Inside the <gasdefinitions> section happens the definition of the different breathing gases used by the <owner> of the UDDF file.
<gasdefinitions> stands after the <divesite> section and before <profiledata>.
The only element inside <gasdefinitions> is <mix> which can be repeated any times.
The <gasdefinitions> section can be omitted. If <gasdefinitions> doesn't occur, the executing software has to set air as the breathing gas to be used. Of course, a breathing gas consumption calculation cannot be performed in case of a missing <gasdefinitions> section.
<uddf>
<gasdefinitions>
<!-- all breathing gases used -->
<mix id="oxygen_pure">
<name>pure oxygen</name>
<o2>1.000</o2>
<n2>0.000</n2>
<he>0.000</he>
<ar>0.000</ar>
<h2>0.000</h2>
</mix>
<mix id="air">
<name>air</name>
<o2>0.210</o2>
<n2>0.790</n2>
<he>0.000</he>
<ar>0.000</ar>
<h2>0.000</h2>
</mix>
<mix id="nitrox6040">
<name>Nitrox60/40</name> <!-- Nitrox 60 % N2, 40 % O2 -->
<o2>0.400</o2>
<n2>0.600</n2>
<he>0.000</he>
<ar>0.000</ar>
<h2>0.000</h2>
</mix>
<mix id="trimix">
<name>Trimix</name> <!-- Trimix 15 % O2, 40 % N2, 45 % He -->
<o2>0.150</o2>
<n2>0.400</n2>
<he>0.450</he>
<ar>0.000</ar>
<h2>0.000</h2>
</mix>
</gasdefinitions>