How to Do SCUBA Dive Calculations

By Edwin Thomas, eHow Contributor

One of the dangers of scuba diving is the absorption of nitrogen into the bloodstream, as too much nitrogen causes decompression sickness. For this reason, dive table calculations are taught in any basic scuba diving course. Through these calculations, a diver can track how much nitrogen her/his body has absorbed on a given dive, and therefore plan her/his time on the surface and her next dive within the bounds of safety.

Instructions

Monitor your bottom time on your first dive, taking note of both the deepest depth and the time spent there. Either you or your dive buddy needs a dive watch to do this.

Round up the depth and time figures to the nearest 5 for safety and utilize these numbers to determine your nitrogen class on the dive table. A dive with 18 minutes of bottom time at 95 feet should be rounded up to 20 minutes and 100 feet, yielding a class of "F."

Monitor your surface interval, or the time spent between the first dive and the second dive of the day. Utilize this number to determine your new, reduced class. After two hours on the surface, you should have dropped to class "D."

Inquire about the maximum depth of the day's second dive and utilize this figure (rounded up for safety) to determine how much bottom time is safe for the second dive. If you spent two hours on the surface and the next dive bottoms at 65 feet, round up to 70 feet. In this example, the table indicates a maximum safe bottom time is 25 minutes and a residual nitrogen time of 20 minutes.

Add your residual nitrogen time to your actual bottom time to determine your new nitrogen class. With 20 minutes of residual nitrogen time and all 25 minutes spent on the bottom, your total nitrogen time is 45 minutes. Fed back into the table, this indicates a new nitrogen class of "I."

Repeat the procedure in Steps 3 and 4 to determine the safe diving parameters for a third dive.

Tips & Warnings

This guide is tailored to match the tables used by the National Association of Underwater Instructors (NAUI). Other organizations, such as the Professional Association of Dive Instructors (PADI) or Scuba Schools International (SSI) have their own tables, but all are based on almost identical principles. Adaptation to another organization's dive table should only require studying how the table is organized and not changing the calculation method.

Dive computers perform the same calculations and with greater accuracy. However, a given dive computer might use a more liberal margin of safety than others. Check a new dive computer's algorithm by comparing its results against those of your dive table calculations for the first few dives, just to see where the dive computer stands.

Thanks to Edwin Thomas, eHow Contributor

Kathy Dowsett
www.kirkscubagear.com

How to Do SCUBA Dive Calculations

One of the dangers of scuba diving is the absorption of nitrogen into the bloodstream, as too much nitrogen causes decompression sickness. For this reason, dive table calculations are taught in any basic scuba diving course. Through these calculations, a diver can track how much nitrogen her body has absorbed on a given dive, and therefore plan her time on the surface and her next dive within the bounds of safety.

Monitor your bottom time on your first dive, taking note of both the deepest depth and the time spent there. Either you or your dive buddy needs a dive watch to do this.

Round up the depth and time figures to the nearest 5 for safety and utilize these numbers to determine your nitrogen class on the dive table. A dive with 18 minutes of bottom time at 95 feet should be rounded up to 20 minutes and 100 feet, yielding a class of "F."

Monitor your surface interval, or the time spent between the first dive and the second dive of the day. Utilize this number to determine your new, reduced class. After two hours on the surface, you should have dropped to class "D."

Inquire about the maximum depth of the day's second dive and utilize this figure (rounded up for safety) to determine how much bottom time is safe for the second dive. If you spent two hours on the surface and the next dive bottoms at 65 feet, round up to 70 feet. In this example, the table indicates a maximum safe bottom time is 25 minutes and a residual nitrogen time of 20 minutes.

Add your residual nitrogen time to your actual bottom time to determine your new nitrogen class. With 20 minutes of residual nitrogen time and all 25 minutes spent on the bottom, your total nitrogen time is 45 minutes. Fed back into the table, this indicates a new nitrogen class of "I."

Repeat the procedure in Steps 3 and 4 to determine the safe diving parameters for a third dive.

Thanks to ehow

Kathy Dowsett
www.kirkscubagear.com

History of the Dive Computer

In a 1951 secret meeting at the Scripps Institution of Oceanography in California, members of the US Navy Committee for Undersea Warfare and Underwater Swimmers discussed improvements to scuba diving gear. Top of their list was a foolproof way of monitoring nitrogen loading.

Two years later two Scripps researchers, Groves and Monk, published a paper that set out the functionalities needed for a decompression device. They said such a device must calculate three things – decompression during the dive, the remaining nitrogen in the human body from previous dives and, based on this information, an optimised, faster ascent rate. Groves and Monk suggested the use of an electrical analogue computer to measure both decompression and
air consumption.

American manufacturer Foxboro came up with the Decomputer Mark I in 1955 (above). It was the first analogue dive computer. A needle indicated danger during an ascent by moving towards a red zone on the display. After several test dives, the US Navy found the device to be too inconsistent – despite their efforts, the use of dive tables was still far more accurate.

It was back to the drawing board. In 1963 the SOS Poseidon 5 was introduced. It automatically calculated how long decompression stops should be based on dive depth and bottom time. However, imprecise calculations for deep and repetitive dives gave it the nickname ‘Bends-o-Matic’ and the US Navy advised against its use for recreational diving.

The same year the market saw the first electric analogue device – the TRACOR. The unit ran on two batteries, but the high power consumption, especially in cold waters, resulted in limited dive times and a worrying rate of system failure.

Next up was the MARK V S developed by the DCIEM (Defence and Civil Institute for Environmental Medicine). It was the first dive computer to tick all the boxes and was sold to industrial and military agencies in the 1960s but was not available to the public.

In the late 1970s dive computers went digital. The XDC-1 was a desktop device designed in 1979 for laboratory purposes and looked like a cash register. The device built the foundation for the improved XDC-2 and XDC-3, or CyberDiver that became the first digital portable computer and 700 units were sold between 1979 and 1982. The later XDC-4 worked with gas mixes, but was too expensive for the mass market.

In the 1980s the technology quickly improved. In 1983 the Orca Edge hit the market as the first commercially viable dive computer. The model was based on the US Navy dive tables but did not calculate a decompression plan. Its design was ahead of its time – it resembled an iPod Nano. However, production capacity was only one unit a day. It was never going to emulate Apple’s sales figures.

A year later, the Decobrain arrived and the modern recreational dive computer was born. It had all the features we have come to expect from a dive computer, including calculated ascent times and an integrated warning system for fast ascents. The Decobrain was also the first dive computer to achieve success in the European market.

DACOR’s follow-up model Microbrain was the first industrial scaled dive computer and the first one using a silicon chip.

In 1986 a little known Finnish company, Suunto, came out with the SME-ML. The computer had all the essential features and was able to store 10 hours of dives, which could be accessed any time. This, and the simple design, was key to its success and marked Suunto’s break into dive gear manufacturing. It took another decade before the Finns became market leaders.

In 1987 the Swiss company UWATEC introduced the Aladin that swept aside all rivals. The unattractive, chunky, grey housing could be seen strapped to wrists from the Red Sea to the Barrier Reef. Soon more divers were using computers than not – and more were using Aladins than anything else.

Thanks to Dive

Kathy Dowsett
www.kirkscubagear.com

Underwater Navigation for the Novice Scuba Diver

The new dive adventurer is similar to being a passenger on a long ride in a car.

You get to looking around and you have arrived at the destination and have no clue of the route taken to get there. The first couple of lessons you may be fine focusing on your breathing, clearing your ears, clearing of the mask and the awesome joy of the underwater adventure. The sooner you start working on your navigation skills the better, especially if you're learning in a lake that has visibility of twenty feet or less. Because just assured as you are reading this, you will be looking around and in an instant your instructor has disappeared into the darkness. Don't panic just stop for a minute and relax. The instructor will be back for you and if he or she doesn't come back ascend slowly to the surface because they haven't taken you far from the boat or shore. It happens don't worry about it, that is when you realize you need to sharpen your navigation skills.

There are some simple rules to navigation that will make it easier to find the way back to the point of entry. The first and most important is to have a dive plan before you and your dive buddy enter the water, decide who will be the lead diver. Follow the lead diver; let that person concentrate on navigating while the other diver focuses on time, air usage, distance and depth. If you are not the leader it is still important to know your dive plan for emergencies or if you and your dive buddy get separated you can get back to the entry point safely.

If the point of entry is a boat follow the mooring or the anchor line down to the bottom and familiarize yourself with the surroundings. From there follow the dive plan your group has put together. If the entry point is from the shore, swim out to the point you have planned to make the decent. Again get familiar with the surroundings and follow the dive plan.

Navigation underwater isn't any more difficult than above ground navigation. Look for markers, rock formations, sand bars and distinguishing corals of any kind or a tree stump. Anything that is memorable can aid you and your dive buddy in returning to the point of entry. These are just a few tips to make the dive more enjoyable and less stressful.


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Kathy Dowsett

www.kirkscubagear.com