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Computer vs. Computerless Diving

Bottom Line:

If you don’t use a dive computer, you must calculate your dives the good old fashioned way[1] to avoid the danger of decompression sickness (DCS). Besides crunching numbers, dive computers offer many other benefits during and after a dive.

SPG with maximum depth indicator

Figure 4. SPG indicates 46 feet as the

maximum depth of previous dive.

At the end of Open Water training, your instructor might have suggested a dive computer as one of your first scuba equipment purchases. If you’ve already purchased one, that’s great! But if you don’t own one yet for whatever reason (like cost or anticipated diving frequency), this—admittedly confusing—article might help explain the value of diving with a computer and why you might want to reconsider your equipment purchasing priorities.

Before we go too deep, a quick review as to why we calculate dives in the first place:

As recreational divers, we are trained to make only NO STOP DIVES, meaning we can ascend directly to the surface any time during a dive without suffering an unusually high risk of decompression sickness.[2]

Conducting no stop diving is no accident—it requires planning and vigilance to ensure depths, dive times, and surface intervals (SI) are within recognized limits.


If you don’t dive with a computer, you must equip yourself with other tools to help ensure you do not violate your no stop limits, also known as NO DECOMPRESSION LIMITS (NDLs). These tools include:

  • A means to record information on each dive (such as a pencil and underwater slate)

  • A dive timer (a dive watch with a rotating bezel or a submersible timepiece with stopwatch function, Figure 3), and

  • A depth gauge; one with a maximum depth needle to indicate your dive’s deepest depth can be helpful.[3]

In Figure 4, the red-tip needle shows current depth as 0 (sea level) and the black “maximum depth needle” shows the previous dive depth as 45 feet. After each dive, the max depth needle can be rotated back to 0 with the twist of a fingernail in the center slot so the next dive’s max depth can be measured. Many new depth gauges do not have this secondary needle because of the overwhelming popularity of dive computers among recreational divers.


We will walk through a two-dive day using a computer, the RECREATIONAL DIVE PLANNER (RDP, Figure 1) and the ELECTRONIC MULTILEVEL RECREATIONAL DIVE PLANNER TOUCH (eRPDml, Figure 2). Our goal is to use the same profile on each device.



Using a dive computer is kind of like taking your mom with you on a dive. Computers watch and record your every move. If you get out of line, it will let you know, just like your mom did (or still does!).


Not only do computers offer warnings during a dive, they keep an eye on you after the dive by timing your surface intervals and calculating “no fly times” by automatically calculating the elimination of theoretical levels of residual nitrogen from your body. With the RDP and eRPDml there is no underwater hand-holding—these devices are used before a dive to help plan depths and times, thereby reducing the likelihood of NDL violations.


Figure 5 shows a typical dive summary using Suunto’s DM5 software and the Eon Steel dive computer (Figure 8). The green graph depicts the diver’s depths throughout the dive. The dive progresses in time from left to right. Here are the dive details as captured by the computer:


  1. Dive #1 maximum depth: 81 feet (average depth: 45 feet).

  2. Dive time: 62 minutes

    • The dive ended because of air limitations, not because an NDL was reached.

  3. Safety Stop: 3 minutes was recommended.

  4. SI: 61 minutes.

  5. Dive #2 maximum depth: 42 feet (average depth: 28 feet)

  6. Dive time: 70 minutes

    • Again, this dive ended because of air supply, not because of an NDL.


Just the thought of using dive tables might send shivers down your wetsuit, but let's try using the RDP on this same dive... But here is our first problem: Meandering dives with limited times at various depths cannot be accurately calculated with the RDP. [4] Because of this, we are forced to calculate the dive to the deepest depth reached (see Figure 6):

  1. Dive #1 must be calculated to 90 feet (we must round up from 81).

  2. NDL time: 25 minutes.

  3. Ending pressure group: Q.

  4. Safety Stop: 3-minute required.[3]

  5. SI: 61 minutes.

  6. New pressure group: F.

  7. Dive #2 maximum depth: 42 feet.

  8. NDL: 80 minutes.

  9. Residual Nitrogen Time (RNT): 24 minutes.

  10. Adjusted NDL: 56 minutes.

  11. Ending pressure group: X.

  12. Safety Stop: 3-minute required.[6]

What’s the Difference?

Because the RDP can only calculate “square dive profiles” (when a diver descends directly to a predetermined depth and safely ascends to the surface from that depth), the dive can only last for 25 minutes (the NDL for 90 feet), in comparison to the 62-minute computer dive. The second dive is limited to 56 minutes due to the adjusted NDL. In comparison, the second computer dive was 70 minutes. Diving to the NDLs of consecutive dives is anything but conservative and is not recommended. Pushing the limits can result in a case of DCS.


Certified enriched air divers must calculate dives using the PADI EANx32 or EANx36 Recreational Dive Planner™. 


Unlike the RDP, the eRPDml can calculate multilevel dives, but our dive was not really not one because the diver did not maintain distinct dive levels. Additionally, the eRPDml can only calculate three dive levels that fall within the parameters explained on p. 82 of the eRPDml Use and Study Guide.[2] Despite these limitations, let’s calculate Dive #1 with the eRPDml as follows (see Figure 7):

1. Multilevel dive profile:

  • Level 1:  81 feet

    • Actual Bottom Time (ABT): 5 minutes

    • Ending Pressure Group: B

    • NDL: 27 minutes

  • Level 2:  60 feet

    • ABT: 29 minutes

    • Ending Pressue Group: Q

    • Multilevel Limit (ML): 35 minutes

  • Level 3:  40 feet

    • ABT: 28 minutes

    • Ending Pressure Group: V

    • ML: 53 minutes

2. Safety Stop: 3-minute required [5]

3. SI: 66 minutes.

  • New pressure group: G.

4. Dive #2 maximum depth: 42 feet

  • ABT: 70 minutes

  • Ending pressure group: Z

  • Safety Stop: 3-minute safety required[3]


What’s the Difference?

Besides being horribly complicated, sucking all the fun out of diving, and not being an accurate representation of our dive, this multilevel eRPDml dive profile is pretty close to the computer dive, sans the required 66-minute SI to complete Dive #2. The second dive ends in the highest pressure group (Z). Pushing NDLs is never recommended.

Oh, but did you want to dive with enriched air? Sorry! The eRPDml can’t help you, but most dive computers handle EANx without a problem.



The RDP—and, in later years, the eRPDml—have successfully guided divers for decades, but not without some effort. The good news is that dive computers can do all this number-crunching for you and more! Some common dive computer capabilities include:

  • ENRICHED AIR (nitrox) compatibility

  • LOGBOOK of dive profiles that can be downloaded to a PC

  • HISTORICAL information for all dives in the logbook

  • SAFETY STOP signal and timer

  • DEEP STOP signal and alarm

  • RAPID ASCENT indicator and alarm

  • PO2 indicator and alarm

  • CNS TOXICITY status and alarm

  • PENDING NO STOP DIVING VIOLATION indicator and alarm

  • NO STOP DIVING VIOLATION indicator and alarm

  • EMERGENCY DECOMPRESSION STOP indicator, alarm, depths and times of decompression stages


  • TOTAL ASCENT TIME during a deco dive

  • NO FLY timer

Dive computers really earn their keep by guiding divers through safe ascents and abnormal situations, i.e. when NDLs are inadvertently violated. And just like mom, computers can put you in a time-out when you've been really, really bad:  The computer will lock you out (i.e. render the computer useless) for 24-48 hours if you violate certain safe diving rules.


Nearly all reputable dive centers have dive computers available for rent, but will you have adequate knowledge on how a rental computer works? Computers can be confusing without adequate preparation. You don’t have to break the bank to outfit yourself with one. About $200 will get you started with a nice, wrist-mount “puck style” computer—and that’s a lot cheaper (and much safer) than a $10,000/day trip to the recompression chamber!

Many dive centers use the Suunto Zoop (Figure 9) in training. It is a solid selection and comes in wrist mount (shown) or multi-gauge console configurations.


Calculating dive tables and using a dive computer can be equally difficult, especially if you don’t dive regularly. There is really no simple way around it. A dive computer can certainly simplify things, but they require a solid understanding on how to operate and what the computer is trying to tell you with its different displays and beeps.


Haven’t purchased a dive computer but want to learn more? Or maybe you have purchased a dive computer, have thoroughly reviewed the user’s manual and still aren’t sure about all its functions. Either way, there is still hope for you!



A company called DiveNav, Inc. hosts a website called®. You might want to check out these free and inexpensive courses (Figure 10) that work on multiple platforms (PC, smartphone and tablet):


How to Choose a Dive Computer (FREE)

Introduction to Dive Computers ($3.99)

Once you’re done with learning the basics, search the site for a course on your specific dive computer. We have found the instruction to be beneficial, especially when illustrating unusual situations that can’t be easily replicated on your own, such as violating an NDL. Classes cost $0.99 to $14.99.

If you haven’t done so already, when will you be purchasing your first dive computer? Let’s go diving!


[1] Calculating dive profiles by using the Recreational Dive Planner™ (RDP, Figure 1), the RDP’s electronic counterpart, the Multilevel Recreational Dive Planner (eRDPml™, Figure 2), or an equivalent device.

[2] PADI Open Water Diver Manual [English], version 3.0, p. 197.

[3] PADI eRPDml™ Instructions for Use and Study Guide (2008), retrieved online at

[4] PADI Recreational Dive Planner Instructions for Use (2004, Dive Table Definitions, p. 32), retrieved online at

[5] PADI Recreational Dive Planner Instructions for Use (2004, Rule 12, p. 7).

[6] PADI Recreational Dive Planner Instructions for Use (2004, Rule 12, p. 7).

[7] Rule 2 of General Rules for using the eRPDml can be found on the inside cover.

This page is provided for informational purposes only. It is solely the opinion of ATA/BAR DIVERS and is provided without compensation, affiliation or consideration of any kind.

Recreational Dive Planner

Figure 1. Recreational Dive Planner.

The eRPDml Touch

Figure 2. The eRPDml Touch.

Dive timer examples

Figure 3. Dive timers include watches with a rotating bezel or a stopwatch function.

Suunto DM5 software dive profile

Figure 5. Dive #1 profile: 62 minutes.

(Dive #2 profile not shown.)

Dive profile calculated using the RDP

Figure 6. Recreational Dive Planner (RDP)

dive profile.

Multi-level dive calculated using the eRPDml.

Figure 7. Multilevel dive profile for Dive #1 calculated using the eRPDml.

Suunto Eon Steel dive computer

Figure 8. Suunto Eon Steel.

Suunto Zoop dive computer

Figure 9. Suunto Zoop.

Dive Computer Training's online Suunto Zoop class

Figure 10. Dive Computer Training's
online Zoop class.

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