Hollis Explorer User Manual - page 77
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THE AUTOMATIC DILUENT VALVE
The EXPLORER also has an automatic diluent addition valve (ADV), which
compensates for loss of gas volume. To activate the ADV either descend
or exhale through your nose and breathe in. The ADV is designed to help
maintain a breathable lung volume in conjunction with the LCV.
The ADV on the Explorer is a simple tilt lever system that in the relaxed
position (no gas applied) will remain open. It seals when gas is applied.
If moisture is present in the system and extreme freezing temperatures are
experienced that are able to form ice within the rebreather this valve could
freeze in the open position. Freezing of ADV’s is a common failure mode
in most rebreathers. The EXPLORER design assumes this and provides an
additional link to the Pre-dive alarm sequence to warn of such a failure.
If the valve is frozen open then gas will freely flow into the circuit. This will
give a ‘HIGH HP’ usage alarm and the unit will fail its Pre-dive sequence.
This is the correct failure mode for this device.
If the system had been pressurized and then it froze and hence the valve
failed closed, again pre-dive will fail momentarily until counterlung collapse
removes the icing as the lever activates, then pre-dive will pass unless
there is a leak due to icing then the pre-dive will again fail (high gas flow
alarm).
The Explorer warning system was developed with the recognition that it
is common for rebreathers to contain moist gas which can lead to icing in
cold environments. The EXPLORER is the first rebreather that fully informs
a diver if an ADV failure occurs from and "iced closed" or "iced open" situa-
tion.
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