By Curt Bowen

On any rebreather, oxygen is the main gas that keeps the diver alive. Eliminate the supply of oxygen for a short period of time is all that is needed to end in tragedy.

For many years, divers have been utilizing a simple device commonly called a “leaky valve”. This device allows a set volume of oxygen to flow freely into the rebreather and is unaffected by increased depth. The amount delivered is controlled by the first stage regulator’s intermediate pressure (IP) and a specific orifice size.

Lower the IP and the volume of oxygen are diminished, increase the IP and the volume is increased. The ideal volume delivered to a diver is adjusted to match their metabolic oxygen consumption at a low workload.

The following test was conducted to help illustrate the importance of a constant flow oxygen orifice.
 
Test A
No Constant Flow Orifice
Test Parameters
•
 Oxygen start set point above 0.7 PPO2
•
 Normoxic Diluent (21% oxygen / 79% nitrogen) used to maintain minimum loop volume during test. (ADV manually added gas as counter lungs pulled a negative pressure)
• Minimum loop volume maintained = (volume required to take a deep breath and bottom out the counter lungs without the ADV adding diluent gas.)
• No Oxygen addition during test
• Test diver walked in place at a steady pace for duration of the test
• Divers nose was blocked to prevent gas leaking or addition
Time (minutes)
Shearwater Triple PPO2 Display
0
Minute
.76 .76 .74
1
Minutes
.65 .65 .64
2
Minutes
.54 .54 .53
3
Minutes
.47 .47 .45
4
Minutes
.36 .36 .35
5
Minutes
.27 .27 .27
6
Minutes
.19 .18 .18
6:36
Minutes
.14 .14 .14
DEAD
 
Test B
Constant Oxygen Flow Orifice 0.8 lpm
Test Parameters
•
 Oxygen start set point above 0.7 PPO2
•
 Normoxic Diluent (21% oxygen / 79% nitrogen) used to maintain minimum loop volume during test. (ADV manually added gas as counter lungs pulled a negative pressure)
• Minimum loop volume maintained = (volume required to take a deep breath and bottom out the counter lungs without the ADV adding diluent gas.)
• Oxygen addition during test at 0.8 lpm through orifice
• No additional oxygen added manually
• Test diver walked in place at a steady pace for the duration of the test
• Divers nose was blocked to prevent gas leaking or addition
 
Time (minutes)
Shearwater Triple PPO2 Display
0
Minute
.76 .76 .73
1
Minutes
.74 .74 .72
2
Minutes
.74 .73 .71
3
Minutes
.72 .72 .69
4
Minutes
.71 .71 .70
5
Minutes
.70 .69 .69
6
Minutes
.68 .68 .65
7
Minutes
.67 .66 .63
8
Minutes
.61 .58 .57
9
Minutes
.58 .54 .52
10
Minutes
.56 .56 .53
11
Minutes
.56 .54 .52
12
Minutes
.49 .49 .45
13
Minutes
.45 .45 .44
14
Minutes
.44 .44 .42
15
Minutes
.39 .39 .37
16
Minutes
.38 .37 .35
17
Minutes
.35 .35 .33
18
Minutes
.33 .33 .31
19
Minutes
.29 .29 .28
20
Minutes
.27 .26 .25
21
Minutes
.24 .24 .23
22
Minutes
.21 .21 .20
STILL ALIVE
 
The above simple test clearly indicates the importance of a continuous flow orifice on any rebreather. Increased workloads or added diver stress will greatly affect the results of oxygen consumption, reducing both test A and B significantly.

Adding a continuous flow device on a solenoid electronically controlled closed circuit rebreather can reduce the workload of the solenoid and add a significant safety net for electronics failure.