Nitrox course. What's the point?

You can't select O2 for the surface. I have a Perdix also.

 

Yes. This is correct. Keep in mind that for a relatively mild EAN mix, the difference in residual N2 off-gassed during ascent is pretty small. However, during a surface interval, the difference is more significant.

In fact, I may be oversimplifying, but this is largely the issue with some early Uwatec Aladdin Air X dive computers that resulted in a lawsuit. The computers in question were manufactured around 1995, so the problem was fixed a long time ago. Anyway, the problem with the algorithm in those computers in Nitrox mode was that they assumed the surface interval was spent breathing the same mix as was used during the dive. On subsequent dives, this resulted in calculating NDL that was longer than it should have been.

 

I don't know how it is in Europe as a whole, but here in Sweden the amount of people who get their certs through clubs connected to CMAS (or other similar) are a drop in the ocean compared to the PADI/SSI ones.
Our club is one of the largest in the country and typically they only get 2 classes done per year, with 4-6 students per class.
To put that in perspective, the local dive shop certifies 4-8 students every other week for 2/3 of the year.
IMO, people interested in diving do not usually choose the cheapest option, they'll go for the option that will get it done the fastest, because most people don't have the patience to spend 1-2 months on a class because the instructor is volunteering his time and can't spend every weekend teaching for free.

 

I wrote about my wish that the computer could be set for the gas you are breathing ocne you are on the surface:

You responded that being on the surface does not matter--you are "'still diving' but on the surface."

I then asked how to tell my computer that I am breathing something other than air when I am on the surface, and you wrote:

OK, so if the computer doesn't care I am on the surface, how is it possible that it assumes I am breathing air and will not let me tell it anything different? You said it was still diving--I can change gases while I am diving.

 

I want to clarify the issue between NDL calculations and ascent rates. In my Excel spreadsheet I use the Schreiner equation to calculate tissue loading for 16 tissue compartments (TC) for each particular depth the user enters. (Another form of the equation which produces the exact results is called the Buhlmann equation.) The algorithm I use for calculating NDL uses the inert gas loading in the TC's to calculate a ceiling. The ceiling is the distance from the surface. For NDL dives the ceiling is negative. For deco dives the ceiling is positive (denoting a stop depth below the surface) and the ceiling for the surface is zero. The algorithm starts at 0 time and keeps adding one minute until the ceiling becomes 0. The total number of minutes accumulated when the ceiling reaches 0 is the NDL time remaining at that depth. For each iteration of increasing time a new TC pressure is calculated and then a new ceiling based on that TC pressure. The TC with the lowest ceiling (less negative for NDL or more positive for deco dives) becomes the controlling compartment.

Notice that in the above description there is no mention of ascent rate, time, or offgassing. The ascent offgassing is taken into account when an NDL is calculated for a newer shallower depth. At the new depth, newly calculated TC pressures are used to determine new ceilings which in turn lead to newly calculated NDL time. So, in effect ascent offgassing is taken into consideration for calculating NDL but the NDL calculation itself involves no ascent rates. I hope this explanation is clearer.

 

It assumes you are breathing air because that is what most if not all divers do when they reach the surface; they take their regulators out and breath air. From the standpoint of the calculations the computer doesn't care that you are underwater, on the surface, or 10,000 ft above sea level. But, it does "know" that the in-water dive has ended and you are on the surface, and it restricts your gas to air. (The Shearwater Perdix manual has some information on this.) I posted my guess why Shearwater doesn't allow another gas on the surface. You should ask them why?

 

I don't think anyone here has claimed that. PADI OW is roughly the same as CMAS 1* or BSAC OD. AFAIK neither of those classes teach staged deco. But unlike PADI, as you climb the ladder, you get to learn deco theory and practice and are certified for simple backgas deco without "going tech" with all those bells, whistles and woo. Both CMAS' and BSAC's equivalent to PADI DM (3* or DL, respectively) qualify you for simple deco dives.

 

One way to add a surface O2 feature would be with a time out. You could designate O2 for say 15 minutes but would have to reaffirm O2 use every 15 minutes when a timer went off. Avoids the left on O2 overnight issue without being too difficult for intentional surface O2 off-gassing. Available in some tech mode only.

 

Yes a timer would help. Or, set the computer for 100% O2, immerse it in 3 ft of water, then breath your O2. The computer will start a new dive but at least it will calculate new tissue loadings while you are on surface O2. The added pressure from the water shouldn't make any difference. I don't know how you could do that traveling in a car though. You could use a small pressurized container to house the computer small enough to take in the car.

 

While there is a benefit to breathing pure O2 at the surface, I can’t see a good reason for a DC manufacturer to add that as an option. Most divers just breathe atmospheric air at the surface. There really is no downside to breathing a higher O2 mix at the surface than the computer thinks. Yes, the NDL on the next dive will show lower, but that just increases safety margin. There is, however, a significant downside to breathing air when the computer thinks you are breathing a higher %.

If the extra time really makes a difference, the diver is being very aggressive, and probably heading for DCS anyway.