Chapter 10 - Communication Systems And Full Face Masks Flashcards
What is the relation ship of sound intensity and distance?
Sound intensity is inversely proportional to the squared distance from the source. This is known as spherical spreading
What are some of the other factors influencing sound intensity?
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
What has the greatest effect on sound transmission?
Variations in water temperature
Temperature decreases as you go down therefore the velocity of sound decreases with depth
What is the shadow zone?
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
Why must divers report thermocline depths?
Because thermocline affect ultrasonic signals.
When does the velocity sound transmission change with temperature?
1% for a 6 degree temperature change. The resultant bending of the sound has great affect over a distance of several hundred metres
What is a zone of silence?
Formed at the backside of a blocking object
What is the description of a full face mask?
It’s ideal for working in polluted waters as it covers the eyes mouth and nose. However air sharing techniques don’t work x
What are the two types of FFM?
Neutral pressure and positive pressure
What’s the difference between neutral pressure and positive pressure full face masks?
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
How do you fit the full face mask?
By the use of a spider. It is a five legged mask strap that is tightened from the bottom up
What does a AGA divator full face mask consist of?
Full face mask with integral second stage
First stage with whips and gauge
Harness and gas supply
What is the construction of the AGA divator?
Inner rubber oro-nasal that minimises the build up of CO2
Separate inhalation and exhalation channels
How does the air flow on the inhalation?
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.
How does air flow on the exhalation?
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
How do you clear your ears in a full face mask?
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.
How does the positive pressure flow switch work?
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
What is a manifold aka bailout block?
Used to combine the pressure whips from the main and bailout cylinders
How does the main air supply pass gas to the diver?
Via a non return valve
Always open to the diver
How is the bailout air supplied?
By opening a valve on the bailout block
What does the non return valve do?
Designed to stop the alternate bailout air supply being lost if there is a problem with the main supply
What does the pressure relief valve do?
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
What are the 2 types of hardwire comms?
Simplex - diver is normally monitored until surface operator presses the push to talk button
Duplex - listen and talk simultaneously like a phone, more costly
What does a transducer do?
Transmits and receives to the diver
