User manual
Poseidon MKVI User Manual
Chapter 3
Page 53
PO
2
setpoint
Every few seconds, the PO
2
value will briey (less than one second) switch to show the current
PO
2
Setpoint. 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. In any case, the PO
2
Setpoint value can be distinguished from the current
PO
2
value by the size of the rst digit (either “1” or “0”). When the displayed value is the PO
2
Setpoint, the rst digit (to the left of the decimal) is shown using only the upper half of the
numeric value.
Figure 3-14. PO
2
Set point value Figure 3-15. Real PO
2
value.
The Poseidon MKVI 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 MKVI 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 dangerous 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 MKVI 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 condence.
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.