What can I say! Thank you to everyone who pledged to buy the book. We have reached our target and a little bit more. Whether you bought one book, more than one book or very kindly donated even more, it is all so very much appreciated, thank you.
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We are so incredibly lucky to have Otter Drysuits helping the book and they have offered us one of their new Atlantic Drysuits in support of the pledges. This is a once only offer and a chance not only for a book and T-shirt but a fabulous new dry suit at less than the normal retail price!
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Possibly my favourite Florida cave. This video is from our January trip. We went back again in May for a trip through Rocky Horror and up through The Courtyard. I will post a video of that amazing dive soon. It is a beautiful cave.
Madison Blue Jan 2018
A few video highlights of our most recent cave trip to sunny Florida. just click on the link below to watch the video.
Spaces are available for this coming winter in Florida in November this year and early January 2019.
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
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