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HSE Scuba > Chapter 8 & 9 - USN Decompression Tables & Commercial Methods > Flashcards

Chapter 8 & 9 - USN Decompression Tables & Commercial Methods Flashcards

1
Q

What is descent time?

A

The time it takes the diver to leave the surface and reach the bottom

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2
Q

What is bottom time (actual bottom time - ABT)?

A

Time the diver leaves the surface and then leaves the bottom

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3
Q

What is the total decompression time?

A

Time diver leaves the bottom and arrives on surface - also known as total ascent time

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4
Q

What is the total time of the dive?

A

Time diver leaves the surface and time they arrive back

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5
Q

What’s the difference between the deepest depth and the maximum depth?

A

Depth recorded on the gauge and then the deepest depth after the correction of the gauge reading for error

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6
Q

What is the decompression schedule?

A

A specific decompression procedure for a given depth and bottom time

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7
Q

What is residual nitrogen?

A

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

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8
Q

What is a repetitive group designator?

A

Indicates the amount of residual nitrogen remaining in a divers body

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9
Q

What is the residual nitrogen time?

A

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

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10
Q

What is an equivalent single dive?

A

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

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11
Q

What is the equivalent single dive time?

A

The sum of the RNT and the bottom time of the repetitive dive. It is used to select the decompression schedule

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12
Q

What is surface decompression?

A

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

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13
Q

What is an exceptional exposure dive?

A

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

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14
Q

What is the descent rate of air decompression tables?

A

Should not exceed 20m/min

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15
Q

What is the ascent rate for air decompression tables?

A

9m/min

Should be between 6-12m/min

Surface decompression ascent rate is 40fsw/min

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16
Q

What is the decompression stop time of the air decompression tables?

A

Begins when the diver arrives at the stop and ends when they leave

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17
Q

What is the last water stop?

A

6m

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18
Q

When is a diver eligible for surface decompression?

A

When they have completed a 12m stop

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19
Q

What does RGD mean on the no decompression table?

A

Repetitive group designation

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20
Q

What do no decompression tables not include?

A

Decompression information or predicted ascent times

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21
Q

What should you do if the diver had been working hard?

A

The schedule should be taken to the next longest time and that RGD used

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22
Q

What depth stops can 100% oxygen be used from on air decompression tables?

A

30 ft and 20 ft stops

23
Q

What do the chamber O2 periods mean on the decompression tables?

A

They are the fraction of a time period on oxygen when each time period is 30 mins

  1. 5 = 15 minutes
  2. 0 = 60 minutes
24
Q

What must happen after every 30 minute period of 100% O2?

A

A 5 minute air break

25
Q

When can shallow water diving tables not be used?

A

When diving in the sea and the diver is contouring the seabed because it is impossible to accurately forecast the exact maximum depth

26
Q

How do you calculate the new RGD for repetitive dives?

A

Using the surface interval tables

27
Q

How do you calculate the new time allowed for a repetitive dive?

A

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

28
Q

What does a double asterisk mean on repetitive dive tables?

A

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

29
Q

What is the residual nitrogen time exception rule?

A

When the surface interval is short the diver can add the bottom times together and take the deepest depth

30
Q

What is a dive profile?

A

A graphical representation of a dive including all relevant decompression information

31
Q

What must you display on a dive profile?

A 
Left surface
Arrived surface
Depth
Deco stops
Left bottom
Repetitive group designation 
Schedule used
Surface interval and RNT
The gas mix
32
Q

What do special procedures exist for?

A

Fast ascents, slow descents and omitted decompressions

33
Q

What is a fast ascent?

A

Rate greater than 40 ft/min

34
Q

What do you do if you have an early arrival at the first decompression stop?

A

Begin timing the first stop when the required travel time had been completed

35
Q

What should you do if your ascent has a delay deeper than 50 ft?

A

Add the delay time to the bottom time of the dive and decompress the diver accordingly

36
Q

What should you do if you have a delay shallower than 50 ft on ascent?

A

Add the delay time to the divers first decompression stop. If on a non stop dive create a 20ft stop for the delay time

37
Q

What is a stage?

A

A decompression stage is a bar on which the diver stops whilst decompressing, usually including reserve air supplies

38
Q

Why do dives at altitude require more decompression?

A

Because if reduced atmospheric pressure

39
Q

What is cross correction?

A

Correcting the depth of the altitude dive to obtain the highest equivalent sea depth then determining the the decompression requirement using standard tables

40
Q

What is equilibration at altitude?

A

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

41
Q

What are lifeline signals?

A

Pre arranged tugs
Consist of bells and pulls
Should be firm but gentle

42
Q

What is a pull?

A

A long steady movement on the line

43
Q

What is a bell?

A

Always come in pairs and are short sharp movements

44
Q

Which signals are not acknowledged by the tender?

A

One pull when the diver has left the bottom

The emergency signal of 5+ pulls

45
Q

What are the qualities of a lifeline?

A

Strong enough to have a safe working load

Broad enough

46
Q

What are Plymouth universities lifelines like?

A 
12mm polypropylene 
30 m in length
Red tape = 15 m
Yellow = 3m
Black = 0m
47
Q

What are the duties of the tender?

A 
Complete full equipment checks
Handle the lifeline
Send and acknowledge signals
Report to the supervisor 
Watch for signs of trouble
48
Q

What must the tender report to the supervisor?

A 
Leak checks
Left surface
Left bottom
Made stop
Left stop
On suface
Emergency signals
Fouled
Depths during ascent and descent
49
Q

What are the duties of the standby diver?

A

Be aware of the number of divers in the water, the task and where the divers are

50
Q

What would the standby do if called in?

A

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

51
Q

What does the standby do if all is well with the diver?

A

Found diver signal on standbys lifeline
All well on divers lifeline
Request to come up

52
Q

What does the standby diver do if the diver is well but seriously fouled?

A

Found diver on standby divers lifeline
Fouling sorted
All well on divers lifeline
Request to come up

53
Q

What does the standby do if the diver is found incapacitated but the lifeline isn’t fouled?

A

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

HSE Scuba (13 decks)

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