RDP vs eRDPml vs Dive Computer

[Matt Beckwith]

During my recent Advanced Open Water certification, we did a multi-level dive, and charted it using all 3 devices: RDP, eRDPml, and dive computer.

• According to the RDP, I should have been dead.
• The eRDPml permitted the dive, made a lot of sense to me, giving me dive groups at each stage.
• The dive computer permitted the dive, but gave no dive groups.

So I liked the eRDPml best, but I can see that it's not as accurate as the dive computer in figuring out your precise no-decompression limit.

How do you log your dive if you used a dive computer? Do you just leave off the dive groups from the log book?

Should I purchase a dive computer?

• I'll probably be going diving about 4 times a year.
• If the dives are those tour dives at resorts, they figure out that they're safe ahead of time, and there's no particular risk of exceeding NDL's.
• I can see that the deeper the dive, the more advantageous the dive computer is, since the RDP won't give you as much bottom time as the computer will (assuming you don't just dive straight down, stay at the same depth the whole dive, and ascend straight up).
• But some of the resorts give you dive computers attached to the regulator, or they can be rented.

So I'm just not sure if it's worth the expense.

[Dirty-Dog]

The RDP provides the largest margin of conservatism, due to the assumption that your dive profile will be square. The ML planner makes the same assumption, but over shorter periods of time. The computer provides real time calculations. Pressure groups, in and of themselves, don't mean anything. They are just a way for you to calculate your NDL on your next dive. Your next NDL is the important number. Computers generally have a planning mode which will show you your current NDL.

There's no need to log pressure groups, unless you're going to need that number to plan your next dive.

Purchasing is a personal choice. Personally, I prefer my own gear. It's easier to operate than rental simply because I use it all the time. I dive computers because I don't see any reason to get back on the boat with 2000PSI left simply because of the square profile assumption.

To me, the most important thing in your post is this line:

"If the dives are those tour dives at resorts, they figure out that they're safe ahead of time, and there's no particular risk of exceeding NDL's."

This frightens me. YOU are responsible for your safety, not the dive op. The dive op doesn't know how many dives you did in the preceeding day, nor what your profiles were. To me, people who rely on others for their safety are one of the most frightening things about diving.

[boulderjohn]

To me, the most important thing in your post is this line:

"If the dives are those tour dives at resorts, they figure out that they're safe ahead of time, and there's no particular risk of exceeding NDL's."

This frightens me. YOU are responsible for your safety, not the dive op. The dive op doesn't know how many dives you did in the preceeding day, nor what your profiles were. To me, people who rely on others for their safety are one of the most frightening things about diving.

Of all the statements in this fine post, this is the one that deserves to be emphasized the most. In some of my most recent OW classes, students have told me that friends who are already certified have told them that they don't need to pay much attention to the dive planning and measuring aspects of the class because the DMs take care of that for you. That is a very dangerous myth, first of all for the reasons stated above. I have also been in situations where the DM frankly led an unsafe profile and I had to have both the knowledge and the courage to tell him that I was not going to follow him any more.

The other factor is that in SOME resorts a DM will plan the dive for you. In MANY others, that will not happen, and you will be totally on your own to plan your dive and dive your plan.

[RTee]

If you are going to dive regularly, a basic computer with Nitrox capability, should you wish to eventually dive nitrox, would be a prudent investment. As to relying on others for your own safety, to me this falls under the trust me dives category that could be very dangerous for a couple of reasons. One, you do not necessarily dive the exact same profile as the DM, you might not necessarily be breathing the same gas and last but not least, your nitrogen loading might end up being very different due to the accumulated number of dives that you may have compare to the DM. As an example, during my last trip south, I did four consecutive days of four dives a day and I occasionally had a different DM in the afternoon. In some cases, that DM was doing his first dive of the day while this may have been my third...therefore, relying on his or her NDL for my safety would have been asking for trouble.

In less than a week I will be leaving for Bonaire with a small group of divers where we will be doing multiple dives a day, not necessarily with same dive buddy and doing so for two weeks. Unless diving square profile, a dive computer becomes a must.

[DivemasterDennis]

In direct response to Matt's post: 1. Yes, get a computer. I always dive with a computer for continuous information and maximizing dive time at depth 2. In logging dives I use the RDP or a neat little multilevel device we used to teach in AOW called "The Wheel." The Wheel allows smaller depth variations and multilevel calculations on a single dive, as is a very useful little device that took a little time to master but never requires a battery.
Responsive to Boulder John, I could not agree more that each diver to be truly competent needs to know how to plan and execute a safe dive. While resorts are not likely to dive questionable profiles, it happens. If you are on such a dive, you can adjust depth or "go home" based upon the information presented.
Last thought- whether you calculate a pressure group or not, log you dives. I always make note ofhhte multilevel aspect of dives, and log as much other info as possible for future reference.
DivemasterDennis

[Matt Beckwith]

"If the dives are those tour dives at resorts, they figure out that they're safe ahead of time, and there's no particular risk of exceeding NDL's." This frightens me. YOU are responsible for your safety, not the dive op. The dive op doesn't know how many dives you did in the preceeding day, nor what your profiles were. To me, people who rely on others for their safety are one of the most frightening things about diving.

I don't think it frightens you. I think it excites you to be able to point your finger at someone.

All I meant is that, when the resort takes a group out for a 45-minute dive to 50 feet, followed by a 45-minute surface interval, and a 45-minute dive to 30 feet, they've calculated that this sequence is not going to exceed the NDL's.

Of course I take responsibility for my own safety. I'm religious about planning my dives, recording my dives, keeping track of pressure groups, etc.

As a beginning diver, however, I have noticed that nobody has ever asked me to verify that what we're about to do is safe. The instructor takes me out for my open-water certification dive, or my advanced-open-water certification dive, and doesn't go over the planned depths and whether they're safe. Since it's an instructor taking me out, I assume he knows what he's doing.

And when I was at Sunset House on Grand Cayman, and they took a group of 10 people out for 2 boat dives, they told us the depths and times. I saw nobody on the boat calculating whether they were safe depths and times. They just did the dives. After the dives, some of them filled out their log books (me included). The instructors did keep close contact with us, however, and made sure that we did our 5-meter 3-minute rest stops.

So I think that there is a danger of assuming that the people taking you on the dive are going to make sure it's safe, when it might not be. That point is well taken. I think from now on I'll get out my RDP before each dive instead of after, even when it's an instructor planning the dive.

[the wart]

How do you log your dive if you used a dive computer? Do you just leave off the dive groups from the log book?

Normally you just write COMP in/ around the pressure group box. Normal procedure is to have a minimum 1 hr surface interval as well.

Should I purchase a dive computer?

• I'll probably be going diving about 4 times a year.

Is this 4 dives or 4 dive trips?

For 4 dives, no it's really not worth it. Computers have this lovely habit of losing battery power when left in the drawer for 6 months, but oddly will function continuously diving for a year or so. Never understood that one.
4 dive trips, different story. Not only do you have piece of mid, but an electronic record of your dives that you can download to your PC.

Should I purchase a dive computer?

• If the dives are those tour dives at resorts, they figure out that they're safe ahead of time, and there's no particular risk of exceeding NDL's.

Ah, they really don't. Fortunately the guy/girl leading the dive is probably doing 2 or 3 dives a day so will monitor to make sure they don't have a problem. Chances are you'll be diving less than them so you'll be ok. But I did say Chances, you're playing the odds.

I've worked at 2 shops that planned the dives out. One assumed that the the 2nd dive of the day was a repetitive one for everybody and adjusted NDL's, the other didn't, they assumed the 2nd dive of the day could be your first. Even though it was the DM's 2nd???

• I can see that the deeper the dive, the more advantageous the dive computer is, since the RDP won't give you as much bottom time as the computer will (assuming you don't just dive straight down, stay at the same depth the whole dive, and ascend straight up).
• But some of the resorts give you dive computers attached to the regulator, or they can be rented.

So I'm just not sure if it's worth the expense.

Resorts are good for Mask, Regs, BCD's, Flippers and Wetsuits pretty much all the time. Computers not so much. Don't rely on one being there. Even if they are, don't expect them to be in large quantities.
Again for 2nd dive resort I mentioned; I rented out my own spare computer to customers as the dive shop didn't have them. Nice for me!

[DevonDiver]

Planning a dive on RDP or eRDPml and then following another diver (using a computer) beyond those limits is nonsensical. It goes back to the most basic of advice; "plan your dive and dive your plan".

If you can't do that, because your plan conflicts with the rest of a computer-using group, then you need to get a computer. Ignoring your plan is not any form of responsible compromise.

As others have stated, following a DM and trusting to their dive plan conflicts with everything taught on your scuba course. Some issues to consider:

1) What if you have a different DM for subsequent dives?
2) What if you lose your DM during the dive?
3) What if your DM is irresponsible and unprofessional? (there's more than a few accounts of that here on Scubaboard).

[Matt Beckwith]

Yes, get a computer. I always dive with a computer for continuous information and maximizing dive time at depth.

I looked at a computer at a local dive shop that's only about $265. Is there a reason to get a fancier computer?

I don't care for the displays of any of the dive computers I've looked at, they're all dark, difficult to read. The only one I've seen that's really readable is bright, multi-colored, and costs $1800. That's not in my budget.

In logging dives I use the RDP or a neat little multilevel device we used to teach in AOW called "The Wheel." The Wheel allows smaller depth variations and multilevel calculations on a single dive, as is a very useful little device that took a little time to master but never requires a battery.

Should I get one of these, or if I have a computer do I really need it? Also, is this wheel equivalent to an eRDPml? I kind of liked that, actually.

It seems to me that a computer should tell you how long you were at each depth. That is, it should spit out a profile after each dive.

[DevonDiver]

I looked at a computer at a local dive shop that's only about $265. Is there a reason to get a fancier computer?

Nope, but there are many reasons to get cheaper!

If you shop around (incl Ebay), you'll find one for $150.

I don't care for the displays of any of the dive computers I've looked at, they're all dark, difficult to read. The only one I've seen that's really readable is bright, multi-colored, and costs $1800. That's not in my budget.

Typically, dive computers are much easier to read in the water. Most have a backlight function, if necessary. OLED displays (which you describe) cost a fortune.

Also, is this wheel equivalent to an eRDPml? I kind of liked that, actually.

The Wheel is the fore-runner of the eRDPml. It's been phased out now, so you can only buy them second-hand (or maybe a few shops might have remaining stock).

You can also get a range of electronic dive planners as software... PC, MAC, Iphone, Android phone etc etc.

It seems to me that a computer should tell you how long you were at each depth. That is, it should spit out a profile after each dive.

More modern dive computers do provide that. Some are even downloadable onto computer software, so that you have an electronic dive log, complete with graphs and other analysis.

In reality though, you just need to know your depth/time/NDL on the dive. When prepping for your next dive, you just need to know your NDL at max depth for planning purposes. Most dive computers have a plan/simulation mode that allows you to determine your max NDL/bottom time after a given surface interval.