Abstract:
In a meter for assessing ventilatory capacity, a piston is free of any sliding contact with the body of the meter except over a region which extends axially not less than about 25% of the piston&#39;s diameter. In this way, tilting of the piston is avoided. Sliding contact is further reduced by the piston or wall having contact ribs extending axially.

Description:
TECHNICAL FIELD 
     This invention relates to meters for measuring the ventilatory capacity of a subject, particularly, but not necessarily exclusively the exhalation capacity. 
     Meters for obtaining a measure of the peak flow rate of exhalation are known (GB-A-1463814) in which the subject blows into one end of a tubular body to displace a piston, against the force of a spring, along the body. The piston is guided slidably for this displacement, out of contact with the body inner wall, on a support rod extending along the axis of the tube. The tube has an open slot running in the direction of piston displacement which provides an exit opening for the air being blown in. A pointer located behind the piston has a light frictional engagement with the slot and is displaced along a scale by the piston as it moves forward against the spring force. The piston is drawn back by the spring when the intensity of exhalation falls, but when it moves back, the pointer remains at the position of maximum displacement of the piston, so giving an indication of the maximum flow rate obtained in the exhalation. 
     Such meters have been developed as reliable and robust instruments. However, the presence of the central support rod for guiding the piston complicates the assembly process. 
     BACKGROUND ART 
     In one of its aspects, the present invention provides a meter for measuring the ventilatory capacity of a subject, the meter comprising a chamber, a piston within the chamber being displaceable axially against a resilient bias by blowing air into the chamber, an exit slot located in a side of the chamber for the escape of the air from the chamber being increasingly opened by the displacement of the piston against the bias, there being a region of sliding contact between the piston and the chamber inner wall at any one position of the piston and the sliding contact region having bounds separated axially by a distance not substantially less than 25% of the transverse dimension of the piston, preferably at least 30%, the sliding contact guiding the movement of the piston and preventing tilting of the piston, and there being means for recording a maximum displacement of the piston. Preferably the axial extent of the periphery is 50% or more of the transverse dimension of the piston, i.e. for a cylindrical piston, its diameter. 
     By giving the contact region a sufficient axial extent it is possible to avoid the need to mount the piston on a support rod, i.e. the piston is a free piston. The assembly of the meter is simplified and its construction costs reduced. 
     The piston periphery may take the form of a cylindrical wall of complementary form to and in sliding contact with the chamber inner wall. However, while such a configuration can prevent tilting of the piston, it can affect adversely the accuracy and repeatability of the meter, particularly at low levels of air flow. One reason for this may be that in use, condensation or other matter may be deposited from the exhalations and can accumulate on the periphery of the piston. 
     These effects could be lessened by increasing the nominal clearance between the piston and the chamber wall. However, the piston is then more likely to tilt and possibly jam within the chamber. The problem can be solved by making the contact between piston and the inner wall peripherally discontinuous e.g. by the provision of ribs on the piston or on the wall: thus the contact area is lessened but resistance to tilting can be maintained. 
     According to another aspect of the invention a meter for measuring the ventilatory capacity of a subject comprises a chamber, a piston within the chamber displaceable against a resilient bias by blowing air into the chamber, an exit slot located in a side of the chamber for the escape of the air from the chamber being increasingly opened by the displacement of the piston against the bias, portions supporting the piston on the chamber wall against tilting and arranged to engage between the piston and the chamber wall over only a part of the axial and/or circumferential extent of the periphery of the piston; and means for recording a maximum displacement of the piston in the chamber. Thus the piston may be a free piston, i.e. be one devoid of support except from the chamber wall. 
     Thus, the contact region can comprise a plain, e.g. cylindrical, wall or a plurality of axially spaced peripheral rims. At least one of the peripheral rims may have a complementary profile to the internal cross-sectional form of the chamber. Alternatively, the region can comprise a plurality of circumferentially spaced axially extending ribs. In another form, an axially restricted disc of a piston is complemented by a plurality of ribs forming axial extensions increasing the axial contact length between the piston and the chamber wall. 
     In these and similar forms of piston, as well as in the case in which the piston has a cylindrical wall, the co-acting wall of the chamber may be formed so that there is contact only at angularly spaced regions. For example, a circular piston may be mounted in a chamber which has a non-circular cross-section or a non-circular piston mounted in a circular cross-section chamber. Alternatively, the chamber may have inwardly extending projections for slidably guiding the piston. 
     The resilient bias acting on the piston should be applied in a manner that does not produce any significant non-axial force that might cause the piston to tend to tilt. If a coil spring is employed as the biasing device, it is therefore preferably attached centrally to the piston. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     By way of example, embodiments of the invention will be described with reference to the accompanying drawings, in which: 
     FIG. 1 is a section through a peak flow meter embodying the present invention with a piston in a first position; 
     FIG. 2 is a section through a peak flow meter embodying the present invention with a piston in a second position; 
     FIGS. 3,  4  and  5  are respectively front, sectioned side, and rear views of the piston of the meter of FIGS. 1 and 2; 
     FIGS. 6 and 7 are respectively front and sectioned side views of an alternative piston for use with the meter of FIGS. 1 and 2; 
     FIGS. 9 and 10;  11 ,  12  and  13 ,; and  14 ,  15  and  16  are respective front, sectioned side, and rear views of three further pistons for use with the meter of FIGS. 1 and 2; 
     FIG. 17 is a cross-section through a second peak flow meter embodying the invention; and 
     FIG. 18 is a cross-section illustrating a modification of the body of the meter of FIG.  17 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The meter  1  shown in FIGS. 1 and 2 comprises a hollow cylindrical body  2  having an inlet end  4  and an apertured closure member  6  at its opposite end. A generally circular-periphery floating piston  8  dividing the chamber formed by the interior of the body is freely movable along the body with sliding contact with the inner surface  7  of its wall. The piston  8  is attached by a coil tension spring  10  to a spider-like retainer member  12  mounted within the inlet end  4 , to which it is drawn by the spring  10  to lie in the rest position shown in FIG.  1 . 
     When a subject blows into the inlet end  4 , air flows through inlet passage  5  into the void  14  between the retainer member  12  and a disc of the piston  8  and the air pressure drives the piston  8  away from the retainer  12 , e.g. to the position shown in FIG.  2 . As the piston moves along the body  2  of the meter, it progressively uncovers a longitudinal slot  16  in the body wall through which the flow can escape. A pointer  18  is mounted in the slot and has a light frictional engagement with its edges. The displacement of the piston  8  away from the retainer member  12  entrains the pointer  18  along the slot but the pointer  18  is not attached to the piston  8 . Thus, when the pressure in the chamber  14  falls as the rate of exhalation through the mouthpiece  4  decreases, the spring  10  draws the piston  8  back but the pointer  18  is retained frictionally at the position of maximum piston displacement. A scale (not shown) along the slot  16  then allows the peak exhalatory flow to be read off. 
     The mode of operation described is similar to that of known peak flow meters of the form described in GB-A-1463814. However, the structure of the embodiment and particularly of its piston, and the relationship of the latter to the inner wall of the body, are quite different. 
     The piston  8 , which is shown also in FIGS. 3-5, comprises a frustoconical disc  20  extending from a hollow cylindrical hub  24 . Spokes  26  projecting from the hub  24  support a steadying rim  28  having substantially the same outer diameter as a rim  30  at the periphery of the disc  20 . The disc and steadying rims  30 , 28  provide two coaxial support surfaces at a spacing of approximately 50% of the piston diameter which engage the body wall to guide the piston  8  as it slides and to prevent it from tilting, while the disc  20  limits flow past the piston  8 . 
     The hub  24  has, at its inner end, a central pip  32  engaging one end of the spring  10  which is thereby attached centrally to the disc  20 . At the other end the spring  10  is attached to a central hub  13  of the retainer member  12 , so the force exerted by the spring  10  on the piston  8  does not tend to apply a torque to the piston. By virtue of the axial distance between the bounds of the region  9  of contact between the wall and the piston, here are represented by the rims  28  and  30 , which should amount to not substantially less than 25% of the diameter of the piston, preferably 30% or more, any tilting moments which may occur do not significantly tilt the free piston and therefore do not have any significant detrimental effect on the freedom of movement of the piston. 
     The piston  8  is arranged in the body  2  of the meter with the disc  20  further from the inlet end of the meter, but it is also possible to arrange the disc and spokes in the reverse orientation, with the disc  20  nearer the inlet end. In either case, as the piston  8  slides along the body  2  against the spring bias, the outer periphery of the downstream end of the piston drives the pointer  18  along the slot  16  to mark the peak flow position. 
     The piston  40  of FIGS. 6 and 7, which can be used instead of the piston  8  shown in FIGS. 1-5, is generally cup-shaped, comprising an end disc  42  with a spigot  44  projecting axially from its base for connection of the spring  10 . Projecting in the opposite axial direction from the periphery of the end disc  42  is an annular skirt  46 . The plain cylindrical outer surface of the skirt  46  is in sliding engagement with the wall of the body  2 , and since at any one time it extends over an axial length  9  which is at least 25% of the piston diameter prevents the piston  40  from tilting within the body. This form of piston is, however, suitable only for applications in which the accuracy and repeatability are less critical, as the large sliding contact area makes it susceptible to variations of performance. 
     FIGS. 8-16 show other alternative pistons that can be used in the meter of FIGS. 1 and 2. All of these pistons are generally cup-shaped, having a closure element in the form of a disc and a skirt extending axially from the disc providing guidance means to counter tilting forces. 
     The piston  50  of FIGS. 8-10 has a cylindrical skirt  52  with a flared peripheral lip  54 , the outer edge of the lip providing sliding contact with the body  2  of the meter. A series of ribs  56  extend from the lip  54  along the skirt  52  from which they protrude radially to the same diameter as the outer periphery of the lip  54 . The ribs are equally spaced around the skirt and extend parallel to the axis of the piston  50 . Together, the ribs  56  and the peripheral lip  54  guide the piston and support it against tilting in the body. A base  57  of the cup provides the reaction face by which exhaled air entering from the inlet  4  drives the piston in the chamber against spring  10 . 
     As in the first example, a central pocket  58  is provided in the upstream side with a pip  59  giving a central connection for the spring  10 . 
     FIGS. 11-13 illustrate some possible modifications of the piston of FIGS. 8-10. In piston  60  a peripheral lip  61  extends perpendicular to the axis of movement and together with ribs  65  is for sliding contact with the body. In place of the pocket  58 , a spigot  62  with a diameter close to the inner diameter of the spring  10  projects from the base  63  of the piston for attaching and supporting the spring. 
     FIGS. 14-16 show a piston  80  which has a series of ribs  82  similar to the pistons  50  and  60 , but the spaces  84  between the ribs  82  are not closed by a peripherally projecting lip such as lips  54  or  61 . Thus, through slots are formed between the ribs  82  allowing a leakage air flow past the piston  80 . The radially outer edges of the ribs  82  guide and support the piston as it slides in the body of the meter. 
     In all of these embodiments, the axial bounds (lip  54  or  61  plus ribs  56  or  65 , or ribs  82 ) of the contact region  9  between the piston and the wall of the chamber is at least 25% of the diameter of the piston, for the reasons explained previously. 
     FIGS. 17 and 18 show how meters embodying the present invention may provide ribs on the inner surface of the body to achieve limited contact between piston and body. 
     In FIG. 17 the body wall  100  of the meter which otherwise is as in FIGS. 1 and 2 is shaped to form three radially inwardly projecting broad ridges  102 , providing equally spaced inner surfaces in sliding contact with a cylindrical piston such as piston  8  or piston  40  to guide and support it as it slides along the body. The ridges  102  extend along the body so as to support the piston through its complete range of movement. Between the ridges  102 , spaces  106  are formed between the piston  104  and the body inner wall which allow a leakage air flow around the piston. 
     FIG. 18 shows a similar arrangement using a series of relatively narrow longitudinal ribs  110  projecting inwardly to act as the contact area of the piston  104  with the inner surface of the body wall  100 . 
     In these modifications, the axial bounds of the contact zone between the piston and the wall are as before spaced apart by at least 25% of the diameter of the piston. 
     It will be understood that although the illustrated examples have been related to a known form of meter, the invention is capable of wider application to other forms of ventilatory capacity meters which rely on the displacement of a piston in a chamber.