Dive computers can be very useful to divers, if these are used with common sense and caution. Since dive computer readings do not guarantee diving safety, it would be best for divers to ensure that they have sufficient education, good equipment and are in good physical health to minimize the risk of accidents.
Less than $1,000 buys a dive computer that requires batteries, brains and common sense. Dive computers follow a model selected by the manufacturer. Since they are machines, they use the assumptions of the model and tend to perform their mathematical calculations accurately. However, what the computer does and what divers think the machine does may be two different things. When a computer performs its calculations perfectly, it is still up to the diver to apply that information. If a diver pushes the computer to its limits, there are no guarantees of avoiding decompression sickness, even though the computer said the dive was OK, The data presented on the face of the dive computer may or may not have any bearing on the individual diver’s susceptibility to decompression sickness.
Computers are a tremendous aid to divers, if used according to each model’s instructions and with common sense and conservatism. Dive computer readings are not guarantees. Every diver is physically unique on every occasion he/she takes the same computer underwater. A dive executed using a computer one day – with no bends symptoms – cannot positively be repeated with the same results, because your body changes every day. A dive computer is merely a sophisticated instrument that tries to keep divers within a depth-time envelope that is safe for most people, most of the time.
The scientific diving community has conducted three workshops since 1989, bringing together doctors, physiologists, hyperbaric chamber operators, physicists, equipment manufacturers, groups conducting scientific diving operations and others. This brain and experience pool has examined dive computers, the bio-mechanics of safe ascents and repetitive diving under the auspices of the American Academy of Underwater Sciences (AAUS).
This article focuses on AAUS’s recommendations for diving safety based on knowledge gathered at all three workshops. If you are interested in more complete information about the thinking-research-experience-gut feelings that went into these recommendations, consider reading the complete reports. Information on how to obtain the workshop material is included at the end of this article. Each diver relying on a dive computer to plan dives and indicate or determine decompression status must have his/her own unit.
Your own experience will confirm that buddies never stay exactly side by side during a dive. This means each person has a slightly different dive profile and requires his/her own computer. If the pair does not have two computers, one diver is on a computer and the other is in limbo. Does the non-computerized diver stay shallower or plan the dive using the Navy Tables or simply fail to take responsibility for his/her own time-depth-decompression situation? Each diver needs to have his/her own computer and be responsible for his/her own time-depth decisions.
On any given dive, both divers in the buddy pair must follow the most conservative computer.
If one computer says it is time to ascend and the other says more time is available, the more conservative computer is the one to be followed – by both divers. It is tempting to separate but then you are making a “same ocean, same day” dive, not a buddy dive.
If a computer fails a t any time during the dive, the dive must be terminated and appropriate surfacing procedures should be initiated immediately.
If your computer fails, you may not know the exact moment it failed in order to read the last “good” data, If you were relying on the computer, you may not have noted your depths and times and probably do not have a set of tables with you to make an accurate manual calculation. In this situation, you should terminate the dive and begin ascending.
Use a computer for each and every dive. If you have made computerless dives prior to using a computer, you must wait 24 hours to let your system outgas and thus make the computer’s computations accurate.
This simply means that when a dive computer is activated it assumes its user is starting with no residual nitrogen. If you have made a dive in the previous 24 hours, the computer has no knowledge of this and there is no way for you to input the needed data.
Once a computer is in use, it must not be switched off until it indicates complete outgassing has occurred, 24 hours have elapsed (whichever comes first) or if no more dives are planned over the next few days.
Some divers try to stretch battery life by switching the computer off at the end of a day’s diving. Batteries are cheap compared to decompression sickness and its treatment. Leave the computer turned on!
When using a dive computer, nonemergency ascents are to be made at the rate(s) specified for the table or the make and model of the dive computer used.
Did you know that some computers utilize a 30 to 40 feet per minute ascent rate and not the 60 the U.S. Navy tables used to recommend? Read the computer’s manual carefully and know your computer’s ascent rate in addition to monitoring the ascent instructions on the display. Some dive computers have variable ascent rates, such as 40 feet for moving from depths of 1 00 feet up to 60. Then, the computer’s programming indicates a 20 feet per minute rate from 60 feet to the surface.
Ascent rates shall not exceed 60 feet per minute.
For many years the U.S. Navy Decompression Tables called for a maximum ascent rate of 60 feet per minute. The current U.S. Navy Tables recommend an ascent rate of 30 fsw per minute. Mark off 60 feet on your sidewalk. Then walk the 60 feet aiming for 1 minute from start to finish. You’ll probably cover the 60 feet in about 30 seconds, as most people do. Then try the experiment again while watching the second hand on a watch. It’s like taking one baby step per second for an incredibly slow trip. Now try doing 20 and 40 feet in a minute. Do you think you have been ascending too fast?
A stop in the 10 to 30 foot zone for three to five minutes is recommended on every dive.
This is practical advice. Stop any place between 10 feet and 30 feet for three to five minutes of outgassing. Holding onto the anchorline or an ascent line makes this easy.
Repetitive and multi-level diving procedures should start the dive, or series of dives, at the maximum planned depth. All subsequent dives should be shallower. This is a re-statement of the old phrase “make your deepest dive first.” This procedure is thought to help reduce decompression stress in divers.
Multiple deep dives should be avoided.
Plan your dives so the deepest dive is first and all other dives in the 24 hour period are to lesser and lesser maximum depths. This procedure is also thought to help reduce decompression stress in divers.
Breathing 100 percent oxygen above water is preferred to in-water compressed air procedures for omitted decompression.
If you complete a dive and miss a required decompression stop(s), AAUS recommends breathing pure oxygen at the surface rather than going back in the water and breathing from a scuba tank. This AAUS recommendation for omitted decompression is different from the U.S. Navy Decompression Table instructions for omitted decompression. This choice of procedure needs to be made by divers for themselves.
Divers can lessen DCS susceptibility by focusing on such things as fitness to dive, adequate rest, hydration, body weigh t, age and especially rate of ascent, which should not be more than 60 feet per minute.
Any computer can be used by any diver. How does your computer know you are tired from a long dive, that you are overweight and out of shape or that you are dehydrated? It doesn’t. Dive computers are built on a theoretical model how a human body absorbs and releases gas. Like the tables, these model “keep most people safe, most of the time,” if used correctly. Originally, the U.S. Navy used male divers for their human tests. How close does the U.S. Navy diver image fit your body? The farther you are “off the mark” in a category, the more important it is to pay attention to the factors you can control – such as hydration, rest and rate of ascent.
Divers are encouraged to learn and remember the signs and symptoms of decompression illness and report them promptly, so as to receive effective treatment as rapidly as possible and prevent residual injury.
Can you name the classic signs and symptoms of the malady called bends, DCS or decompression sickness?
Joint and limb pain and numbness are the most commonly reported symptoms. Initially, the area may feel numb, then the pain increases in intensity. Some victims describe the pain as “deep and throbbing.” Shoulders are commonly affected but wrists, elbows, hips, knees and ankles are frequently affected.
If the bubble formation occurs in the tissues of the nervous system, the symptoms may be blurred vision, stomach pain, vertigo, nausea and vomiting, numbness, headache, confusion, lack of balance, backache or general weakness.
If the symptoms are minor, the affected diver may not realize he/she has a problem. In some instances the buddy has noticed the person did not seem his/her normal self. After recompression in a chamber, the patient realized the treatment stopped the feeling of being “fuzzy.” He or she may not have been able to characterize this “fuzzy feeling” until it resolved with treatment.
The use of oxygen on the surface – whenever possible via a demand regulator Mask System – to ensure the highest percentage of oxygen to the patient, is recommended while awaiting treatment if decompression illness is thought to be present. The use of 100 percent oxygen in the water while awaiting treatment is not recommended for recreational diving.
If you have any inkling you decompression sickness, the sooner you start breathing pure oxygen at the surface, the better. Breathing oxygen on the way to a hyperbaric chamber is beneficial. Owing to the problems of oxygen toxicity, inability to monitor the patient and additional physiological stress, it is not recommended to treat decompression symptoms in the water while breathing oxygen.
The physics and physiology of diving have not changed in recent years but how people look at the modeling has. Much of the research dates back to the Doppler bubble studies and is still valid. Computers are becoming more sophisticated and have increasing capabilities, however, the underlying mathematical models are merely that, models. A model may not necessarily have a lot in common with an individual’s body. And, how the computer’s information is used is still up to the diver.
Divers need to understand that a computer read-out is not a guarantee of safety. Different brands and models utilize different decompression theories and decompression calculation procedures. Most manufacturers consider their model’s proprietary and consumers have difficulty trying to research more than the “barest facts” about a particular computer. The documentation and instructions vary tremendously with each manufacturer. See how long it takes to read the manual and find out what rate of ascent your meter specifies.
Divers need to take responsibility for their own actions. Computers are only machines and need the human brain and batteries to work. There is risk every time you scuba dive but education, good equipment, good physical health and common sense reduce the risk. There is no magic potion, procedure or policy that will totally prevent DCS.