hard to break into. Finally come the agencies. Now, as an agency your job is to create skilled divers. These divers become your brand, your face out in the world if you will, which implies that an agency should be carefully monitoring its divers and ensuring that they are in fact representing the right image. One would expect that agencies (both sport and technical) would pro-actively discipline its divers and instructors when they flagrantly violate the rules and standards. But they seldom do which creates an impression that (once again) this behavior is condoned. The result is that we have created a community where there are simply NO CONSEQUENCES.
So I was pleasantly surprised with ISDA. For those of you not on the chat list, this is a new, local agency that has received a lot of flak for breaking away from the older, more established agencies. It now has the unenviable title of being a rogue agency. Johan Beukes (a controversial figure in himself) decided that he was fed up with the lip service being paid to standards and to the poor quality of diver that was being created by our locally established agencies (both sport and technical), so he created his own. The recent incident where a sport instructor took open water divers on what is a full Cave, Trimix dive saw the usual blame shifting… except for ISDA, who suspended the open water diver who did the dive! In fact, they recently suspended an instructor for taking experienced open water students to a mere 40 odd meters in open water. ISDA divers know that they are their agencies brand and their agency is serious about its standards. They now know that if they break those standards there are consequences, which means that there are probably going to be a whole lot less standards being broken by those divers. Which leads me to ask the question, isn’t it time that the old, complacent agencies started to catch a wake up! Here is an agency that takes its standards and reputation seriously. Here is an agency that understands that its reputation is based on what their divers actually do AFTER the course is over!
I find myself asking the question, should I be paying more attention to ISDA and getting involved ? It would blow my neutral stand completely….but maybe it is time to stand for something, rather than against something.
We are the diving community ! Who you are and the service you demand from agencies and dive sites is what has created and what will create the community in which we dive. If you want to improve the standards out there, you need to commit to creating an expectation of action from the agencies and an expectation of compliance from the divers. And while that happens, I am going to start paying more attention to those so called rogue agencies!

Wednesday, October 8, 2008

Wondergat Fatalities

These stats are compliments of Mike Beresford, a fellow WUC (Wits Underwater) diver and long time friend...... Total Number Counted to date - 13.

1971: A diver from Wits Underwater Club died after a free ascent. The diver was using a cylinder with a reserve valve (J-valve), and no SPG. The J-valve usually had an actuation rod / pull rod down the side of the cylinder, to allow the diver to pull the reserve lever. In this case the diver did not have one, and was unable to access the reserve. He tried to do a free ascent, but suffered from pulmonary barotrauma. I spoke to Roly Nyman about this one years later, and he added a detail that is quite relevant to divers even today. The victim had done many dives with the cylinder, without the reserve rod, and always been able to actuate it. On this particular weekend he was diving in a brand new wetsuit – apparently a birthday present. The wetsuit was far stiffer than his old gear (probably a thick rugby jersey, knowing the times), and on this occasion he was unable to trigger the reserve. Even small changes in equipment can have a major effect. Don't take untried equipment on a deep dive.

1975: A diver died during a club trip with the Transvaal Sub Aqua club. I was later a member of this club, and had access to all the old records. I also spoke to older members about this accident, but no-one could shed much light on what actually happened. The diver had been trained overseas, in Germany, and had never dived with the club or in South Africa before. He was also suffering from flu at the time. The club Diving Officer advised him not to dive that weekend, both because of his condition and because the DO was not happy with his training records. The diver decided to go anyway, and the club took a “it’s your risk” approach. Afterwards this was a major discussion point at the club, and the committee minutes in the aftermath note that the club would, in future, be more “positive” in controlling who could and could not dive. In terms of the actual accident, it appears that the victim lost consciousness and drowned. Whether it was narcosis, medical, or a combination was uncertain.

1977: A bad year at Wondergat. Early in the year a diver from Eskom Sub Aqua died. Again the details are not too clear, as he separated from his buddies. His body was later found on the bottom.

Later in the year two divers from Wits Underwater Club died on a night dive, Wondergat’s first double fatality. The divers had been doing a shallow night dive , using a line and float to keep them at a safe depth, and out of overhangs. The club also had a surface marshal, logging each group in and out. The two divers surfaced after the dive, a few metres from the entrance. The surface marshal logged them as surfaced, but then some time later noted that they hadn’t climbed out. There was no sign of them at the water, or at the camp. A search was launched, and the bodies were recovered from the bottom. According to the theories on this accident, the two divers decided to swim underwater from where they had surfaced back to the exit point. However, on descent they managed to pull the surface float underwater, and ended up going all the way down. Inexperience was certainly a factor in this accident. It is also important to see how the systems they thought would protect them proved inadequate.

1983: The Niewenhuizen twins. Probably the most talked about accident at Wondergat. The twins were on a night dive, linked by a buddy line and with a single torch between them. At some point they separated from the group, and lost the single torch. The torch bobbing to the surface was actually the first sign that the rest of the group had of the problems. The bodies were recovered from the bottom the following morning. Although there was a lot of criticism of the dive group for allowing two divers to share a torch in this manner, it was not an uncommon practice. My first night dive at Wondergat two years before this was in exactly the same situation! It was only after this that it became an absolute rule that each diver had to carry their own torch.

1984: A diver died after becoming entangled in a rope near the grid during a night dive. I never got many details of this accident, as it involved a dive school at a time when there was tremendous animosity between schools and clubs. The dive school later denied that there had been an accident, my only lead was that I knew the divers who did the body recovery.

1985 or 1986: A diver from the RAU club died in the main cave. Again many details were unclear as the victim had separated from the rest of the team. His mask was found some distance from his body. There was speculation that he’d lost his mask, and tried to do an ascent with his eyes closed, or at least with minimum opening. He is thought to have hit his head on the cave roof and lost consciousness as a result. Nuno Gomes and Malcolm Keeping recovered the body.

1991: Dick Grace incident. A very experienced instructor (Instructor Trainer) was killed during a deep rescue. I haven’t got the full details, I was living in the UK at this time and only heard about it much later. As I recall a student also died. The whole dive was basically a mistake – inexperienced divers being taken too far. The fact that the leader was a very well experienced and qualified diver didn’t prevent the accident, the leader was killed trying to rescue one of the victims.

1996: A free diver died, presumably from breath-hold blackout. He had been free diving to the bottom of the main shot line, was seen to surface and then immediately sank. Sadly this took place right at the end of a weekend, so all scuba gear had already been packed away. By the time a team could be kitted up and sent down, it was too late. Despite lengthy resuscitation efforts, the diver died. The diver in question was very experienced, but perhaps showed poor judgement by free diving so deep with no cover available.

2004: A lady diver died during a dive. From what I could gather, the victim had a medical problem that resulted in her losing the regulator. She subsequently drowned before she could be rescued.

Wednesday, October 1, 2008

Diving Deep on Air

Last weekend a young sport instructor (who only recently completed her Advanced Nitrox course), deemed herself fit to take an even less experienced diver (open water 2 I believe, so what, 20 dives ?) to the back of the cave at Wondergat. This is a cavern dive to 52 meters. It looks easy. First you descend 35 or so meters to the cave grid. Then you head off on one of the permanent lines, down a fairly sleep slope that slowly levels out as you enter the gloom of the cavern. If the vis is good you can see the whole cavern mouth from the back. Normally the vis is trashed thanks to inexperienced sport divers speeding in on short bottom times (to avoid decompression) and with inadequate buoyancy skills.

This is not the first time that this instructor has done this. Last time a complaint was lodged with her agency (nothing happened) and she was talked to by experienced technical instructors so I can only conclude that she sees nothing wrong with her actions and deems herself to be a superior diver that can manage any situation at any depth with any number of following divers. This is interesting because one of the fatalities from the back of the cave was a highly experienced national instructor who took open water 2’s to the back of the cave and paid for it not only with his life but also the life of one of his students (the student was on a single cylinder and ran out of air at the back of the cave. The instructor was on twins. They both ran out of air before reaching the mouth of the cave).

The thing about Wondergat is that it is deceptive. Going in is easy thanks to the slope which means that there is little effort involved. Going out is a swim thanks to that slope. This means divers are having to work, which increases gas consumption… all of which is a recipe for disaster. Especially if the divers are sport divers with marginal buoyancy (yes, pretty much every diver we get thinks they have buoyancy control but turns out when they have to do the standard technical drills they do not), sport equipment (one cylinder, one first stage, no back up gas, no decent lights) and no gas planning. Oh yes, and no experience or build up to mitigate narcoris.

I have a number issues with this dive. Firstly let me point out that this lady is not the only instructor doing this. She was just silly enough to brag about it on the net, so she gets to be the lesson.

My first is the arrogance of instructors who think that they are capable of dealing with the situation just because they are instructors. That is the kind of attitude that has killed people in the past and will kill people in the future. It is called ego and it seems that women are not immune to it. An instructors does not give you immunity or god like powers (and this applies to highly qualified, experienced technical instructors as well).

The second is the blind naivette of the student who believed her and followed her (see my blog on trusting experienced divers). As a diver you need to take responsibility for your own safety and not just trust divers who seem to know better. The odds are they do not! And you pay the price, often with your life.

Lastly, I am annoyed and concerned with the blasé attitude of sport divers and instructors who simply DO NOT KNOW how restricted their world is and break the rules without bothering to find out how. Now I guess that last statement could be deemed to be contradictory. After all, I am stating that I do not condone breaking of the rules (divers should not dive outside of the depth they are trained to) and then I say if you are going to break them at least know how ? Training is one way to learn how to break the rules. It is there to teach you what you do not know and how to to safely extend your limits. It is not the only way to learn however it is the safest.

I should at this point say that there is a reason PADI does not have courses that take you deep…depth changes the rules significantly. Sport diving teaches you to dive on a single cylinder with no decompression. It does not teach proper dive planning, proper gas management, proper buoyancy control and any appropriate risk assessment or management that will allow you to undertake a deep dive safely. Why ?Because that is not what sport diving is about. Sport diving is about the path of least resistance. It is about the least amount of knowledge that will get the most people divin, safely enough to avoid legal action (perhaps a bit cynical, but not so far from the truth). That is why technical dive training exists.

So, why is the back of the cave so different ? And how would a technically trained diver do things differently ?

The first hurdle to get over is the anti-air brigade. It has become kewl to diss air and recommend Trimix. Yes, air is bad and yes, you should be diving Trimix. The problem is Trimix is not always available and when it is, it is ludicrously expensive (think R600 for a dive). Based on these two facts alone, air is NOT going to go away easily. Add to that the fact that air is easy to come by never mind easy to breathe and there is no real hard limit to stop a diver from just going deeper and deeper. Now throw in the handy dive computer and any limits a sport diver used to have disappear entirely. They don’t have to think! They don’t have to know anything about decompression. All they need is a full cylinder and a dive computer and they can go anywhere for as long as they like…… or so they think.

If you are going to dive air you need to understand the characteristics of air and the risks entailed so that you can actively PLAN for these. In a nut shell dive planning firstly identifies what risks are unique to the dive you are doing. Then it allows you to consciously determine if you can avoid these and if not, how you are going to manage them. All dive planning amounts to ensuring you have enough gas to breathe for the whole dive (and never touch your reserve), you have enough light to see by, you have the mental ability to think and that you can manage any probable situation underwater.

For the back of the cave the list has things like:
- Narcosis, it is 50m
- It is dark, and gloomy so if you lose the line you are lost
- It is dark and gloomy, so your narcs is going to be higher as you are going to be more stressed
- It is at least 5 minutes (that is optimistic) from the surface, probably more so what happens if you have a dv failure, or blow an o-ring ?
- It is going to take at least 5 minutes to get there, which means you have descended at speed, which means your narcosis is going to be way worse.

Lets look at narcosis first!
Now every diver thinks they know about narcosis. But what do they know ? That it starts at 30 meters ? What divers tend to be unaware of is that the increase in narcosis with depth is not linear but rather exponential. Narcosis at 50 meters plus is very, very different to that at 30 or even 40 meters. To give you an example - as an instructor with Wits Underwater club I regularly had over confident open water 2 divers who (because they had been trained at 30 meters) had NO respect for narcosis simply because they had never really experienced it ( 30 meters is the start of narcosis so the effects are hard to notice). I learnt quickly that talking about narcosis had not effect. The only time these divers got respect for how bad they are at thinking and ‘doing’ at depth was when they were deliberately taken on a dive designed to narc them out (that would be a fast descent to 35, 40 meters). Then and only then did some (not all ) of them realise how debilitating narcosis really is, which is the point at which they started to take it seriously.

The next thing you learn is that there is no cure for narcosis. If you dive deep enough on nitrogen you will always experience narcosis (adding helium only extends that limit, it does not cure narcosis ). To get rid of narcosis you have to reduce the partial pressure of the nitrogen in your mix, which practically means ascending. Acclimitisation makes a difference. In fact, divers who regularly dive deep air spend inordinate amounts of time training for deep air and acclimatising.

Now the concept of acclimitisation is in itself controversial. I am a deep air diver, so my experience is that if you build up for depth your ability to manage task loading (think and do) is greatly improved. In fact, if I have not dived deep for a while my first deep dive will be a nightmare. I am totally narced, so I spend as little time as possible there and get shallow as fast as I can. The next dive is normally better. My approach to managing high levels of narcosis is to slowly build up to them. What do I mean ? Well if I was to move from 30 meters to 50 meters I would spend a weekend just doing 30 meter dives -. 3 on the Saturday (all with decompression because it is time at depth that gives you acclimitisation) and then a 30 and 40 meter on the Sunday. The next weekend (not more than 2 weeks later) I would then do 4 40 meter dives. Only then would I deem myself to be fit to do a 50 meter dive (and then only if the dive is within 2 weeks of the build up sequence). On the actual weekend I would do a 30 and a 40 m on the Saturday followed by the big dive first thing on Sunday. Oh, and I would have had no alcohol for the entire build up sequence and be avoiding things like caffeine and coke.

There are two other aspects a technical diver would look at, having the right equipment and being 100% familiar with that equipment. Being familiar with equipment means being able to lay your hand on a dv or a cylinder or a fin or a knife within seconds of thinking you need it… without spending valuable seconds searching for it. It takes effort and lots of practice to build up that kind of unthinking familiarity with your equipment.

Which brings us to equipment. From our dive planning we know that this dive is at least 10 minutes, if not 15 minutes long which means gas is going to be a concern. It is also deep, so you are going to be using more gas than on your normal 20 meter dive. You are also going to have to swim, are going to be narced and it is dark and guaranteed as a sport diver that means you have insufficient light (last Time I heard this dive done with one torch between 4 divers), so you are going to be stressed. As a technical diver you know what your gas consumption rate is so you can work out for a 15 minute bottom time (that means leaving the surface, getting to the back to the cave and leaving on 15 minutes) exactly how much gas you are going to need.

You are also not working on a standard 50 bar reserve rate, but on critical pressures (which is a whole blog on its own). In a nutshell your critical pressure is the number on your spg at which you must turn around. If you turn around when your gas content gets to that pressure you have enough gas to get you to the surface and still have 50 bar left. This is completely different to the sport divers who turn around on 50 bar without knowing if 50 bar is enough to get them to the surface. The assumption is it is and in most cases that is true, because they are diving within very tight limits. Another difference is that leaving on 50 bar means you are still using gas to get to the surface and are planning on reaching the surface with 20 bar. We plan to reach the surface still with all our reserve…totally untouched. If you are not using critical pressures (which requires conscious dive planning), you should be using at the very least the rule of thirds, i.e. you breathe a third of your gas mix into the cave and when that is gone REGARDLESS OF WHETHER YOU HAVE REACHED THE BACK OR NOT, you turn around and come back out.\

There is a good chance that calculating your actual gas requirements will indicate that this dive is NOT feasible on a single cylinder. In any event, as a technical diver you always have at least two cylinders with you. There is a very good reason for this and it is called redundancy. The last place you want to lose your entire gas supply is at 52 meters. Now every good sport diver just shrugs and says, my buddy is right next to me…except he is stressed and sitting at 80 bar, so now you have to get two divers all the way out of the cave on 80 bar ? Not a situation I would rate my chances in. Sport rigs also have only one pillar valve, which means if you blow an 0-ring or have a problem with a first stage you lose BOTH dv’s, the octo as well. Not a situation I would like to be in at 52 m with only a novice buddy to look to for assistance.

At the very least on a dive like this every diver should have access to staged gas. They should be carrying it with them or have placed it at the grave stones and then the cave grid and still they should have something with them.

This is the abbreviated version of a dive to the back of the cave. It does not even touch on decompression, just the basics of making sure you are not narced out of your tree and have enough gas to get yourself and a buddy all the way back out again. There is simply no way I could put into a single blog all the information you would need to be able to do this dive safely…which is why we have technical training. It is also why I will strongly berate divers who do this kind of thing. If you take only one thing away from this, let it be this…. It is never what you know that kills you, it is what you did not know…and what you did not plan for…..oh and sport diving is highly, highly restricted and limited.