Abstract:
A demountable pellet is disclosed for use as an electrode contact assembly. The pellet comprises a base  1  upon which is mounted a plurality of electrodes  18, 19, 20  arranged in respective spaced parallel planes and having respective contacts  23, 24, 25  protruding from them. A closure  2  for the base  1  has a member  11, 12, 13  engaging at least one of the electrode contacts. In an unassembled condition of the pellet, with the closure  2  separate from the base  1  and each closure member disengaged from its respective electrode contact, the contacts  23, 24, 25  lie in respective spaced parallel planes. In the assembled condition of the pellet with each closure member engaging its respective electrode contact, at least one of the contacts  23, 24, 25  lies in a plane other than that in which it lay in the unassembled condition of the pellet.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to a demountable electrode contact assembly. Such self-contained sub-assemblies are conveniently referred to as “pellets”. The present invention is particularly suitable for use in a photoionisation detector (PID) which enables gases, particularly air, to be sampled for gases which form ions when exposed to light emitted from a UV light source within the PID. These photoionisable gases can be harmful or dangerous to health, can present a fire risk, and/or can indicate the failure of equipment designed to contain them in a gaseous or liquid state. 
       BACKGROUND OF THE INVENTION 
       [0002]    PIDs usually incorporate a chamber through which a sample gas is passed by means of suitable tubing, pumps, fans and the like. The chamber is exposed to photons. A proportion of the photons has sufficient energy to disassociate any photoionisable gas molecules within the sample into ions. This process is known as photoionisation and the chamber itself is described as a photoionisation chamber. Each photoionisation event engages one photon and causes equal numbers of positively and negatively charged ions to form, usually one of each. An electric field is applied across the chamber by means of two or more electrodes which are a part of, or are contained within, the chamber walls. The ions are attracted to the electrodes, causing a current to flow between the two or more electrodes within the chamber. This current is amplified and displayed, providing an indication of the presence of the target gas molecules. 
         [0003]    PIDs also incorporate means to produce UV light of sufficient energy to ionise gases of interest, such as volatile organic carbon compounds, without ionising common constituents of clean air, such as nitrogen, oxygen, argon and/or carbon dioxide. UV discharge electrodeless lamps of up to a few centimetres length and up to about one centimetre diameter are commercially available. These lamps contain a gas at a few millibars of pressure, typically a noble gas such as krypton. Soda glass is a suitable material for the body of the lamps, terminating at one end with a wall which typically comprises a flat round disc of magnesium fluoride and lithium fluoride which are transparent to photons in the energy range of interest and which typically forms one wall of the ionisation chamber. 
         [0004]    It is desirable to remove and thereby detect photogenerated ions in a photoionisation chamber by means of electrodes located no more than a millimetre or two from the lamp face and from each other. The reasons for this are fourfold. First, photoionising UV light may be adsorbed not only by photoionisable gases but also by other gaseous constituents of air, most significantly, moisture. Thus an electrode network extending more than one or two millimetres from the lamp face gives rise too readily to a response to ionisable gas within air which varies with humidity. Second, the absorption of light by ionisable gases increases with distance from the lamp, causing the response to an organic gas not to increase in proportion to its concentration, even at quite modest concentrations (tens of parts per million of ionisable gas) in chambers having an electrode gap of more than a few millimetres. Third, a few hundred volts per mm of electrode gap must be applied between the electrodes in order to strip out all the photo-ions between the electrodes before they are conveyed by sample flow away from the electrodes or recombine to form neutral molecules which are not attracted to the electrodes and do not contribute to the photoionisation current. Generating such a voltage becomes more problematic as the gap across the electrodes is increased. Finally, positive and negative ions are more likely to encounter each other and recombine as the electrode gap is increased, again causing poor proportionality between response and ionisable gas concentration. 
         [0005]    The electrodes are typically of planar form and have thicknesses less than a few tenths of a millimetre. One electrode, namely the anode, is at a positive electric potential relative to the other electrode, namely the cathode, and abuts or is otherwise located very close to the lamp window, attracting negatively-charged ions. The anode includes means for admitting light through it, for example, by slots or holes. The cathode is at a negative potential relative to the anode and is placed parallel to, and within a millimetre of, the anode attracting positively-charged ions. This arrangement assures that positive ions, which form more contaminating products than do negative ions, are conveyed away from the lamp window. 
         [0006]    However, some ionisable gases form negatively charged ions which are attracted towards the lamp windows and, are adsorbed thereon. Further, collisions between such negative ions and neutral molecules within the ionisation chamber can generate ion clusters which, being of high molecular weight, are very prone to deposition on surfaces, such as the lamp window. Further, the sample gas might contain non-volatile material in aerosol form which can also deposit on the lamp window. In short, the lamp window can be fouled by species within the sampled air, whereby the sensitivity of the PID to a given concentration of ionisable species is reduced. Most ionisable gases are organic and, therefore, it is reasonable to anticipate that window fouling is usually caused by a film of organic material. Sometimes, a photoionisation chamber may also be fouled by liquids being drawn into it. 
         [0007]    It will be apparent that photoionisation chambers are frequently of small dimensions and, in certain applications, prone to contamination or mechanical damage during use and servicing. It is thus desirable to design the photoionisation chamber to be readily replaceable. Such replaceable chambers will include electrical terminals to the photoionisation electrodes described above. Other gas sensing ionisation chambers might also be advantageously rendered easily replaceable for similar reasons. 
         [0008]    However, there has frequently been difficulty in assuring good electrical contact between the electrode terminals in the replaceable chamber and terminals from supporting electronic circuitry contained within a sensing member which accommodates the replaceable ionisation chamber. In particular, spring-loaded retractable pins, which ensure a force is applied between the contacts, do not always retain their spring loading. Also, pins inserted into metal or metal-coated holes are prone to failure after multiple engagement and removal of the ionisation chamber. 
         [0009]    The invention seeks to mitigate the problems and disadvantages associated with the known electrical contacts of disposable PID electrode assemblies discussed above. 
         [0010]    Needs dictate that the small detector pellets of such assemblies have five external connections located within close proximity of each other, to allow the working electrodes, as previously described, maximum space to collect the charged ions in the operating chamber part of the detector pellet. Five connections from an external printed circuit board (PCB) to the respective connections located on this pellet have to be constructed in such a manner that manufacturing tolerances in contact alignment in three dimensions are readily overcome. 
         [0011]    One form of prior art arrangement uses spring-loaded telescopic pins which can change their length to take up manufacturing tolerances in one dimension and are designed so they depress further to provide firm contact on preformed metal contacts located within the pellet against spring pins. They rely upon an internal, suitably small, coil spring located within the assembly. This technique requires much space and if specialised miniature telescopic pins are used, then the costs may be prohibitive. 
         [0012]    Preformed contact surfaces in the pellet may be used to bring all the differently placed electrode contacts on to one common plane for the spring-loaded telescopic pins to press upon. 
         [0013]    There are several technical problems with this approach. 
         [0014]    First, the expensive telescopic pins are, for practical reasons, located within the retained PCB exterior to the disposable pellet. The whole assembly can be used in aggressive and potentially corrosive atmospheres and thus, after some repeated use, the pins may become damaged or corroded, thereby losing their effectiveness in communicating electrical signals between the pellet and the externally retained PCB. The prior art arrangements make no allowances for this and thus disposal of the pellet together with the whole externally retained PCB must be effected. 
         [0015]    A further problem with this prior art arrangement is at the points of contact between the pins and contacts. 
         [0016]    These points of contact are generally between the pins and contacts which are perpendicular to each other and, thus, have no means for wiping the points of contact clean whilst making contact between the pins and contacts. Oxide or external debris if not wiped away during the contact-making generation may impair good ohmic contact for electrical signals. 
         [0017]    Another prior art arrangement employs a pin and slide socket arrangement. The pin is located within the electrode pellet parallel to the direction of pellet placement with respect to the mounting PCB and slides into a suitably placed socket located on the PCB. This has the advantage of being slightly less expensive than the prior art arrangement discussed above and the barrel action of the interference spring connection within the socket allows the slide sockets to be packed more tightly together. 
         [0018]    Advantages of this prior art arrangement over the previous one described above will now be explained. 
         [0019]    First, there is a wiping action while contact is being made which aids oxide and debris removal and, hence, enhances ohmic contact. 
         [0020]    Second, provided the socket is long enough in its sliding contact length, then a common pin may be connected to the differently placed grids, thus relying upon the comparatively long travel of contact engagement to take up both manufacturing tolerances and electrode grid position. 
         [0021]    However, this particular prior art arrangement does have some disadvantages. 
         [0022]    The main disadvantage is that the socket and pins must be parallel to each other and to the direction of their relative travel. Clearly, this is not too much of a problem if it is just one or two pins, as in the prior art example, but when, say, five pins are used then this becomes more difficult to achieve. Further, this option still tends to be unduly expensive in relation to unit costs and assembly requirements. 
       OBJECT OF THE INVENTION 
       [0023]    It is an object of the present invention to provide a pellet, and an associated method of its assembly, for use as an electrode contact assembly in a PID which overcomes, or at least substantially reduces, the disadvantages discussed above in relation to the prior art arrangements. For example, the pellet can be low cost and therefore readily replaceable. 
       SUMMARY OF THE INVENTION 
       [0024]    Accordingly, one aspect of the invention resides in a demountable pellet for use as an electrode contact assembly, comprising a base upon which is mounted a plurality of electrodes arranged in respective spaced parallel planes and having respective contacts protruding therefrom, and a closure for the base having a member engaging at least one of the electrode contacts, wherein, in an unassembled condition of the pellet, with the closure separate from the base and the at least one closure member disengaged from its respective electrode contact, the contacts lie in respective spaced parallel planes and wherein, in the assembled condition of the pellet with the at least one closure member engaging its respective electrode contact, at least one of the contacts lies in a plane other than that in which it lay in the unassembled condition of the pellet. 
         [0025]    Advantageously, the closure member(s) are protrusions from the closure but other means, such as the shape of the closure, can readily be employed for engaging the electrode contact(s) and thereby changing the plane thereof when the pellet is in an assembled condition. 
         [0026]    In an embodiment of the pellet to be described hereinbelow, the pellet base has three electrodes mounted to it, with three corresponding electrode contacts lying in respective, spaced parallel electrode planes when the pellet is in its unassembled condition. When the closure is fitted to the base, closure members engage their respective contacts and deform two of them into the same plane as the third contact. 
         [0027]    The pellet base may be provided in one wall thereof with a plurality of holes corresponding to the number of electrode contacts, so that, in the assembled condition of the pellet, the contacts overlie respective ones of those holes through which fixed terminal pins, preferably of the same height, can be received to engage respective ones of the contacts, to provide ohmic contacts therebetween. As the fixed terminal pins are received through the base holes, they may deform the contacts in scraping engagement therewith, to enhance such ohmic contacts. In this embodiment, the free ends of the contacts, which are spaced from the closure members engaging them, are deformed in a direction which is generally opposite to that in which the contacts are deformed initially when the closure is fitted to the base. 
         [0028]    A second aspect of the invention is directed to a method of assembling a pellet for use as a demountable electrode contact assembly, which method comprises providing a base upon which is mounted a plurality of electrodes arranged in respective spaced parallel planes and having respective contacts which protrude therefrom and lie in respective spaced parallel planes; providing a closure having at least one member for engaging one or more of the electrode contacts when the closure is fitted to the base; and fitting the closure to the base, to cause the at least one closure member to engage its electrode contact and to deform the said electrode contact into a plane other than that in which it lay prior to the closure being fitted to the base. 
         [0029]    In the embodiment mentioned above and to be described in more detail hereinbelow, three electrode contacts are provided and, in the assembled condition of the capsule, all three contacts lie in a common plane, preferably overlying respective holes through which fixed terminal pins, preferably of the same height, are receivable from the exterior of the pellet. 
         [0030]    Such terminal pins, when received through their respective holes in the base of the pellet, deform their corresponding electrode contacts in a direction generally opposite to that of the initial deformation of the contacts when the capsule is assembled. 
         [0031]    The fixed terminal pins may be those of a PID capsule, in which case, a further aspect of the invention resides in an assembly comprising a PID capsule to which is fitted, preferably detachably, a pellet in accordance with the invention and as defined and described above, wherein the terminal pins of the PID capsule engage respective ones of the electrode contacts in ohmic contact therewith. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: 
           [0033]      FIG. 1  is an exploded perspective view of a partially-assembled electrode pellet with a moulded base body and lid; 
           [0034]      FIG. 2  is a perspective view of the underside of the lid of the pellet; 
           [0035]      FIG. 3  is a plan view of a base body of the pellet similar to that shown in  FIG. 1 ; 
           [0036]      FIG. 4  is a diagrammatic, partial sectional elevation of the pellet of  FIG. 1  in a partially-assembled condition; 
           [0037]      FIG. 5  is a diagrammatic, partial sectional elevation of the pellet shown in  FIG. 4  but in its assembled condition; 
           [0038]      FIG. 6  is a perspective view of the body of a photoionisation detector capsule; 
           [0039]      FIG. 7  is a plan view of the capsule shown in  FIG. 6 ; 
           [0040]      FIG. 8  is a diagrammatic, partial sectional elevation of the pellet of  FIGS. 1 and 5  fitted to the capsule of  FIGS. 6 and 7 ; 
           [0041]      FIG. 9  is an exploded sectional elevation of the pellet of  FIGS. 1 to 5  and the capsule of  FIGS. 6 and 7  before assembly; 
           [0042]      FIG. 10  is a sectional elevation of the pellet of  FIGS. 1 to 5  assembled to the capsule of  FIGS. 6 and 7 ; 
           [0043]      FIG. 11  is a diagrammatic, partial sectional elevation of an electrode pellet with an electrode contact, in an assembled condition of the pellet; 
           [0044]      FIG. 12  is a diagrammatic, partial sectional elevation of the pellet of  FIG. 12  in its assembled condition and with the electrode contact engaged by a terminal pin of a PID capsule; and 
           [0045]      FIG. 13  is a plan view of part of the pellet shown in  FIGS. 11 and 12 , showing the electrode contact located therein. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0046]    All references to downward and other directions are relative to the base of the pellet, as shown in the drawings, and do not imply any particular orientation of the pellet. 
         [0047]    Referring first to  FIG. 1  of the accompanying drawings, an electrode pellet in accordance with the invention, and for use with a photoionisation detector (PID) capsule, comprises a moulded plastics base body, generally designated  1 , and a moulded plastics lid, generally designated  2 , which can be fitted and secured to the pellet base body  1 . 
         [0048]    The base body  1  comprises a circular aperture  3  for alignment with one end of an ionisation lamp of a PID capsule, as will be described hereinbelow. 
         [0049]    A circular recess  4  concentric with the aperture  3  is arranged to receive therein three electrodes (not shown) separated by respective electrically-insulating spacers (also not shown), as will also be described hereinbelow. 
         [0050]    Each electrode has a contact (not shown) which extends into a respective one of three recesses  5 ,  6 ,  7  in the base body  1 , to overlie a respective one of three holes  8 ,  9 , extending through the base body  1  from the recesses  5 ,  6 ,  7 . Those three electrode contacts lie in spaced parallel planes within the base body  1 . 
         [0051]    Referring now to  FIG. 2  of the accompanying drawings, the underside of the pellet lid  2  comprises three protrusions  11 ,  12 ,  13  which, when the lid  2  is fitted to the base body  1  with the electrodes and associated contacts in place, engage respective contacts to deform them into a common plane within the base body  1 , while still overlying respective holes  8 ,  9 ,  10 . 
         [0052]    This arrangement is such that the protrusions  11 ,  12 ,  13  of the lid  2  are spaced from the respective holes  8 , 9 , 10 , with the free ends of the electrode contacts still overlying the respective holes  8 ,  9 ,  10 . 
         [0053]    As will be described hereinafter, the holes  8 ,  9 ,  10  are arranged to receive therethrough fixed terminal pins of equal height of a PID capsule. 
         [0054]    In  FIG. 3  of the drawings, there is shown a more detailed plan view of the base body  1 , illustrating the circular aperture  3 , circular recess  4 , recesses  5 ,  6 ,  7  for receiving the electrode contacts and the respective terminal pin-receiving holes  8 ,  9 ,  10 . 
         [0055]    The base body  1  is also provided with two further holes  14 ,  15  through which are receivable two further terminal pins for contacting a contact strip (also not shown) extending between the holes  14 ,  15  as part of a circuit for determining the presence of the pellet when associated with a PID capsule. 
         [0056]    Two opposed sides of the base body  1  are provided with respective elongate lugs  16 ,  17  which provide a snap-fit of the assembled pellet to an associated PID capsule, as will be described hereinbelow. 
         [0057]    When the assembled electrode contact pellet is snap-fitted to an associated PID capsule, the two further terminal pins thereof make contact with the respective ends of the contact strip extending between the two holes  14 ,  15 , to confirm that the pellet is correctly fitted to the capsule. 
         [0058]    On assembly of the pellet, as shown in  FIGS. 4 and 5 , three electrodes  18 ,  19 ,  20  and respective insulating annular spacers  21 ,  22  are located in the base body  1 , such that their respective contacts,  23 ,  24 ,  25  lie, along with their respective electrodes  18 ,  19 ,  20 , in spaced parallel planes, as shown in  FIG. 4 , with the respective free ends of the three contacts  23 ,  24 ,  25  overlying respective holes  8 , 9 , 10  in the base body  1 . 
         [0059]    The lid  2  is aligned with the open top ( FIG. 4 ) of the base body  1 , with the lid protrusions  11 ,  12 ,  13  depending downwardly in alignment with their respective electrode contacts  23 ,  24 ,  25 . 
         [0060]    It is to be appreciated that for reasons of clarity, the further holes  14 ,  15  in the base body  1  are not shown and, also, that  FIGS. 4 and 5  are purely diagrammatic and not to scale. 
         [0061]    Then, the lid  2  is fitted to the base body  1 , as shown in  FIG. 5 , such that the lid protrusions  11 ,  12 ,  13  engage their respective electrode contacts  23 ,  24 ,  25 , such that the upper two contacts  23 ,  24  are deformed downwardly into the plane of the lowermost electrode contact  25 . 
         [0062]    In the arrangement shown in  FIG. 5 , with the lid  2  fitted to the base body  1 , the uppermost contact  23  in  FIG. 4  now comprises a downwardly depending portion  23 ′ and another portion  23 ″ extending in the plane of the lowermost contact  25 , with its free end portion  23 ′″ overlying its respective hole  10  in the recess  7  of the base body  1 . Similarly, a portion  24 ′ of the electrode contact  24  ( FIG. 4 ) depends downwardly, with a portion  24 ″ also extending in the plane of the lowermost contact  25 , with its free end portion  24 ′″ overlying its respective hole  9  in the recess  6  of the base body  1 . 
         [0063]    In this manner, the two upper electrode contacts  23 ,  24  of  FIG. 4  have been deformed downwardly, such that portions  23 ″,  24 ″ thereof lie in a common plane with the lowermost electrode contact  25  whose free end portion  25 ′″ also overlies its respective hole  8  in the recess  5  of the base body  1 . 
         [0064]    The lid  2  is then secured firmly in position on the base body  1 , for example, by ultrasonic welding or any other suitable means. 
         [0065]    Thus, there is provided a compact integral pellet for subsequent fitment to a PID capsule, such as that generally designated  30  in  FIGS. 6 and 7 . 
         [0066]    This PID capsule  30  comprises a generally cylindrical body  31  with a top  32  having a recess  33  whose shape and depth correspond generally with those of the pellet described above in relation to  FIG. 5 . 
         [0067]    In the recess  33 , there is provided a circular aperture  34  and associated annular recess  35  for receiving an ionisation lamp, as will be described below. 
         [0068]    Also, the recess  33 , as shown in  FIG. 6 , has three holes  36 ,  37 ,  38  through which are receivable respective fixed terminal pins  39 ,  40 ,  41  of equal height, as shown in  FIG. 7 , and which, when the pellet is assembled to the top  32  of the capsule body  31 , lie in register with corresponding holes of the base body  1 . 
         [0069]    Similarly, the circular aperture  34  in the recess  33  of the top  32  of the capsule body  31  lies in register with the circular aperture  3  in the base body  1  of the assembled pellet. 
         [0070]    Further, additional holes  42 ,  43  provided in the recess  33 , as shown in  FIG. 6 , lie in register with corresponding holes  14 , 15  in the base body  1  of the pellet. 
         [0071]    As shown in  FIG. 7 , these additional holes  42 ,  43  also receive therein additional fixed terminal pins for engaging a contact (not shown) extending between the holes  42 ,  43 , which forms part of a circuit for determining the presence or absence of the assembled pellet with respect to the PID capsule  30 . 
         [0072]    In  FIG. 8 , the assembled pellet, now indicated generally at  50 , is shown fitted in the recess  33  of the top  32  of the PID capsule base  31 . As the pellet  50  is fitted into that recess  33 , the rounded ends of the terminal pins  39 ,  40 ,  41 , which are of equal height and which extend through respective holes  36 , 37 , 38  in the recess  33 , engage the free ends  23 ′″,  24 ′″,  25 ′″ of the electrode contacts  23 ,  24 ,  25  with a scraping action, to provide good ohmic contact therebetween as the free contact ends  23 ′″,  24 ′″,  25 ′″ are deformed in a direction opposite to that in which the contact portions  23 ′,  24 ′ extend. 
         [0073]    In this manner, good ohmic contact is made between the circuitry associated with the PID capsule  30  and the electrode contacts and the electrodes  18 ,  19 ,  20  themselves. 
         [0074]      FIG. 9  illustrates an exploded sectional view of a PID assembly comprising the unassembled pellet  50  including the base body  1 , lid  2 , electrodes  18 ,  19 ,  20  and respective insulating spacers  21 ,  22 . An ionisation lamp  60  and associated O-ring  61  for fitting into the PID capsule body  31  via the circular aperture  34  is also shown. 
         [0075]      FIG. 10  shows the assembled PID capsule  30  including the pellet  50 , ionisation lamp  60  and O-ring  61 . 
         [0076]    Referring now to  FIGS. 11 ,  12  and  13  of the drawings, here there is shown a modified electrode  119  and associated contact  124  arrangement. For reasons of clarity, the other electrodes and associated contacts are not shown in these two Figures. 
         [0077]    As in the case of the three electrodes  18 ,  19 ,  20  and associated contacts  23 ,  24 ,  25  discussed above, before the pellet is assembled, for example as shown in  FIG. 4 , the electrode  119  and a first portion  124 ′, an intermediate portion  124 ″, a second portion  124 ′″, a clamping portion  125  and a circular portion  126  of the contact  124  all lie in a common plane, with the portions  124 ″,  124 ′″ extending backwards with respect to and adjacent the portion  124 ′ and connected integrally thereto by the clamping portion  125 . 
         [0078]    The base body  1  is provided with a protrusion  114 , whilst the lid  2  has a corresponding protrusion  113 . 
         [0079]    On assembly of the pellet and when the lid  2  is applied to the base body  1 , for example, as shown in  FIG. 5 , the lid protrusion  112  engages the portion  124 ″ of the contact  124 , thereby deforming downwardly the portion  124 ′ and, to some extent, the portions  124 ″,  124 ′″,  125  of the contact, as shown in  FIG. 11 . 
         [0080]    Also, the lid protrusion  113  engages the portion  125  of the contact  124 , which portion  125  is also deformed downwardly to engage the base body protrusion  114 , so that the contact portion  125  is effectively sandwiched between the lid and base body protrusions  113 ,  114 . 
         [0081]    Subsequently, and when the so-assembled pellet is applied to a PID capsule, such as that shown at  30  in  FIGS. 6 ,  7  and  9 , the corresponding terminal pin  140  ( FIG. 12 ) of that capsule engages the circular portion  126  of the electrode contact  124  with its rounded end, to deform the portions  124 ′″,  126  in a direction opposite to that in which the portion  126  extends. In this manner, the rounded end of the terminal pin  140  engages the portion  124 ′″ with a scraping action, to provide good ohmic contact therebetween during such oppositely-directed deformation. 
         [0082]    In this manner, the portion  125  of the contact  124  is sandwiched firmly between the lid and base body protrusions  113 ,  114 , so that when the circular contact portion  126  is engaged by the terminal pin  140  of the PID capsule only the contact portions  124 ′″,  126  are deformed by the terminal pin  140 , without unwanted deformation of the remaining portions  124 ′,  124 ″,  125  of the contact  124  which might otherwise detract from the integrity of the components of the contact assembly. 
         [0083]    Although the other electrodes and associated contacts are not shown in  FIGS. 11 and 12 , it is to be assumed that they are configured, prior to and during assembly of the pellet, as discussed above in relation to  FIGS. 4 ,  5  and  8  and, subsequently, when fitted within a PID assembly, as also described above in relation to  FIGS. 7 ,  9  and  10 . 
         [0084]    The invention is not limited to any particular configuration of electrodes. 
         [0085]    As discussed above in relation to  FIGS. 6 and 7 , the electrodes can be configured to include a circuit determining or indicating correct installation of the assembled pellet on a photoionisation detector or other module. 
         [0086]    Our granted patents EP 1474681 and U.S. Pat. No. 7,046,012 describe and claim an ionisation detector including a third or fence electrode wherein, inter alia, current drawn from the fence electrode provides a measure of the degree of contamination or condensation within the detector and of lamp efficiency. Such electrode arrangement can advantageously be incorporated into the present invention. 
         [0087]    It is to be appreciated that the invention provides a demountable and replaceable pellet and an associated pellet/PID capsule assembly which have distinct advantages over the prior art arrangements discussed above. 
         [0088]    The electrode contacts are deformable in such a manner that when they are further deformed by engagement with the fixed terminal pins, they are scraped by the rounded ends of the terminal pins as they slide thereover, to remove oxide and other debris between the respective contact/terminal surfaces. This can be done in one action as the pellet is fitted into place upon the PID capsule. 
         [0089]    Physical alignment is not crucial in any dimension but this sliding and scraping action of the terminal pins upon the flat surfaces of the electrode contacts is able to take-up movement and self-alignment in all three dimensions. 
         [0090]    Also, costs are reduced to a minimum, in that there are only two parts, one part being the fixed terminal pins and the other being a mere extension contact of the associated electrode. 
         [0091]    Environmental effects are also minimised, due to the wiping action discussed above. The pellet represents the disposable part of a PID capsule and can thus be inexpensively exchanged to retain original performance. 
         [0092]    While the invention has been described with reference to photoionisation detectors, it is applicable to other demountable electrode assemblies. 
         [0093]    Other manufacturing benefits are forthcoming from this invention. 
         [0094]    First, the electrodes require no further manufacturing processes to cause permanent deformation prior to their being loaded into the pellet, thus saving on separate inventory control and additional tooling. 
         [0095]    Second, because the electrodes of the pellet are assembled in their flat, undeformed state, visual inspection is readily accessible to identify rogue parts which is more difficult to achieve with preformed components. 
         [0096]    Third, flat electrodes are much more easily loadable into the pellet, preformed components being more likely to tangle when together, technically called “fish-hooking”. 
         [0097]    Fourth, two of the electrodes can be designed to be mirror images. Thus, the same electrodes may be placed within the pellet because the pellet body during assembly creates the left-hand part and right-hand part simultaneously in the one closing action.