All the stations I was at,had Fog Sirens powered by Kelvin Diesel Engines,either K2 Two Cylinder(44hp.)or K3 Three cylinder (66hp) driving Sentinel Air Compressors.Working pressure about 24 psi.
Each fog signal has it's own sequence eg.three blasts every ninety seconds.
Visibilty closing in,time to start the Fog Horn.
Prime the carburettor with petrol.
Press the starter button
Activates alarm system (Via time delay ),engages starter motor.
Hand over carburetter bowl,as engine starts sucking in petrol gradually lift hand away as cylinders create enough vacuum to draw own petrol in.
After a few minutes operate the petrol/diesel changeover lever to allow running on diesel and isolate high tension leads
Disengage magneto impulse starter
Adjust throttle to appropriate setting
Open Valve B (Allows air from common air pipe to receivers)
Open Valve C (allows air from individual compressors to common air pipe
Close valves on Compressor(allows compressor to operate)
Operate the valves in the wrong sequence and you would have safety valves operating with an ear piercing whistle.
Watch the Air Receiver pressure
When it reaches the appropriate level
Open Valve D (allows air to the coding(timing)mechanism)
Open Valve A (Allows air to the siren)
This was general practise at most stations,with variations at some:-
Maughold Head had Air Receivers outside the engine room connected via another valve to Air Receivers by the tower
Which meant opening Valve A then opening the Valve to the other Receivers and only when pressure had built up in all Receivers opening Valve D to allow coding mechanism to start.
Point of Ayre had Receivers near the engine room and further Receivers by the horn,
In this instance when Valve A was opened,air was fed via a one way valve to the horn Receivers and also to the coding mechanism.
Which meant when the siren started,pressure was still low,which used to give a very low note to start off with..
There was a chart recorder connected to the air pipe supplying the horn,so we had a permanent record of Fog Signal operation.
If we had not operated the fog signal that day,we moved the pen out at midnight,so we had a record of not blowing as well.
On monday mornings,if we had not blown in the previous seven days we used to have a practise run for half an hour.
The Coder Mechanism.
When air first arrives at the coder it powers an air rotor which winds up a weight(just out of sight) on the chain
The weight falling then powers the rest of the clockwork mechanism as well as operating a valve to rewind the weight at the appropriate times
Coder speed governed by a pendulam escapement.
One valve opens to allow air to an air rotor which powers the siren and brings it up to speed.
Cams on the coder operate valves to allow air to the siren,where it modulates the air flow to give the required note.(The siren has an adjuster so that you can tune it to the right note.)
And the horn did sound.
When visibility improves
Press stop button-switches off alarm system
open compresessor valves
close valves C and B
wait until end of sequence
close valves A and D
move petrol/diesel changeover lever to center position
when engine stops. set lever to petrol position
engage magneto impulse starter.
clean and oil as required.
Remember to change Fog Recorder chart at midnight.
Point of Ayre 2010
Horns about 4 meters long
Horn diameter 1.37 meters
Point of Ayre 1992
Siren bases + valve chest + coder mechanism
Geen support shown in position to lower siren and swing out to service siren.
There are two cylinders with slots,one fixed and one rotated by the air rotor,the air rotor is supplied with air via the coding mechanism,timed to reach correct speed as the main valve is operated,the main air supply is modulated(chopped)by the air passing through the revolving rotor and out through the horn to give the sound.
The pitch(tone) of the siren is varied by cork pads on adjustable spring arms to control the speed of the rotor.
A siren last used in1987 these pictures taken 2004.
Spring Tension Adjustment Lever
Siren Drive Rotor-showing air supply pipe which has a nozzle to direct air onto blades.
Siren Speed Control(Slight Govenor)
As the rotor speeds up centrifugal force throws the govenor pads out into contact with the stationary rim and slows the rotor down,spring tension is varied by the drive adjuster rotating the tension adjuster arm-adjusting effective spring length.
As far as I remember the alarm system monitored water temperature and oil pressure.
The alarm system provided "semi-automatic" operation which allowed a combined lightroom and fog signal watch.