RDP question

[Fdge]

Doing my ow course, and reading my book, I came across some basic rules for the RDP. I say basic, since we now learn to use computers instead of tabels or wheels.
Luckily I've read some older PADI OW books and I have a basic understanding of them, but I have a question regarding one of those rules.

In my book it first say you pick the depth you are going to be diving or a depth somewhat deeper, to calculate the no-deco time limit, which sounds perfectly reasonable to me.
But the next rule says 'you pick the time you are going to be diving, or the next, somewhat longer time'

There's really no explanation to what do mean by this - do they mean you pick the time and then check what the depth limit is, or do they mean you pick the depth, and then check the time/somewhat longer time?
To me, the first options seems more reasonable diving conservative wise, but since there is no explanation, I'm not sure...
Any ideas? Wouldn't it make more sense to say 'you pick the time you're allowed according to the RDP, or the somehwat SHORTER time? Just to be sure?
Yes, I will see my instructor tonight but maybe there are people online willing to answer my question in the meantime

 

[DevonDiver]

Depth and time are the two factors that primarily dictate the rate at which your body absorbs nitrogen. The RDP utilises those two factors to assign a 'Pressure Group' which is a theoretical designation of the nitrogen absorbtion state in your body tissues.

When planning a scuba dive on an RDP, you need to be aware of two factors; the maximum depth that you will reach on the dive and the maximum bottom time (descent + time underwater up until the start of your direct ascent to the surface).

On occasions, there may be limitations upon your maximum depth and/or bottom time, because of the no-decompression limits imposed by the RDP. The RDP is calculated to ensure that your body is never nitrogen saturated to a degree where you couldn't ascent directly to the surface within a maximum ascent speed 18m/60ft per minute.

Your dive depth may be dictated by the actual depth of the dive site you are visiting (for instance, a specific ship wreck), or it may be possible to just 'pick' your maximum depth if the topography of the dive site consists of a shelving or sloping profile of increasing depth.

Dive time can be decided unilaterally (providing that it fits within RDP NDLs). However, in reality there are probably going to be other factors which will cause limits to the dive time. First and foremost will be your air consumption. There's no point planning a dive for 60 minutes, if your air will only last you for 20 minutes. Ask your instructor how to calculate your air consumption and include gas management with your dive planning - this isn't covered within the PADI Open Water course - but it should be!

Other factors that may determine time limits are water temperature (you might get cold), boat schedules, boredom thresholds or tidal patterns (avoiding diving in high current during tidal changes) etc etc etc. Also, if you will be doing subsequent dives that day, you may opt to reduce the dive length to control the residual nitrogen that might otherwise limit the duration/depth of those subsequent dives.

In reality, you will probably start your dive planning with a specific depth in mind. From there, simply track down the depth column on your RDP to gain an awareness of the maximum time allowed (that corresponds to the NDL). If you have a set duration of dive that you would like to do, just confirm that it is within the NDL.

Where possible, divers should add conservatism to their dive planning, so diving right up to the NDLs isn't regarded as 'best practice'. I, personally, like to keep at least 10 minutes away from any no-decompression limit when diving recreationally.

 

[knotical]

Nitrogen absorption increases with greater depth and with longer times.
During actual dives you can ensure conservatism by staying at a depth less than planned for a time less than planned.

When planning a dive, "rounding up" to the next greater depth and to the next greater time are both conservative.

The RDP is designed for you to first select a depth and then a time.

For example if you plan a dive to 55 feet for 22 minutes, on Table 1 enter the 60 foot column and go down to the first time equal to or greater than 22 minutes (23 minutes).

During the dive, you ensure conservatism in this case by going no deeper than 60 feet for a time no greater than 23 minutes (such as your original intent of 55 feet for 22 minutes).

 

[Fdge]

Thanks for the answers I 'kinda' understand the longer dive time, but not quite how that is more conservative than picking a somewhat shorter time.

I also understand all the side factors that have influence on your no-deco limit, eventhough not all of it is explained in the book.
I understand why gasmanagement isn't a part of OW, since we already have a lot on our minds with all this new stuff going on, but I find myself having these questions which the books don't answer, eventhough I want to be able to at least understand the basics of how it all works, so I agree on that it probably should be a part of the OW course. I know what SAC-rate is from reading up on these forums, but I don't know how to calculate it.

 

[Garrobo]

I use a computer. Lots easier and less prone to make any mistakes.

 

[currier]

Thanks for the answers I 'kinda' understand the longer dive time, but not quite how that is more conservative than picking a somewhat shorter time.

Choosing to dive to your desired depth for a [literal] shorter period of time is clearly 'more conservative' - but we dive because we want to be underwater for intervals as long as possible so long as the dives are safe and conform to recreational limits.

By choosing a longer time (rounding up), and diving your initial plan (a shorter period of time), you are crunching numbers representative of MORE theoretical nitrogen intake than that to which you are exposing your body.

Extra safety!

 

[DivemasterDennis]

Select your maximum depth. The RDP assumes your bottom time is the time at which you begin your decent until you Begin your ascent. I dive with computer, but I also calculate pressure groups for each dive, following the dive, and use the RDP. If you select a time, the RDP will tell you if you can do it. If not, you shorten your time ( or decrease your depth) in planning a dive. In calculating your ending pressure grroup after a dive, use your actual time. The RDP will then tell you a pressure group, and based upon your next planned dive, how long a surface interval you need. I believe EVERYONE should know how to use the RDP as a back up for the computer. Also, that familiarity will get you more knowledgeable about dive theory should you wish to go pro. We used to use "the Wheel" for multilevel daive- but you can do multilevels with the RDP too. Ask and instructor or divemaster who has been around 10 years or so to show you.
DivemasterDennis

 

[FuzzyNutz]

Perfect example of why they should teach both. Gotta learn addition before you can use a calculator.

 

[ScubaWulf]

When I took my OW course back in Feb I bought an RDP and brought it to class. Since I had it and asked about it the instructor took the time to explain the basics of using one. He actually seemed kind of glad to be teaching it.

 

[geogator]

I just completed my knowledge reviews this weekend & my instructor spent a good amount of time teaching us how to use the RDP. In addition to the problems that were on the knowledge reviews in the PADI book, he also had problems on the quizes and 5 or 6 problems on the final exam as well. Some were fairly easy such as just finding a pressure group or only planning a single dive but others were more complicated where we had to figure out the minimum surface interval time for 2 different dives along with their pressure groups. Although it took me a couple of minutes on the complicated ones, I got them correct but my classmate had some difficulty on a couple but the instructor was really good about going over them with him until he understood how solve the problems. The instructor told us that even though we'd probably be using a computer in the future when we dive, he was adamant about us learning the tables too as a backup. The hardest part was using the magnifying glass to read the dang card!!