A few minutes of video from our most recent sidemount cave trip to France. Click on the link to go to the video.
In my last post (Choosing Trimix Diluent) I discussed one way of calculating and choosing your CCR diluent for a trimix dive. The process used did not consider O2 to be narcotic but how would you calculate the mix if you prefer to consider oxygen to be narcotic.
Our start point is exactly the same, we would choose the fraction of oxygen (FO2), based on a safe maximum PO2 of the diluent at our Maximum Operating Depth (MOD).
So using the same depth as the previous post as an example, on a dive to 70m and a maximum PO2 of our diluent at our MOD of 1.0 we would have a FO2 of: -
70m = 8 Bar
PO2/P (where P is the depth as an absolute pressure) = FO2
1.0/8 = 0.125
So we would probably choose an FO2 of 0.12 (12% O2) for the oxygen in our diluent.
Having calculated the FO2 we want, the next step is to calculate how much helium we will need in the loop to meet our chosen Equivalent Narcotic Depth (END). We need to calculate the loop fraction of helium first as this will then tell us what we need to have in our diluent. Remember that because we are on a constant PO2, our loop gas will not be the same as our diluent and in fact will contain a little more oxygen than our diluent. As we are considering oxygen to be narcotic, we need to take this slight increase in FO2 in to account.
So, the fraction of helium needed in the loop (FHe loop) =
1 - (END/P) where END and P (pressure at actual dive depth) are in absolute pressure.
If we chose a 30m END = 4 Bar, and have already planned our dive depth to be 70m, P = 8 Bar
1 - (4/8) = .50
So, the FHe in the loop is .50 (or 50% He)
In order to calculate how much helium we need in the diluent we simply use:-
FHe diluent = 1 - [(1 - FO2 loop - FHe loop) + FO2 dil)]
Firstly, let us check the FO2 in the loop.
Setpoint/P will give us this so:-
1.3/8 = .1625 (rounded down to .16 or 16%)
Therefore, using the above formula, the helium we want in our diluent cylinder is:-
FHe dil = 1 - [(1 - .16 - .50) + .12]
This leads to a diluent FHe of .54 or 54% in our diluent cylinder.
Some of you might have slightly different figures depending on if or how you have rounded up or down your calculations. I tend to round down the O2 and round up the He for conservatism.
Please bear in mind that none of this is a substitute for proper CCR and trimix training.
© Eau2 & Martin Robson 2017
Choosing a trimix diluent for your CCR is a relatively straight forward process but there are a few ‘rules’ or reasonably well accepted safe diving practices that can help you to ensure your gas is safe and suitable for the planned diving depth.
In a previous blog post ‘Choosing OC Trimix’ I referred to some of the well established limits for the PO2 of a gas, be it for your bottom mix or decompression gas. In that post I said that we could consider a PO2 of 1.4 as our ‘working’ limit and the start point for calculating the FO2 for the best mix for our trimix dive.
When diving on OC, the moment we get shallower, the PO2 will fall with a consequent reduction in CNS% per minute and OTUs per minute.
That is not the case with CCR. On a constant partial pressure of oxygen our CNS% per minute and OTUs per minute will also remain constant irrespective of a reduction in ambient pressure. This means as CCR divers we are exposed overall to a higher or potentially very high CNS% and OTU loading by the end of the dive.
This is one reason that our start point for the PO2 of our diluent at maximum operating depth (MOD) is much lower than on OC.
Another reason is that if we have a higher FO2 in our diluent, for example one that matches our setpoint at 1.3, it would be difficult if not impossible to flush the loop down should you need to reduce the loop PO2 and/or check the response of your cells.
Depending on which manual you might read or one particular training organisation’s preference compared to another it is generally considered to safe to apply one of the following rules:-
Max PO2 of diluent at MOD ≤ 1.0 - 1.1
Max PO2 of diluent at MOD = Dive setpoint - 0.2
Most of us probably dive with a setpoint in the region of 1.3 - 1.2 PO2 so either rule will put us roughly at the same start point.
So, for example, on a dive to 70m and a maximum PO2 of our diluent at our MOD of 1.0 we would have a FO2 of: -
70m = 8 Bar
1.0/8 = 0.125
So we might well choose an FO2 of 0.12 (12% O2) for the oxygen in our diluent.
The next step is to choose the level of narcosis with which we are comfortable. In this example I am going to elect that oxygen is not narcotic. In a previous blog post, Is Oxygen Narcotic I discussed this and of course it is a personal decision and divers can factor in oxygen as causing narcosis if they prefer.
As outlined in the post about choosing OC trimix, what we are actually doing is calculating the amount of nitrogen (N2) we want in the mix and whatever is ‘left over’ is the helium (He) content.
So as an example let us have an equivalent narcotic depth (END) of 30m.
The FN2 in air is 0.79 so the PN2 at 30m (4 bar) is:-
0.79 x 4 = 3.16
If we want to keep the same PN2 at our Mod of 70m we simply divide 3.16 by the absolute pressure at 70m so:-
3.16/8 = 0.396
So, the fraction of nitrogen in the mix should be 0.395 (we can round it to 0.40). We now have:-
FO2 = 0.12
FN2 = 0.40
The remaining 0.48 (48%) is made up with helium. So our onboard trimix diluent is going to be 12/48.
I hope this post is useful. If you have any questions or I can help in any way you are very welcome to email me at firstname.lastname@example.org
© Eau2 & Martin Robson 2017
I really would like to say thank you to all those at this years Tek Camp who came to listen to my presentation. Unlike many of the other (excellent) presentations, mine did not include exciting wrecks or caves or big diving expeditions and was perhaps a little bit scientific. Nonetheless I had a lot of very positive feedback from a very interested audience. The talk focused on oxygen and some of it's effects on our physiology when diving. So, following on from a previous post looking at choosing OC gasses and including one of the subjects I covered at Tek Camp, let us have a quick look at the subject of whether oxygen should be considered as narcotic for the purposes of diving.
There has been some research done on the subject and I will try to summarise some of the keys points. I will also try to dig out some of the references and post them at some point so you can look at the complete documents if you want.
We all know that the site for the narcotic effect is the brain, specifically the cell wall of the cells within the brain. Given the same partial pressure of oxygen or nitrogen at this site they will both be equally narcotic as predicted by the Meyer Overton law relating to fat solubility and to some extent the molecular size and weight to narcotic potency.
This holds true for all substances exerting an anaesthetic effect. The only difference between anaesthesia with an inhaled modern medical agent and nitrogen narcosis is the partial pressure required for the effect. This is about 0.01 of a bar of isoflourane and about 15 bar for N2 to produce the same level of unconsciousness!
However although the brain is very well supplied with O2 it is also a big consumer of O2. The brain uses oxygen at a high rate and so the tissue partial pressure remains low when breathing air at 1 bar. In order to supply the brain with sufficient O2 we have the compound haemoglobin in blood cells which increases the amount of oxygen carried from 0.3 mls per 100mls dissolved in plasma to 20mls/100mls in red cells with haemoglobin. When breathing air the haemoglobin is fully saturated with oxygen in blood going to the brain in healthy individuals.
This means that increasing the PPO2 in the inspired gas has little effect on the amount of O2 delivered to the brain and so the tissue PO2 (our effect site for narcosis) does not rise until quite high PPO2s, probably in the region of 1.5 - 2.0 bar. This is also the region of CNS O2 toxicity and hence not often breathed underwater or if so, not usually for extended periods of time or at great depth.
Thus whilst O2 is narcotic and can be demonstrated to be so under controlled chamber conditions at a PPO2 of around 3.0 bar, the risk of CNS O2 toxicity also limits the narcotic effect in practical terms underwater.
Incidentally, the effect of increased CO2 (from skip breathing, poor breathing patterns or an issue with CO2 removal or component failure in a CCR) increases blood flow to the brain. CO2 is an epileptogenic, i.e. high levels of CO2 lower the threshold at which a seizure may occur. Increased CO2 is a far more effective way of increasing O2 flow to brain tissue and helps explains an increase in narcotic effect and heightened risk of CNS O2 toxicity and convulsions in the ‘borderline’ region of 1.5 - 2. 0 bar inspired PPO2.
So, both points of view are essentially correct in different circumstances.
So, if you want to include O2 in your equivalent narcotic depth (END) calculations, choose your preferred narcotic depth then:-
FN2 = (END/P) - FO2
where FN2 is the fraction of nitrogen you will have in your trimix, the END is the Equivalent Narcotic Depth you have chosen and P is the absolute pressure of the dive depth.
If we follow on from the example used in the previous post where we used a dive PO2 of 1.35 Bar to calculate our FO2 for a 60m dive we had an FO2 of .18 or 18%.
Our chosen EAD for that dive was 30m so for an END of 30m we use:-
END = 33m = 4.3 Bar
P = 60m = 7 Bar
FN2 = (4.3/7) - FO2
FN2 = (4.3/7) - .18
FN2 = .434
In the previous post our mix was calculated to be 18/37 (i.e. .FHe of 37 or 37% HE). Using the formula to include O2 as narcotic gives us a mix of 18/39, so just a little bit more helium in the mix when counting O2 as narcotic.
FHe = 1 - FO2 - FN2
FHe = 1 - .18 - .43 = .39
© Eau2 & Martin Robson 2016
It's almost that time again. If you have not already booked then get in touch and reserve your place. It is going to be another amazing event.
Quite honoured to be asked to be the International Training Director for the National Association for Cave Diving (NACD).
There are several reasons for taking the time to make sure we choose the right gas when diving on trimix. It allows us to ensure we are diving on a safe PO2 and to control our levels of narcosis.
When we do choose our OC gasses there is guidance on what might or might not be a safe or appropriate gas, so let’s just review some of the generally accepted OC practices for choosing trimix. At the moment we will just look at our deep gas. Intermediate and decompression gasses will be looked at in a separate post.
If we look at the ‘pressure T’ we can determine the best mix, MOD or TOD or safe depth for a particular gas.
PO2 is the partial pressure of oxygen, FO2 is the fraction of oxygen in the gas mix and P is the depth expressed an an absolute pressure. Whatever we wish to determine, we simply ‘cover that up’ and then do the remaining sum.
PO2 = FO2 x P (to determine the PO2 of a gas at a depth)
FO2 = PO2/P (to determine the best mix for a gas at a a particular partial pressure and depth)
P = PO2/FO2 (to determine the MOD or TOD of a gas at a particular partial pressure)
For example, the absolute pressure ‘P’ at 30 metres is 4 bar, Nitrox 32 or 32% has an FO2 of .32 and 32% at 30 metres will have a PO2 of 1.28 (.32 x 4 Bar).
When choosing a Nitrox for shallower diving the staring point is usually the PO2 of the gas at our Target Operating Depth (TOD) or Maximum Operating Depth (MOD), depending on the nature of the dive.
When choosing a Trimix we would have the same start point but with some adjustments. So let’s quickly recap the ‘starting PO2s’ we might opt for.
A PO2 of 1.6 is, as we know the maximum limit. Normally used in technical diving rather than recreational and typically only for decompression gasses.
A PO2 of 1.5 is often called the recreational limit.
A PO2 of 1.4 is similarly called the working limit and very commonly used as the start point for calculating the FO2 for the best mix for a trimix dive. There are some other adjustments which you might choose to adopt, again depending on the nature and exposure even of the dive.
In it’s simplest form with no adjustments or safety factors applies an example for a 60m dive would look like this.
60m = 7 Bar absolute
PO2 of 1.4/7 = .20 (or 20% O2)
So we now have an FO2 for your trimix.
We all know that there are a number of factors which have the potential to make us more susceptible to CNS oxygen toxicity. I’m not going to review them all but included amongst them is getting cold, increased CO2 production and the length of exposure to the elevated PO2. Hard work underwater will increase our CO2 production. So for cold, a hard working dive or a long dive we can reduce our start point PO2 for calculating our FO2. The reduction factor is 0.25 for each.
From the example above we might be planning a relatively straight forward 60m dive as far as bottom time goes but water is a bit chilly cold and there might be a current flowing so we could choose to reduce ur starting PO2 by 0.25 for each of these factors. This would give us a starting PO2 for our calculation of 1.35.
1.35/7 = .193 rounded up so we might choose .18 or .19 (18% or 19% O2) as our FO2 in our trimix.
As an aside, it is fairly standard within the dive industry that when a dive centre mixes nitrox or trimix then a variation of + or - 1% is acceptable. It is better to err on the side of caution and ask for a slightly lower FO2 so that if it comes out a tiny bit rich it is still going to be a safe gas.
We can, if wanted, apply a ‘safety factor to our decompression gasses too, of -0.05 Bar PO2 for each of cold or long dive time (there shouldn’t really be any hard work on deco!) This is an additional safety margin we can put in to control our CNS% and OTU loading on a longer dive.
Having chosen our FO2 we then need to calculate how much Helium we want in order to control our levels of narcosis. We perhaps should set aside for another debate the question of O2 being narcotic. Yes, the mathematics and physical properties of the gas suggest it should be but there are equally compelling agreements, based on scientific investigations, that suggest in real diving, practical terms, it is unlikely to actually cause narcosis. If I can dig out some research I can post a synopsis of some of the theories. Based on these documents I personally fall on the side of not taking in to account the potential for O2 to be narcotic so I will leave that out of the process of calculating the He to add in our mix.
The start point should be your own comfort levels as far as narcosis is concerned. Some are happy to dive to a deeper level of narcosis than others so it really can be a very person choice but we should try to keep some level of team compatibility if we can.
So let us choose 30 metres as our equivalent air depth (EAD). We need to calculate what the partial pressure of nitrogen (PN2) is at a depth of 30m. So, the absolute pressure at 30m is 4 Bar, so the PN2 is 4 x .79 = 3.16.
3.16 is the peak PN2 we are aiming for at our actual dive depth of 60m. If we divide 3.16 by the absolute pressure at 60m of 7 Bar (3.16/7 = 0.451) we get an FN2 of 0.45. This is how much N2 we will allow in the mix.
To calculate the FHe we simply subtract the FO2 and FN2 from the whole and what is left is the FHe.
So, 1 - .18 - .45 = .37. Our FHe in our mix is .37 (37%). Our gas mix is now calculate as Trimix 18/37. (It is common to give the FO2 first then the FHe when deciding a trimix gas).
So, all fairly straight forward. Bear in mind that this process is used for OC gas calculations. We would adopt a slightly different approach and apply slightly different rules when calculate a trimix for use in a CCR both for onboard active diluent and bailouts.
© Eau2 & Martin Robson 2016
The team! Many congratulations to Tim Nottage, Federico Mentegazzi and Luke Shepherd. NSSCDS & IANTD CCR Cave Divers. 18 dives and over 1300 minutes of cave time!
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.
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