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60
BRITISH KINEMAIOGRAPHY
Vol. 23, No. 3
The cable itself is standard B.I. camera cable consisting of 4 coaxial cables and 28 single conductors, the only addition being an extra 100 mil P.V.C. sheath over the standard sheath ; the diameter of the cable is slightly under one inch. P.V.C. although very durable, has the disadvantage that the cable is difficult to handle in cold weather due to the hardening effect — a condition often found in under water photography.
Design of suitable Pressure Casings
The design of pressure casings appears at first a simple task solvable by following the old well-tried principles, but certain special features more or less govern the design.
Having laid down the maximum depth of operation and margin of safety required, the container thickness and shape can be calculated. A spherical container is the ideal, but as it has to house a camera rectangular in
correctly proportioned give the best results. Porosity in the glanding material cannot be tolerated.
From the two figures in Fig. 3 it will be seen that b is purely a variant of a, the difference lying in the spigot shape ; the glanding material must not be of the type that gives a permanent set when mated for long periods and the shore hardness should be carefully chosen.
The grooved spigot at B ensures a longer water path in the event of leak and allows the glanding material room to move with increasing pressure ; when the joint is made initially it need not be fully tightened, the pressure at increased depth giving a self sealing joint.
It will be noted that both spigots have a " lead " in the glanding groove and that the groove is only partly filled with glanding
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Fig. 2
shape, this results in a casing with a fair amount of waste space which is larger than desirable. A cylindrical container is easier to manufacture and gives less wasted space internally. Although tides will have greater effect the housing can be finned so that it takes position in the tide.
Glanding
All glands must be watertight to a pressure exceeding the working depth and to the
irgin of safety desired. This can be considerable and at a 1,000 feet working is to the order of 440 lbs. per square inch. Many types have been tried but the two types shown
(a)
(b)
Fig. 3
material. This ensures concentricity so that mis-mating which would be liable to happen were this precaution not taken, cannot occur.
P.V.C. and rubber of requisite shore hardness have been used as glanding materials. Although P.V.C. has a slow recovery from permanent set it does not shear so readily as some rubbers.
Underwater Visibility
Visibility under water can vary over a wide range. Most British coastal waters are rather poor in this respect, particularly in the neighbourhood of estuaries such as the Thames and the Mersey, where visibility is