The present invention relates to respiratory protection equipment (RPE), which expression encompasses military and industrial respirators (both unpowered and powered types), gas masks, dust masks, surgical masks, compressed air and oxygen-fed breathing apparatus, and the like.

RPE typically incorporates a sealing member adapted to form a seal against the face of the user (whether around the whole periphery of the face or at least around the nose and mouth) to isolate his respiratory organs from the external environment and, where appropriate, conserve the breathing gas supply. The integrity of the seal thus formed is crucial to the protection afforded by the equipment and it is important that the sealing member is correctly fitted to the face when the equipment is donned and remains so throughout the period of use. It can be compromised by such factors as incorrect alignment with the face, variations in face shape between different users, incorrect strap tension, debris between the face and sealing member, and exacerbated by exertion of the user. The seal integrity is typically checked by performing a pressure test when donning but it would be useful for some means to be provided for monitoring the integrity of the seal thereafter so that if a gap is discovered the user can take corrective action, e.g. by adjusting the harness or other means by which the sealing member is held to his face, or even abandoning a mission if there is a risk of substantial leakage.

With the foregoing in mind, in one aspect the present invention resides in respiratory protection equipment comprising a sealing member adapted to form a seal against the face of the user, one or more electrodes disposed upon or within the sealing member, and means for monitoring the integrity of said seal by monitoring the electrical capacitance across the sealing member between said electrode(s) and the face of the user.

The invention likewise resides in a method of monitoring the integrity of a seal formed by a sealing member of respiratory protection equipment against the face of the user, which comprises monitoring the electrical capacitance across the sealing member between one or more electrode(s) disposed upon or within the sealing member and the face of the user. It will be appreciated that the monitoring of capacitance "across the sealing member" will involve only part of the thickness of that member when the electrode(s) are disposed within that member.

The electrode(s) of such RPE are preferably formed from an electrically-conductive elastomer, which may be filled with nickel-coated carbon.

There may be an array of the electrodes embedded in a moulded matrix material which is bonded to the sealing member.

The electrode(s) may alternatively be formed by printing onto a substrate with an electrically-conductive ink, and the substrate may be the sealing member itself.

The invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a pictorial view of one form of respiratory protection equipment to which the invention can be applied, as worn by a user;

Figure 2 is a schematic cross-sectional view of the RPE of Figure 1 ;

Figure 3 is a pictorial view of the RPE of Figures 1 and 2 with the face piece

sub-assembly removed;

Figure 4 illustrates a capacitance model useful for understanding the invention;

Figure 5 illustrates a circuit for measuring the capacitance; and

Figure 6 illustrates a multiple electrode structure which may be employed with the invention.

Referring to Figures 1 to 3 these show one form of RPE to which the invention can be applied, being a respirator hood for service personnel to provide protection against nuclear, biological or chemical (NBC) hazards. It is of a form described in

WO03/095031 comprising a hood sub-assembly 1 and a demountable face piece sub-assembly 2 so that the respirator can be worn in the fully assembled condition of Figures 1 and 2 or with the face piece sub-assembly removed as shown in Figure 3, the latter to permit free breathing of ambient air e.g. when standing by for action or when there may be a threat of a hazard but no actual hazard encountered.

The hood sub-assembly 1 comprises a flexible head covering 3 which is also extended downwards over the shoulders of the user, and a substantially rigid profiled ring 4 attached to the head covering so as to encircle the face of the user at an appropriate spacing when the head covering is donned. The ring 4 and covering 3 are secured together around the whole of their mating edges and additional adjustable ties 5 are provided between these elements to assist in supporting the ring 4 and the face piece sub-assembly 2 when the latter is added. The ring 4 also carries a sealing member in the form of a profiled elastomeric gasket 6, typically of butyl rubber or other suitable elastomer, which is adapted to extend into sealing engagement around the periphery of the user's face when the head covering is donned.

The face piece sub-assembly 2 comprises a moulded face plate 7 with inset lens 8 and a fitted air supply hose 9. Internally it carries an oronasal mask 10 (Figure 2) with a soft sealing edge to engage around the mouth and nose of the user when donned. The face piece sub-assembly 2 can be demountably attached to the hood sub-assembly 1 and in the illustrated embodiment is retained by lugs (not shown) on the face plate 7 engaging in sockets 11 (Figure 3) formed on the ring 4, one of which includes a

manually-releasable latch 11 A. The periphery of the face plate 7 is profiled to match the contour of the ring 4 and carries a seal 12 (Figure 2) to ensure a gas-tight connection between those elements when attached.

In the fully assembled condition of the respirator shown in Figure 2 a facial cavity 13 is formed, bounded by the face plate 7 and gasket 6, in which the user's nose, mouth and eyes are isolated from the external environment. The integrity of the peripheral face seal formed by the gasket 6 is crucial in this respect. Within the facial cavity 13 a smaller subdivision 1 is formed by the interior of the oronasal mask, but the integrity of the latter is of secondary importance to the gasket 6.

In use of the respirator clean filtered air is supplied via the hose 9 (Figure 1 ) and enters the oronasal mask cavity 14 through a one-way inlet valve 15 (Figure 2). Exhaled air is vented to atmosphere through a one-way outlet valve 16. A stream of air is also directed through ports (not shown) in the mask 10 to pass over the interior face of the lens 8 for demisting purposes, in accordance with known practice. Although not illustrated as such, the valves 14 and 15 may be of the form described in our copending United Kingdom patent application no. 1012205.9.

In accordance with the present invention the gasket 6 bears an electrode 17 which in the illustrated embodiment (Figures 2 and 3) comprises a length of electrically-conductive elastomer extending around the gasket on its surface opposite to that which seals against the user's face. The electrode typically comprises a strip of silicone rubber with a nickel-coated carbon fibre filler, that is bonded or co-moulded to the gasket.

Alternatively the electrode may be embedded within the thickness of the gasket. A conductive elastomer is preferred for this purpose because it is flexible and stretchable and its presence on the gasket will not significantly stiffen the latter or impair its ability to conform to the contours of the user's face.

The electrode 17 is effectively used as part of a sensor system for monitoring the integrity of the seal formed between the gasket 6 and the user's face by monitoring the electrical capacitance across the gasket between the electrode and face. In this respect the arrangement can be visualised as a classic capacitor comprising a pair of plates separated by a dielectric as depicted in Figure 4, where the plates are represented by the electrode 17 and the corresponding portion of the user's face U respectively and (in the case where the gasket is fully sealed against the face) the dielectric is represented by the thickness of the gasket (plus any layer of adhesive between the electrode and gasket). It is well known that in such an arrangement capacitance is given by:-

C = keoA

~d ^~

where:-

C is capacitance in Farads (F)

A is the area of the plates in square metres (m ² )

d is the distance between the plates in metres (m)

k is the dielectric constant of the material separating the plates

e ₀ is the permittivity of free space (8.85 X 10 ^"12 F/m)

It follows from this model that if at any location along the length of the electrode 17 the gasket 6 is not properly sealed against the face, i.e. a gap exists between the gasket and face, then at that location d will be greater than for the fully sealed condition and the aggregate k will be lower than for the fully sealed condition due to the existence of air between the gasket and face, so due to both of these factors the total capacitance will be reduced in comparison to the fully sealed condition.

In practice therefore if a baseline capacitance measurement is taken after donning the respirator and checking that the gasket 6 is fully sealed against the face - e.g. with a pressure test and/or by employing an experienced respirator fitter - subsequent monitoring of the capacitance can be used to monitor the integrity of the seal and a signal can be given to the user, so that he can take corrective action, in the event that the capacitance falls to a level which indicates the presence of a gap somewhere around the gasket.

A circuit which can be used for measuring this capacitance is indicated schematically in Figure 5. A low voltage (typically 5V peak-to-peak) AC waveform is produced by a generator 18 and applied to the electrode 17 of the above-described capacitive arrangement which is indicated at C in Figure 5, the user's face U acting as a virtual ground. A load resistor 19 is used to set the frequency to a low RF value, typically 120khz. The capacitor acts as a voltage divider and the AC signal is rectified and converted to a DC level by a detector 20, this level accordingly being proportional to 1/C. A low pass filter 21 is used to reduce noise in the DC level which is read as notionally indicated at 22.

It will be appreciated that while a single electrode such as 17 can be used to identify if there is a gap in the seal between the gasket 6 and the user's face at any location along its length it cannot indicate at which actual position around the gasket the gap exists. For this reason there may instead be a plurality of shorter electrodes disposed around the circumference of the gasket and indeed a plurality of rows of such electrodes across the width of the gasket, all addressed in turn by use of a suitable micro controller so that it is possible to identify the location of any gap both circumferentially and widthwise of the gasket. The distribution of such electrodes may also be varied at different regions of the gasket so that the resolution of the arrangement is greater in regions of greater leakage potential (e.g. temples and jaw) than in regions where a reliable seal is more easy to maintain (e.g. across the brow).

When a multiplicity of electrodes are required, structures such as the one illustrated in Figure 6 may be employed. This comprises an array of patch electrodes in the form of pieces of conductive elastomer 23 embedded in a moulded matrix of polyurethane 24 which can be bonded to the surface of the gasket 6 opposite to that which seals against the user's face, particularly in regions where high resolution for the location of possible gaps is required. The matrix has a lattice structure with numerous apertures 25 for maximum flexibility, and individual matrices can be moulded to follow the contour of the respective regions of the gasket to which they are to be bonded. They can be overlaid with a flexible PCB to connect the individual electrodes 23 to the requisite

driver/reader/micro controller. The elastomer electrodes are typically 6mm x 4mm in area and 2mm thick and it has been found that with this small size the sensitivity of the system is sufficient to identify the presence of a single human hair across the seal, which is typically the smallest and most common contaminant that leads to a respirator seal leak. Instead of the illustrated arrangement where the electrodes 23 are "line abreast" in each of the four rows a staggered or "brickwork" pattern can be employed so that at least one electrode will be encountered across the width of the gasket at any circumferential location. Individual conductive elastomer patch electrodes of this type could alternatively be moulded directly onto or into the gasket.

In an alternative embodiment the electrode(s), and tracks to connect them with a suitable driver/reader/controller, can be formed by printing with an electrically-conductive ink, either directly onto the surface of the gasket opposite to that which seals against the user's face or onto a thin flexible substrate of e.g. Mylar ^® which is bonded onto the gasket.

The type of signal that is conveyed to the user by the system for monitoring the integrity of the seal may be of any suitable visual, audible or tactile form. For example in the simple case where there is a single, universal electrode 17 such as illustrated in Figures 2 and 3 there may be a green light signal when the measured capacitance remains at or close to its "baseline" level and a red light signal when the capacitance falls below a certain threshold, or a succession of different light signals corresponding to a succession of diminishing capacitance thresholds. Where there are a plurality of electrodes disposed around the gasket a visual display could take the form of a dot pattern in the shape of the gasket where the respective dots are illuminated or change colour at certain capacitance thresholds to represent the status of the seal at respective locations, or a set of bar graphs of variable magnitudes could be used. The above visual displays could for example appear in the user's field of view through the lens 8 using known helmet- mounted display technology, or could appear on a wrist-mounted or other suitably located unit.

Although the invention has been described above with reference to a respirator featuring a demountable face piece assembly it will be appreciated that it is equally applicable to more conventional respirators with a fixed face piece and to all other forms of RPE where it is required to monitor the integrity of a seal formed between a sealing member and the user's face.

Furthermore although described above in terms of an aid to the user of a respirator throughout the period of use the invention may also or alternatively be used to aid the initial fitting of the equipment, for example employing a plug-in unit to indicate variations in the measured capacitance as the fit of the equipment is adjusted, and/or as an aid to training in the fitment of such equipment.