Pressure compensation device for an immersed volume of gas, in particular for an underwater electroacoustic transducer

This pressure compensating device for a volume of immersed gas is equipped with mobile or deformable walls and includes an immersed compensation bladder. The bladder is secured in a housing and communicates with the volume to be compensated. It is arranged by mounting means roughly at the level of the volume. According to the invention, the housing mounting means for the compensation bladder are arranged to allow the housing to move vertically relative to the gas volume so as to keep the mean front of the bladder folds at the level of the surfaces to be compensated, as the volume may be immersed to different depths. The housing mounting means include an arm hinged about a horizontal axis, and a spring exerting its force on a cable connected to the hinged arm. Depending on the preferred application, the mobile walls enclosing volume to be compensated consist of the acoustic drives of underwater electroacoustic transducers.

BACKGROUND OF THE INVENTION 
TECHNICAL FIELD OF THE INVENTION 
This invention relates to a device for compensating the pressures inside 
and outside of an immersed volume of gas, and chiefly of air. It is 
equipped with mobile or deformable walls whose positions are not to be 
affected by the depth variations of the volume. 
A typical example of application comprises of underwater electroacoustic 
transducers used for sweeping acoustic mines. 
The pressures inside and outside of the diaphragms forming the drives of 
these electroacoustic transducers must be kept equal in order for them to 
operate correctly. This is done by mounting a flexible compensation 
bladder in a housing on the rear of the minesweeping vehicle, with the 
bladder in communication with the volume to be balanced. The housing 
mounting means positions the housing at the general level of the volume to 
be compensated. 
The principle of the pressure compensation device is as follows. Any 
variation in the depth at which the vehicle is navigating varies the 
hydrostatic pressure applied to the diaphragms of the electroacoustic 
transducers, and simultaneously compresses or expands the compensation 
bladder until the internal and external pressures of this volume are 
equalized. This compensation is performed independently and automatically, 
and therefore seems to be satisfactory; but it does suffer from one 
limitation, in that it allows a nominal navigation depth of only about 8 
m, with possible 4 m variations above and below this nominal depth. 
SUMMARY OF THE INVENTION 
The purpose of the invention is to propose an immersed volume compensating 
device compatible with deep immersions and large variations above and 
below the nominal immersion depth. As an example, a nominal immersion 
depth of about 60 m is considered, with variations of 30 m above or below 
that depth. 
It is not enough to simply adapt the technique that exists already for 
shallow depths, because the great differences in immersion depth 
correspond to large variations in hydrostatic pressure and hence require 
relatively large bladder volume variations. Yet a flexible bladder of 
large capacity first deforms, when immersed, in its lower part where the 
hydrostatic pressure is highest. This lower part will then move with 
respect to the fixed housing and therefore to the volume, so that there 
will be a variable difference in height between the mean level of this 
lower part and the fixed mean level (with respect to the compensated 
volume) of the moving walls to be compensated. This will cause a 
difference in the variable internal and external pressures at the level of 
the walls to be compensated, so that the equal pressure desired will not 
be achieved. 
This difficulty is solved in accordance with the invention by the fact that 
the housing mounting means securing the bladder is arranged to allow the 
housing to move vertically relative to the volume of gas, so as to keep 
the mean front of the bladder folds at the level of the walls to be 
compensated as the volume of gas is immersed more or less deeply. 
Advantageously, the housing mounting means includes a guiding means along 
at least one vertical component, and one elastic recall means. The former 
advantageously includes an arm, integral with the housing and hinged about 
a horizontal axis, while the latter means includes a spring exerting its 
force on a cable connected to the hinged arm, preferably through a sector 
integral with the arm and about which the cable bends partially. 
It is advantageous to provide two housings, one on each side of the volume 
to be compensated. 
The preferred application of the invention is the compensation of the 
acoustic drives of underwater electroacoustic transducers.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The immersed volume of air 1 is watertight and includes one mobile wall 
that remains unaffected by any variations in depth above or below a 
nominal depth of immersion. Such a mobile wall may consist of the 
diaphragm of an underwater electroacoustic transducer. 
The horizontal axis of the wall to be compensated is designated by 0 and 
the horizontal plane containing this axis by H. 
To either side of the volume 1, two mobile housings 2 are provided 
containing a flexible bladder 4 (FIG. 2) whose internal volume is 
connected by a flexible communication, diagrammed in FIG. 3, to the inside 
of the air volume 1. 
The housings 2 include orifices, not shown, in their walls that communicate 
with the outside medium, i.e. with the environing water, in such a way 
that the bladders 4 they contain are subjected to the ambient hydrostatic 
pressure. As the hydrostatic pressure exerted on the bottom of the bladder 
4 is greater than that exerted on the top, the bladder 4, subjected to a 
pressure that increases with the depth of immersion, will contract from 
bottom to top and, in so doing, will form folds 4' defining a roughly 
horizontal front. 
According to the principle of the invention, illustrated very schematically 
in FIGS. 1A, 1B and 1C, the mounting means of housings 2 with respect to 
the volume 1 are such that the mean lower front 4' of the bladder 4 is 
always located roughly in the plane H containing the axis 0 of the mobile 
walls of volume 1 to be compensated, and remains so through a range of 
variation of immersion depth to either side of the nominal immersion depth 
of volume 1. 
To do so, it is best, as FIGS. 1A and 1C show, that the housings be mobile 
between: 
a high position (FIG. 1A) where the bladders are not contracted in the 
housings 2, i.e. the low point of the housings 2 is roughly tangent to the 
plane H; 
and a low position (FIG. 1C) where the bladders are completely compressed 
(or nearly so) in the housings 2, which are then filled with water 5, and 
are roughly tangent to the plane H at the level of their upper part. 
Between these two extreme positions exist intermediate positions such as 
those of FIG. 1B, where the bladders are partly contracted in the housings 
2, still in such a way that the mean front of the folds 4' of the bladder 
are roughly in the plane H. 
Under these conditions, it is understood that, in conformity with the 
invention, the air pressure in the compensation bladders 4 is always equal 
to the hydrostatic pressure at the level of the plane H, and that 
therefore the compensating pressure offered by the said bladders to the 
internal volume 1 is precisely the pressure that exists outside the mobile 
walls of the volume, so that these walls are perfectly balanced and do not 
move despite the variations in immersion depth. 
The mobile mounting means of housings 2 on volume 1 can be constructed in 
various forms. 
It is therefore possible to design the housings 2 to be mounted mobile in 
vertical translation, and supported by a spring action device. 
However, in a preferred embodiment (FIG. 3), each housing 2 is mounted on a 
system of hinges 6 of horizontal axis, by arm 7. 
A circular reversing sector 8 linked to the arm 7 of the housing 2 is 
centered on the hinge 6 and receives one end of a cable 9 in a groove 
running around its circular edge. The other end of the cable is connected 
to a recall spring 10, itself integral with a fixed point on the chassis. 
As the system to be compensated is lowered in the water, the bladder 4 
contracts and the buoyancy exerted upon it therefore decreases. Its 
apparent weight in the water increases and the torque applied to the arm 7 
(tending to pivot the housing clockwise) increases. 
This torque is centered by the recall moment applied to the arm 7 by the 
spring 10, which gradually compresses, ensuring the proper descent of the 
housing 2, while holding it. 
When the system to be compensated rises, the reverse operation takes place. 
It is possible to provide high and low stops to limit the possible 
displacements of the housing. 
Also, to modulate the recall countertorque applied by cable 9, a reversing 
sector 8 can be provided that is not of circular pitch but of variable 
radius, and/or a recall spring 10 of variable stiffness. 
FIG. 4 illustrates the layout of the device in conformity with the 
invention, in an underwater vehicle 11 for hunting acoustical mines. 
It is known that such a vehicle is designed to affect the acoustic 
activation devices of the mines, for the purpose of destroying them by 
exploding them. It constitutes the sound emitter of the minesweeping 
system, and can be towed at a distance (e.g. of 450 m) behind the carrier 
vessel, or can navigate independently. 
The volume 1 includes the emittor, mainly consisting of a dual 
electroacoustic transducer. The acoustic drives are two hemispherical 
diaphragms 12 arranged laterally and moving in phase opposition. These 
diaphragms are sealed by deformable rubber circular joints. 
The vehicle 11 includes two housings 2 of about 80 liters fore and aft of 
the volume 1. Each housing consists of an elongated cylinder with rounded 
base walls, held around two generatrices by two dual arms 7, mounted 
pivotably on a hinge shaft 6 (or two half-shafts) provided in a yoke 
integral with the vehicle chassis 12. An integral sector 8 is mounted 
against the inside of one of the dual arms 7. One end of a cable 9 is 
fastened to the sector 8 and its other end to a spring fastened to the 
vehicle chassis 12. 
The bladders 4 of the two housings 2 communicate with each other through a 
flexible link 3, itself connected, in a way not shown, to the inside of 
volume 1. 
The housings 2 include a rectangular antipitching flap 13 fastened on a 
generatrix, to damp the rapid parasitic oscillatory motions of the housing 
by setting up a viscous resistive torque due to the drag of this flap in 
the water. 
FIG. 4 shows the housings 2 in upper position, and a sketch 2' of the 
housings in lower position. 
These positions correspond to a depth of between 35 m and 90 m, 
approximately.