RonR
Contributor
A few observations:
Because to the relative conservatism of Suunto computers, many people have the impression that the RGBM algorithm is inherently conservative- it isn't inherently anything, RGBM gives computer designers the ability to tune the conservatism to match statistical risk factors derived from clinical experience. Suunto chose to be fairly conservative, but that has little to do with RGBM. It's a choice made about how RGBM was implemented on a particular platform. Our Cobalt, which also uses RGBM, is very much in the middle overall for conservatism.
A dive computer is much more than a timer with alarms. Bubble models like RGBM model the physics of bubbles and attempt to give you profiles that minimize the potential for bubble formation. That’s complex. That may mean that they penalize multi day or repetitive diving more than other models. This does represent a physical reality: that such repetitive diving carries higher risks of DCS. Specific implementations, like Suunto’s, may penalize certain sorts of violations, such as ascent rates, more than others. This is indeed a proprietary aspect of these computers, but it is related in a logical, mathematical way to the algorithm. It's not something you can tweak without access to the handles in the algorithm.
There is also much more going on with any algorithm than just the NDL’s. You cannot adjust the FO2 and expect the results to map in any predictable way to what other computers would give for a variety of profiles, particularly over multiple days. It’s not like changing the settings on a thermostat. If you want your computer to match other divers, and you can’t get there through the built in conservatism settings on yours, you should use a computer that is the same as the one you want to match. But don’t try to game the system- if you do, and if at some point you need to press limits in the middle of your trip, or end up on a deeper dive, it won’t be giving you useful information.
I’ve made some of the points about conservatism below in previous posts, but will copy here.
Conservatism in dive computers is hard to nail down, but differences between computers tend to seem more extreme in shallower, longer dives, because we treat deco/ no deco as a binary function when in reality is is a gradually increasing slope- very gradually increasing at shallow depths. So on shallower dives even a slight divergence in the algorithm conservatism can translate into many minutes more or less of no-deco time. The same computers might show much less difference- in minutes of no-deco time- at deeper depths. That probably accounts for some of the subjective differences in experience of how conservative a particular computer is.
Computers that show only a minute or two difference on a deep dive might diverge by many minutes on a shallow one. What you don't see is that one is "almost" in deco, and the other is "barely" in deco. They may not be, in mathematical terms, very far apart at all, but because they treat deco/ no-deco and a binary switch, one seems much more conservative. If it were displayed in analog terms, as say a % of risk, they might not seem very different.
Metaphorically, a lot of divers see no-stop limits as falling off a cliff. But it's a lot more like hikers climbing a very gradually increasing slope, and deciding at what % grade to turn back. If one person decides to turn back at 30% and another at 32% grade, and they both head straight uphill (analogous to a deep dive), they will turn around at almost the same time. If they traverse the slope at a very shallow angle (analogous to a shallow dive), the 30% hiker may turn around long before the 32% hiker. His turnaround point is no more conservative than it was going straight uphill, his risk no greater, but the difference in minutes between the two is greater because of the angle at which they approached the hill.
Erik Baker observed that "all decompression algorithms are attempts to draw a bright, clear line through a fuzzy gray area". Something to bear in mind. Your question comes up regularly, and probably some reading on decompression would help make the case that just tweaking FO2 doesn't get you where you want to go.
Ron
Because to the relative conservatism of Suunto computers, many people have the impression that the RGBM algorithm is inherently conservative- it isn't inherently anything, RGBM gives computer designers the ability to tune the conservatism to match statistical risk factors derived from clinical experience. Suunto chose to be fairly conservative, but that has little to do with RGBM. It's a choice made about how RGBM was implemented on a particular platform. Our Cobalt, which also uses RGBM, is very much in the middle overall for conservatism.
A dive computer is much more than a timer with alarms. Bubble models like RGBM model the physics of bubbles and attempt to give you profiles that minimize the potential for bubble formation. That’s complex. That may mean that they penalize multi day or repetitive diving more than other models. This does represent a physical reality: that such repetitive diving carries higher risks of DCS. Specific implementations, like Suunto’s, may penalize certain sorts of violations, such as ascent rates, more than others. This is indeed a proprietary aspect of these computers, but it is related in a logical, mathematical way to the algorithm. It's not something you can tweak without access to the handles in the algorithm.
There is also much more going on with any algorithm than just the NDL’s. You cannot adjust the FO2 and expect the results to map in any predictable way to what other computers would give for a variety of profiles, particularly over multiple days. It’s not like changing the settings on a thermostat. If you want your computer to match other divers, and you can’t get there through the built in conservatism settings on yours, you should use a computer that is the same as the one you want to match. But don’t try to game the system- if you do, and if at some point you need to press limits in the middle of your trip, or end up on a deeper dive, it won’t be giving you useful information.
I’ve made some of the points about conservatism below in previous posts, but will copy here.
Conservatism in dive computers is hard to nail down, but differences between computers tend to seem more extreme in shallower, longer dives, because we treat deco/ no deco as a binary function when in reality is is a gradually increasing slope- very gradually increasing at shallow depths. So on shallower dives even a slight divergence in the algorithm conservatism can translate into many minutes more or less of no-deco time. The same computers might show much less difference- in minutes of no-deco time- at deeper depths. That probably accounts for some of the subjective differences in experience of how conservative a particular computer is.
Computers that show only a minute or two difference on a deep dive might diverge by many minutes on a shallow one. What you don't see is that one is "almost" in deco, and the other is "barely" in deco. They may not be, in mathematical terms, very far apart at all, but because they treat deco/ no-deco and a binary switch, one seems much more conservative. If it were displayed in analog terms, as say a % of risk, they might not seem very different.
Metaphorically, a lot of divers see no-stop limits as falling off a cliff. But it's a lot more like hikers climbing a very gradually increasing slope, and deciding at what % grade to turn back. If one person decides to turn back at 30% and another at 32% grade, and they both head straight uphill (analogous to a deep dive), they will turn around at almost the same time. If they traverse the slope at a very shallow angle (analogous to a shallow dive), the 30% hiker may turn around long before the 32% hiker. His turnaround point is no more conservative than it was going straight uphill, his risk no greater, but the difference in minutes between the two is greater because of the angle at which they approached the hill.
Erik Baker observed that "all decompression algorithms are attempts to draw a bright, clear line through a fuzzy gray area". Something to bear in mind. Your question comes up regularly, and probably some reading on decompression would help make the case that just tweaking FO2 doesn't get you where you want to go.
Ron
