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Suunto D4i User Guide

Suunto RGBM

Suunto's decompression model development originates from the 1980s when Suunto implemented Bühlmann's model based on M-values in Suunto SME. Since then research and development has been ongoing with the help of both external and internal experts.

In the late 1990s, Suunto implemented Dr. Bruce Wienke's RGBM (Reduced Gradient Bubble Model) bubble model to work with the earlier M-value based model. The first commercial products with the feature were the iconic Suunto Vyper and Suunto Stinger. With these products the improvement of diver safety was significant as they addressed a number of diving circumstances outside the range of dissolved-gas-only models by:

  • Monitoring continuous multiday diving
  • Computing closely spaced repetitive diving
  • Reacting to a dive deeper than the previous dive
  • Adapting to rapid ascents which produce high microbubble (silent-bubble) build-up
  • Incorporating consistency with real physical laws for gas kinetics

The Suunto RGBM predicts both dissolved and free gas in the blood and tissues of divers. It is a significant advancement on the classic Haldane models, which do not predict free gas. The Suunto RGBM provides additional safety through its ability to adapt to a variety of situations and dive profiles.

Diver Safety

Because any decompression model is purely theoretical and does not monitor the actual body of a diver, no decompression model can guarantee the absence of DCS. Experimentally it has been shown that the body adapts to decompression to some degree when diving is constant and frequent. Personal adjustment settings are available for divers who dive constantly and are ready to accept greater personal risk.


Always use the same personal and altitude adjustment settings for the actual dive and for the planning. Increasing the personal adjustment setting from the planned setting as well as increasing the altitude adjustment setting can lead to longer decompression times deeper and thus to larger required gas volume. You can run out of breathing gas underwater if the personal adjustment setting has been changed after dive planning.

Altitude diving

The atmospheric pressure is lower at high altitudes than at sea level. After traveling to a higher altitude, you will have additional nitrogen in your body, compared to the equilibrium situation at the original altitude. This 'additional' nitrogen is released gradually over time and equilibrium is restored. It is recommended that you acclimatize to a new altitude by waiting at least three hours before making a dive.

Before high-altitude diving, you need to adjust the altitude settings of your dive computer so that the calculations take into account the high altitude. The maximum partial pressures of nitrogen allowed by the mathematical model of the dive computer are reduced according to the lower ambient pressure.

As a result, the allowed no decompression stop limits are considerably reduced.


SET THE CORRECT ALTITUDE SETTING! When diving at altitudes greater than 300 m (1000 ft), the altitude setting must be correctly selected in order for the computer to calculate the decompression status. The dive computer is not intended for use at altitudes greater than 3000 m (10000 ft). Failure to select the correct altitude setting or diving above the maximum altitude limit will result in erroneous dive and planning data.

Oxygen exposure

The oxygen exposure calculations are based on currently accepted exposure time limit tables and principles.

The dive computer calculates separately the Central Nervous System oxygen toxicity (CNS) and the Pulmonary Oxygen toxicity, the latter measured by the addition of Oxygen Toxicity Units (OTU).

Both fractions are scaled so that the diver’s maximum tolerated exposure for each is 100%.

Suunto D4i does not display CNS% or OTU% but instead displays the larger of the two in the OLF% field. OLF% value is the Oxygen limit fraction or Oxygen Toxicity Exposure.

For example, if the diver’s maximum tolerated exposure for CNS% is 85% and the maximum tolerated exposure for OTU% is 80% the OLF% displays the largest scaled value, here 85%.

Oxygen related information displayed by the dive computer is also designed to ensure that all warnings and displays occur at the appropriate phases of a dive.


WHEN THE OXYGEN LIMIT FRACTION INDICATES THAT THE MAXIMUM LIMIT IS REACHED, YOU MUST IMMEDIATELY TAKE ACTION TO REDUCE OXYGEN EXPOSURE. Failure to take action to reduce oxygen exposure after a CNS/OTU warning is given can rapidly increase the risk of oxygen toxicity, injury, or death.

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