+44 7932 735 619
Martin Robson Tec Diving
  • Home
  • Contact
  • OC Tec
    • Intro Tec
    • Tec
    • Advanced Tec
  • CCR
    • Rec Rebreather
    • Tec CCR
    • Advanced Tec CCR
  • Cave
    • Cavern & Basic Cave
    • Tec Cave, CCR Cave & Mine diver
    • Advanced Cave
  • Eau2 Exclusives
  • Instructor & Trainer
  • Equipment
  • Dive Industry Consultancy
  • Course calendar
  • Blog

Choosing Trimix Diluent (Part 2) by Martin Robson

3/8/2017

0 Comments

 
Picture
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.

Safe diving.

© Eau2 & Martin Robson 2017

0 Comments

Your comment will be posted after it is approved.


Leave a Reply.

    RSS Feed

    Martin's Blog

    A new website and a new blog to keep you up to date with courses and trips.

    Categories

    All
    Latest News
    Tec Diving Stuff
    Training

    Archives

    February 2018
    August 2017
    May 2017
    July 2016
    May 2016
    April 2016
    December 2015
    October 2015
    August 2015
    July 2015
    June 2015
    May 2015
    April 2015
    March 2015
    September 2014
    July 2014
    June 2014
    May 2014
    March 2014
    February 2014
    January 2014

Copyright © 2014   Eau2 Advanced Diver Training   +44 (0) 7932735619   info@eau2.com
✕