How do we team dive with the many different types of dive computers and decompression algorithms on the market? It’s not often that we see a team of divers all diving the exact dive computer. In this months blog, I thought it would be a good idea to have a discussion about procedures that should be considered when you are team diving with dissimilar computers.
First off, lets talk about the why this could be an issue. Not all dive computers are created equal. Most dive computers have similar basic functions. They display current depth, NDLs (no decompression limits), they track nitrogen loading, ascent rate, and log the dive. However, different computers use different algorithms/decompression modules and this can give divers different NDLs times than their teammate's computer. This can translate to some confusion while diving if we do not plan appropriately.
Here's why your dive computer may be giving you more or less time than your teammate's computer. Different algorithms/decompression modules calculate dive data differently. While all of these algorithms are theory, most of them have gone through testing by the Navy, NOAA, and other various organizations. This doesn't mean they are flawless, just tested! The most common decompression modules that are in use today are Bulhlmann, VPM, and RGBM. Do a google search and learn about them. Cool stuff! There are also variations and conservatisms to each of these models such as gradient factors, deep stops, etc. Some dive computer manufactures add in there own unique variations to these deco modules such as Pelagic’s DSAT or Z+. There are many possibilities! The chances are that your computer is running a different deco model than your teammates.
Recreational Computer Dive Planning.
So what should computer dive planning look like to a recreational diving team that stays within NDLs (no decompression limits)? I believe that basic computer dive planning should start with a conversation about maximum depth, maximum time, minimum air pressure, and a dive leader. Most dive computers that run any sort of decompression model has planning software within its menus. When planning out a dive with your team, every team member should find their NDLs on their personal dive computer for the maximum depth of the dive. What you may notice is that some of your teammates computers will be more or less conservative than yours. This means that your teammate may be given more or less bottom time for the dive. This is because their computer may be running a different algorithm or it may be set on a different conservatism for the same algorithm. Many dive computers today have user adjustable algorithms to closely match your teammates computer. You can find out if your computer has this capability by simply reading the user manual. Yes, you should always read your computers user manual. If the team cannot match NDLs, you should simply dive to the more conservative computer. If the diver ignores the NDLs of his computer because the other team member is getting more bottom time out of his, you will bend your computer causing it to enter into emergency decompression mode and locking you out for the next 24 or so hours. Dive within the limits of the more conservative computer.
Technical Computer Dive Planning.
How should computer dive planning work for Technical Divers? First off let me say that I am a proponent of cutting dive tables for every technical dive and keeping them in a pocket or something easily accessible. Even when diving multiple computers. Cutting tables can make you more intimate with the dive plan and provides an extra margin of safety. It’s too easy with all the software available such as ultimate deco, v-planner, and the planning software available in most dive computers today to have this information. Your smart phone is capable of this as well. I understand that computers give more realistic dive data than planning from a square profile. However, they are still electronic devices being used underwater to lead you through a decompression schedule attempting to keep you from getting bent.
That being said, I conducted an online research of how different diving teams plan their dives when the team is using dissimilar dive computers and dissimilar algorithms. As expected, I received many different opinions. Some diver teams said they plan their own dive based on their personal preferences of algorithms and conservatisms and some use identical plans.
Here are two examples below:
A tech diving team made up of two divers uses their own software and preferences to plan out their individual decompression schedule. Diver 1 uses Bulhmann ZHL-16 with a GF (gradient factor) of 30/70. Diver 2 also uses the same Bulhmann ZHL-16. However, he chooses to use a GF of 50/80. Their deco schedules look totally different. They dive and stay together for the same depth and bottom time. However, when it is time to leave they go at decompression alone diving their independent deco schedules. Diver 1 is making deeper stops along the way up while diver 2 is stopping shallower and surfacing much sooner than diver 1.
You cannot call this type of diving team diving. If something was to go wrong they are unable to assist one another because they are solo diving. Loss of deco gas, toxicity issues, and every other situation that may appear must be handled alone. However, in overhead diving environments, sometimes scattering the team at different places may be beneficial. This is not due to the fact of opposing views on decompression theory. It is due to restriction and over-crowding issues in a small space. Doing that requires a lot of training and pre-planing to make sure that the divers are able to assist each other if the need arises.
A tech diving team made up of two divers begins to plan out their dive. Diver 1 is diving a computer that uses VPM while Diver 2 is diving a computer that uses Bulhmann with GF. They run a few deco schedules on their planner and agree on the schedule that Diver 1’s VPM computer puts out. Diver 2 adjusts his GFs to closely match the agreed upon deco schedule. Diver 2’s computer has only a small variance from the agreed upon schedule by a 10ft deeper stop for 1 minute. The divers agree to stay together while diver 2’s computer requires the extra stop. They stay together through the entire dive working as a team and aiding one another when needed.
I believe that this is the best way to plan dives when diving in a team with dissimilar dive computers using dissimilar algorithms. During my online studies and conversations on this subject, most divers use the planning method from example 2.
Dive safe and within your limits!