Flow Rate Calculation He/EAN

 

I have managed to finally get some good numbers on the Draeger 1st stage. I have found that normal pressure gauges are not very good even over time. I had to make a special adapter for my 6 inch gauge. The normal gauges are not linear across their range. Contrary to what I thought, I found little variation with temperature. Previous inconsistencies were due to the measuring anomalies with the normal dive SPG's.

I also found that flow meters are not reliable flow indicators for He mixes. Two different gauges (different manufacturers) give different results with He and the same with air!  Below are the values I have come up with:

40 % Orifice Diam 0.236 mm

Nominal flow 10.75 lpm

 He 50% 0.459 cfm 13.0 - 13.4 lpm measured
Air 0.380 cfm 10.7 lpm measured
50 % Orifice Diam 0.203 mm

Nominal flow 7.8 lpm

 He 50% 0.348 cfm 9.8 - 10 lpm measured
Air 0.293 cfm 8.2 measured

 

I was able to get good tracking on observed flow values with the application of the following two formulae:

Flow in lpm  =  11 * P D2   where P is in bars and D is diam. of orifice in mm. (1)

Flow will follow the relationship of the molecular weights of the gases as:
Square root of          G ref  /    G product                                                                                                         (2)

 

Molecular weight

Air

 30/30 21/50
16 O2 .21 = 3.36 .3 = 4.8 .21 = 3.36
14 N2 .79 = 11.06 .4 = 5.6 .29 = 5.6
4 He _____

14.42

 .3 = 1.2

11.6

 .5 = 2.0

9.42
Ratio

1.1

1.24

Whether we use balanced drive regulators for bottom mix or not seems to make very little difference on deco time. The carrying of the intermediate mix puts us back on track.

Using the 40% orifice and a VO2 of 1 lpm with 21/50 and the Draeger reg would give an FiO2 of 15.4% and with a balanced reg of 16.7%. Voyager shows only 5 mins difference between 12/56 - 16/53 and 18/50 on our normal profile.

The oxygen at 125 psi drops the flow to 6.5 lpm.

 

Last update  1 Jan 2008