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Suunto EON Core User Guide 3.0

Gas mixtures

By default, Suunto EON Core has only one gas (air) available. The default oxygen percentage (O2%) setting is 21% (air) and oxygen partial pressure (pO2) setting is 1.6 bar (23 psi). You can modify O2 percentage and pO2 settings in Gases menu.


When diving with a nitrox gas mixture, Suunto recommends changing the partial pressure to 1.4 bar (20 psi).

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.

If you need more than one gas, activate multi-gas option in your device. Go to Dive settings » Parameters and turn on Multiple gases option.

You need to define all gas mixtures intended to use during the dive in the Gases menu because during your dive the decompression algorithm calculates ascent time using all gases available in the Gases menu.

If you want to use trimix gas mixtures (with helium activated), you need to turn helium on under Dive settings » Parameters. When this is done, you can change helium percentage (He%) for the selected gas in the Gases menu.

You can activate multi-gas diving and helium, configure dive modes and change gas settings using Suunto app, too.

Changing gas during a dive

It is important to understand how your Suunto EON Core device works when you dive with multiple gases. For example, you may have the following gases when diving to 55 m (180.5 ft):

  • tx18/45, MOD 62.2 m (pO2 1.3)
  • tx50/10, MOD 22 m (pO2 1.6)
  • Nx99, MOD 6 m

While ascending, you are notified to change gas at 22 m (72 ft) and 6 m (20 ft) according to the maximum operating depth (MOD) of the gas. To take a better gas into use, you need to change gas manually by completing the following steps:

  1. Press any button to acknowledge the gas change notification.
  2. Long press the middle button to see gas options.
  3. Scroll with the upper or lower button to the desired gas.
  4. Press the middle button to confirm gas selection.

If you press any button while Change gas notification is visible on the device screen, the notification disappears. By pressing a button, you only confirm the notification, but gas is not changed automatically. You should always change gas manually. To change gas, you must follow the above listed steps.


When you select CCR, gas mixtures are divided to open-circuit and closed-circuit gases. See Rebreather diving.

Modifying gases during a dive

Modifying the gas list in your device is for emergency cases only. For example, due to unforeseen events, you might lose a gas mixture, in which case you could adjust to the situation by deleting that gas mixture from the gas list of Suunto EON Core. This allows you to continue diving and get correct decompression information calculated by the dive computer.

In another case, if for some reason you run out of gas and need to use a gas mixture from a dive buddy, it is possible to adapt Suunto EON Core to the situation by adding the new gas mixture to the list. Suunto EON Core re-calculates decompression and shows the correct information.


This feature is not enabled by default, it must be activated and it creates an additional step to the gas menu during the dive. It is only available if multiple gases are selected for the dive mode.

To enable modifying gases, turn the feature on in the settings menu under Dive settings » Parameters » Modify gases.

When enabled, during a multi-gas dive, you can add a new gas as well as select an existing gas from the gas list to remove it.


You cannot modify or remove the gas currently in use (active gas).

When Modify gases is turned on, you can remove gases which are not in use from the gas list, add new gases to the list, and modify parameters (O2, He, pO2) of nonactive gases.

Isobaric counterdiffusion (ICD)

Isobaric counterdiffusion (ICD) occurs when different inert gases (such as nitrogen and helium) diffuse in different directions during a dive. In other words, one gas is being absorbed by the body while the other is being released. ICD is a risk when diving with trimix mixtures.

This may happen during a dive, for example, when trimix gas is switched to nitrox or light trimix. When the switch is made, helium and nitrogen rapidly diffuse in opposite directions. This produces a transient increase in total inert gas pressure which can lead to decompression sickness (DCS).

Currently there are no algorithms that can address ICD. Therefore, you need to take it into account when planning trimix dives.

You can use Suunto EON Core to plan your trimix usage safely. Under the Gases menu, you can adjust oxygen (O2) and helium (He) percentages to see the change in partial pressure of nitrogen (ppN2) and the partial pressure of helium (ppHe) values.

An increase in partial pressure is indicated by a positive number, and a decrease by a negative number. The changes in ppN2 and ppHe are displayed next to each gas mixture that you want to switch to. Maximum operating depth (MOD) of a breathing gas is the depth at which the partial pressure of oxygen (pO2) of the gas mix exceeds a safe limit. You can define pO2 limit for the gas.

An ICD warning is generated when:

  1. Gas switch depth is greater than 10 m (33 ft).
  2. Geometric mean of the partial pressure change of N2 and partial pressure change of He is bigger than 0.35 bar.

If these limits are exceeded with a gas switch, Suunto EON Core indicates the risk of ICD as shown below:

isobaric eon

In this example, the available gas mixtures for a deep trimix dive are:

  • Trimix 15/55, MOD 76.7 m (pO2 1.3)
  • Trimix 35/15, MOD 27.1 m (pO2 1.3)
  • Trimix 50/10, MOD 22 m (pO2 1.6)
  • Oxygen, MOD 6 m

Suunto EON Core highlights the dangerous ICD condition when the gas mixture switches from 15/55 to 35/15 at a depth of 27.1 m.

If this gas switch is made, the change in ppN2 and ppHe are far beyond the safe limits.

One way to avoid the ICD risk is to increase helium content in the first decompression gas mixture (trimix 35/15) to get a 35/32 trimix mixture. Due to this change, the second decompression gas (trimix 50/10) needs more helium to avoid ICD risk. The second decompression gas mixture should be trimix 50/12. These modifications would keep the changes in partial pressure at a safe level and remove the danger of sudden ICD.

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