Deep diving apparatus

A pressure regulatory valve system for a breathing system in a deep diving apparatus comprises a pair of pressure relief valves, one adapted for connection to a gas supply line extending from a source of breathable gas provided by a diving bell to the diving helmet of a diver working out of the bell, and the other being adapted for connection to a gas return line extending from the diving helmet to the bell. The relief valves are adapted to be controlled by differences between the pressure in the helmet and the pressure in the bell so that when the diver moves to levels above that of the bell the relief valve connected to the supply line responds to reducing helmet pressure in an opening manner and vents gas from the supply line to the bell, while the relief valve connected to the return line responds in a closing manner. When the diver moves to levels below that of the bell the return line relief valve responds to increasing helmet pressure in an opening manner to allow gas to pass from the bell into the return lines and the supply line relief valve moves towards closing to raise the supply line pressure. This action of the pressure regulatory valve systems enables the pumps of a push-pull pump system circulating the breathable gas between the bell and the helmet to operate at substantially the optimum pressure rise for the pertaining ambient pressure conditions as sensed in the helment.

THIS INVENTION relates to deep diving apparatus of the type comprising a 
diving bell from which a diver receives life and environmental support via 
umbilical connections and is particularly concerned with breathing 
systems, for such deep diving apparatus, in which gas is supplied to and 
withdrawn from the helmet of a diver by a push-pull pump. 
A deep diving breathing system incorporating a push-pull pump for 
circulating a breathable gas mixture including helium through the system 
by way of the diver's helmet provides a recirculation system whereby the 
loss of helium from the system is minimal. However, an operational problem 
arises in regard to gas conservation when the diver is operating out of a 
diving bell which provides the breathable gas source for the push-pull 
pump. During vertical excursions the pressure difference across the wall 
of the diver's helmet is maintained substantially constant by means of a 
return pressure control valve; however the absolute pressure within the 
helmet varies with the depth at which the diver is located. Consequently, 
at levels increasingly below that of the bell the absolute pressure and 
thus the density of the gas in the helmet increase relative to that in the 
bell and at levels increasingly above the bell reverse conditions occur. 
Thus, as the diver rises above the level of the diving bell the required 
gas pressure in the diver's helmet falls below the pressure in the bell so 
that gas delivered by the push pump expands on entering the helmet, 
eventually attaining a volume beyond the capacity of the pull pump to 
return to the bell and requiring provision of arrangements to relieve the 
excess pressure that would otherwise develop in the diver's helmet. In 
view of the high cost of helium, it is uneconomic to allow the excess gas 
to be discharged to the sea by a pressure relief valve on the helmet. 
Our U.S. Pat. No. 4,442,835, issued Apr. 17, 1984 discloses a deep diving 
breathing system that aims to overcome this problem, the system of that 
patent including helmet pressure control means comprising inlet valve 
means for controlling flow of gas from a gas supply line into the helmet, 
outlet valve means for controlling the flow of gas from the helmet to a 
gas return line and bleed valve means operable in response to the 
difference in pressure of gas flowing from the outlet valve means and 
water pressure ambient to the helmet, i.e. absolute pressure, for bleeding 
gas from the gas supply line upstream of the inlet valve means when the 
absolute pressure in the helmet is less than that in the bell. 
Whilst this system provides highly effective means of conserving expanded 
breathable gas, by preventing such gas from being discharged to ambient as 
a diver makes excursions to and from locations above the diving bell, this 
system requires a relatively large bleed hose in the umbilical and is 
somewhat uneconomic in meeting the power requirements of both the pressure 
side and of the suction side of the push-pull pump in contending with the 
line-losses and the varying pressure/suction requirements in delivering 
gas to and returning gas from the diver's helmet as he moves from one 
level to another, above and below the bell. 
When the diver is operating below the diving bell the pressure of the gas 
fed by the pump to the supply line must accommodate the total pressure 
loss within the umbilical conduit system (line losses) plus the difference 
in ambient pressure between the location of the diver and that of the 
bell. However, when the diver is operating above the bell, the difference 
in ambient pressure between their respective locations compensates line 
losses so that the supply line pressure requirement is less. The 
suction-pressure at the pump for the return gas is also affected by the 
relative location of the diver and the bell, but inversely in comparison 
with that of the supply gas pressure and must accommodate the line losses 
plus the ambient pressure difference when the diver is above the bell, and 
the line losses minus the ambient pressure difference when he is below the 
bell. 
With one exception, in all the contemporary push-pull pump systems known to 
us, the supply gas relief valve and the return gas relief valve, which 
respectively determine the pressures at the pump ends of the supply and 
return lines, are normally set to accommodate the maximum pressures 
required in moving both the supply gas and return gas to and from the 
helmet by the push-pull pump, as dictated by the maximum permitted 
vertical operational locations above and below the diving bell that the 
diver may reach during an excursion therefrom. This means that under 
conditions where the diver's location is intermediate the highest and the 
lowest, relative to the bell, at least one of the pumps of the push-pull 
pump is operating at a higher pressure rise, and drawing more driving 
power, than that which is actually required. 
The exceptional system known to us is that disclosed in U.S. Pat. No. 
3,965,829, in which it is proposed that push-pull pump means having the 
capacity to return all the gas to the bell from the helmet during any part 
of a diver's excursion above the bell shall include valve means that 
off-load the pull pump in conditions of less than maximum operating load. 
More explicitly, a system according to U.S. Pat. No. 3,965,829 provides, 
within the diving bell a control valve in the gas return line for 
minimizing the power requirement of the return pump. This valve controls 
the negative pressure in the return line in response to the difference in 
pressure between the supply line and the return line. Preferably, the 
system also includes, within the bell, a regulating valve for maintaining 
the volume rate of flow to the supply line from the push-pump at a 
substantially constant value. The regulating valve operates in response to 
the difference in pressure across a flow restrictor positioned adjacent 
thereto at its connection to the supply line. 
A breathing system such as disclosed in U.S. Pat. No. 3,965,829 provides 
control of pressure in the helmet that affords breathing conditions that 
are acceptable to the diver at, substantially, only one preselected level 
relative to that of the bell. In one practical utilisation of such a 
system breathing system has been developed in which the regulating valve 
is provided with a means of manual adjustment whereby the volume rate of 
flow of gas supplied to the helmet may be varied or reset by a supervisor 
in the bell. This enables more acceptable breathing conditions to obtain 
in the helmet for different levels at which the diver may be working, but 
requires that the relative levels of the diver and bell are known to the 
supervisor and continually monitored by him for wholly successful 
adjustments to be made. 
However even the most practised supervisor would be unable to make the 
continual fine adjustments necessary to correct for the slight variations 
in absolute pressure in the diver's helmet as he changes his body 
attitudes and shifts his position while working, and whilst the diver 
would not consciously notice these slight variations and the consequential 
changes in his breathing effort, it being instinctive, for example, to 
reduce the depth of breathing while increasing the rate in response to an 
increase in gas density, they nevertheless tax his strength slightly and 
in consequence reduce the period for which he can sustain a constant 
workload. 
The present invention aims to provide a pressure regulatory valve system 
for a deep diving apparatus breathing system which will substantially 
overcome these problems and especially that of excessive power being drawn 
by the pumps of a push-pull pump system during much of the working 
schedule of a diver when operating from a diving bell, while automatically 
providing in the diving helmet optimal conditions for breathing. 
According to the present invention a pressure regulatory valve system for 
relief of overpressure in a gas supply line extending from a gas source on 
a diving bell to the diving helmet of a diver working outside the bell, 
and underpressure in a gas return line extending from the diving helmet to 
the bell, a breathing system in a deep diving apparatus, comprises a pair 
of pressure relief valves, one of said valves being adapted for connection 
to said gas supply line and the other said valve being adapted for 
connection to said gas return line, said pressure relief valves 
respectively being adapted to vent to and from the bell and to be 
controlled by differences between the pressure in the diving helmet and 
the pressure in the diving bell, respectively, so as to maintain 
respective supply and return line pressures individually appropriately 
related to the respective helmet and bell pressures. 
With such a system, when the diver is at the same level as the bell, the 
relief pressures of the two relief valves are at substantially the same 
value. However, as the diver moves to levels above that of the bell from 
which he is operating, the relief valve connected to the supply line and 
thus controlling communication between the supply line and the bell 
interior (bell pressure) responds to reducing helmet pressure in an 
opening manner, while the relief valve connected to the return line 
responds in a closing manner. On the other hand, if the diver moves to 
levels below the bell the return line relief valve responds to the 
increase in helmet pressure by further opening the communication with the 
bell interior whilst the supply line relief valve moves towards closing to 
raise the supply line pressure as required. Such action on the part of the 
pressure regulatory valve system enables the pumps of the push-pull pump 
to operate at substantially the optimum pressure rise for the pertaining 
ambient pressure conditions as sensed in the diver's helmet. 
The present invention also resides in a breathing system for deep diving 
apparatus comprising a diving bell providing a source of breathable gas 
for a diver working out of said bell, said breathing system including a 
push-pull pump for circulating breathable gas between the bell and a 
diving helmet via gas supply and gas return lines, and valve means in the 
vicinity of the diving helmet for maintaining the gas pressure therein at 
a value related to the ambient water pressure, characterised by a pressure 
regulatory valve system for relieving overpressure in the said gas supply 
line and underpressure in the said gas return line, such regulatory valve 
system comprising a pair of pressure relief valves, one connected to the 
gas supply line and adapted to vent gas therefrom to the bell, and the 
other connected to the gas return line and adapted to admit gas thereto 
from the bell, said relief valves respectively being controlled by 
differences between the pressure in the diving helmet and the pressure in 
the diving bell, respectively, so as to maintain respective supply and 
return line pressures individually appropriately related to the respective 
helmet and bell pressures.

FIG. 1 illustrates a deep diving apparatus breathing system 10 that 
includes a diving helmet 11 having an inlet valve means, an outlet valve 
means, and shut-off valve means associated with it and situated on, or in 
the close vicinity of, the helmet 11. The helmet has an inlet deflector 12 
terminating a breathable gas supply line 13 at its entry to the helmet 11, 
the other end of the supply line 13 being connected to the push pump of a 
push-pull pump assembly (not shown) that draws a breathable gas mixture 
from a source of gas on a diving bell 17. A manually adjustable pressure 
relief valve 14 branches from a helmet outlet connection 15, or from a gas 
return line 16 which extends from the outlet connection 15 to the pull 
pump of the push-pull pump assembly. The breathing system 10 further 
includes a pressure regulatory valve system that is housed within the 
diving bell 17 and is connected into the gas supply and return lines 13 
and 16 for the purpose of preventing unnecessary pressure rises in the 
push and pull pumps while being responsive to the diver's breathing state. 
In this embodiment the inlet valve means comprises an inlet flow control 
valve 18 included in the supply line 13, the outlet control valve means, 
which controls pressure in the helmet 11, comprises an outlet gas flow 
pressure regulator valve 19 included in the return line 16 close to the 
connection 15, and the shut-off valve means comprises a self functioning 
safety valve 20 disposed in a helmet pressure sensing line 21 which is 
tapped into the return line 16, at the entry to the outlet gas flow 
pressure regulator valve 19. 
The pressure regulatory valve system comprises a pair of pressure relief 
valves 23, 24 connected in a manner such that the relief valve 23 controls 
communication between the supply gas line 13 and the interior of the bell 
17 and the relief valve 24 controls communication between the return gas 
supply line 16 and the interior of the bell. The helmet pressure sensing 
line 21 connects with a datum pressure control chamber of each of the 
relief valves 23, 24. 
A non-return valve 27 is preferably included at the inlet to the inlet flow 
control valve 18, whilst an optional pressure responsive indicator 28 
tapped into the helmet pressure sensing line 21 indicates within the bell 
the vertical position of the diver. At least one bottle 29, containing 
pressurised breathable gas, is connected to the helmet 11 by way of a 
manually operated control valve 30 to provide an emergency breathable gas 
supply for enabling the diver to return to the bell in the event of a 
failure of the system feeding gas via the supply line 13. 
The relief valves 23 and 24 may be provided by valves of the simple poppet 
form shown in FIGS. 2 and 3, in which a valve head moves to open against a 
closing force that is pneumatically and automatically variable. 
Referring to FIG. 2, the breathable gas supply relief valve 23 in this 
embodiment comprises a hollow valve body 41 within which a poppet-type 
valve member 43 is constrained to slide. This valve member 43 has a body 
portion carrying a sealing ring in slidable contact with the interior 
surface of the valve body 41 and a valve head 44 which is co-operable with 
a valve seat 45 arranged axially of and in one endwall of the body 41. The 
valve head 44 spans a ported relief chamber 46 communicating with the 
interior of the bell 17 and is urged towards seating on the valve seat 45 
by a compression spring of appropriate rate that is reacted against a 
spring adjuster 47 slidably located in a leakproof manner within the valve 
body 41 and adjustable by a turnscrew carried in the other endwall. The 
valve seat 45 circumscribes a conduit connection with the breathable gas 
supply line 13 so that the valve head 44, in co-operating therewith, can 
control a gas path connecting the supply line 13 to the interior of the 
bell 17 by way of the ported relief chamber 46. A conduit connection on 
the valve body 41 connects the helmet pressure sensing line 21 with a 
datum pressure chamber 48 formed within the valve body 41 between the 
valve member 43 and the spring adjuster 47 so that helmet pressure 
supplements the spring force tending to close the valve. 
Referring to FIG. 3, the return gas relief valve 24 has a hollow body 51 of 
generally stepped cylindrical form that contains a valve member 52 
comprising a piston 53 and a valve head 54 which are spaced apart by an 
annular undercut section. The valve body 51 provides a datum pressure 
chamber 56 for the piston 53 and a relief chamber 57 that is divided by a 
wall having an annular seat 58, with which the valve head 54 co-operates. 
The datum pressure chamber 56 is connected to the helmet pressure sensing 
line 21 whilst the relief chamber 57 is connected, on the side of the 
valve head 54 remote from the datum pressure chamber 56, with the gas 
return line 16, and on the other side of the valve head 54 with the 
interior of the bell. The valve head 54 thus controls a gas path 
connecting the return line 16 to the interior of the bell 17 by way of the 
relief chamber 57. The valve head 54 is urged towards seating on the valve 
seat 58 by a compression spring of appropriate rate that is reacted 
against a spring adjuster 59 which is slidably located in a leakproof 
manner in the body 51 and adjustable by means of a turnscrew carried in an 
endwall of the body 51. 
FIG. 1 shows a modification of the relief valves 23, 24 illustrated in 
FIGS. 2 and 3, in which, the sealing rings carried by the body portions of 
the valve members 43, 52 respectively are replaced by rolling diaphragms 
connected between the body portions of the valve members and the internal 
surfaces of the valve bodies 41, 51, respectively. 
The compression springs urging the respective valve heads 44 and 54 towards 
seating in the relief valves 23 and 24, respectively, are of such a rate 
as to ensure that, together with the pneumatic load effected by the sensed 
helmet pressure, the required maximum pressure rise is developed in the 
push pump when the diver is, at his lowest permitted location relative to 
the bell 17 and, likewise the required maximum pressure rise is developed 
in the pull pump when the diver is at his highest permitted location 
relatively to the bell: whereas when the diver is substantially at the 
level of the bell the minimum pressure rise in the pumps sufficient to 
overcome line losses is obtained. 
The inlet flow control valve 18, as used in this embodiment, is provided by 
a pressure balanced valve which in general terms is a combination poppet 
and spool valve having a pressure datum reference obtained from the 
ambient water pressure. One form of this valve is shown in FIG. 4, the 
inlet flow control valve 18 of that Figure being similar to that disclosed 
in our U.S. Pat. No. 4,442,835, issued Apr. 17, 1984, and comprising a 
hollow valve body 61 having a differential pressure-sensing device 62 
attached to one end. 
The hollow body 61 interiorly provides, in axial spaced relationship, an 
annular valve seat 63 and an annular land 64. A lightweight combination 
poppet and spool valve member 65 is freely supported within the body 61 by 
two impermeable flexible membranes 66, 67 that are disposed outboard of 
the annular valve seat 63 and land 64, respectively. The membrane 66 
closes one end of the body 61 and provides part of a wall of a control 
pressure chamber 68 of the pressure-sensing device 62, whilst the membrane 
67 provides a wall separating a balancing chamber 69 from a flow chamber 
70 formed between the two membranes 66, 67. The control pressure chamber 
68 and the balancing chamber 69 are interconnected by a balancing duct or 
tube 71. The combination valve member 65 provides a valve head 72 and a 
raised annular land 73 that are co-operable, respectively, with the valve 
seat 63 and the annular land 64 provided within the flow chamber 70. 
The pressure-sensing device 62 comprises a differential pressure chamber 
formed by the control pressure chamber 68 and an ambient (immersing water) 
pressure chamber 74, which two chambers are separated by an impermeable 
flexible diaphragm 75 that is peripherally trapped between the rims of a 
perforated cover 76 and a flared portion 77 of the valve body 61. The 
valve member 65 is mechanically secured to the diaphragm 75 by a stud 
arrangement 78 that spans the control chamber 68 as an axial extension of 
the valve member 65. 
When the valve head 72 is seated the land 73 is just entered within its 
associated land 64 of the flow chamber 70. A small radial clearance is 
provided between the lands 64, 73. The valve member 65, within the length 
of the flow chamber 70, is of hollow construction and has cross drillings 
at each end outboard of the valve head 72 and land 73. 
An inlet 81 for connection to the breathable gas supply line 13 is provided 
in the wall of the body 61 at a position between the valve seat 63 and the 
land 64, whilst an outlet 82 is positioned in the wall to the side of the 
land 64 remote from the seat 63. The perforated cover 76 carries a 
threaded spring adjuster 83 that is aligned with the axis of the valve 
member 65 and holds a low rate compression spring 84 against the stud 
arrangement 78. Another low rate compression spring 85 may be provided in 
the balancing chamber 69 in axial opposition to spring 84. A helmet 
pressure sensing tube 86 is connected to the control pressure chamber 68. 
The outlet gas flow pressure regulator valve 19 in this embodiment may be 
provided by an anti-suction valve, of the form shown in FIG. 5 , which may 
be similar in detailed construction to the gas flow regulator valve 
disclosed in our UK Patent Application GB 2088726A published June 16,1982. 
The outlet gas flow pressure regulator valve 19 shown in FIG. 5 comprises 
a rigid outer body member 91 of tubular form, having an inlet end 92 and 
an outlet end 93. Housed within the outer body member 91 is a tubular 
member 94 having radial slots which provide a weir-like flow path towards 
the outlet end 93. The tubular member 94 comprises a plurality of weir 
elements 95 each formed by an annular plate having one plain face and one 
face provided with two raised rings (not shown) that are concentric with 
the axis of the plate. The weir elements 95 are secured by equally spaced 
bolts and spacer means (not shown) to form between their opposed faces a 
plurality of slots which provide radial flow paths between the exterior 
and interior of the tubular member 94. The bolts (not shown) pass through 
the outlet end 93 and the weir elements 95 into threaded engagement with a 
member 96 which closes that end of the tubular member 94 facing the inlet 
end 92 of the body 91. A thin shim plate 99 formed to the curvature of the 
outside diameter of the weir elements 95 is secured thereto so as to 
occlude a small arcuate area of the entry to each of the slots formed 
between the weir elements. A thin elastomeric tubular sleeve 97 of 
substantially the same diameter as the outside diameter of the weir 
elements 95 is fitted about them and is stretched between the inlet end 92 
and the outlet end 93 at which ends it is secured by suitable clamping 
means (not shown). This sleeve 97 is arranged to lift from the tubular 
member 94, when in use, by the pressure rise in the helmet created by a 
diver's exhalation in breathing. The outer body member 91 is perforated by 
holes 98 so that absolute (immersing water) pressure is effective on the 
outer surface of the elastomeric sleeve 97 to hold it in contact with the 
tubular member 94 whenever the pressure within the member 94 is less than 
the ambient water pressure. 
For the best performance of the system of the present invention it is 
preferred in practice to integrate the structures of the inlet flow 
control valve 18 and the outlet gas flow pressure regulator valve 19 so 
that by very close positional relationship of their pressure sensing 
elements they respond to, substantially, the identical absolute ambient 
pressure and conveniently operate as a single rigid valve unit as 
represented in FIG. 1. To facilitate integration and close positioning of 
the respective valves 18, 19 it is useful to modify the means shown in 
FIG. 4 for adjusting the low rate spring of the inlet flow control valve 
18 by arranging that the threaded spring adjuster 83 acts through spring 
84 in the form of a cranked lever (FIG. 1) whereby the spring adjuster 83 
can be positioned in a sidewall of the integral structure. 
The safety valve 20 in this embodiment is similar in principle to the 
associated outlet gas flow regulator valve 19, being in the form of an 
anti-suction valve utilising an impermeable sleeve over a perforated tube. 
Tnus as shown in FIG. 6, the safety valve 20 comprises a rigid tubular 
outer body member 101 providing connections 102, 103 for inclusion in the 
helmet pressure sensing line 21 (FIG. 1). Housed within the outer body 
member 101 is a waisted body element 104 of circular transverse 
cross-section which is provided with two sets of internal ducts 105, 106, 
and 107, 108 of which the ducts 105, 106 join the connection 102 with the 
periphery of the body whilst ducts 107, 108 similarly join the connection 
103 with the periphery of the body. A thin elastomeric tubular sleeve 109 
of substantially the same diameter as the outside diameter of the waisted 
portion of the body element 104 is fitted thereon and retained at each end 
thereof by suitable retention means 110. The outer body member 101 is 
perforated by holes 111 so that immersing water pressure can be effective 
on the outer surface of the elastomeric sleeve 109 to hold this in contact 
with the body element 104. 
In operation of this embodiment, assuming that the breathable gas supply 
line 13 and the gas return line 16 are appropriately and respectively 
connected to the push and the pull pumps of a push-pull pump (not shown), 
breathable gas is delivered to the helmet 11 by the push pump by way of 
the supply line 13 which includes the inlet flow control valve 18 and a 
non-return valve 27. Gas is returned to the pull pump from the helmet 11 
by way of the gas return line 16 and the outlet gas flow pressure 
regulator valve, or anti-suction valve, 19. The non-return valve 27 
prevents back flow from the inlet control valve 18 in the event of a 
failure of the supply hose. The pressure relief valve 14 prevents pressure 
rising in the helmet above a predetermined level of, say, 2.76 kPa (0.4 
psi) above ambient pressure. Sensing of the gas pressure within the helmet 
11 is obtained in the respective datum pressure chambers 48 and 56 of the 
two relief valves 23 and 24 by way of the helmet pressure sensing line 21 
which includes the safety valve 20. The safety valve 20 is situated in the 
vicinity of the helmet 11 and whilst permitting flow in either direction 
ensures that, should the helmet pressure sensing line 21 become ruptured 
while the diver is below the bell, gas will not flow from the helmet to 
the extent of causing a depression therein. 
Breathable gas passes through the inlet flow control valve 18 to the helmet 
11 from the supply line 13, entering and leaving this valve by connections 
81, 82 (FIG. 4) respectively. Helmet pressure obtains in the pressure 
chamber 68, being sensed by way of the sensing tube 86 and is effective 
upon the diaphragm 75 so as to oppose ambient pressure applied by the 
immersing water in chamber 74. Helmet pressure is also effective upon the 
spool-supporting membranes 66 and 67 by way of balancing tube 71, whereby 
the spool is axially balanced. The diaphragm 75 responds to ambient 
pressure and to the effect of spring 84 which is adjusted to bias the 
combined poppet and spool valve 65 so as to seek to maintain a small 
positive datum pressure, determined by the anti-suction valve 90, in the 
helmet, relative to the ambient pressure. When the valve member 65 is in a 
steady controlling mode at constant depth it passes a small flow of gas to 
the helmet for ventilation of the diver. Even when the valve head 72 is 
seated upon its seat 63 a small (ventilation) flow through the valve 17 is 
ensured by way of the annular flow path between the two lands 64 and 73, 
so that the diver is not denied totally a supply of gas into the helmet at 
any time. 
The ambient pressure is effective upon the elastomeric sleeve 97 of the 
anti-suction valve 19 by reason of immersing water entering the body 
through the holes 98 and tending to hold the sleeve 97 on to the tubular 
member 94, whereas helmet pressure is effective in the inlet end as far as 
the upstream end of the tubular member 94 at the face or the closure 
member 96. Pull pump suction pressure applies at the outlet end of the 
valve and interiorly of the slots formed between the annular elements 95 
of the tubular member 94. The resistance to flow of this valve establishes 
a positive datum pressure in the helmet by predetermined relationship of 
the restrictive area of the slots and the tension of the elastomeric 
sleeve 97. In response to changes in helmet pressure, owing to the effect 
of inhalation and exhalation on the part of the diver, the open cross 
sectional area of the slots varies as the pressure difference across the 
elastomeric sleeve 97 varies accordingly within a working range. 
As the diver rises or descends the ambient pressure on the elastomeric 
sleeve 97 respectively decreases or increases the clamping load holding it 
to the tubular member 94 and the helmet pressure consequently follows that 
of the ambient pressure while maintaining the positive datum pressure. 
Obviously, the pressure and density of gas in the helmet 11 relative to 
that existing in the diving bell 17 is less when the diver is above the 
bell and greater when he is below the bell. Consequently the push pump 
delivering breathable gas from the bell 17 to the helmet 11 requires less 
power to accomplish this task, when the diver is at a level above the bell 
than when he is at or below the level of the bell. On the other hand, the 
pull pump requires more power and sufficient volumetric capacity to return 
gas to the bell, if gas is not to be wasted to ambient when the diver is 
above the bell than when he is at or below the level of the bell. These 
pump power requirements are obtained by operation of the relief valves 23, 
24 as a consequence of helmet pressure being effective in the respective 
datum chambers 48 and 56 of the valves 23 and 24, as a variable datum 
reference pressure, and thereby regulating communication between the 
interior of the bell 17 and supply line 13 and return line 16. 
The reference helmet pressure together with the effect of the compression 
springs urges the valve heads 44 and 54 of the relief valves 23, 24 
respectively towards closing, such that when the differential pressure 
value is zero, or substantially so, (i.e. the diver and bell are at the 
same level) the two valves are open to an extent where gas is bled from 
the supply line 13 into the bell 17 and also therefrom into the return 
line 16 to an amount sufficient to off-load both the push pump and the 
pull pump to a condition where their power requirement is only that which 
is sufficient to contend with the line losses of the gas flow circuit. 
As the diver rises above the level of the bell 17, the reference pressure 
obtaining in the datum chamber 48 of the supply gas relief valve 23 is 
reduced and consequently the combined closing pressure of this and the 
exertion of the compression spring in the chamber 48 reduces whereby, the 
valve tends to open, so that more gas from the supply line is allowed to 
pass into the interior of the bell 17 by way of the ported chamber 46 so 
as to relieve the pressure rise of the push pump and also reduce the 
amount of gas delivered to the helmet to match the lower density of the 
gas at helmet pressure. The reduced helmet or reference pressure, being 
present also in the datum chamber 56 of the gas return relief valve 24, is 
effective upon the piston 53 therein and provides reduced opposition to 
the force exerted by the compression spring housed in relief chamber 57 
such that the combined force of the sensed pressure and the spring produce 
a greater force on the valve member 52 towards closing its valve head 54 
on to seat 58. This reduces or prevents gas flow from the bell 17 to the 
pull pump by way of the ported relief chamber 55, relief chamber 57 and 
duct 16, thereby enabling the pull pump to develop a greater pressure rise 
as required to contend with the reduced density of the gas in the return 
line. 
On the other hand, when the diver moves to levels below the bell the effect 
of the two relief valves 23, 24 and the operation of their respective 
valve members 43 and 53 reverses. Increasing helmet or reference pressure 
sensed in the datum pressure chamber 48 causes the spring force in the gas 
supply relief valve 23 to be supplemented and so increasingly urge the 
valve head 44 towards contacting valve seat 45, thereby decreasing the 
relieving gas flow passing from the supply line 13 into the bell interior 
and causing increasing pressure rise of the push pump in order to overcome 
the increasing ambient pressure gradient between the bell 17 and the 
diver. The reference pressure sensed in the datum pressure chamber 56 of 
the valve 24 opposes the force of the compression spring in relief chamber 
57 so that as the reference pressure increases the valve head 54 lifts and 
permits gas to pass from the interior of the bell 17 to the suction of the 
pull pump by way of ported relief chamber 55 and relief chamber 57, 
whereby the pressure rise of this pump is reduced and advantage taken of 
the decreasing ambient pressure gradient between the diver and the bell. 
Thus the two relief valves 23, 24 operate in a manner relieving the working 
load on the pumps to that actually required of each one at the pressure 
conditions pertaining in the system and by directly sensing the pressure 
in the helmet the two valves are enabled to be more responsive to the 
difference in helmet absolute pressure and bell absolute pressure than in 
a system where the pressure difference is determined indirectly, such as 
by sensing the supply line and bell pressures. Because of the better 
responsiveness of the relief valves in systems according to the present 
invention, small pressure variations in the helmet caused by changes in 
the diver's breathing pattern are acted upon quickly and more accurately. 
Thus the supply of gas to the helmet is automatically maintained at, or 
very substantially, at, the optimum for the diver's breathing requirements 
over the full range of his permitted vertical excursions and changes of 
attitude and position in working. 
FIG. 7 is a graph illustrating typical datum pressure settings of the 
relief valves for a helmet to bell pressure difference range of -89.6 kPa 
(-13 psi) to +310.2 kPa (+45 psi), corresponding to a diver's vertical 
excursion between 9.1 meters (30 feet) above and 30.4 meters (100 feet) 
below a diving bell operating within its limits. The valve 23 for 
relieving gas flow from the supply line 13 opens at pressures within the 
range 137.8 kPa (20 psi) to 586 kPa (85 psi), whilst valve 24 for 
admitting gas into the return line 16 opens at pressures within the range 
275.7 kPa (40 psi) to zero. When the diver and the bell are at 
substantially the same level both valves open to establish the same 
pressure value of 224 kPa (32.5 psi), which value is appropriate to the 
system line losses. 
In the graph of FIG. 7, the `x` axis represents pressure at which the 
relief valves 23, 24 relieve and the `y` axis represents helmet pressure 
relative to bell pressure, the broken vertical lines `A` and `8`, 
respectively, representing the diver positions at 9.1 meters (30 feet) 
above and 30.4 meters (100 feet) below the bell. The sloping lines `S` and 
`R` indicate the relieving pressure values of the supply line pressure 
relief valve 23 and the return line pressure relief valve 24, 
respectively.