PADI wheel vs PADI RDP

[Eileentk]

I did a search for this topic but did not find an answer to my question, so here goes. If you have to solve a minimum surface interval problem where the depth of both dives is given in 10 ft increments (60 ft or 50 ft versus 65 ft or 55 ft); will you get the same answer no matter whether you use the wheel or the table? I came across a problem in my practice theory exam while getting ready for my IDC where the instructions said to use the wheel to solve the problem. Just out of curiousity, I worked the problem with both the wheel and the table and came up with the same answer (but got there quicker with the table). I was discussing this with a couple of instructor friends and asked if I would always get the same answer from wheel or table as long as the depths were in 10 ft vs 5 ft increments. One of them told me no and the other wasn't sure. Any opinions would be welcome but please remember that this is purely a test type question (I use redundant computers when I dive and always go with the more conservative one).

 

[MikeFerrara]

I can't work any examples because I loaned out all my weels but you won't always get the exact same answer as on the table.

Also keep in mind the "calibration" of the weel. I've seen borderline examples where you might not even get the exact same answer on two different weels.

 

[The Kraken]

Based upon my experience, one will get the same answer with both the RDP and "The Wheel" if using 10' dive increments.

My only problem with the wheel is that I occasionally get a different pressure group than the given answer for a problem.

I think this has to do with the "calibration" as Mike mentioned previously.

I suppose there could also be some variations in the printing registrations on differing wheels.

I wish the "line" for determining the pressure group on "The Wheel" was a bit more definitive.

the K

 

[diverdown247]

By design, the RDP is more conservative. Utilizing 10' increments, you should remain relatively the same on everything but the pressure group. If you look, your surface interval may also dictate an even larger difference in pressure group, provided you use the exact same numbers to reach that point for the Wheel and the RDP.

The wheel was designed for more advanced divers...people on their way to Divemaster, Assistant Instructor, and Instructor. This is why you should understand that with every advanced level..be it RDP, Navy Tables, Wheel, Radar enhanced RDP, or dive computer there will be differences in all of them due to the conservatability of the lower levels.

Don't be surprised if the question comes up in your IDC about the difference in the RDP and the Wheel.

 

[SteveAD]

The wheel and the table both us the same "algorithm" just a different "interface" because of the "calibration issue" of the wheel there is a potential for vaiation between the two which will increase with the number of steps in the calculation. In theory the two should always give the same answer, in practice there are occasional differences. during the IE, all wheel questions are subject to review. if on your test, your answer doesn't agree with the answer key, show the proctor how you got your answer with your wheel, if you procedure is correct and your results repeatable, it is marked right.

 

[riguerin]

Another source of possible variation with the Wheel is the human element. That is, since the Wheel is basically an analog calculator, there is an element of subjectiveness introduced by the user. For example, you and I could both be looking at the same exact device and in some cases may reach different conclusions regarding which an arrow is before, touching, or crossed a line. It's possible to differ by a whole pressure, especially on the steepest curve segments.

As others have stated, the Wheel and RDP should be based on the same algorithm. Therefore, you should get the same answers for square profiles, so long as you are consistent with depths. As you probably know, some advantages of the Wheel over the RDP are the finer granularity (5ft depth increments), the elimination of arithmetic calculations, and the ability to plan multi-level dives.

Good luck with your IDC.

 

[wilkie]

The wheel and the table both us the same "algorithm" just a different "interface" because of the "calibration issue" of the wheel there is a potential for vaiation between the two which will increase with the number of steps in the calculation. In theory the two should always give the same answer, in practice there are occasional differences. during the IE, all wheel questions are subject to review. if on your test, your answer doesn't agree with the answer key, show the proctor how you got your answer with your wheel, if you procedure is correct and your results repeatable, it is marked right.

I have a wheel with a "calibration issue", used it during a multilevel course. My answers often varried from the "right" answers, after the review process discribed above, I was graded 100% for (as I recall) 25% incorrect answers! (circa 1991)
????? My instructor called PADI. To make a long story short, the answer was essentially, "Don't worry it's close enough!" (no exchange, no refund)
So I think to myself....$65. wheel, $75. class= 100% qualified to plan and make multilevel dives with a tool that delivers precise calculations about 75% of the time COOL!!!:light:
Like I said, that was 1991, I sincerely hope that the "calibration issue" has been resolved, or at least minimised in the years passed. I would also say, in all fairness, that my wheel is a bit more consertive than most computers that I have experence with, however this issue does not inspire comfidence in the wheel.
I'm not even going to mention the well known "snotty" nickname...:mooner:

 

[Charlie99]

The PADI RDP and the Wheel are both based upon the same deco model, a multicompartment aka neo-Haldanian model using the DSAT M-values. So if you restrict the profile to 10' depth increments, the RDP and the Wheel should give identical answers.

I used a spreadsheet to generate a table essentially equivalent to the PADI RDP (values differ +/1 minute due to roundoff errors), but with 5' increments. I also added to the table the multilevel NDLs (ML numbers) and the rules for allowable levels in multilevel dives. The table is in my photo gallery.

Another interesting feature of the wheel is that a line of the wheel does all of the SI calculations. Looking closer at the RDP, it turns out that all of the SI table could be replaced by just the row that goes Z down to A.

 

[Eileentk]

Thanks all. I appreciate the opinions. I was out at the quarry all day today, helping with dive classes and I asked this question of several more instructors. I received a variety of answers ranging from "they should give the same answer as long as you use 10 ft increments" to "don't even try to answer a wheel question using the table during your IE, because you will be wrong". So I guess that I'll just have to find a source of "answered" wheel questions (to remove the uncertainty factor of "is it really piercing or is it actually just touching) and run the comparative calculations with the table. If nothing else, I'll get more practice with the wheel. A special thanks to Stevead for the clarification regarding wheel questions in the IE. My IDC starts July 2 with the IE scheduled for July 15,16. Wish me luck.

 

[texdiveguy]

I got a PADI wheel I would love to unload---I mean sell to someone!