By default, Suunto EON Core has only one active gas (air). You can modify O2 percentage and pO2 settings in Gases menu.
If you need more than one gas, activate multigas option on the device menu under Dive settings » Parameters.
If you want to use trimix gas (with helium activated), you need to turn helium on under Dive settings » Parameters. When this is done, you can change helium percentage (He%) for selected gas in the Gases menu.
You can activate multigas diving and helium, configure dive modes and change gas settings using DM5, too.
When you have analyzed your gas, you should round the result down when entering it for Suunto EON Core. For example, if the analyzed gas is 31.8% oxygen, then define the gas as 31%. This makes the decompression calculations safer.
If you need to adjust the computer to provide more conservative calculations, use the personal adjustment feature to affect decompression calculations or reduce the pO2 setting to affect oxygen exposure according to the entered O2% and pO2 values.
THE DIVE COMPUTER WILL NOT ACCEPT FRACTIONAL PERCENTAGE VALUES OF OXYGEN CONCENTRATION. DO NOT ROUND UP FRACTIONAL PERCENTAGES! Rounding up will cause nitrogen percentages to be understated and will affect decompression calculations.
What you see under the Gases menu can be customized. See Customize dive modes with DM5.
It is important to understand how the Gases menu works when multiple gases and helium are activated. For example, you may have the following gases when diving to 55 m (180.5 ft):
In the screeshot below, the menu has three gases and tx18/45 is selected as the active gas. Even though only one gas is active, the decompression algorithm calculates ascent time (during dive) using all these three gases.
To select another active gas:
If you are diving with only one gas, ensure you have only that one gas in the Gases menu. Otherwise, Suunto EON Core expects you to use all gases in the list and notifies you to change gases during the dive.