Lesson 131

Lesson two: Planning a Minimum Surface Interval

Course Complete

Learning Objectives

By the end of this session, I should be able to answer these questions:

1. Why would I need to find a minimum surface interval?

2. What are three ways I can find a minimum surface interval with a dive computer without using dive tables?

3. How can finding minimum surface intervals with the RDP Table and eRDPML help me plan them with dive computers?

Planning a Minimum Surface Interval

When making two or more dives, you commonly plan your dives by checking
no stop times after your surface interval. You decide how long you want to dive
and choose your maximum depth based on the no stop times, or you choose the depth you want and plan your time. When you can plan multilevel dives, you have more flexibility.

Sometimes, though, you want to make a repetitive dive to a specific depth for a specific time. Your objective may be very specific, and you need a minimum time to do it, such as photographing a wreck at 18 metres/60 feet. Or, when the dive site is flat (i.e., you can’t ascend to a shallower depth to increase no stop time), you may simply want enough time to make the dive worth it.

Planning a repetitive dive for a specific depth and time requires finding a minimum surface interval. This is determining how long to wait after the first dive to have the no stop time you want at the planned depth of the repetitive dive.

Here’s an example: You and your buddy are planning a repetitive dive to 15 metres/50 feet. The bottom will be relatively flat, so you can’t plan a multilevel dive to increase your no stop time. Based on experience, you know that with a full cylinder, you can dive to that depth for about 50 minutes, ascend, make a safety stop and surface with 50 bar/500 psi reserve.

You and your buddy decide that you want at least 45 minutes of no stop time for a reasonably long dive within your air supply limit. Sticking to conservative dive practices, you also want your dive to be well within the no stop limits. You decide you want a surface interval that gives you a no stop time of at least 55 minutes at 15 metres/50 feet. To dive well within your dive computer or dive table limits, you also know that it’s a good idea to plan your surface interval to be longer than the absolute minimum.

There are three ways to find a minimum surface interval with a computer: wait and check, use the dive computer plan mode and use a tablet/smartphone.

Wait and check. Just as it sounds, after your dive, scroll your no stop times and check the no stop time for your planned repetitive dive depth. If it’s too short, you recheck every so often until you’ve been at the surface long enough to have the desired no stop time, plus some extra to stay well within the limits.

Although this method sounds imprecise, with experience you’ll get a feel for about how long you have to wait. You’ll notice that during a surface interval, your repetitive no stop times increase quickly at first, but the increase rate gradually slows as the interval gets longer.

Use the dive computer plan mode. Some higher-end dive computers have a sophisticated plan mode that allows you to enter a surface interval. The computer tells you what the no stop time will be after you’ve been up that long. If your computer has this feature, enter surface intervals (this may take a bit of trial and error) to find out how long you need to wait to have the desired no stop time, plus extra time so you’re well within limits, at your planned depth.
See the manufacturer literature or your instructor for specifics on how to use this function with your computer, if it has it.

Use a tablet/smartphone app. In Section Four, you learned that you can get dive planning apps that calculate no stop times. Tec divers use dive planning apps that calculate no stop times, but recreational divers also use them to estimate minimum surface intervals. With such an app, you enter your first dive maximum depth and time, a surface interval and the desired second depth and time. If the app shows that the second dive requires decompression stops, enter longer surface intervals until it shows the desired no stop time, plus a conservative margin, at the planned depth.

Your app probably won’t exactly match your dive computer, but it will give you a good estimate. Be sure to scroll your computer’s no stop times before diving to confirm you have the time you want.

Using Tables

You can also use the RDP Table or eRDPML to find minimum surface intervals with your computer. Again, don’t expect the times to exactly match your dive computer, but you will get a good estimate, and scroll your computer’s no stop times before diving. Also, if the first dive was a multilevel dive, you may not be able to use the RDP Table to get a good estimate because you have to use a single depth. The eRDPML is generally more useful, but you need to track your depth levels and times to do this.

The steps for finding a minimum surface interval are in the Instructions for Use included with the RDP Table and eRDPML. Your PADI Instructor can also help you.

Quick quiz

Comments