VR Technology Limited
Warning!
Diving beyond the MOD of the diluent in closed circuit mode / or beyond the
PO2 of the unit is less that the diluent PO2 at depth
Dive computers calculate decompression based on the inert gas partial pressures. This
is the only accurate way to estimate decompression requirements. Decompression
computers are an aid to decompression and backup tables and depth/time
instrumentation should always be carried.
When diving beyond the MOD in closed circuit mode and if the setpoint PO2 of the
unit is less than the diluent PO2 at depth, then the diluent PO2 will be used to drive
the decompression. This is a correct assumption as it is the partial pressures of the
inert gas in the diluent that drives decompression. Alarms on the VR showing the
actual diluent PO2 will display once the diluent/setpoint PO2 level has been exceeded.
As the main concern at this stage is oxygen toxicity then the increased PO2 will
display and the CNS % calculation will be updated until it in turn warns of dangerous
levels. Additional alarms are displayed on our Rebreather product.
If you remain at depth with no additional gas injection, then the PO2 in the loop will
drop (due to metabolism)and the inert gas partial pressures may rise (dependant on the
bodies gas absorption rates) a small amount dependant on the gas volume (and hence
total partial pressure within the loop). At some point the resultant volume drop will
make the loop volume become un-breathable until diluent is added. At this point the
decompression may vary slightly from the decompression displayed on the VR.
However due to gas volumes being absorbed by the body and other factors any
variations that may occur will be minimal. If diluent were added during this static
phase of the dive (as it should be) then the decompression will be as accurate as
possible.
The rise in PO2 (drop in inert gas pressures) will reduce the total time to surface
calculation on the assumption that the PO2 (hence PN2/PHe) will remain constant to
the surface as it is only being predictive based on current facts. Once the depth is
decreased and the PO2 reduces then the calculation and prediction will update.
In the above example, If the decompression is being driven via a sensor linked system
as in a rebreather then the PO2 is always monitored and providing a breathable
volume is maintained, then the decompression will be accurate. However, if the PO2
falls below that of the equivalent for the diluent at that depth, then a warning will
appear. The VR will assume the o2 cell has not been calibrated correctly. Again this is
the most likely result as training agencies and normal dive practice do not allow for
turning of the diluent in a rebreather. Exceeding the MOD/setpoint number will
generate an alarm, warning of this unsafe practice.
Having a diluent that has a sufficiently low PO2 at the MOD to allow the rebreather to
be flushed to confirm sensor operation or reduce the PO2 in the loop is a safe practice.
Decompression computers calculate decompression requirement based on the inert
gas partial pressures. Whether you are above the MOD of a gas or below it this
calculation is the same. In both scenarios additional safety factors are applied to the
decompression calculations to allow for variations in diving practice.
VR Dive Computer Operations Manual 2009 rev E 28/01/2009
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