Chapter 8 & 9 - USN Decompression Tables & Commercial Methods Flashcards
What is descent time?
The time it takes the diver to leave the surface and reach the bottom
What is bottom time (actual bottom time - ABT)?
Time the diver leaves the surface and then leaves the bottom
What is the total decompression time?
Time diver leaves the bottom and arrives on surface - also known as total ascent time
What is the total time of the dive?
Time diver leaves the surface and time they arrive back
What’s the difference between the deepest depth and the maximum depth?
Depth recorded on the gauge and then the deepest depth after the correction of the gauge reading for error
What is the decompression schedule?
A specific decompression procedure for a given depth and bottom time
What is residual nitrogen?
The excess nitrogen in a divers tissues after surfacing from a dive. If a second dive is made the nitrogen must be considered in computing the decompression requirements of the second dive
What is a repetitive group designator?
Indicates the amount of residual nitrogen remaining in a divers body
What is the residual nitrogen time?
The time that must be added to the bottom time to compensate for the nitrogen still in the divers tissues. Expressed in minutes.
RNT can be subtracted from the No D limit to calculate the repetitive dives NDL
What is an equivalent single dive?
Determines the decompression requirement for a single dive equivalent. The bottom time of the SD is equal to the sun of the RNT of the ABT
What is the equivalent single dive time?
The sum of the RNT and the bottom time of the repetitive dive. It is used to select the decompression schedule
What is surface decompression?
The decompression stops in the water are skipped and made up using a recompression chamber. The surface interval is the time the diver leaves the 40fsw water stop to the time he arrives at the 50 fsw in the recompression chamber.
Surface interval should not exceed 5 minutes
What is an exceptional exposure dive?
The risk of decompression sickness, oxygen toxicity and exposure to the elements is greater than a normal working dive
Requires approval of the CNO (chief of naval operations)
They are…
Dives deeper than 190 fsw
Total decompression time greater than 90 minutes
SurDo2 dive with a chamber oxygen time greater than 120 minutes
What is the descent rate of air decompression tables?
Should not exceed 20m/min
What is the ascent rate for air decompression tables?
9m/min
Should be between 6-12m/min
Surface decompression ascent rate is 40fsw/min
What is the decompression stop time of the air decompression tables?
Begins when the diver arrives at the stop and ends when they leave
What is the last water stop?
6m
When is a diver eligible for surface decompression?
When they have completed a 12m stop
What does RGD mean on the no decompression table?
Repetitive group designation
What do no decompression tables not include?
Decompression information or predicted ascent times
What should you do if the diver had been working hard?
The schedule should be taken to the next longest time and that RGD used
What depth stops can 100% oxygen be used from on air decompression tables?
30 ft and 20 ft stops
What do the chamber O2 periods mean on the decompression tables?
They are the fraction of a time period on oxygen when each time period is 30 mins
- 5 = 15 minutes
- 0 = 60 minutes
What must happen after every 30 minute period of 100% O2?
A 5 minute air break
When can shallow water diving tables not be used?
When diving in the sea and the diver is contouring the seabed because it is impossible to accurately forecast the exact maximum depth
How do you calculate the new RGD for repetitive dives?
Using the surface interval tables
How do you calculate the new time allowed for a repetitive dive?
RNT must be calculated a.k.a penalty time
Need to know new RGD and anticipated depth for next dive
This is subtracted from the time you would be allowed to have on a single no deco stop dive
What does a double asterisk mean on repetitive dive tables?
If the dive is 20ft or shallower the nitrogen loading associated with the depth is actually higher than what would actually be achieved. If there is a double asterisk you can still perform the repetitive dive but at the end of the dive the repetitive group remains unchanged and the no deco time is unlimited. They can spend any amount of time there
What is the residual nitrogen time exception rule?
When the surface interval is short the diver can add the bottom times together and take the deepest depth
What is a dive profile?
A graphical representation of a dive including all relevant decompression information
What must you display on a dive profile?
Left surface Arrived surface Depth Deco stops Left bottom Repetitive group designation Schedule used Surface interval and RNT The gas mix
What do special procedures exist for?
Fast ascents, slow descents and omitted decompressions
What is a fast ascent?
Rate greater than 40 ft/min
What do you do if you have an early arrival at the first decompression stop?
Begin timing the first stop when the required travel time had been completed
What should you do if your ascent has a delay deeper than 50 ft?
Add the delay time to the bottom time of the dive and decompress the diver accordingly
What should you do if you have a delay shallower than 50 ft on ascent?
Add the delay time to the divers first decompression stop. If on a non stop dive create a 20ft stop for the delay time
What is a stage?
A decompression stage is a bar on which the diver stops whilst decompressing, usually including reserve air supplies
Why do dives at altitude require more decompression?
Because if reduced atmospheric pressure
What is cross correction?
Correcting the depth of the altitude dive to obtain the highest equivalent sea depth then determining the the decompression requirement using standard tables
What is equilibration at altitude?
Two effects taken into account before diving at altitude. The first process is called equilibration (12 hours) and the second part is called acclimatization which takes a lot longer than 12 hours
What are lifeline signals?
Pre arranged tugs
Consist of bells and pulls
Should be firm but gentle
What is a pull?
A long steady movement on the line
What is a bell?
Always come in pairs and are short sharp movements
Which signals are not acknowledged by the tender?
One pull when the diver has left the bottom
The emergency signal of 5+ pulls
What are the qualities of a lifeline?
Strong enough to have a safe working load
Broad enough
What are Plymouth universities lifelines like?
12mm polypropylene 30 m in length Red tape = 15 m Yellow = 3m Black = 0m
What are the duties of the tender?
Complete full equipment checks Handle the lifeline Send and acknowledge signals Report to the supervisor Watch for signs of trouble
What must the tender report to the supervisor?
Leak checks Left surface Left bottom Made stop Left stop On suface Emergency signals Fouled Depths during ascent and descent
What are the duties of the standby diver?
Be aware of the number of divers in the water, the task and where the divers are
What would the standby do if called in?
Gets incident and instructions from supervisor
Enters the water and moves to the lifeline
Standby leaves surface
Follows the line to the diver freeing minor fouling
What does the standby do if all is well with the diver?
Found diver signal on standbys lifeline
All well on divers lifeline
Request to come up
What does the standby diver do if the diver is well but seriously fouled?
Found diver on standby divers lifeline
Fouling sorted
All well on divers lifeline
Request to come up
What does the standby do if the diver is found incapacitated but the lifeline isn’t fouled?
Found diver signal is passed
Bailout Valve opened on full face mask
Standby holds diver
5+ pulls
Hoist diver to the surface at a controlled rate
Standby rides the lift
Topside team takes over when reached surface
