VR Technology VR Series - A Note on the MOD Setting and Gas Switches; A Closed Circuit Dive Planning Example

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A Note on the MOD setting and Gas Switches
The MOD setting of an ON gas is important as this is the depth at which the VR will
prompt you to switch to that gas. The look-ahead Time To Surface (TTS) and
decompression profile will also assume a switch to deco gas at that depth. If you
mistakenly set the MOD shallower than the preferred switch depth, you can still
switch and tell the VR you have done so. Switching to a decompression gas too deep
will trigger a PO2 warning. Incorrectly setting the MOD, or leaving gases ON which
will not be used, means that the optimum look-ahead TTS and decompression profile
will not be calculated or displayed. What you actually do during the ascent and
any gas switches you make and tell the VR about will still be factored into the
calculation but the on-screen prediction may not be the most efficient profile.
The TTS includes ascent time and time at all decompression stops based on the diver
switching to all the ON gases in his gas list at the MOD selected for each gas. If you
plan to switch to a deco gas on ascent but subsequently do not in fact switch, the stop
times shown will remain the same but the VR minutes will take longer to elapse in
real time. If at any point during the dive you modify a gas content on the ON list,
the decompression profile prediction will alter to take this into account. When
you change to a different gas, in a few moments the VR will modify the
decompression list and re-calculate all decompression. The decompression screen
stays inactive for 20 seconds. Every time you push a switch whilst in this screen the
20 second timer starts again and the decompression calculations are temporarily
frozen.
A Closed Circuit Dive Planning Example
A Trimix rebreather dive is planned with 18/35 as the diluent. The diver will use
closed circuit Trimix throughout and then switch to open circuit surface-supplied
oxygen at the 6m stop. In the event that an open circuit bailout is required the diver
plans to carry an 18/35 cylinder and a Nitrox 36 cylinder. 18/35, Nitrox 36 and
oxygen are selected as the three gases. The Nitrox 36 and oxygen remain OFF leaving
Trimix 18/35 as the only ON gas. This will be seen as the DIL when the diver chooses
the CLOSED CCT screen. In the CLOSED CCT screen the diver selects a PO2 of 0.7
for the start of the dive, then exits and re-enters the screen to select a second PO2
setpoint of 1.3 which can then be switched to at depth. If the diver were to leave
Nitrox 36 and PO2 ON then they would be factored into the decompression
prediction. Having said this, even if they were left ON accidentally the actual
decompression would still be based on the gas selected as the DIL. In other words the
prediction would be wrong but the actual decompression would be correct. If the
required diluent is not displayed, in the PO2 select screen press <DIL> (short push,
both switches) to get to the gas select screen. This is similar to the gas select screen
for Open Circuit. Once there, by pressing you can scroll through the range of
programmed diluents until the one required is found. Pressing confirms your selected
diluent and returns you to the PO2 SELECT screen.
Note: Setting CCR Bailout gases is covered in detail in the Gaslist section.
VR Dive Computer Operations Manual 2009 rev E 28/01/2009
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