Click on this Youtube link below to check out the video. I was asked to help out with the filming of the latest promo video for cave diving in the park. We will be back there in a few weeks (sadly no spaces left on that course) and again in early December (we do have one space left for that cave course. It can be for OC Tech Cave or CCR Cave.
Gas matching is an important part of the dive planning process and an essential component of any pre-dive safety checks. If you and your team have a bad day and end up having to share air it could make the difference between a comfortable swim out of the cave and a rather frantic race, worrying that your gas reserves might not last. Gas matching is nothing more complicated than adjusting the size of your reserve to compensate for factors that might affect it. We are basically making sure that the reserves we plan are actually large enough to support our exit in an air sharing emergency.
So, when do we gas match? I am sure others could add to the list but here are a few examples for guidance.
Any time when in a team of two divers, the diver with the largest RMV is also using the largest cylinders.
Anytime when the team has to negotiate restrictions.
Anytime, even in a team of three or more, where there is a breathing rate difference of more than double between two divers.
When don’t we need to gas match?
In a team of three or more divers (apart from the breathing difference mentioned above)
A team of two divers using the same sized cylinders (exception as above)
What does all this mean? Let’s look at them more closely.
If a diver with a larger breathing rate uses larger cylinders (which, let’s admit, is quite common) then if something goes wrong at the point of furthest penetration and it is the bigger breather who needs to support the smaller breather then his larger cylinders plus the smaller breather should mean ample gas to swim home. If it is the other way around however, then it is quite likely that the amount of gas in reserve in the smaller breather’s smaller cylinders might not support both of them out of the cave.
When negotiating restrictions with all other things being equal, it should take the same time to travel in through the restriction as it does out. However a considerable amount of extra time might be needed when divers are sharing air through a restriction, hence the need to check that the reserves are going to be big enough.
If you have go someone on the team with a very large SAC then it would be a good idea to check the numbers to ensure the reserve volume is sufficient to support them out of the cave if you have to donate gas.
For my imperial friends we have the delights of what are known as ‘dissimilar tank calculations’. I am going to leave that for a separate short article. Those who have to use these will no doubt understand!
How do we do the maths for gas matching. One way is to cheat a little and get a gas matching table. IANTD certainly have them and they are easy to use.
Another way is to do some simple maths just to make sure the reserves are large enough. It mainly boils down to adjusting your turn pressure, turning sooner than would be expected, using less gas on the way in and out and keeping more than 1/3 in reserve.
As an example if my buddy has an SAC of 24 litres per minute and mine is lower at 12 lpm I can look at a chart at mine and my buddies SAC rates and quickly read the SAC Ration Factor (SRF). The SRF is nothing fancier than a new percentage of my starting pressure at which I need to turn. The usual 1/3 is roughly 66%. Looking at the SRF for mine and my buddies SACs I need to turn earlier at 76%. So for a start pressure of 230 bar I will turn at about 175 bar, thus making sure my reserve third is big enough to support my buddy if needed. I didn’t do the maths, I just flipped the chart over to check the numbers where my SRF crosses my start pressure.
If you dive with the same team then it is easy to pop the numbers down in your wet-notes, either from the a chart or from sitting down with a calculator!
Why is this important? Well if you don’t match gas, this could happen.
Diver A has an SAC of 12 litres per minute and is diving on twin 12s charged to 200 bar
Diver B has an SAC of 15 litres per minute and is diving on twin 15s charged to 200 bar.
They will reach their turn pressure at the same time. If, at that moment Diver B has a catastrophic gas loss and needs to share gas with Diver A…..well Diver B will have used 2000 litres of gas going in. Diver A will have used 1600 litres. Diver A will need the same volume to get out, 1600 litres leaving his 1/3 in reserve at 1600 litres. Diver B still needs 2000 litres of gas so that is a 400 litre shortfall. This isn’t going to have a happy ending.
If on the other hand the dive team match gas, and calculate that Diver A needs a reserve big enough to match the gas volume needed by his buddy…..well Diver A just needs 2000 litres as a reserve, leaving his usable gas at 2800 litres, 1400 litres for the dive in, 1400 litres for the dive home. Depending on the depth of the dive that extra 200 litres from each leg of the journey now put aside for the reserve might only be just a few minutes further in but puts the gas plan back in to the realms of being safety first.
1400 litres from the starting pressure means Diver A turning the dive at 145 bar rather than 135 bar.
All of this has been simplified slightly so it does not take in to account deeper cave diving with a significant decompression obligation or any other multi-stage extended range penetration dive. That is probably better left to the classroom as part of a Deep Cave Diver or Multi-Stage Cave Diver course but the main principles are very similar in concept.
I consider myself to be very lucky to have the opportunity to regularly visit Florida to teach Cave Diver classes. My trips are often a few months apart and so I notice changes as soon as I see them. It might be different if I dived the caves every day or every week. Familiarity might make me miss subtle changes until they become more profound.
Over the last couple of years one of the changes (and not for the better) that has become very noticeable is the standard of line laying. Now, don't get me wrong, this article is not trying to point fingers and say 'he or she did this wrong', but just to discuss why this might be the case, what the problems are and maybe even offer some pointers that might help the next time you lay a line.
As a Cave Instructor Trainer not only do I want to ensure that cave students get well rounded training but also that new Cave Instructor candidates continue to pass on these skills and their importance.
Years ago I was a Trainer in a Fire Department. It might surprise you to know that, at least in the UK, fire-fighters use guidelines! They are a bit thicker than cave lines and have tactile markings on them, indicating direction, in or out, which can be felt whilst wearing thick fire-fighting gloves but they are deployed to help Fire-fighters safely enter, search and exit a smoke filled building. They are not necessarily used in smaller domestic properties but in larger commercial and industrial premises. I am sure that we can all understand the hazards of getting lost in a smoke-filled building that is on fire. I used to teach the procedures and protocols for laying these guidelines and the important role they play in Fire-fighter safety. I stress the same things to my cave diving students and perhaps this background is the origin of why I feel that line laying is such an important part of a cave diver's skills. So, back to the caves. It would be difficult to accurately pin-point a particular reason for the decline in line laying skills but I think there are a few factors that have contributed. I am not going to say whether I think any of these are good or bad, but just highlight some changes that have had an impact on this particular skill.
The first, and to me most obvious, is that in a number of caves some divers believe we don't strictly need to run our own primary line in to the cave to get to the permanent line. A number of cave sites have seen the 'gold line' (a term used a lot in Florida for the permanent main line in a cave due to their colour) move progressively closer and closer to the cave entrance. Whilst they may not be quite out to open water, their proximity does mean that fewer and fewer divers bother to run their own primary line. Whether we like this or not it does mean, for most cave divers, less practice aying lines.
Next, there are more participants in the sport. More cave divers means busier systems and regardless of where a permanent line might be inside the cave, a great many cave divers choose not to lay their own line. This decision might be because there does not appear to be enough space to lay their own, or they think they know the cave well enough not to need one, or they want to leave space for others to lay a line, for example a team of experienced divers choose not to lay a line in order to leave space for an Instructor with students to lay a line.
Dare I say it, but Instructors might also be contributing toward the decline of line laying skills. Lines nearer to open water means that students might only have to lay a few feet of line to the 'gold line’. Instructors don't need to take the time to teach good line laying skills and can get their students further back much quicker and so the students get more 'instant gratification' of longer penetrations on their cave course without the boring bit of having to lay a long and difficult line
So why lay a line if we don't think we need to? The golden rules of cave diving are as relevant today as they always have been. 'Always maintain a continuous guideline to open water/the surface/a safe area (choose which ever you think is best) is not just an old adage. Choosing any one of these, preferably one of the first two options, so that there is little room for confusion, sounds like a good idea to me, even if you think you know the cave really well.
I recall exiting from the upstream side of Cow Spring one sunny afternoon to see another diver very obviously 'lost' and somewhat stuck where he had taken a wrong turn, just a few feet from open water and found himself in amongst the boulders from where he couldn't figure out an exit route....
Just a few weeks ago,whilst cave diving in France, a solo diver swam past us on his way out of the cave. He was less than 10m/30ft from being in the head pool and that included 6m/20ft of vertical ascent through a boulder chimney. As he swam past us (we were busy assembling our under water habitat at the time) he gave us a cheery wave and then looked up, banged his head on the ceiling and realised he couldn't remember his route through the boulders to open water. We were filming our 'construction site' at the time and the look on his face, captured on video, is an absolute picture! Needless to say we pointed to our line and exit route and he went happily on his way. (I did the same for the chap a bit stuck in Cow Spring, so happy endings all round).
I can't think of a better reason to have that bit of string in place, even if it's just for piece of mind. You might think know your way around really well but do the rest of your team? Are they happy that they don't have a primary line to the outside world? It only takes a few minutes.
So why else? It does also let other divers know roughly in which direction you have headed. A god-send if you are hoping someone might come looking should you get a little lost further back. From an Instructor's view point, it does the same. I know there are divers ahead of me and if I am planning some drills, skills and fun and games I can go somewhere else so that I don't interfere with their pleasure dive. If I do head your way then I will have an expectation of meeting another team and can use this as a learning exercise for my students.
Is there anything you can do to improve your line laying skill? Well for a start, if you see me in North Central Florida, France or anywhere else for that matter and want some hints or tips, just ask. Or even jump in on a dive with me. Just come and ask, you would be very welcome. There is always a fair amount of line laying done on most of my teaching dives. I am pretty sure that there are a number of other Instructors who would do the same.
On top of that, when you do lay your line, just stop for a second and look into the cave to see where (if there are any) the other lines have been laid. This gives you the chance to pick the best spot for you own primary tie-off. Be honest, how many times have you made a primary then secondary tie-off, then headed in to the cave and thought **%%*^&, I wish I had tied off on this/that side instead! (Of course we always go back out and start again, don't we?)
Try to remember some of the rules and protocols that your Instructor taught you. Trying, where possible, not to parallel another line too closely, not using the same tie-off point as another line, not using another line as a tie-off point(!) I have this seen quite a few times recently– nowhere to tie-off, just make a few tie-offs on other lines in the cave!. I also recently saw a line zig-zagging through the cavern zone at Peacock 1 tied alternatively between points on the main line and the opposite side of the cave! Not only do poor lines like these cause potential entanglements but put stress on the gold line and make it very difficult for other divers to lay their lines! The only time we should be tying our line onto another is to secure our spool or reel onto the line we are now going to follow.
Keep your line tight and out of the way of other divers' lines. This will make it easier for you when you turn and reel out of the cave, and will help to keep the others away from your own line, making it less likely that any of your tie-off points will be accidentally kicked or pulled off.
Take your time and look ahead. Try and plan the route you will take and look for possible tie-off points well in advance. This will also make buoyancy control whilst tying-off much easier as you will have anticipated your actions.
Don't forget the team work aspect. The second divers should be helping to illuminate possible tie off points then lighting the one you choose if the cave configuration allows and checking for line traps.
Safety should be the paramount thought for any cave diver. I would encourage all of us to think about this before you decide whether or not to lay a line, be it a primary line or a jump or gap. Just for a second also think about the signals you send out to newer cave divers. 'Our Instructor made us lay a line all the time, but once you get a bit more experienced you don't need to, because, hey, take a look, none of these guys do and they are really experienced'.
Of course this experience won't prevent them becoming a statistic!
Have fun, but most of all, be safe.
Oh, and lay a line!
Apparently I do have too many under suits! So, in an attempt to slim down the collection some of them have to go. If there is no price listed then I am open to sensible offers. As before, I'm happy to meet at Vobster for collection or post out but shipping costs will need to be paid for. There are also some gas connection sets for Sentinel CCR at the bottom of the page.
Otter Arctic 200. I think it is a Large-Tall. £ offers
Weezle Extreme Plus in very good condition but no bag. Size large-tall. £75
Fourth Element Xerotherm Arctic top. Size L. £ offers
Rule of Thirds
Ask any group of tec divers what is the rule of thirds and you will probably get a number of different answers. In all probability they will more than likely be close in concept and close to the actual rule but with a few variations in wording. But what do they actually mean?
“1/3 in, 1/3 out and a 3rd in reserve”, “surface with a third of all your gas”, “save a third of your gas for emergencies” are typical statements. You might also hear divers say that they dive ‘to’ the rule of thirds but is that going to be the conservative option if you do need to utilise your reserve gas supply?
The rule of thirds is variously attributed to either the pioneers of cave diving in the UK and the founders of what was to become the Cave Diving Group (CDG) or to their slightly more modern counterparts exploring the springs of North Central Florida.
Regardless of origin, the rule was used by cave divers, allowing one third of the gas supply to be used on the inward journey, one third for the dive out and a third held in reserve in case things did not go according to plan.
In simple terms it seems to work. Looked at more closely there are flaws, albeit only minor, which need to be addressed as part of your dive plan in order to stay safe if things go wrong.
First let us look at the common belief that we should dive ‘to’ the rule of thirds. For this example we will assume identical breathing rates and identical cylinder sizes between two dive buddies. Should one diver have a catastrophic loss of gas at the furthest point of penetration the other diver has a third of his gas ready to donate to the out of air diver. However if the process of the first diver trying to deal with the gas lost in the first instance takes any time or the act of sharing air and sorting themselves out for the exit takes more time, the diver donating gas will have been using some of his or her exit gas already. Those few minutes to get organised for the swim out could leave gas supplies perilously close to the limit or even being exhausted prior to the team reaching safety.
What if the air sharing swim out of the cave takes much longer just because the long hose has been deployed? What if the two divers did not have identical breathing rates? If the out of air diver has a measurably larger breathing rate then the donor diver’s reserve third might simply not be enough gas!
How can we plan to avoid these potential pitfalls?
Firstly we can look at how we might apply the rule of thirds before we discuss other planning considerations. When I said that many divers will dive to the rule of thirds the implication is that they will turn the dive when the hit the point of having used 1/3 of their gas. But turning around takes time. Making sure everyone on the team has seen the signal and they too have turned takes time. All this is eating in to the gas for the swim out and as so in to your reserves as well.
So, why don’t we look ahead. Rather than wait until you reach the exact turn pressure, consider turning just a few bar/psi earlier. If your turn pressure is 140 bar consider turing when you are at 145 bar. (For my non-metric friends a similar example might be a turn pressure of 2000 psi so perhaps turn at 2100 psi). Let’s be honest, we aren’t really going to get that much further in for another 5 bar and we can always come back another day with more gas!
Think too about where you might turn the dive. Right now might be a good time because if you swim another minute or so you and the team could be in a smaller cave passage making turning more difficult and if you keep going to where it opens out again it is almost certain you will have gone past your gas turn pressure.
So, rather than diving to the limit of the rule of thirds, we should try to dive within it.
Perhaps now would be a good time to have a look at how we go about calculating our turn pressure based on the rule of thirds.
Let’s say we have a starting pressure of 210 bar. That one is nice and easy, 210 divided by 3 is 70 bar, so each third is 70 bar. 70 bar from our start pressure would mean the turn pressure is 140 bar.
(A simple imperial example would be a starting pressure of 3000psi. Again, nice and easy, 3000 divided by 3 is 1000psi, so each third is 1000psi. 1000psi from our start pressure would mean the turn pressure is 2000psi).
What if the stat pressure is not a nice easy number to divide by 3? I know a few divers who can seemingly instantly divide almost any starting pressure by 3 and calculate their turn pressure. I have also seen diver then use this very precise pressure during the dive when they have digital readings or pressure. However I have also noticed how this seems to entice those divers to go right to the limit of their gas.
Simpler and safer would be to err on the side of caution and make the maths easier. Just for fun I will look at a couple of examples.
A starting pressure of 220 bar is not easy to divide by three in your head whilst floating in the head pool of a cave discussing the dive plan and turn pressures. Far easier is to round the number down to the next lowest number that is easily divisible by 3. In this case that would be 210 bar. The usual turn pressure for 210 bar would be 140 bar (210 - 70) but in this case we are starting with 10 bar more, so very simply, calculate how big 1/3 is from our rounded down pressure (70 bar here) and subtract that from your actual starting pressure. 220 bar - 70 bar would give a turn pressure of 150 bar. 70 bar in and 70 bar out would leave the ‘largest 1/3’ as the reserve, 80 bar.
(Imperial - 3400psi start pressure, round down to 3300psi. 3300 divided by 3 is 1100. 1100 psi subtracted from your actual start pressure of 3400psi would give a turn pressure of 2300psi)
What about 200 bar start pressure. Well 195 bar is easily divisible by 3. Each 1/3 would be 65 bar. 65 bar subtracted from your actual start pressure would give a turn pressure of 135 bar.
For some that calculation might not be so easy but there is no reason why you can’t write a list of start pressures and turn pressures in your wet-notes so you don’t have to worry that your maths isn’t so good or that you might get it wrong.
When you have calculated or looked up your turn pressure don’t just tell the team what your turn pressure is, tell them how much gas you have and then what your turn pressure will be. That way you get a few other brains to check that you have got it right. After all, it is their safety too that could be jeopardised if someone gets it all wrong.
Remember too that for wreck penetration dives, while still using the same approach with the rule of thirds, it is applied a little differently. We have to take in to account the fact that having exited the wreck, in most cases, we still have an ascent to make and possibly decompression too. The same might apply to a very deep cave dive. Perhaps that will be for a another post in the future.
What else can we do to to add conservatism and safety to how we use this rule?
There is something called gas matching which we can apply to make sure we don’t get caught short on our gas supplies.
How, when and where to gas match will be up soon.
How to choose your bailout gas (part one)
Off on a CCR dive? You will need to take some bailout with you then. It’s not so deep so air should be good, yes? Maybe it is a bit deeper and you will have trimix as your diluent. To keep things simple you could just have the same onboard gas as your deep bailout gas. That’s probably OK but there could be a better way. A friend of mine who is a mathematician told me it was a more fun way too but the rest of the course students were not entirely in agreement!
Don’t worry it isn’t too difficult. Let’s start with a few simple guidelines first.
What ever gas you have as your bailout, it needs to be a life supporting fraction of oxygen (FO2) for the depth of the dive your are planning. For the majority of open ocean dives in the shallower than normoxic trimix depth range this often means just a single cylinder, providing guideline number two below is considered.
You need to have life sustaining volume. Bailing from your CCR on to your used (you used it during the descent) 2 or 3 litre onboard air diluent at 40 msw/131 fsw is not going to get you back to the surface. Even a second 3 litre strapped to the rebreather might not do it, especially if you have incurred any deco.
So as well as choosing the right FO2, have enough of it with you.
One of the ways in which we might choose the ‘right FO2’ is to consider another couple of general guidelines.
The first of these is that ideally you don’t really want your bailout gas to have any undesired physiological impact. By way of an explanation, let’s look at what we are actually breathing when on the loop of a CCR, breathing a setpoint of 1.3 PO2, using air diluent at a depth of 36 metres (118 feet). The actual fraction of inspired O2 would be a touch over .28, or 28%. For some divers or in some diving environments, bailing out from a warm, moist breathing loop or 28% on to a cold dry regulator supply air could bring on an uncomfortable level of narcosis.
Just to recap, divide the PO2 by the depth as an absolute pressure will give you the fraction of inspired oxygen (FiO2)p
So rather than air as a bailout gas, we could take Nitrox 28. It is a life supporting FO2 for our depth and will perhaps lessen any physiological effect of bailing out on to open circuit.
We will have a look at choosing diluent and using an off-board cylinder plugged in to your rebreather in a future post.
The final guideline dovetails nicely with the previous one, in that our bailout gas should’t have any adverse affect on your decompression obligation or no-decompression limits. In some diving circumstances it would be easy to go from a no-stop dive in to a decompression obligation just by bailing out to a poorly chosen gas. In the event that the reason for the bailout becomes or is unrecoverable the subsequent ascent is never usually immediate. Time can often spent at depth initially trying to resolve the problem, then alert your buddies or dive team, maybe send up a surface marker buoy; all of these can easily take a dive from no-stop in to deco or add to an existing deco requirement, avoidable with careful gas selection.
So, our bailout gas should have no appreciable effect on our no-stop or decompression time and no adverse physiological effects.
If you are using trimix, then choosing your bailouts and any necessary decompression gasses can be a bit more involved but nothing too taxing. Yes, another post for the future.
If you fancy doing something a little different over Christmas, we will be running an NSSCDS Deep Cave course in Florida. It will all kick off on 20th December and run though until 25th/26th although their is an option to stay for more diving combined with an IANTD Self-Sufficient Diver programme until 28th.
Conditions permitting we will be diving in some of the less visited sites such as the amazing Morgan Sink and we are planning at least one upstream and one downstream dive in the magnificent Eagle's Nest.
The programme can be done on CCR or Open Circuit, you just need to be cave certified with a few cave dives under your belt and CCR or OC Trimix. If you don't yet have your trimix qualification we can easily incorporate that in to the schedule and you would get to do your trimix course in caves!
Or...you could stay at home and watch the same Christmas TV repeats.
If you are interested or would like to know more please get in touch via the website contact form, vie email (email@example.com) or call +44 (0) 7932 735619.
I thought I would try and make these posts a regular feature on the blog so, following on from the previous article, some more thoughts on bailout strategies.
When to use different bailout strategies
In a previous post I talked about different CCR bailout strategies that are available for us to use but when would we chose one over the other and if necessary how might we combine strategies to optimise our dive.
If we think back to the easiest of the plans, individual bailout, this is straight forward and simple. Each diver carries their own bailout you just need to check your numbers to make sure you have the right gasses and sufficient volume, then off you go diving.
The downside is that the deeper or further you want to dive the more each diver will have to carry. It won’t be long before you could be met with the law of diminishing returns. Some fellow cave diving friends wanted to go a little further in a particular cave system. They planned their dives and were encouraged by their initial success to add another bailout cylinder and go a little further. With each dive came more equipment and they realised their progress was getting slower. They went back a step, left a few stage cylinders behind and started to get experience using DPVs in this cave system. They had already done a Cave DPV course and had used DPVs in cave systems before so again initial progress was good. Then they started to add in those extra stage cylinder and a few more on top, having realised their bailout needs had increased due to the greater distances being covered using DPVs. With all the extra stages slowing them down they thought to add spare scooters to the ensemble just in case a DPV failed. They were not getting not much further in to the system than when they first started out with their rebreathers, a couple of stages and old fashioned fin power!
In their case they would perhaps have been better served planning their dive using staged bailout rather than trying to carry it all. Their original thoughts against doing this were the time it might take to set everything in place, the additional time to then retrieve it all after the main dive and their lack of practice and therefore confidence in knowing how and where to stage their bailout gasses.
Before you start using staged bailout gasses you must be able to ensure you can get back to them. If you are not able to guarantee this then you need a back-up to the staged cylinders or you need to choose a different strategy. In many circumstances but certainly not all, a cave is a reasonably safe bet for getting back to your staged cylinders.
The trick is to look at swim pace (or DPV speed), gas consumption rates, decompression obligations, cave passage configuration and then carefully plan where you will stage your gas. When you drop cylinders take the time to check your travel pace back out and see if it matches your estimates. Take account of the fact that you might be swimming or scootering wearing more cylinders than during the set-up dive if you have to pick them up on the way having bailed out due to an unrecoverable CCR failure. Remember that using a DPV has the capacity to put you a long way back in a cave and if the DPV fails, has your team practiced team towing skills or do you have a spare DPV that can be towed or possibly staged, if so how far is the swim or tow back to it?
So outside of the cave environment is there anywhere else we might consider using staged bailout cylinders? The open ocean is not really the best place to leave cylinders. Sadly there have been instances of divers being injured through leaving cylinders on the sea bed as part of their plan, only to find they have either been moved by the ocean currents and tides or they simply couldn’t find their way back to their gas. Not enough gas, omitted decompression and injured divers was the end result.
We could use the drop cylinder approach. Using what has now become the accepted norm in tec diving of using a yellow surface marker buoy to signal some kind of gas emergency, a dive boat can drop cylinders down your SMB line to you. The cylinders need to be set up first on their own sturdy line with a flotation device attached to the other end and ideally the signal needs to be sent to the surface at a deeper depth than that at which you will access the gas, just so the thing doesn’t hit you on the head when the boat crew drop it down your line to you!
This really needs to be discussed with the skipper and crew and practiced, although some boats I have used are very familiar with this system and will even ask before the diving commences if anyone onboard has any drop cylinders, how they want them rigged and how they would like the boat to deploy them if needed.
Remember you will still need to carry enough gas to get you to a point where you can access the drop cylinder.
It is just a personal opinion but I do think that with practice and when you know the boat, skipper and crew this can be a really good option but I do tend to limit the depths at which I will consider this to no deeper than the Normoxic CCR range. I still need to carry a bailout I can access at the deepest point of the dive but if the bottom time is not excessive there are many normoxic level dives from which you could bailout and decompress on your off board trimix stage. Yes you will be in the water a lot longer than with your deco gas but if the boat takes longer to get to you than expected you're probably not going to get in to immediate difficulties.
Deeper than normoxic or shallower range dives I would choose a different option.
You can have similar strategy to the drop cylinder system if you have the luxury of support divers to help you. By using support divers you can plan how much gas you ned to carry for the deeper sections of the dive and arrange to meet your support team who can exchange cylinders with you for more depth appropriate mixes.
The use of staged gas, the drop cylinder system or using support divers can be combined with the concept of individual bailout (definitely with the drop cylinder system so that you know you have your own cylinder on the boat if you need it) or on deeper more complex dives, especially involving a larger team than just you and a buddy, can be combined with the concept of team bailout.
As discussed in a previous post (Bailout Strategies), team bailout gives us the capacity to spread the gas load amongst the team. Yes, we have to be presumptuous and say that we don’t think all the rebreathers in the whole team will all fail on the same dive but personally, I think that is a reasonable stance to take given the track record of reliability most modern rebreathers now have. Couple that with rigorous pre-dive checks, a comprehensive pre-breathe and monitoring of all the CCR systems under water we should have some trust in our rebreather to do its job.
I did own a CCR that had done a significant number (well in to four figures) of dives. A statistician friend told me that whilst it’s track record of reliability was proven, it did not make it any more or less likely to fail the next time I used it. That gave me some food for thought and was a reminder of the need to go through all the pre-dive things we do with CCR with no less diligence just because I have been doing it for some time. It does also give me trust in my rebreather. Not blind faith but a lot of trust.
I guess I also thought that whilst I too had a track record of reliability I was no more or less likely to fail the next time I went diving, so every dive incorporates a mental pre-dive check of myself!
So, in addition to thinking about what gasses you are going to take (I think that will be a topic for a future post) and how much you are going to take, have a think about how best to manage all these cylinders!
Copyright © Eau2 2015
Over the years I have accumulated a lot of scuba stuff and I have acquired even more via a friend who has decided to hang up his fins. He has asked me to help sell it, so I have a few things that really do need a new home. All of it works but some items could probably do with a service. I didn't check to see if the fins work! The prices are as stated. I am not sure I have the time to get in to a bidding exchange but I think there are some real bargains.
I have not measured the accuracy of the length of lines on the reel. Where stated this is the model of reel, I can't guarantee the line is still full length.
Meet at Vobster to collect. I might be able to post within UK but buyer will have to cover P&P costs.
You can get in touch via the contact form on the website here:
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Scubapro Twin Jets Fins. Size L
A new website and a new blog to keep you up to date with courses and trips.