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Suunto EON Steel User Guide - 1.6

Multi-gas diving

Multi-gas diving

Suunto EON Steel allows gas changes during a dive between the gases defined in the Gas(es) menu. When ascending, you are always notified to change gases when a better gas is available.

For example, you may have the following gases when diving to 55 m (180.5 ft):

  • tx18/45, MOD 58m
  • tx50/10, MOD 21m
  • oxygen, MOD 6m

While ascending, you are notified to change gas at 21 m (70 ft) and 6 m (19.7 ft) according to the maximum operating depth (MOD) of the gas.

A pop-up notifies you when to change gases, as shown below:



When diving with multiple gases, remember that the ascent time is always calculated with the assumption that you use all the gases found in the Gas(es) menu. Always check that you have only the gases for your current planned dive defined before you dive. Remove the gases that are not available for the dive.

Modifying gases during a dive

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

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


This feature is not enabled by default, it must be activated and 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.

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 Steel 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 that you want to switch to. Maximum Operating Depth (MOD) is assumed to be the depth when start to use the gas mixture.

An ICD warning is generated when the gas switch depth is greater than 10 m (30 ft) and either:

  1. The change ppN2 increases by more than +0.5, or
  2. The change in ppHe increases by more than +0.5 and ppN2 decreases by more than -0.25.

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

isobaric new

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

  • Trimix 15/55
  • Trimix 35/10
  • Trimix 50/10
  • Oxygen

Suunto EON Steel highlights the dangerous ICD condition when the gas mixture switches from 15/55 to 35|10 at a depth of 34.4 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 35/10 gas mixture to a 35/25 Trimix mixture. This would keep the changes in partial pressure at a safe level and remove the danger of sudden ICD.