Decompression Devices

Today diving has progressed into the computer age. There are all types of computer operated gauges on the market but most significant to the safety of divers could be the DIVE COMPUTER. Dive computers are instrument used by divers to help prevent decompression sickness, but many divers are confused as to the following: What they are? and How do they work? To answer these questions we will first need to review what decompression sickness is and how one gets it.

Due to the gas laws that affect the diver, he cannot go to any depth whenever he pleases. During every dive made on compressed air, nitrogen is absorbed by the body. How much is absorbed is dependent on the depth and amount of time spent there.

Henry's Law of physics shows that a gas will dissolve in a liquid in proportion to the partial pressure of that gas. As the pressure increases the amount of nitrogen that the tissues and blood will hold also increases. If a quantity of nitrogen exceeds certain amounts and the diver surfaces without following a strict procedure of stops, decompression sickness will result.

The classic example of a gas coming out of solution is opening a carbonated drink. With the cap on no bubbles are visible because of the held in pressure. If one pops the top the pressure is suddenly reduced and bubbles start flowing. The bends comes on about the same way. The diver relieves the pressure too suddenly from his body and bubbles develop in the blood. Following any dive at least twelve hours is required for nitrogen levels to reach normal. If another dive is made before the twelve hour period, the nitrogen in the body must be taken into consideration. The greater the time between dives, the less nitrogen there well be in the body. This is the reason dive computers were made, to try to determine and monitor the amount of nitrogen in the body, to avoid harm to the diver, and increase bottom time. Every diver must at all times be aware of the amount of nitrogen in the tissues.

Before the invention of the Self Contained Underwater Breathing Apparatus the only divers were surface supplied, or hard hat divers. These divers preformed task at set depths, meaning they spent their entire dive at one depth. All of their decompression status, and any decompression stop execution, was preformed by diver tenders. Once freedom of the surface was achieved, the diver was free to move about at any depth he wanted, for any time he fancied, within reason. This produced the need of the diver to monitor his own bottom time, and depth, to determine his decompression status. In surface supplied diving, depth, and bottom time were all controlled by the surface support tenders. Now the SCUBA diver has to monitor all this. This produced the need for the SCUBA diver to monitor, or determine decompression status underwater. Without the unlimited air supply of the surface compressor, the SCUBA diver has to return to the surface for fresh tanks, thus repetitive diving, rarely done by hard hat divers.

To keep divers within the safe limits of nitrogen the Dive Tables were developed. These tables take into consideration the amount of nitrogen absorbed by the body at different depths for different times. The United States Navy (USN) had Decompression Tables made to allow for stage decompression and no-decompression diving. The U.S. Navy Dive Tables and all others, are based on one depth diving. They do not account for SCUBA divers do not stay at the same depth throughout a dive. When using dive tables the diver always takes his deepest depth obtained during the dive, and uses it as if the whole dive was done there. Many divers believe that this is cheating them out of bottom time. A diver made a thirty-seven minute dive, and during that dive spent ten minutes at seventy five feet. With proper use of the tables, the diver will be charged with a eighty foot dive for forty minutes. Even though it is a no-decompression dive, the diver is pushing the tables. Because SCUBA divers do not dive the way the Tables are meant to be used - one dive to a set depth - modern technology has stepped in.

The dive computer was produced to do multi level decompression calculations for the diver. It maximizes bottom time by calculating the nitrogen absorbed based on multi-level, multi-tissue dive profiles, rather than the deepest depth, with a total bottom time

With the advent of microprocessors the computer can be taken underwater to calculate decompression. The basic theory of operation is information from a pressure sensor is fed to a programmed, battery operated computer, which calculates data, and provides a digital readout. They are known generically as Electronic Dive Monitors (EDM) or Underwater Decompression Computers (UDC), but the dive industry has agreed to call them Dive Computers.

Dive computers use basic designs that incorporate the highly advance technology of computers today. A pressure transducer is used to convert the ambient pressure, or the pressure the diver is at, to a voltage level that is input to an analog to digital (A/D) converter. This A/D converter changes the pressure transducer voltage to a digital signal that is read by the microprocessor. The microprocessor controls the signal flow and performs the mathematical and logic operations.

There is a Read Only Memory or ROM, as it is known by computer people, that contains the program. The ROM is the brain of the computer to put it simply, When the computer is turned off any information here stays. It is the permanent program that determines the decompression status of the diver.

The Random Access Memory is called RAM. Unlike ROM, RAM holds temporary information. It is here that the variable data for doing the decompression calculations is done. If the computer is turned off, any information in the RAM will be lost.

A display is needed for the user to access, or read the information. An internal clock is used to synchronize the operation steps, and time inputs. The power supply must be able to run the computer for the maximum time of the program. And finally a waterproof housing is needed to protect the internal components.

The programs used to run a dive computer can be one of two types, table based or decompression model based. Table based computers read an established set of decompression tables programmed into them. They use the tables based on the maximum depth achieved and the total bottom time of the dive. There is no benefit to these computers other than the diver does not have to figure his decompression status. The same information can be obtained from a set of plastic dive tables. The reason to dive with a decompression computer is to compute nitrogen levels based on multi-level diving.

In the decompression model based computer the diver's decompression is based on a model program in the computers permanent memory. With this type the diver's depth is read into the computer at set time increments, and the decompression model is used to compute changes in status. The time increment on reading the ambient pressure is normally five seconds. This allows for maximum multi-level credit, by basing the decompression status on an exact profile, and not an assumed profile as is used in the tables.

When comparing the dive tables with the dive computer the flexibility of the computer wins out. Most tables use depth increments of ten feet of seawater, and in most cases time increments of five minutes. These limit the number of depth/time combinations available. In the US Navy No-Decompression Dive Table, for the depth range ten feet to 130 feet, there are only thirty - two possible depth/time combinations. The number of possible depth/time combinations that a computer can resolve is almost limitless.

The major advantage of a dive computer over standard tables is the ability to compute decompression status based on the actual dive. This is what allows for the multi-level, dive profiles capabilities. Users of standard tables use the rule of next greater depth and next longer time to figure decompression status, while dive computers use the exact depth and bottom time. What this does, is it allows the diver to use sixty two feet for thirty - six minutes as such, and not seventy feet for forty minutes as he would by using the tables.

Flexibility in diving is one of the most useful application of the dive computer. If for any reason the diver deviates from a planned dive, the computer will adjust. For rescue divers the use should be apparent, seeing that in several circumstances the rescuer will be doing multi-level diving. In the Dive Tables, the Repetitive Group Designator is based on only one tissue group, while the decompression computer uses up to fifteen tissue groups.

An ascent rate monitor is found in most models. This tells the diver if he is ascending too fast. Most of the computers ascent rates are about thirty feet per minute, far slower than the normal rate of sixty feet per minute.

Like anything the computers are not God, so are not perfect. There are some limitations that must be noted. First it tends to eliminate any safety factor that a diver might get with the tables. By using the next greater depth and time in working the tables the diver is giving himself a safety margin. This is because on most dives the diver does not stay long at the maximum depth. When using a decompression computer, and the diver goes the limit of the program, he may be more than pushing it. Because of this the program used should be more conservative than the tables. The other big drawback is that the computer does not sense water temperature or work load of the diver. The computer works the same in forty degree water as in eighty. When planning cold, and/or active dives remember the rule of decompression concerning them: be conservative.

Like any gauge it must be monitored. Divers run out of air because they do not check their Submersible Pressure Gauge. When a diver ignores his dive computer he could get decompression sickness. Lastly, the user needs to be aware that some computers will say that it is all right to go deeper than the previous dive. The diver must know that no matter what the computer says dives must get progressively shallower. One should never dive deeper than the last dive

Some people use the tables and a computer, this is wrong, and foolish. Anyone who teaches this does not understand how a dive computer operates. Using the dive computer with the tables defeats the purpose of having one. The limits are derived differently from those of the tables. It constantly monitors the depth and provides information based on multi-level dive computations.

Once a diver starts diving with a computer he is committed to use only that system until he becomes a new diver again. A diver cannot switch between the tables, and computers the same way he changes a tank. Also divers using the computer must follow the manufacturer's guide to operation of the device.

When using a dive computer certain rules should be applied:
  • Dive computers are only intended to be used for no-decompression diving and should be to avoid any needed decompression stop.

  • When using a dive computer only the diver equipped with one can dive using it. In other words it is not to be shared.

  • If given an ascent rate, this specified rate must be used.

  • Never switch off your dive computer until it has completely out-gassed

  • If the dive computer fails underwater come up immediately and perform a five minute safety stop at ten feet.

  • Failure, or accidental switching off poses a problem due to lost dive history. The diver should delay any diving for the time specified by the manufacture, normally 24 hours.

  • It has been found that even with dive computers the diver needs to dive his deepest dive first. Some computers may say it is all right to go deeper on a repetitive dive than the previous dive, do not.

  • The diver should still carry his regular depth gauge, timer, and plastic tables in case of malfunction.

Dive computers could change the face of diving as long as the general safety rules are followed and the diver follows the manufacture's instructions. They also make good depth gauges since they are within +/-1 to 2 fsw (feet of seawater). Most even have a self check program built into them, every time you turn it on it checks itself. Care is simple, just rinse after every dive, and avoid shock, prolong sunlight, extreme heat and, do not use any chemical cleansers.

Remember that people get decompression sickness when well within the limits of no-decompression, whether they are using the tables, or a dive computer. No device or table removes that possibility. A diver should trust no one who says this system or tables prevents decompression sickness.

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