Dive computer compared to RDP

How conservative are dive computers.

Have had this discussion a few times and have read articles here, with statements in regard to some computers are too critical when it comes to being conservative. I compared my Suunto Cobra 2 by running it in dive plan mode to the PADI RDP and here’s what I found.

Computer settings, RGBM 100 % - Deep stops set to off - Computer mode Dive Air (21%) Desat, Time of 0.00


Suggested NDL
Depth --------- Suunto Cobra 2 (min) ------ RDP (min)
50’ ----------- 69 ---------------------- 80
60’ ---------- 50 ---------------------- 55
70’ ---------- 36 ---------------------- 40
80’ ---------- 28 ---------------------- 30
90’ ---------- 22 ---------------------- 20
100’ --------- 17 ---------------------- 16
110’ ---------- 13 ----------------- ----- 13

What I can gather and please someone correct me if my assumptions are wrong. It seems the differences between Suunto and the DRP in allowable dive time would almost equal the 3-5 minute safety stops at 15 feet being Suunto has subtracted the allowable safety stop time from the dive time, thus increasing the safety stops to almost double the time required compared to the RDP. Yet when you get to the deeper depths of 90 – 110 they compare the same allowable time at depth. It almost seems the computer is more conservative at shallower depths than deeper. Could this be because the average depth most recreational diver’s dives are at these depths? Could anyone explain this, am I reading this wrong. I would greatly appreciate any information or facts relating to this. I am just trying to better understand my dive computer.

The biggest difference is that the RDP assumes that the dive is to the deepest level for the entire dive (a square profile) & it does not accompany you on the dive, nor does it keep track of time & depth. The computer in planning mode is pretty much the same, but once doing the dive, the computer will continuously monitor depth & time, an RDP can not do that. With this constant information it is constantly updating itself, giving you credit to your time as you ascend, this extending your bottom time & taking away time as you descend, thus shortening it if you decide to stay deep. Computers & the RDP are generally based on one of a few different algorithms of tissue nitrogen loading with different conservative settings. Some computers will even let you set the conservatism from bare bones to being VERY conservative. Although I do not have one, Suuntos tend to be rather conservative, but, like I said above, there are models in which the conservatism can be set by the diver. How the times are set depends greatly on the algorithms used in either the RDP or the computer.

tstormdiver:

The biggest difference is that the RDP assumes that the dive is to the deepest level for the entire dive (a square profile) & it does not accompany you on the dive, nor does it keep track of time & depth. The computer in planning mode is pretty much the same, but once doing the dive, the computer will continuously monitor depth & time, an RDP can not do that. With this constant information it is constantly updating itself, giving you credit to your time as you ascend, this extending your bottom time & taking away time as you descend, thus shortening it if you decide to stay deep. Computers & the RDP are generally based on one of a few different algorithms of tissue nitrogen loading with different conservative settings. Some computers will even let you set the conservatism from bare bones to being VERY conservative. Although I do not have one, Suuntos tend to be rather conservative, but, like I said above, there are models in which the conservatism can be set by the diver. How the times are set depends greatly on the algorithms used in either the RDP or the computer.

Click to expand...

Thanks
I understand all RPD allowable time at depth assumes your staying at that exact depth. Perhaps thats why the difference are not as much for deeper depths between the two planners being the computer and RDP. Thanks for the feed back.

I don´t know anything about dive computers algorithms but, is it possible that Suunto use this depth as an average depth and not the max depth as on the RDP?

If you have a dive computer that uses the same deco model as the PADI RDP, then the 1st dive NDLs will be very close to the PADI RDP numbers. This is true for the various Pelagic (Oceanic, Aeris, Sherwood, ...) computers that use the PADI/DSAT deco model.

The Suunto Cobra 2 uses a model which is essentially the same as the DSAT model used in the PADI RDP, but with different values for the limits. The RGBM elements are added ad-hoc by adjusting these limits downward for repetitive dives based upon the previous dive profile.

The NDLs for deeper dives are determined by the limits ( M-values) of the faster compartments. THe NDLs for shallow dives are determined by the m-values of the slower compartments.

It is clear from the NDLs that you posted that the medium speed compartments (around 40 minute halftime) have lower limits in the Suunto model than does the DSAT model. That's why the Suunto has shorter NDL limit for 50'. OTOH, the limits for the 5 to 20 minute compartments are similar for both models, so the NDLs for deeper dives are similar. As a rule of thumb, the controlling compartment -- the one that reaches its limit first -- will generally have a halftime around 1/2 or 2/3 of the NDL value.

Unfortunately, the Suunto RGBM model is poorly documented, with little available public information. The compartment halftime and limits of the PADI/DSAT model, as well as other models such as the Buhlmann ZHL-16 and USN/Workmann are readily available public info. A search for Erik Baker's article "Understanding M-Values" will pop up that info.

The fact that a computer will calculate multilevel dives is not relevant to this discussion, since the NDL numbers the Suunto displays in the planning mode are for square profiles.

Thanks Charlie99

Explains why I can't find any documentation on this from Suunto or the designers in regards to the calculations made in the suunto program. Very informative information you've provided and once again thank you for helping me understand and where to direct my studies further.

Dive Safe!

Rob