Chapter 10 - Communication Systems And Full Face Masks

Question 1

What is the relation ship of sound intensity and distance?

Answer 1

Sound intensity is inversely proportional to the squared distance from the source. This is known as spherical spreading

Question 2

What are some of the other factors influencing sound intensity?

Answer 2

Some energy will be absorbed and converted into heat (attenuation)
Some of the energy will be scattered by objects
Surface and bottom may reflect sound back into the water
Bending of sound from temperature
Water density - temperature, salt content and static pressure

Question 3

What has the greatest effect on sound transmission?

Answer 3

Variations in water temperature

Temperature decreases as you go down therefore the velocity of sound decreases with depth

Question 4

What is the shadow zone?

Answer 4

A point beyond which sound is not received. It is determined by the rate of change of temperature with depth, the depth of the source and the depth at which the reception is made

Question 5

Why must divers report thermocline depths?

Answer 5

Because thermocline affect ultrasonic signals.

Question 6

When does the velocity sound transmission change with temperature?

Answer 6

1% for a 6 degree temperature change. The resultant bending of the sound has great affect over a distance of several hundred metres

Question 7

What is a zone of silence?

Answer 7

Formed at the backside of a blocking object

Question 8

What is the description of a full face mask?

Answer 8

It’s ideal for working in polluted waters as it covers the eyes mouth and nose. However air sharing techniques don’t work x

Question 9

What are the two types of FFM?

Answer 9

Neutral pressure and positive pressure

Question 10

What’s the difference between neutral pressure and positive pressure full face masks?

Answer 10

Neutral - pressure in the mask is the same as that in the surroundings. If it leaks water will enter the mask

Positive - pressure in the mask is slightly higher than the ambient pressure but it is normally retained in the mask seal around the face. If the mask leaks air escapes rather than letting water in

Question 11

How do you fit the full face mask?

Answer 11

By the use of a spider. It is a five legged mask strap that is tightened from the bottom up

Question 12

What does a AGA divator full face mask consist of?

Answer 12

Full face mask with integral second stage
First stage with whips and gauge
Harness and gas supply

Question 13

What is the construction of the AGA divator?

Answer 13

Inner rubber oro-nasal that minimises the build up of CO2

Separate inhalation and exhalation channels

Question 14

How does the air flow on the inhalation?

Answer 14

Flows from the second stage through the demister ports and over the inside of the visor which helps to cool the diver and prevent condensation.

Question 15

How does air flow on the exhalation?

Answer 15

The chamber pressure rises and the diaphragm disc is pressed outwards engaging the inner diaphragm with the positive pressure diaphragm. The outlet is opened to allow exhaled air out

Question 16

How do you clear your ears in a full face mask?

Answer 16

The standard manoeuvre isn’t possible so a rubber block is inserted into the oro-nasal mask. The diver places their nostrils against the block and blows gently to clear the ears.

Question 17

How does the positive pressure flow switch work?

Answer 17

Should be in the off position
When diver takes their first breath the flow switch is triggered

Before the mask is removed the flow switch should be put back to the off position

Question 18

What is a manifold aka bailout block?

Answer 18

Used to combine the pressure whips from the main and bailout cylinders

Question 19

How does the main air supply pass gas to the diver?

Answer 19

Via a non return valve

Always open to the diver

Question 20

How is the bailout air supplied?

Answer 20

By opening a valve on the bailout block

Question 21

What does the non return valve do?

Answer 21

Designed to stop the alternate bailout air supply being lost if there is a problem with the main supply

Question 22

What does the pressure relief valve do?

Answer 22

If there is a free flow in the first stage gas will escape via this valve. It is kept close during normal diving

If the main pressure falls to below 50 bar you can manually open the valve and breathe from the bailout supply but let the supervisor know and normally ascend

Question 23

What are the 2 types of hardwire comms?

Answer 23

Simplex - diver is normally monitored until surface operator presses the push to talk button

Duplex - listen and talk simultaneously like a phone, more costly

Question 24

What does a transducer do?

Answer 24

Transmits and receives to the diver

Question 25

How would you call up a diver?

Answer 25

Diver one this is topside

Topside this is diver one go ahead

Question 26

How can the diver better hear communications from topside?

Answer 26

Stop work
Control breathing
Look down
Concentrate

Question 27

What is the bottom call for when diver has made bottom?

Answer 27

Made bottom
Depth by gauge
Bottom type
Visibility 
Current

Question 28

What are the bottom calls for when the diver is ready to leave the surface?

Answer 28

Made shot
Max depth by gauge
Contents main and bailout
Ready to leave bottom

Question 29

How do you check comms?

Answer 29

Topside this is diver 1, comms check
Diver one this is topside comms good how me
Topside this is diver 1 loud and clear

Question 30

How do you do a callout in ffm?

Answer 30

Turn on air 
Buoyancy
Clips 
Turn off divers main
Breathe down air
Open bailout block
Check NRV
On bailout, bailout reads, non return valve working
Close bailout block
Indicate when out of air 
Restore main air supply 
On main, main reads

Question 31

What should you do when figures need to be passed over comms?

Answer 31

Use the word decimal

For example 6.5 would be six decimal five meters

Question 32

What should you check on the full face mask?

Answer 32

Spider straps
Fittings
Faceplate for scratches
Oro nasal
Nose dam
Mushroom valves