Dive Theory: Decompression Theory and the Recreational Dive Planner Flashcards
actual bottom time (abt)
Used in repetitive diving. ABT is added to residual nitrogen time, the sum gives you the total bottom time.
In essence it is the total time actually spent underwater from the beginning of descent until leaving the bottom for a direct continuous ascent to the surface or safety stop.
adjusted no decompression limit
The time limit for a repetitive dive that accounts for residual nitrogen. Your actual bottom time should never exceed the adjusted no decompression limit.
ascent rate
The proper speed for ascending, which is no faster than 18 meters/60 feet per minute. A slower rate is acceptable and appropriate.
bottom time
Time from the beginning of descent until beginning a direct, continuous ascent to the surface or safety stop.
decompression diving
Diving that requires planning stops during ascent to avoid decompression sickness.
In rec diving, a decom stop is considered an emergency procedure only, and never an intentional part of the dive plan.
multilevel diving
tbd
dive profile
A drawing of your dive plan including depth, time at depth, before and after pressure groups, surface intervals, etc.
multilevel limit (ML)
Max allowable no decom time you have on each level of a multilevel dive.
no decompression limit (NDL)
Max time that can be spent at a depth before decompression stops are required. Also called “no-stop time”
no stop dive
A dive made within no decompression limits because you don’t have any required emergency decompression stops. Also called a “no decompression dive”
pressure group
A letter used on the recreational Dive Planner to designate the amount of theoretical residual nitrogen in your body.
repetitive dive
A dive that follows another while there’s still a significant amount of residual nitrogen in your body.
Using the Recreational Dive Planner, generally this is a dive made within 6 hours of a previous dive.
residual nitrogen
The higher than normal amount of nitrogen remaining in your body after a dive.
residual nitrogen time (RNT)
An amount of nitrogen, expressed in minutes, for a specific depth added to the actual bottom time of a dive to account for residual nitrogen from a previous dive.
safety stop
A stop usually at 5 meters/15 feet for three or more minutes at the end of a dive for additional safety. Recommended after all dives.
Required for dives below 30m/100ft and those coming within 3 pressure groups of the no decompression limit.
surface interval (SI)
The time spent on the surface between two dives - usually recorded in hours:minutes.
total bottom time (TBT)
The sum of Residual Nitrogen Time and Actual Bottom Time of a repetitive dive used on Table 1 of the RDP Table to determine the pressure group following the repetitive dive.
You don’t use TBT eRDPML because it does the addition and displays the new pressure group automatically.
Who is credited in coming up with the basic decompression model we use today?
John Haldane
What are the seven basic decompression concepts developed by Haldane?
- Upon descent, nitrogen dissolves from the air - higher pressure - into body tissues - lower pressure
- Nitrogen continues to dissolve into the until the body saturates and absorbs no more nitrogen at that depth.
- Upon ascent, nitrogen dissolved in the body - tissue pressure - is now higher than surrounding pressure. Nitrogen dissolves out of the tissue into the breathing air and is exhaled from the body
- The difference between the dissolved nitrogen pressure and the surrounding pressure - whether ascending or descending - is called the pressure gradient.
- On ascent, tissues can tolerate some gradient of high tissue pressure without DCS occurring. Nitrogen in solution dissolves harmlessly out of tissues.
- If the pressure gradient exceeds acceptable limits, then dissolved nitrogen comes out of solution faster than the body can eliminate it through respiration and circulation. If acceptable limits are exceeded, nitrogen bubbles form, causing DCS.
- DCS can be avoided by keeping the gradient within acceptable limits. Gradient too great, decompression stops needed or slower ascent.
What is meant by “compartment” in relation to Haldanean decompression model.
Different parts of the body absorb and release dissolved nitrogen at different rates.
Haldane constructed a mathematical model consisting of multiple theoretical tissues.
Theoretical tissues don’t correspond to any particular body tissues, they simply model that the body doesn’t absorb and release nitrogen on a singular time scale.
What is the relationship between the human body and Haldane decompression model and how far can you rely on the model?
There is no direct relationship between the model and the body. Actual dive data imply and support the relationship but like all models, decompression models have limits of reliability.
pressure gradient
Difference between the dissolved nitrogen pressure and the surrounding pressure.
What is ‘halftime’ in relation to Haldanean decompression model?
Time in minutes for a particular compartment to go halfway from its beginning tissue pressure to equilibrium - saturation - at a new depth.
50% equilibrated after one halftime, 75 after two, 87.5 after 3, etc.
After six, the compartment is considered 100 percent equalized.
Haldanean Half-time ranges.
Haldanean’s original model had half times ranging from 5min to 75min.
Modern models 3min to 600min.
Often called slow (600min) and fast (3min) compartments.
What is ‘M-Values’ in relation to Haldanean decompression model?
Maximum tissue pressure allowed in the compartment when the diver surfaces, so as to prevent exceeding the maximum acceptable gradient.
Exceeding the M-value
Unacceptable risk of decompression sickness
M-Value and no decompression diving.
Only concerned with the surfacing M-Value (M0-value).
M-Value relationship to compartments?
The faster the compartment (halftime length) the higher its M-Value. With slower compartments having lower M-Values.
Haldanean model consists of several _______, each with a _______ in minutes that represent how fast is absorbs and releases nitrogen, and a(n) ________ that sets the max nitrogen pressure it can have when a diver surfaces.
compartments
halftime
M-Value
A 60-minute tissue compartment requires _______ to fill or empty completely, while a 120-minute compartment requires _______.
6 hours
12 hours
60 min x 6 = 6 hours
120 min x 6 = 12 hours
halftime repeated 6 times, compartment considered 100% full or empty.
There is _______ relationship between the Haldanean model and the human body; you can rely on it only as far as it is supported by _______.
no direct
test and field data
The time limit for a repetitive dive that accounts for residual nitrogen remaining in your body from a previous dive.
Adjusted no decompression limit.
Used in repetitive diving and added to the Residual Nitrogen Time to equal the Total Bottom Time.
Actual Bottom Time
The time from the beginning of descent until beginning of a direct, continuous ascent to the surface or safety stop.
Bottom Time
The time spent on the surface between two dives - usually recorded in hours:minutes.
Surface Interval
An amount of nitrogen expressed in minutes, for a specific depth added to the actual bottom time of a dive to account for the residual nitrogen from a previous dive.
Residual nitrogen time
The sum of the Residual Nitrogen Time and the Actual Bottom Time of a repetitive dive used on Table 1 of the RDP table to determine the Pressure Group (PG) following repetitive dives.
Total Bottom Time (TBT)
A stop usually at 5m/15ft for three or more minutes at the end of a dive for additional safety
safety stop
The higher-than-normal amount of nitrogen remaining in your body after a dive
Residual nitrogen
A letter used on a rec dive planner (RDP) to designate the amount of theoretical residual nitrogen in your body
Pressure Group (PG)
Non rec diving that requires planning stops during ascent to avoid decompression sickness
Decompression Diving
A dive made within no decom limits because you don’t have any required emergency decom stops
No stop dive
The proper speed for ascending, which is no faster than 18m/60ft per minute
Ascent rate
The max time that can be spent at a depth before decompression stops are required.
No decompression limit (ndl)
A dive that follows another while there is still residual nitrogen in your body.
Repetitive dive
Why was the 1950’s version of the US Navy table at one time the standard for rec diving?
Developing a table was tedious.
Many early divers and instructors came from the military.
The tables were widely available.
What is basis for the US navy table’s repetitive diving surface interval credit?
Navy designed an easy to use surface interval credit table based on the worst-case scenario - a repetitive dive preceded by a decompression dive.
At the time of the original development, the navy based repetitive credit on the slowest compartment, the 120 minute halftime.
The newest version of the U.S.N tables (2008rev) uses even longer halftimes.
Testing based upon all male, fit and in their 20s and 30s. Test criteria = DCS or no DCS. Doppler checking for bubbles didn’t exist then.
Based on Decompression Diving. Might be to conservative for recreational diving, because rec diving is no decom diving.
The early version of the US Navy tables were the standard for rec diving because?
Before the rise of computers, it was beyond the practical abilities of most people to develop a table.
They were widely available.
Many early divers were former military divers used to them already.
The basis of surface interval credit with the 1950s version of the US Navy tables is the ________ minute compartment.
120
Why does the recreational dive planner use a different basis for repetitive dive credit from that used by the 1950s US Navy table?
1) 120 minute halftime for calculating washout is overly conservative for exclusively no decom diving. US Navy tables developed for decompression diving. Rec diving is no decom or no stop diving.
2) Test group used by Navy (20s and 30s fit male) doesn’t represent rec diving population.
3) Doppler ultrasound flow meters came into being. This tech found “silent” asymptomatic bubbles in test subjects diving to US Navy table limits. Made clear tables could be overly conservative or insufficiently conservative, depending on the circumstances.
Note: surface interval credit can be less conservative when limited to no decom diving.