Computer Diving

Caisson wokers

Caisson wokers

Upon its invention the literal creators of Scuba Diving were aware that extended times underwater (pressure) could bring about painful and crippling side effects because of Caisson’s disease witnessed in workers on bridge foundations. Workers spending up to 12 hours a day working in pressurized vaults on river bottoms were plagued by muscle seizures and extreme joint pain when brought to the surface, and only found relief when brought back to the pressurized environment. Some sadly were crippled for life.

Realizing that ascending from depth too rapidly can aggravate this condition, a rule for speed of ascent was contrived on the set of the 1950’s TV show Sea Hunt, where the Hollywood rule for scuba ascent became “Never ascend faster than the smallest of your bubbles” As the sport grew graphs and tables were developed that guided divers through safe procedures, with the industry standard being Padi’s Recreational Dive Planner.

One of the most difficult aspects of learning to Scuba dive is the dreaded Recreational Dive Planner (RDP). Developed and manufactured by PADI, the Professional Association of Diving Instructors, this hard plastic chart is used to calculate bottom times, and no-decompression limits for recreational divers, but it is inconvenient, and very analog, in what is now a very digital world.

In a nutshell; divers underwater are exposed to the pressure of the water around, and over them. Underwater the regulator delivers air to the diver at the same pressure the rest of the diver’s body is exposed to, and the air enters the diver’s bloodstream through the lungs under pressure. As the diver ascends he/she is exposed to less and less pressure as the depth diminishes, and the air that has been absorbed into the bloodstream begins to expand creating bubbles. In a properly executed dive the air expands to micro-bubble size during a controlled ascent, and the bubbles are readily dispersed through the lungs and other respiration tissues safely. However going too deep, staying at depth too long or ascending too fast can create pressure induced physiological problems such as embolisms and Decompression Illness (the Bends).

Resembling, and using data from the US Navy Decompression tables, the RDP allows scuba divers to calculate safe bottom times, plan repetitive dives, and avoid the previously mentioned discomforts. To use this you simply figure out the depth of your dive, and the RDP will show you the maximum bottom time for that depth, and in subsequent dives it will show safe times for each. One down side to this method of calculating is that it does not take into account the time spent at shallower depths and assumes that the entire dive is at maximum depth. So, an 80 foot deep dive allows for 29 minutes of bottom time without having to make decompression stops upon ascent, but the 29 minutes is total dive time from submerging to surfacing, not just the time spent at 80 feet. Add to this the fact that the RDP is just so analog, I mean it is a piece of plastic with a graph printed on it. To use it you have to find a value, track it across the card to a new value, and then flip it over, find the new value, track it to the corresponding group… get the idea, you may as well have a Slide Rule (ask an old person).

Enter the Dive Computer. In 1957 the US Navy funded the design and manufacture of a proto-type analog “diving computer”. Mechanical failures and inconsistencies forced abandonment of this project and fueled the development of the Navy Dive Tables, the grandfather of Padi’s RDP. However in 1959 a couple of Italians developed and marketed the “Decompression Meter” distributed by their company “SOS” and sold by Scubapro and Cressi. The device functioned so poorly it was soon nick-named the “Bend-o-Matic”.

Various models of decompression meters, and planners were developed in the analog format before 1983 when the “DecoBrain” designed by the Swiss company Divetronic AG was developed and the first digital dive computer with modern features looked out upon the ocean.

Talking to anyone at all interested in the inner workings of their dive computer will bring up the word ‘algorithms’ sometimes with the voice of supposed superiority. Algorithms, not to be confused with logarithms (historically perplexing math tables gone the way of the slide rule and abacus), are calculations and formulas using varied data to arrive at conclusions, in our case the divers exposure to pressure, and the different time and depth scenarios available. From Wiki here is some eye-crossing text with links about that: “Examples of decompression algorithms are the Bühlmann algorithms and their variants, the Thalmann VVAL18 Exponential/Linear model, the Varying Permeability Model, and the Reduced Gradient Bubble Model”. So there, this is all your normal diver really needs to know about the inner workings of a digital dive computer.

The actual use of a dive computer is pretty easy. You strap it on your arm, or any other permanent and accessible appendage, hit the “on” button (some actually turn on when they get wet) and go diving. Under the pressure of the water the computer will display your depth, your time underwater, and your “No Decompression Limit” or the time you have left underwater without going into forced, and extended decompression stops. These are displayed as numbers or even simpler, colored bar graphs allowing the diver to know his/her nitrogen loading, and dive time remaining at a glance. You may never look at the RDP again.

For many the purchase of a dive computer is one of the first investments made in the accumulation of their dive kit. It gets rid of the need of the RDP, gives you more time underwater, it is far more convenient and with most divers using them having one allows you to stay at the same dive profiles as your dive buddies.

Computerized appendages

Computerized appendages

As you may imagine most divers showing up here on Roatan bring their computers with them, and we get exposed to quite a few different varieties. The computers all show the same basic information, depth, no-decompression time, & etc. but bottom times remaining can vary widely. There are many different makes and models of dive computers and each manufacturer has its own set of engineers and programmers using their proprietary algorithms (formulas, recipes). Here on dives like “Texas” where the dive is long and deep, we notice that the time remaining at depth on our diver’s computers can vary 15 minutes or more between devices.

Independent testing of dive computers has shown that the various algorithms used by the different manufactures can lead to a wide variety of no decompression times. Let me offer this data copied directly from Wiki showing the difference in dive computer NDL times:

These are truly alarming numbers taken at first glance, but this is a simple data sheet, and real dives are not like this. Your computer takes into account the entire dive, calculating the time spent at shallower depths and adjusting accordingly in real time. So, while the difference on the chart between the Veo250 and the Mosquito is 31 minutes, on an actual dive the computers may be just a few minutes apart at the surface.

Typical loading graph

Typical loading graph



This being said we dive a lot here on Roatan. The Divemasters and Instructors working here are diving up to three dives a day, six days a week, and keeping an eye on your nitrogen loading is a must. Discussing this BLOG with one of the long-time locals and an extremely active diver who shall remain nameless (come here and I will point him out), brought forth the comment; “Well, I gotta say that after using my Veo for a couple of deep dives in one day, I really pay for it at night with achy joints and a bit of skin bends. (!!)

My recommendations for dive computers are:

Find one you like, and that is easy for you to read (for me, big numbers).

Make sure it doesn’t clash with the rest of your gear.

Choose one geared to your type of diving. (You may not need a Trimix Technical computer)

If you are old, and somewhat sedate (like me) get one that leans to the conservative side with regards to No decompression Limits. If you are young and enthusiastic……well, you deal with it.