Poseidon SE7EN User Manual
Appendix 3 Page 107
PO
2
setpoint
Below the PO
2
value the PO
2
setpoint vill be displayed. Normally, this value will be the same as
the current PO
2
, because the system will normally maintain the correct PO
2
(i.e., Setpoint PO
2
).
In some cases, however, the value may be slightly different.
Figure 3-14.
PO
2
Set point 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 first 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 flood 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 first 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 confidence.
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.
