POSEIDON SE7EN+ - PO Setpoint; Hyperoxic Linearity Test; Chapter 3 Page 54

Poseidon SE7EN+ User Manual Chapter 3 Page 54
PO
2
setpoint
The oxygen partial pressure (PO2) in the breathing loop is displayed prominently in the upper
left corner of the Primary Display. This is perhaps the most important number on the entire
screen, as maintaining an appropriate oxygen partial pressure in the breathing gas is critical to
ensure safe diving. If the value departs substantially from the current PO2 setpoint, the value
will ash. If the value becomes dangerously high or dangerously low, the diver will be prompted
to switch to open circuit mode and terminate the dive. A low PO2 alarm will be triggered when
PO2 < 0.30 bar. A high PO2 alarm will be triggered when PO2 > 1.6 bar for more than 60
seconds or immediately when PO2 > 1.8 bar.
Figure 3-14. PO
2
Set point value Figure 3-15. Real PO
2
value.
The Poseidon SE7EN+ incorporates a dynamic PO
2
setpoint value, which means the setpoint
changes depending on depth and decompression status. Two setpoint settings control what
the range of setpoint values will be during the dive. A “surface” setpoint value (default/minimum
0.5 bar / atm) establishes the PO
2
setpoint when at the surface, and a “deep” setpoint (default
1.2 bar / atm) establishes the PO
2
setpoint when at a depth greater than of 15 m / 50 feet.
Between these two depths, the setpoint changes in small increments between these two
values. Thus, when the depth is less than 15 m / 50 ft, the setpoint will be some value between
the “surface” setpoint and the “deep” setpoint, proportional (but not linearly so) to current
depth. This dynamic setpoint method helps prevent excessive PO
2
“spikes” during descent,
and excessive oxygen wastage during ascents from no-decompression dives.
There are two exceptions to the dynamic setpoint method described above. The rst is that
whenever a decompression ceiling exists, the setpoint will not drop below 0.9 bar / atm during
ascent. The second involves the Hyperoxic Linearity test on the primary oxygen sensor, as
described below.
Hyperoxic linearity test
One of the important new features in the Poseidon SE7EN+ is the Hyperoxic Linearity test.
When the oxygen sensors are calibrated during the pre-dive routine (Chapter 2), the linearity of
the oxygen sensor response is only validated up to a PO
2
value of 1.0 bar / atm (i.e., 100%
oxygen at sea level). Most rebreathers assume that the sensor response remains linear at
higher values (operational PO
2
setpoint values often exceed 1.0 bar / atm). However, in certain
situations the sensors may not be linear above 1.0 bar / atm, which can lead to a very danger-
ous situation. For example, if the sensor is not capable of responding to PO
2
values greater
than 1.2 bar / atm, and the PO
2
setpoint is 1.2 bar / atm, the control system may ood the
breathing loop with dangerously high levels of oxygen while attempting to achieve a PO
2
value
that the sensors are not capable of registering.
To overcome this problem, the Poseidon SE7EN+ performs a test on the primary oxygen
sensor the rst time a depth of 6 m / 20 ft is achieved. The test injects a short burst of oxygen
directly onto the primary sensor to ensure the sensor response is linear up to a PO
2
value of
1.6 bar / atm. If the test passes, then the dynamic setpoint performs as described previously
(i.e., using up to the “deep” PO
2
setpoint value when the depth exceeds 15 m / 50 ft.). However,
if the Hyperoxic Linearity test fails, then the maximum allowable setpoint is set at 1.0 bar / atm.
The reason for this is that the primary oxygen sensor is known to be linear to at least 1.0 bar /
atm, based on the successful completion of the pre-dive calibration process. Thus, as long as
the PO
2
does not exceed 1.0 bar / atm, the response value is known with condence.
Using the default “surface” and “deep” PO
2
setpoint values, a setpoint of 1.0 is not achieved
until the depth exceeds 6 m / 20 ft, so there is no consequence on dives shallower than this
depth, even if the Hyperoxic Linearity test is never performed. Until the Hyperoxic Linearity test
passes successfully, the PO
2
setpoint value will be limited to 1.0 bar / atm.