Abstract:
The invention relates to a retrofittable pressure gauge for a spray gun comprising a pressure sensor, located in a pressure chamber, a power source and a display unit, all enclosed within a case, a passageway providing fluid communication between the pressure chamber and a pressurised interior space of a spray gun when the gauge is mounted on a spray gun and a means for attachment to a spray gun, wherein the means for attachment comprises a mounting fixture configured for mounting the pressure gauge at a spreader valve or a trigger operated fluid valve of a spray gun.

Description:
[0001]    This invention relates to an air pressure gauge, preferably a digital air pressure gauge, adapted to be retrofitted to a compressed air spray gun and a compressed air spray gun comprising the said air pressure gauge. In particular the invention discloses an air pressure gauge adapted to be retrofitted to a compressed air spray gun at a location down stream of a compressed air inlet pressure reducing valve or trigger operated shut off valve.  
         BACKGROUND OF THE INVENTION  
         [0002]    Traditionally the compressed air inlet pressure of a compressed air spray gun is adjusted to suit a particular compressed air spray gun air cap or spraying medium by a compressed air pressure reducing valve and air pressure gauge mounted upstream of the compressed air inlet, mounted either adjacent to the source of the compressed air near a spray booth or mounted pendant of the handle of the compressed air spray gun. One problem associated with the aforementioned arrangements of air pressure gauge and compressed air spray gun is that the location of the air pressure gauge is such that the air pressure upstream of the compressed air pressure reducing valve is measured. It is preferable to measure the air pressure downstream of the compressed air pressure reducing valve and all other compressed air valves which can affect a variable pressure reduction and lead to inconsistent compressed air pressure reaching the air cap.  
           [0003]    It is possible for a compressed air inlet pressure reducing valve to be included as part of the spray gun thus eliminating the need for a separate compressed air pressure reducing valve, however a problem with this arrangement is that there is at present no means of displaying the compressed air pressure, and if a separate compressed air pressure reducing valve is used upstream with an air pressure gauge, the pressure reducing valve is downstream and if not fully open can cause the pressure setting to vary.  
           [0004]    A solution to the aforementioned problem is provided by EP 526 525 which discloses a compressed air spray gun comprising a miniature digital electronic air pressure gauge located down stream of the compressed air inlet pressure reducing valve for measuring the air pressure at the air cap and downstream of the trigger operated shut off valve. As such compressed air spray guns are often used in hazardous areas, the air pressure gauge comprises a sealed unit with an aperture leading to a pressure sensitive transducer connected to an amplifier. The signal from the amplifier is input to an analogue digital converter and thence to a decoder/driver and display. On exhaustion of the cell powering the amplifier, the sealed unit is discarded and replaced with a new sealed unit.  
           [0005]    The miniature digital electronic air pressure gauge of EP 526 525 is threaded into a threaded aperture bored into the compressed air spray gun. The air pressure gauge is arranged to be in communication with a bore leading to a region downstream of the compressed air inlet pressure reducing valve and downstream of the trigger operated shut off valve.  
           [0006]    A similar solution is offered by the ITW OMX-610 spray gun except the threaded aperture receives an analogue pressure gauge and communicates with a region downstream of the compressed air inlet pressure reducing valve but upstream of the trigger operated shut off valve.  
           [0007]    DE 100 31 857 A and DE 100 31 858 A disclose spray guns comprising pressure gauges which are integrated into respectively the handle of the spray gun and the compressed air inlet valve. Embodiments are disclosed with the compressed air pressure being measured upstream of the compressed air inlet, between the compressed air inlet pressure reducing valve and the trigger operated shut off valve and downstream of both the compressed air inlet pressure reducing valve and the trigger operated shut off valve.  
           [0008]    Given the foregoing, a need has been identified to provide an air pressure gauge which can be retrofitted to a wide range of existing compressed air spray guns without the need to modify the compressed air spray gun body and thereby measure the air pressure downstream of a compressed air inlet pressure reducing valve and trigger operated shut off valve where the mounting of the gauge does not affect the ease of use of the existing spray gun controls.  
         SUMMARY OF THE INVENTION  
         [0009]    In a first aspect, the invention provides a retrofittable pressure gauge for a spray gun comprising a pressure sensor, located in a pressure chamber, a power source and a display unit, all enclosed within a case, a passageway providing fluid communication between the pressure chamber and a pressurised interior space of a spray gun when the gauge is mounted on a spray gun and a means for attachment to a spray gun, wherein the means for attachment comprises a mounting fixture configured for mounting the pressure gauge at a spreader valve or a trigger operated fluid valve of a spray gun. The mounting fixture can comprise a mounting fixture bore through which at least a part of the spreader valve or trigger operated fluid valve can pass thereby mounting the pressure gauge at the spreader valve or the trigger operated fluid valve. The passageway can provide fluid communication between the pressure chamber and the mounting fixture bore. Furthermore the mounting fixture bore and the part of the spreader valve or trigger operated fluid valve which can pass through the mounting fixture bore can define an annular space in communication with the passageway.  
           [0010]    The pressure gauge can comprise an analogue or digital pressure gauge. The pressure gauge optionally comprises a movement detector switch which activates the pressure sensor only when the pressure gauge is subject to movement.  
           [0011]    In one preferred embodiment, the pressure sensor comprises a semiconductor strain device. Typically the display unit is a liquid crystal display. The pressure gauge preferably further comprises a printed circuit board mounting a microprocessor.  
           [0012]    In a second aspect of the invention, a kit is provided comprising the pressure gauge hereinbefore described and a spreader valve bushing or a trigger operated fluid valve bushing, the spreader valve bushing or trigger operated fluid valve bushing comprising a channel, in substantial alignment with the longitudinal axis of the spreader valve bushing or trigger operated fluid valve bushing, which, when the spreader valve bushing or trigger operated fluid valve bushing is fitted into a spray gun, allows fluid communication between the pressurised interior space and the annular space.  
           [0013]    In a third aspect of the invention, a spray gun is provided comprising the pressure gauge hereinbefore described. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0014]    Embodiments of the present invention will now be described with reference to the figures in which:  
         [0015]    [0015]FIG. 1 shows a side view of a partly sectioned compressed air spray gun comprising a digital electronic air pressure gauge according to the invention attached to the compressed air spray gun at a spreader valve;  
         [0016]    [0016]FIG. 2 shows an exploded view of a digital electronic air pressure gauge according to the invention;  
         [0017]    [0017]FIGS. 3 a - 3   d  show respectively a view on A-A indicated in FIG. 3 d , a view on B-B indicated on FIG. 3 c , a front view and a side view facing left of a case of a digital electronic air pressure gauge according to the invention;  
         [0018]    [0018]FIG. 4 shows a part sectional view of part of a compressed air spray gun illustrating a digital electronic air pressure gauge according to the invention mounted on a compressed air spray gun at a spreader valve;  
         [0019]    [0019]FIG. 5 a - 5   b  show respectively a side view of a sectioned and a view on A-A indicated in FIG. 5 a  of part of a movement detector switch of a digital electronic air pressure gauge according to the invention;  
         [0020]    [0020]FIGS. 6 a - 6   b  show respectively a side view of a partly sectioned compressed air spray gun and a part side view of a sectioned compressed air spray gun illustrating a digital electronic air pressure gauge according to the invention mounted on a compressed air spray gun at a trigger operated fluid valve; and  
         [0021]    [0021]FIG. 7 shows a side cross-section of a compressed air spray gun comprising concentric trigger operated shut off and trigger operated fluid valves. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0022]    [0022]FIG. 1 shows a spray gun comprising a gun body  101 , defining an interior space  102  through which compressed air can be passed, having an integral handle  111  depending from adjacent a first end of the gun body  116  and an air cap  112  secured to an opposing second end of the gun body  117  by a retaining ring  113 .  
         [0023]    A trigger  105  is secured to the gun body  101  by a screw  114  to pivot towards the handle  111  when manually squeezed to turn on the spray gun. A fluid reservoir  115  is reversibly attached to a fluid inlet  108  protruding from an upper part of the gun body  101 . A compressed air hose (not shown) is reversibly attached to a compressed air inlet  104  located on an end of the handle  111  remote from the gun body  101 .  
         [0024]    The volume of compressed air entering the spray gun is controlled by a compressed air inlet pressure reducing valve  110  operating within the handle  111  of the spray gun. During use, as the trigger  105  is manually squeezed, compressed air from the compressed air hose (not shown) passes up through the handle  111  and through a trigger operated shut off valve  106 , located in the first end of the gun body  116 , into the interior space  102  of the gun body  101 . The interior space  102  communicates separately with atomisation air outlets (not shown) and spray pattern shaping air outlets (not shown) in the air cap  112  by respectively, an atomising air passage (not shown) and a spray pattern shaping air passage  118 . During use, the volume of compressed air in the interior space  102  which passes through the spray pattern shaping air passage  118  to the spray pattern shaping air outlets (not shown) is controlled by a spreader valve  103 , also located in the first end of the gun body  116 .  
         [0025]    On manually squeezing the trigger  105 , a trigger operated fluid valve  107  is also opened permitting fluid, such as a paint, to pass from the fluid reservoir  115  through the fluid inlet  108  into a fluid passage (not shown) within the gun body  101  and out through a fluid discharge orifice (not shown) as a stream. Compressed air exiting the atomising air outlets atomises the fluid into particles as the stream of fluid leaves the fluid discharge orifice (not shown) producing a spray of atomised fluid particles. Compressed air exiting the spray pattern shaping air outlets (not shown) controls the shape of the said spray of atomised fluid droplets.  
         [0026]    An adjustable threaded end cap  119  sited at the first end of the gun body  116  regulates the maximum volume of fluid which can pass through the fluid discharge orifice (not shown) by acting as a stop on the degree to which the trigger operated fluid valve  107  can be opened.  
         [0027]    A pressure gauge  109  is secured to the first end of the gun body  116  by the spreader valve  103  thereby to measure the air pressure downstream of the compressed air inlet pressure reducing valve  110 .  
         [0028]    The pressure gauge  109  comprises a case  201 , as shown in FIG. 2, including a recess  202 , set into a side wall of the case  201 , for a printed circuit board (PCB)  203  mounting a single chip microprocessor. The PCB  203  is operatively linked to a two digit, seven segment liquid crystal display (LCD)  204  by a contact strip  205 . A cover  206  is provided for the recess  202  comprising a window  207  through which the LCD  204  may be viewed.  
         [0029]    In the floor of the recess  202  are set a first well  208  for a pressure transducer  209 , and a second well  210 . for a battery  211 , the pressure transducer  209  and the battery  211  being independently operatively linked to the PCB  203 . The pressure transducer  209  is controlled by the PCB mounted microprocessor. The first well  208  and the pressure transducer  209  together define a pressure chamber (not shown).  
         [0030]    The case  201  additionally comprises a mounting fixture  212  pendant from a lower surface of the case  201 , the mounting fixture  212  comprising a mounting fixture bore  213  substantially in alignment with the LCD  204  axis of view. The case  201  also comprises an air passage  301 , as shown in FIGS. 3 a  and  3   b , providing communication between the first well  208  and the mounting fixture bore  213 .  
         [0031]    [0031]FIG. 4 shows a detail of the mounting of the pressure gauge  109  onto the first end  116  of the gun body  101  by the spreader valve  103 . The spreader valve  103  comprises a male thread  401  on part of an exterior surface of a shank  402 , the shank  402  terminating at a first end  403  with a tip  404 , which can progressively block the entrance to the spray pattern shaping air passage  118 , and at a second end  405  by a knurled nut  406  rotation of which rotates the shank  402  causing translation of the tip  404  thereby blocking the said entrance. The male thread  401  are located approximately between the knurled nut  406  and a point midway between the knurled nut  406  and the tip  404 .  
         [0032]    The spreader valve  103  additionally comprises a spreader valve bushing  407 , comprising a female thread (not shown) located on an inside surface of the spreader valve bushing  414 , in which the shank  402  is retained by interaction between the female thread (not shown) and the male thread  401 . The spreader valve bushing  407  is threaded into a threaded aperture  408  bored into the first end  116  of the gun body  101  terminating at the interior space  102 . The spreader valve bushing  407  is provided with an annular channel  409  formed on the inside surface of the spreader valve bushing  414  substantially in axial alignment with the longitudinal axis of the spreader valve bushing  407 . A first end of the annular channel  410  terminates at a first end of the spreader valve bushing  411  located within the aperture  408  and a second end the annular channel  412  terminates approximately mid-way between the first  411  and a second  413  end of the spreader valve bushing. The second end of the annular channel  412  communicates with an opening  416  through the spreader valve bushing  407  connecting the inside  414  and outside  415  surfaces of the spreader valve bushing. The female thread (not shown) lies approximately between the second end of the spreader valve bushing  413  and a point midway between the first  411  and second  413  ends of the spreader valve bushing.  
         [0033]    The pressure gauge  109  is mounted onto the spray gun by passing the spreader valve bushing  407  through the mounting fixture bore  213  of the pressure gauge  109  and threading the spreader valve bushing  407  into the threaded aperture  408 . The pressure gauge  109  is thereby clamped between a first surface defined by the exterior surface of the gun body adjacent the aperture  408  and a second surface defined by a spreader valve bushing shoulder  417 . The spreader valve bushing  407  and shank  402  of the spreader valve  103  are adapted to be longer in the present invention than typically used when not mounting the pressure gauge  109  of the invention. In another embodiment, the spreader valve bushing  407  and shank  402  of the spreader valve  103  remain unchanged in length in the present invention. In the latter case, only the spreader valve bushing  407  need be adapted and modified to mount the pressure gauge  109  to the spray gun by, for example, the provision of a removable ring located between the knurled nut  406  and the second end of the spreader valve bushing  413 . The pressure gauge  109  is mounted on the spray gun in place of the removable ring.  
         [0034]    Furthermore, the inside diameter of the mounting fixture bore  213  is larger than the outside diameter of the spreader valve bushing  407  thereby creating a spreader valve annular space  418  between the opposing surfaces defined by the inside surface of the mounting fixture bore and the outside surface of the bushing  415 .  
         [0035]    On mounting the pressure gauge as described hereinabove, compressed air in the interior space  102  can pass through the channels  409  formed on the inside surface of the spreader valve bushing  414  and through the openings  416  and thence through the air passage  301 , via the spreader valve annular space  418 , into the first well  208  of the pressure gauge  109  wherein resides the pressure transducer  209 .  
         [0036]    The pressure transducer  209  comprises a semiconductor strain gauge device bonded to a glass substrate mounted on a ceramic base whose characteristics change when subjected to a physical strain induced by, in this case, compressed air. The pressure transducer  209  generates a signal which is input to the PCB mounted microprocessor which includes an analogue digital convertor. The output of the PCB mounted microprocessor is fed to the LCD  204  which indicates the measured air pressure.  
         [0037]    The pressure gauge  109  operates in the following manner. The pressure gauge  109  can be in any one of two operational states, namely an active mode and an idle mode. In the active mode, the pressure gauge  109  actively measures and displays the current compressed air pressure. Should the compressed air pressure remain at zero for longer than a predetermined period, the pressure gauge  109  will then automatically switch to the idle mode, in order to conserve battery  211  life, at which point the LCD  204  will go blank. In the idle mode, the PCB mounted microprocessor will activate the pressure transducer  209  for a short period at regular intervals. If a compressed air pressure above atmospheric pressure is detected, the pressure gauge  109  will immediately switch to the active mode again.  
         [0038]    To further increase the life of the battery  211 , the pressure gauge  109  can additionally comprise a movement detector (not shown) which signals the microprocessor to de-energise the pressure transducer  209  after the spray gun has been stationary for a predetermined period thereby conserving battery life. When the spray gun is moved, the movement detector (not shown) detects this movement and signals the microprocessor to energise the pressure transducer  209 .  
         [0039]    A suitable movement detector switch, shown in FIGS. 5 a  and  5   b , comprises a metal ball  501  trapped in a slot  502  set into the surface of a metal housing  503 , an open side of the slot  502  covered by the PCB  203 . FIG. 5 a  shows that the PCB  203  comprises a first through contact hole  504  permitting electrical contact between a first copper track  505  and a first copper contact pad  506 . In FIG. 5 b  a second copper contact pad  507  is shown adjacent the first copper contact pad  506 , the second copper contact pad  507  in electrical contact with a second copper track (not shown) via a second through contact hole (not shown). The slot  502  is shown in FIG. 5A angled slightly downwardly to ensure the metal ball  501  remains in contact with at least one of the first or second copper contact pads  506 ,  507  when the spray gun is held upright with the trigger operated fluid valve  107  in a horizontal position.  
         [0040]    The metal ball  501  completes a first and second electric circuit between respectively the first  506  or second  507  copper contact pad and the metal housing  503  thereby producing input signals to the single chip microprocessor mounted on the PCB  203 . The metal ball  501  can either complete the first electric circuit between the first copper contact pad  506  and the metal housing  503 , or the second electric circuit between the second copper contact pad  507  and the metal housing  503  depending on the location within the slot  502  of the metal ball  501 . When the spray gun is being used, the metal ball  501  can move within the slot  502  successively completing the first and second electric circuits. Completion of each electric circuit generates different input signals which the single chip microprocessor recognises. When the microprocessor receives successive alternating inputs from each of the two circuits, the microprocessor energises the pressure transducer  209 . When the alternating input signals cease for a predetermined time, due to the spray gun being motionless, the microprocessor de-energises the pressure transducer  209 .  
         [0041]    As shown in FIGS. 6 a  and  6   b , the pressure gauge  109  can be mounted on a compressed air spray gun in a similar manner at the trigger operated fluid valve  107 . The trigger operated fluid valve  107  comprises a needle  601 , including a first needle end (not shown) terminating at an apex and a second needle end  602 , and a first shoulder (not shown) located approximately mid-way between the first (not shown) and second  602  needle ends of the needle. The trigger operated fluid valve  107  further comprises a fluid valve bushing  603 , which fits over the needle  601 , including first  604  and second  605  fluid valve bushing ends. The second fluid valve bushing end  605  is capped with the adjustable threaded end cap  119 . The first fluid valve bushing end  604  terminates adjacent the trigger  105 . Around a portion of the needle  601  bounded by the adjustable threaded end cap  119  and a second shoulder  606  on the needle  601  lies a spring  607 . The fluid valve bushing  603  extends through the gun body  101   
         [0042]    When the spring  607  is in a relaxed state, the first needle end (not shown) blocks the fluid discharge orifice (not shown). On manually squeezing the trigger  105 , a face (not shown) on the trigger  105  impinges on the first shoulder (not shown) forcing the needle  601  to move away from and thereby to progressively unblock the fluid discharge orifice (not shown) compressing the spring  607 .  
         [0043]    [0043]FIG. 6 b  shows that the fluid valve bushing  603  is provided with a fluid valve bushing channel  608  on the exterior surface of the fluid valve bushing  609  permitting communication between the interior space  102  and a fluid valve annular space  610  formed in identical fashion as described hereinabove for the spreader valve  103 .  
         [0044]    The pressure gauge  109  is mounted onto the spray gun by passing the fluid valve bushing  603  through the mounting fixture bore  213  and threading the fluid valve bushing  603  into a threaded bore  611  located in the first end of the gun body  116  below the spreader valve  103 . The pressure gauge  109  is thereby clamped between a first surface defined by the exterior surface of the gun body adjacent the threaded bore  611  and a second surface defined by a fluid valve bushing shoulder  612 . The fluid valve bushing  603  and needle  601  are adapted to be longer in the present invention than typically used when not mounting the pressure gauge  109  of the invention. In another embodiment, the fluid valve bushing  603  and needle  601  remain unchanged in length in the present invention. In the latter case, only the fluid valve bushing  603  need be adapted and modified to mount the pressure gauge  109  to the spray gun by, for example, the provision of a removable ring located between the knurled nut  406  and the second end of the spreader valve bushing  413 . The pressure gauge  109  is mounted on the spray gun in place of the removable ring.  
         [0045]    The pressure gauge  109  can also be mounted, in a similar manner as described hereinabove, on spray guns of alternative design and layout. Such spray guns include that illustrated in DE 100 31 858 A where the longitudinal axis of the spreader valve is substantially orthogonal to the plane of the spray gun. The trigger operated fluid valve of the spray gun is in a similar location to that shown in FIG. 1. The compressed air inlet pressure reducing valve is, however, sited on the gun body between the trigger operated fluid valve and a handle pendant from one end of the gun body. Another design of gun on which the pressure gauge of the invention can be mounted is that shown in FIG. 7 where the trigger operated shut off valve and trigger operated fluid valve are concentric. A further spray gun layout suitable for accommodating the pressure gauge of the invention is a spray gun combining concentric trigger operated compressed air and trigger operated fluid valves with a spreader valve whose longitudinal axis is substantially orthogonal to the plane of the spray gun.  
         [0046]    It will be appreciated that various modifications and changes may be made to the above described preferred embodiments of the invention without departing from the invention as set out in the following claims.