Attenuated (50%) RGBM

I'm happy to move this to the computer/gauges board if it is not appropriate here.

I've got a Suunto D9 computer. The D9 uses the Reduced Gradient Bubble Model (RGBM) algorithm. One of the settings for the computer is an "attenuated (50%) RGBM mode". The only thing the manual says about it is the following:

The D9 also allows experienced divers who are willing to accept a greater level of risk to adjust the RGBM model. The default setting is 100% which gives full RGBM effect. Suunto strongly advises to use full RGBM effect. Statistically very experienced divers have less incidents with DCI. The reason for this is unknown, but it is possible that some level of physiological and or psychological accommodation can take place when you are very experienced as a diver. Thus for certain divers and diving conditions you may want to set attenuated (50%) RGBM mode.

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That doesn't tell me much about what risk I'm taking on if I switch into attenuated mode. Does anyone know what the technical story here is?

(So far, I've turned on attenuated mode when I've gone diving in 80° water in the Caribbean, figuring I'm at lower risk then, but leave it full on when I do my usual cold water diving in the US.)

Any info appreciated to help me decide what I should do.

Hello resnick:

It took a bit to find an answer to this – and I am still not certain.

Modification

According to Dr Weinke’s book, there are two ways to modify the output. For some dive computers, the M-values are changed ,i.e., the gradient is increased.

For other computers, the properties of the bubbles are also modified. The equation of state [pressure volume relationship] of the membrane at the bubble surface is changed. All is without testing.:11:

How Much?

This modification is, in a sense, a do-it-yourself table. People who dive with tables are always [were always] able to modify the dive conditions; they can add or subtract bottom time as they wish. It was taught that you should read the table with your dive rounded down to the next deepest depth and the next longest interval.

Depending on your nerve, you could refrain from this practice. The same is true with the dive-fly interval. Some people are “wet hair divers.” If the plane needed to make a forced landing for medical reasons, someone might check and that diver receive a bill from the airline. Ouch!

Dr Deco :doctor:

Dr Deco:

According to Dr Weinke’s book, there are two ways to modify the output. For some dive computers, the M-values are changed ,i.e., the gradient is increased.

For other computers, the properties of the bubbles are also modified. The equation of state [pressure volume relationship] of the membrane at the bubble surface is changed.

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So it sounds like the short answer is, "It depends on the computer what they mean by '50% effects'". :ermm:

I'll see if the folks over on the computer board have any more details specific to the D9.

As you know Suunto computers track a number of factors in addition to how long you are underwater. When you exit the water you will notice the diver alert symbol, an exclamation point (!) inside a triangle. This means that the computer strongly recommends that you extend your surface interval until the symbol disappears. In most cases the recommended surface interval is around 45 to 75 minutes, depending on depth and time. Should you do anything "naughty" during your dive the recommended surface interval gets longer.

Naughty includes: going faster than 10m/min (getting a SLOW warning and a mandatory safety stop), reverse profile (current dive deeper than the last), decompression dives, insufficient surface interval, etc. All of these things will cause the diver alert symbol to stay on longer.

If you re-enter the water while the diver alert symbol is active the computer will penalise you and reduce your no decompression times. This is why many people think suunto computers are very conservative. The reason is that the computer thinks micro-bubbles may still be present and extra caution should be taken.

Choosing the 50% RGMB model means that the computer only applies HALF of the penalty to subsequent dives, giving you slightly longer bottom times, should happen to do something naughty (see above). Please note that in 50% RGMB mode the diver alert symbol stays on all the time as it is considered less conservative.

Regards,
David

sucnip:

As you know Suunto computers
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Regards,
David

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Nice to see that my work is appreciated. A link to the source, which is my page at atlas.hasselbalch.com/suunto-rgbm.html, would have been good form, though.