34
VI
Diving with more than one gas mixture
VI UWATEC
®
Smart dive computers
Switching off gas mixtures
Tanks which are not going to be used during the next dive (even though their
fraction of oxygen has been defined) must be "switched off". This is achieved
by switching off the appropriate gas mixture at TEC.
Gas mixtures which have been switched off cannot be selected during the dive
and TEC will ignore their transmitters.
Tank 1 cannot be switched off. If tank D (deco mix) is switched off, tank 2 will
automatically be switched off as well.
Procedure:
1. Select the tank symbol of the gas mixture you wish to switch off and the set-
ting of the O
2
mix, according to the instructions on page 18 (points 1-3).
2. Reduce the fraction of oxygen with B and –, until "– – %O
2
" appears.
3. Confirm your settings with B and E.
The pairing of transmitter and tank symbol will remain valid even if the gas mix-
ture has been shut off.
The selected depth for the change of gas mixture (MOD) is deleted once the
gas mixture is switched off.
Functions during a dive with several gas mixtures
Diving with more than one gas mixture represents a much higher risk (than
diving with a single mixture), and mistakes by the diver may lead to serious
injury or death.
During dives with several gas mixtures, always make sure you are breathing
from the tank you intended to breath from.
Mark all your tanks and regulators, so they can under no circumstances be
mixed up! Before every dive and after changing a tank, make sure that each
gas mixture is set to the correct value for the corresponding tank.
Decompression prognosis
The calculation of decompression data (e.g. the total time of ascent) is based on the assumption that
changes of gas mixture will be performed at the previously selected depths (MOD gas mixture 2, MOD gas
mixture D). If a diver ignores a required change or changes the mixture belatedly, TEC will readjust the
decompression calculation accordingly. The dive computer will then base its calculations on the assumption
that the diver will either use the indicated gas mixture until he/she reaches the depth for the next change
or – if no further changes are planned – that he/she will ascend to the surface with the current mixture.
Remaining Bottom Time
In addition to calculating the decompression prognosis for all the activated tanks, in the background TEC
also calculates the decompression prognosis for tank 1 by itself, and (when diving with three tanks) for
tanks 1+2 only and for tanks 1+D only. The Remaining Bottom Time (RBT) while breathing from tank 1
considers that you will finish the dive with tank 1, using the decompression prognosis for tank 1 alone.
The RBT while breathing from tank 2 considers that you will finish the dive with tank 2 using the decom-
pression prognosis for tanks 1 and 2 only (see -> 35, 36). This allows, when starting the ascent before
RBT=0, to always be able to finish a dive if by accident you were to lose the travel and/or deco bottles,
find that they are empty or in case of regulator malfunction. Note that for any tank (1, 2 or D) when you
are shallower than 6m/20ft the RBT indicates the duration of the gas supply (minus the reserve) at the cur-
rent depth.
Since TEC needs to evaluate your breathing pattern and the corresponding
pressure drop in the tank, it takes about 2 minutes after you start breathing
from a tank before TEC shows the RBT.
WARNING
%
Mixture 2
"switched off"
