If Nitrox or Trimix dive mode is selected, you need to define the gas(es) for the decompression algorithm to work properly. You define the gases under Gas(es). In Nitrox mode, gas only has only oxygen (O2%). If helium is turned on in the dive settings, He% is available.
Trimix dive mode is disabled by default, so helium is not available. Activate Trimix in DM5 and then tou can choose multiple gases by turning on multiple gases option in the dive settings.
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. Oxygen calculations (pO2, OTU, CNS%) are also kept conservative, since the oxygen percent used for those are O2% + 1.
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.
You can customize what you see in the Gas(es) menu. See Customize dive modes with DM5.
It is important to understand how the Gas(es) menu works on Trimix mode, where you can make use of multiple gases. 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 Gas(es) menu. Otherwise, Suunto EON Core expects you to use all gases in the list and notifies you to change gases during the dive.