Suunto Fused RGBM 2 vs Bühlmann 16 GF: Which Algorithm to Use

Which Suunto computers have Fused RGBM 2, Bühlmann 16 GF or both?

It depends on the model. The D5 runs only Fused RGBM 2. The Ocean, Nautic and Nautic S run only Bühlmann 16 GF. The EON Core, EON Steel and EON Steel Black can run either, with two catches: the EON Core got Bühlmann only with software 4.0 in December 2024, and the EON Steel needs an upgrade at a service centre, paid for units older than two years. The Zoop Novo uses the older Suunto RGBM and has no choice at all.

Both options are Suunto's own code. Fused RGBM 2 comes from the bubble models Suunto developed with Dr. Bruce Wienke; Suunto says it moved Fused RGBM in a less conservative direction after customer feedback and introduced the result on the D5. Bühlmann 16 GF is Suunto's implementation of ZHL-16C with gradient factors. How the two model families differ in theory is covered in dive computer algorithms explained. This guide stays with the Suunto menus and what each choice does to your dives.

How to switch between Fused RGBM 2 and Bühlmann on an EON

Hold the middle button to open the main menu, then go to Dive settings > Parameters > Algorithm and pick one. The computer restarts to apply the change. You can also set the algorithm per dive mode in the Suunto app: its dive mode customizer offers personal levels for RGBM 2 and GF Low and GF High for Bühlmann, and software 4.0 added Bühlmann support to the customizer on the EON Core.

An EON Steel without the option needs the upgrade first, and you cannot install it yourself. You book it through Suunto's Online Service Request form, choose Algorithm update, and the unit gets a free maintenance check on the way. Suunto lists the price as 80 € or the local equivalent for computers bought two or more years ago; younger units get it under warranty terms.

After the restart, check these before the next dive:

• The conservatism control of the algorithm you are now on. The personal setting belongs to Fused RGBM 2 and gradient factors are used only with Bühlmann, so the other one does nothing.
• Altitude, under Dive settings > Parameters > Altitude. It has three ranges up to 3,000 m (9,800 ft) and is set separately from the algorithm.
• Deco profile, stepped or continuous. Since software 4.0 both algorithms offer both.
• The dive planner, because the no-stop times change with the algorithm.

What each setting changes on a Suunto

The part you handle is the conservatism control. Fused RGBM 2 has a five-step personal setting: +2 and +1 are conservative, 0 is the default for ideal conditions, and -1 and -2 are aggressive. Suunto tells you to move toward +1 or +2 when risk factors apply, such as water colder than 20 °C (68 °F), age over 50, fatigue or a patent foramen ovale.

Bühlmann 16 GF uses two numbers instead. On the EON Core and EON Steel Black the default is 30/70, and Suunto calls every other value out of recommendation: change it and the number turns red with a warning on screen. The Ocean and Nautic start at 40/85 and offer 45/95 and 35/75 as presets, so the same brand ships two different Bühlmann defaults.

Underneath, the models track different things. RGBM 2 uses 15 tissue compartments with nitrogen half-times of 1-720 minutes and reduced, variable M-values that depend on your diving habit and on violations, tracked for up to 100 hours after a dive. Suunto says its RGBM family was built to handle multi-day diving, closely spaced repetitive dives, a dive deeper than the one before and fast ascents. The Bühlmann model has 16 compartments of 4-635 minutes and assumes all gas stays dissolved, so on that setting your margin comes from the gradient factors you chose.

The EON guide also describes deep stops, on by default for dives deeper than 20 m (66 ft) and prompted halfway up from your maximum depth. They cannot be switched off when helium is enabled. The guide does not say whether they also run in Bühlmann mode, where GF Low already sets the depth of the first stop.

No-stop times: Fused RGBM 2 vs Suunto's Bühlmann presets

Suunto does not publish a no-decompression table for Fused RGBM 2. None of the current guides we read (D5, EON Core, EON Steel, EON Steel Black) prints one, and the support pages describe RGBM 2 only in words: for recreational divers it 'may offer slightly longer no-deco times', depending on the personal setting, without saying longer than what. So the RGBM column below holds the one number Suunto prints, and we do not estimate the rest.

That number is the dive planner example in the D5 guide, repeated in the EON Core, EON Steel and EON Steel Black guides: 18 m on air, first dive of a series, 51 minutes of no-decompression time. None of them states the personal setting behind it.

The Bühlmann columns come from the same code as our NDL calculator: ZHL-16C with an 18 m/min descent counted as bottom time and a direct ascent at 9 m/min, with only the gradient factors changed to Suunto's documented values. Suunto's own code will not match to the minute.

At 18 m the EON default of 30/70 allows 33 minutes. That is 14 fewer than the Ocean's 40/85 and 18 fewer than the D5 example. If that example is at personal setting 0, moving an EON from RGBM 2 to Bühlmann without touching the gradient factors costs about a third of your first-dive no-stop time at that depth.

Which one should you choose?

Keep the default your model ships with unless one of these applies:

• Your buddy dives a Shearwater or a Garmin. Both run Bühlmann ZHL-16C with gradient factors, and Shearwater's default is Med, 40/85. Set the EON to Bühlmann 16 GF at the same numbers and both computers start from the same model and margin, though not the same code. Left at 30/70, you reach your no-stop limit first: 33 against 47 minutes at 18 m in our model. The red numbers on the EON are Suunto's warning, not a fault.
• You are training for decompression or technical diving. Use the algorithm and settings your course uses. If your instructor plans with gradient factors, Bühlmann lets the computer and the plan share a model. For helium, RGBM 2 on the EON supports trimix and closed circuit to 150 m; the D5 has no trimix or rebreather support, and the Ocean takes air and nitrox up to NX99 with a 60 m depth limit.
• You dive on holiday, several dives a day for a week. Fused RGBM 2 adjusts for repetitive and multi-day series by itself. On Bühlmann, pick gradient factors that leave room for the whole week, because Suunto describes no such adjustment for that model.
• You own a D5, Ocean or Nautic. There is nothing to choose, so spend the decision on the conservatism level, compared across brands in dive computer conservatism settings.

What not to do: switching mid-trip and other shortcuts

Do not switch algorithms in the middle of a dive series. Suunto's guide says changing between dives is possible but not recommended while the no-fly time is counting down. DAN's guide to dive computers puts the principle more strongly: a decompression model is invalid unless you start using it when you are clean. Suunto does not say how residual gas from one model carries into the other, so switch at home, after the no-fly time has ended. Buddies on different models should agree before the dive to follow the more conservative computer; DAN warns that each diver following a very different computer can break down the buddy system.

Do not use a gauge dive to get around a setting. After a dive in Gauge mode the EON locks decompression calculation for 48 hours, and the lock goes back to 48 hours if you dive again while locked.

Do not reset tissues to clear a lock or a long no-fly time. The EON Steel Black has a Reset tissues option, but Suunto's warning limits it to a computer used by someone who has not dived in the past two days or longer, and the reset cannot be undone.

Do not read Bühlmann's missing lock as permission. On RGBM 2, staying more than 0.6 m (2 ft) above the ceiling for three minutes locks the algorithm for 48 hours. On Bühlmann 16 GF the EON keeps showing the original plan with a Ceiling broken warning, marks the dive and the next one in the log, and the planner warns that the violation affects its accuracy. The Bühlmann-only Ocean does not lock either. To plan the stops of a decompression dive in advance, use our scuba deco planner.

Keep the same personal and altitude settings for planning and diving. Suunto warns that raising them after you plan lengthens decompression and the gas you need, and you can run out of gas underwater.