CH 5– Decompression, Dive Tables, and Dive Computers Dr. Yusheng M. Huang Department of Marine Sports and Recreation.

CH 5– Decompression, Dive Tables, and Dive Computers減壓，潛水減壓表與潛水電腦

Dr. Yusheng M. HuangDepartment of Marine Sports and

Recreation

Learning Goals (學習目標 )• Learn about ingassing and offgassing nitrogen and

how it affects the amount of time you can spend underwater ( 氮氣的溶入與析出 )

• Understand the concept of residual nitrogen (餘氮 )

• Learn about dive table terms and rules (減壓表名詞與規則 )

• Learn how to use the NAUI Dive Tables to plan your dives and find your Letter Group designation after each dive (學習如何使用減壓表 )

• Be introduced to the NAUI Dive Planning Worksheet and how to use it to keep track of the information from your dives and the NAUI Dive Tables (潛水作業表使用 )

• Learn about precautionary and mandatory decompression stops (安全減壓與必要減壓 ) and how important they are in minimizing the risk of decompression sickness

• Be introduced to dive computers and their benefits (潛水電腦的優點 )

Nitrogen and Diving (氮氣與潛水的關係 )

Air is 78% nitrogen, 20.9% oxygen, and 1.1% other gases.•Ingassing Nitrogen (氮氣的溶入 )-Nitrogen is a metabolically inert gas-Your body ingasses nitrogen until the pressure of nitrogen in your body equals the pressure of nitrogen in the air your breathe.

-You do not experience any negative effects when ingassing occurs at moderate depths.

- The different tissues of your body (fat, muscle, bone, and so on..) absorb nitrogen at different rates when you are under water. (不同的身體組織吸收氮氣的速率不同 !!)

- Dive tables and dive computers consider all these varying rates and eliminate the need for you to make complex calculation.(減壓表與潛水電腦協助你簡化複雜的計算 )

• Offgassing Nitrogen (氮氣的析出 )- Dissolved Nitrogen in your body tissue will be

released as you ascend. - This process occurs rapidly during the first few

minutes after your ascent, but it takes many hours for your body’s nitrogen level to return to normal

- If the change in pressure is sudden, the nitrogen in your tissues can come out of solution so rapidly that bubbles form in your body.

- These bubbles can damage tissues and cause painful condition known as decompression sickness (DCS) or classically, as the bends.

- You must control the two factors that affect in- and offgassing: time and pressure.

- Factors that can increase your change of suffering DCS: dehydration, fatigue, injuries, hard work during or shortly after a dive, effects of drugs or alcohol, obesity, or advanced age.

- DCS might be occurring to depths of 6 m.- For depths of 6.4 m or deeper, dive time limits or

no-decompression limits have been established.- Recreational diving uses shorter time limits than

those in the US Navy tables.- Any dive you make puts you at risk for DCS

- Depth limits (深度限制 ): Basic Scuba Diver (18 m); advanced Scuba Diver (30 m); the maximum recommended depth for all recreational divers is 40 m.

- Nitrogen offgassing occurs at different rates among your various body tissues.

• Residual Nitrogen (餘氮 )- It takes hours of fully absorb nitrogen into your

body or fully eliminate nitrogen from your body.- You must always take the nitrogen remaining in

your system from any previous series of dives into account when planning your next dive.

Dive Table Overview

The NAUI Dive Tables use a Letter Group designation to express the amount of residual nitrogen in your body.Different dive tables and computers provide vary information. You should agree to use the most conservative dive planning information.•Dive Table Terms (減壓表專有名詞 )-Dive Schedule/profile (潛水計畫 /剖面 ): this is an abbreviated statement giving the depth and time of the dive. It is expressed as depth/time.

- Maximum Dive Time (MDT; 最大無減壓潛水時間 ): this is the time you can spend at a given depth without having to do a required decompression stop during ascent (no-decompression limit)

- Decompression Stop(減壓停留 ): this is a point in a dive where you stop at a specified depth for a specified time during ascent to allow nitrogen offgassing before continuing your ascent or surfacing.

1. Precautionary decompression stop (安全減壓 )2. Required decompression stop (必要的減壓 )

- Actual Dive Time (ADT) (實際潛水時間 ): this is the elapsed time from the moment you begin your descent from the surface until the time you return to the surface.

- Residual Nitrogen (餘氮 ): this is the excess nitrogen remaining in your body from any dive or dives made before you have completely offgassed.

- Letter Group Designation (餘氮等級 ): This letter symbol identifies the amount of residual nitrogen you have in your system.

- Surface interval Time (SIT; 水面休息時間 ): this is the time spent on the surface between dives.

- Repetitive Dive (重複潛水 ): this is any dive that you make before you have completely offgassed from any previous dive or dives.

- Residual Nitrogen Time (RNT; 餘氮等量時間 ): this is the amount of time you must consider as already having been spent at a given depth for a planned repetitive dive. This this is based on the residual nitrogen remaining in your body from a previous dive or dives.

- Adjusted Maximum Dive Time (AMDT; 調整後最大潛水時間 ): This is the maximum Dive Time minus the Residual Nitrogen Time for a repetitive dive to a given depth.

- Total Nitrogen Time (TNT; ): This is the sum of your Residual Nitrogen Time and your Actual Dive Time following a repetitive dive. You use this total to obtain your new Letter Group at the end of the dive.

• Dive Table Rules (減壓表的規則 )- Ascend no faster than 9 meters per minute.- Use the exact or the next greater number listed in

the table for your depth- Use the exact or the next greater number listed in

the table for your time- Use the deepest depth you reached during your

dive to determine the dive schedule for your dive- Always make your deepest dive first when making a

series of dives. Plan each of your repetitive dives to a shallower depth than your previous dive

- Consider any dive shallower than 12 meters to be a 12-meter dive when planning your dives

- Surface interval time (SIT) must be at least 10 minutes between dives. If your SIT is less than 10 minutes, you must consider you second dive as a continuation of the first dive. NAUI recommends a SIT of at least one hour between dives.

- Use the next greater dive time if your dive is particularly cold or strenuous.

- Avoid dives that take you right to the no-decompression limit for any given depth and time combination

Dive Table Organization(減壓表的組成 )

• Table 1 – End-Of-Dive Letter Group (餘氮等級表 )• Table 2 – Surface Interval Time (SIT) Table (水面休息時間餘氮等級修正表 )• Table 3 – Repetitive Dive Time (重複潛水時間表 )

Dive Planning Worksheet(潛水計畫作業表 )

• Planning Your Dive• Finding your Letter Group• Recording your surface interval and finding your new

letter group• Planning your repetitive dive• Recording your repetitive dive

Maximizing Your Underwater Time

(最大化你的滯底時間 )• Limiting your dive time- You can maximize your underwater time and depth

to which you want to dive by adjusting your SIT.

• Planning your surface intervals

• Limiting your depth

Special Rules(特別規則 )

• Decompression diving- Intentionally exceeding the Maximum Dive Time is

unwise, unsafe, and discouraged. As you will learn in advanced or specialty training for deep diving, you must meet many requirements to carry out decompression dives properly. In normal recreational diving decompression dives are still discouraged, even if you meet all the requirements. Diving is only a sport – why risk injury?

• Precautionary decompression stops- You should stop at 5 meters for 3 minutes at the end

of each dive for a precautionary decompression stop or safety stop.

• Required decompression- Stop at 5 m as table 1 indicates- Minimum physical activity- It is useful to have a means of support to maintain a

constant depth during your stop.- Time spent decompressing is considered neutral

time.

• Omitted decompression- Discontinue diving for 24 hours no matter how well

you feel- Breathe 100% oxygen, if available.- Drink plenty of fluids- Rest- Watch for symptoms of DCS- You must seek medical help if you suspect that you

have DCS.**Never go back into the water to try to make up for

the omitted decompression stop.

• Cold or strenuous dives- If you get cold, do strenuous physical work during or

after a dive, or both, use the next greater time for your dive schedule.

• Flying after diving- Commercial planes pressurize their cabins to an

altitude equivalent to about 2400 m or 0.75 atm or less.

- 12 and 24 hour rules• Altitude diving

- You need to use altitude conversion tables or the NAUI Altitude Tables and follow special procedures.

Sample Problems

Using Dive Computers

• Dive computer terms• Displays• Initialization• Scrolling• Dive mode• Decompression

Conclusion (結論 )• Be familiar with using dive table (了解並會使用減壓

表 )• Dive computer and dive table can be used

alternatively• Be sure to read the manual for your dive computer

so you are completely familiar with its operation• Computers and dive tables will not eliminate the risk

of DCS. Always make a precautionary decompression stop at the end of every dive and be conservative in the use of your dive computer or dive tables.