CCR Rebreather Loop Gas Simulator

 
Dave T and I have been playing around with a bit of simple maths.  Have a look at this spreadsheet.  It is intended to model the changes in loop gas partial pressures in various CCRs as you descend from the surface to depth.

SO GO ON, HAVE A PLAY!  PLUG  SOME NUMBERS INTO THE RED BOXES AND WATCH THE GRAPHS CHANGE!!!  INTERACTIVE AND EDUCATIONAL!!!!

Note that this is only an attempt at modelling by two guys who are too clumsy to waterproof a multi gas analyser and find out what actually happens. One of us has never used a rebreather, is a mathematical simpleton and is by preference drunk most of the time.  If you are silly enough to actually use this to base real diving on, your well-deserved death would obviously be no loss to the human gene pool (yada yada yada, see any other rebreather page for further taunting).

 
INSTRUCTIONS:

There are 3 worksheets.  One is for the KISS (see assumptions below), the other 2 are for an eCCR with either single or dual O2 set points

 

The (fairly large) assumptions made are:
  1. The diver's lungs contain the same gas composition as the loop (stop laughing now)
  2. for the KISS, the unit replaces exactly the amount of O2 metabolised (I.e. the KISS CMF injection rate is exactly the same as the diver's VO2). It has been pointed out to me by Andrew Fock that the CMF device is usually set to deliver a little less than VO2.  Furthermore the KISS fails to deliver the set CMF volume of 02 somewhere below between 50 and 70 m as a function of the pressure differential between the first stage IP and ambient pressure.  See HERE for the explanation.  
  3. uptake of inert gases by the diver has no effect on loop concentrations (obviously incorrect but I'm guessing that very little He and relatively little N2 is lost)
  4. the unit's ADV maintains the loop volume constant regardless of depth

 

For the KISS worksheet, just plug in the following information in the red boxes in the top left section:  For example :
  1. diluent FO2 (eg .1)
  2. diluent FHe (eg.5)
  3. loop FO2 on the surface prior to descent (eg .21 of it's full of air, 1.0 if you and it are flushed with 100%O2 etc)
  4. loop He on the surface if you flush first with dil and than manually flush with 02.  In this situation you'll need to manually type in your starting loop F02, and also your FN2 and FHe.  Just put them in in the ratio they're in in your dil.  So say you're using 10/50 dil, the ratio of helium to nitrogen is 5:4.  So if you top the loop up to an F02 of .4, you'll have about .33 of He and .27 of N2.
 

For the eCCR worksheet you can specify setpoint(s), dil FO2 and Dil FHe.

 
We're not sure if we've got it right.  Please feel free to email me to point out any glaring errors.