Patent Publication Number: US-10779429-B2

Title: Method of forming a flameproof housing

Description:
CROSS-REFERENCE TO THE RELATED APPLICATIONS 
     This is a Divisional Application of application Ser. No. 14/419,180, with a filing date of Feb. 2, 2015, now U.S. Pat. No. 10,225,942, entitled ‘FLAMEPROOF HOUSING WITH DISPLAY,” which is a National Stage entry of International Application No. PCT/TJS2012/052632, with an international filing date of Aug. 28, 2012 entitled “FLAMEPROOF HOUSING WITH DISPLAY.” 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a flameproof housing, and more particularly, to a flameproof housing with a display. 
     Statement of the Problem 
     Vibrating conduit sensors, such as Coriolis mass flowmeters and vibrating densitometers, typically operate by detecting motion of a vibrating conduit that contains a flowing material. Properties associated with the material in the conduit, such as mass flow, density and the like, can be determined by processing measurement signals received from motion transducers associated with the conduit. The vibration modes of the vibrating material-filled system generally are affected by the combined mass, stiffness and damping characteristics of the containing conduit and the material contained therein. 
     A typical Coriolis mass flowmeter includes one or more conduits that are connected inline in a pipeline or other transport system and convey material, e.g., fluids, slurries, emulsions, and the like, in the system. Each conduit may be viewed as having a set of natural vibration modes, including for example, simple bending, torsional, radial, and coupled modes. In a typical Coriolis mass flow measurement application, a conduit is excited in one or more vibration modes as a material flows through the conduit, and motion of the conduit is measured at points spaced along the conduit. Excitation is typically provided by an actuator, e.g., an electromechanical device, such as a voice coil-type driver, that perturbs the conduit in a periodic fashion. Mass flow rate may be determined by measuring time delay or phase differences between motions at the transducer locations. Two such transducers (or pickoff sensors) are typically employed in order to measure a vibrational response of the flow conduit or conduits, and are typically located at positions upstream and downstream of the actuator. The two pickoff sensors are connected to electronic instrumentation. The instrumentation receives signals from the two pickoff sensors and processes the signals in order to derive a mass flow rate measurement, among other things. Vibratory flowmeters, including Coriolis mass flowmeters and densitometers, therefore employ one or more flow tubes that are vibrated in order to measure a fluid. 
     In some environments, electrical signals may need to be conducted through a flameproof physical barrier or housing. For example, a housing may surround and enclose electrical circuits of a meter electronics or transmitter. Process control transmitters designed for use in hazardous atmospheres often utilize a combination of protection methods, including flameproof housings and/or barriers, to avoid uncontrolled explosions of flammable gases. International standards define the compliance requirements for flameproof devices and structures. 
     In the case of Coriolis flowmeter transmitters, it is well known to enclose the active electronics components within a flameproof compartment or housing so that an explosion of gases that might occur as a result of electrical energy within the electronics will not propagate beyond the enclosure. Consequently, the housing is desired to be sealed off, including display components that need to remain externally visible. 
       FIG. 1  shows a prior art flameproof display panel interface using a prior art curable sealing material. A potting material or adhesive is applied to one or both of the housing and/or the glass panel before assembly and comprises a curable sealing material. When the glass panel is moved into position in the housing, as shown, the potting material or adhesive is at least partially compressed between the glass panel and a portion of the housing. The potting material or adhesive therefore is desirably spread over essentially an entire interface region between the glass panel and the corresponding portion of the housing. The curable sealing material may then cure over time, or may be subject to heating or other processes to cure the material. The potting material or adhesive not only may seal the glass panel to the housing, but may also bond the two components together, thereby providing a cementing function. 
     The prior art has drawbacks. The potting material or adhesive may be subject to environmental and/or workplace regulations and may therefore be expensive and costly to handle, to apply, and to dispose of. The potting material or adhesive may not be allowed by regulation in some locales. The potting material or adhesive can be improperly or incompletely applied. After installation, the potting material or adhesive may include air bubbles, cracks, furrows, or irregular boundaries or may be too narrow to form a flame path of a desired length. The potting material or adhesive may shrink and/or crack with age, wherein the glass panel may exhibit leakage over time. The potting material or adhesive may lose adhesion to one or both of the glass panel and the housing. 
     What is needed, therefore, is a glass display panel that does not require cementing in order to achieve a flameproof seal with a corresponding housing. 
     ASPECTS OF THE INVENTION 
     In one aspect of the invention, a flameproof housing comprises:
         a display aperture formed in the flameproof housing;   a shoulder adjacent to the display aperture;   a transparent panel including an outer face and a perimeter; and   a fastener element configured to engage an interior surface of the flameproof housing and hold the transparent panel against the shoulder;   wherein a perimeter interface region between the perimeter of the transparent panel and the interior surface of the flameproof housing creates a perimeter gap that does not exceed a predetermined flameproof gap limit and wherein a face interface region between the outer face of the transparent panel and the shoulder creates a face gap that does not exceed the predetermined flameproof gap limit.       

     Preferably, the flameproof housing further comprises a seal groove formed in the shoulder and a seal positioned in the seal groove, wherein the seal prevents moisture from entering the flameproof housing at the display aperture. 
     Preferably, the shoulder includes a predetermined shoulder width that defines the face interface region. 
     Preferably, the transparent panel includes a predetermined panel thickness that defines the perimeter interface region. 
     Preferably, a flamepath length comprises a predetermined panel thickness plus a predetermined shoulder width. 
     Preferably, the perimeter interface region provides a first flamepath span L 1  and the face interface region provides a second flamepath span L 2 , wherein the first flamepath span L 1  plus the second flamepath span L 2  provides a flamepath length that equals or exceeds a predetermined minimum flamepath length. 
     Preferably, the perimeter interface region provides a first flamepath span L 1  and the face interface region provides a second flamepath span L 2 , wherein the first flamepath span L 1  plus the second flamepath span L 2  provides a flamepath length that equals or exceeds a predetermined minimum flamepath length, wherein the second flamepath span L 2 , comprises a shoulder width of the shoulder minus a seal groove width of the seal groove. 
     Preferably, the perimeter interface region provides a first flamepath span L 1  and the face interface region provides a second flamepath span wherein the first flamepath span L 1  plus the second flamepath span L 2  provides a flamepath length that equals or exceeds a predetermined minimum flamepath length, wherein the second flamepath span L 2  comprises an outer shoulder portion that is located outward from the seal groove. 
     In one aspect of the invention, a method of forming a flameproof housing comprises:
         providing a display aperture in the flameproof housing;   providing a shoulder adjacent to the display aperture;   providing a transparent panel including an outer face and a perimeter; and   providing a fastener element configured to engage an interior surface of the flameproof housing and hold the transparent panel against the shoulder;   wherein a perimeter interface region between the perimeter of the transparent panel and the interior surface of the flameproof housing creates a perimeter gap that does not exceed a predetermined flameproof gap limit and wherein a face interface region between the outer face of the transparent panel and the shoulder creates a face gap that does not exceed the predetermined flameproof gap limit.       

     Preferably, the method further comprises providing a seal groove formed in the shoulder and providing a seal positioned in the seal groove, wherein the seal prevents moisture from entering the flameproof housing at the display aperture. 
     Preferably, the shoulder includes a predetermined shoulder width that defines the face interface region. 
     Preferably, the transparent panel includes a predetermined panel thickness that defines the perimeter interface region. 
     Preferably, a flamepath length comprises a predetermined panel thickness plus a predetermined shoulder width. 
     Preferably, the perimeter interface region provides a first flamepath span L 1  and the face interface region provides a second flamepath span L 2 , wherein the first flamepath span L 1  plus the second flamepath span L 2  provides a flamepath length that equals or exceeds a predetermined minimum flamepath length. 
     Preferably, the perimeter interface region provides a first flamepath span L 1  and the face interface region provides a second flamepath span L 2 , wherein the first flamepath span L 1  plus the second flamepath span L 2  provides a flamepath length that equals or exceeds a predetermined minimum flamepath length, wherein the second flamepath span L 2  comprises a shoulder width of the shoulder minus a seal groove width of the seal groove. 
     Preferably, the perimeter interface region provides a first flamepath span L 1  and the face interface region provides a second flamepath span L 2 , wherein the first flamepath span L 1  plus the second flamepath span L 2  provides a flamepath length that equals or exceeds a predetermined minimum flamepath length, wherein the second flamepath span L 2  comprises an outer shoulder portion that is located outward from the seal groove. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The same reference number represents the same element on all drawings. The drawings are not necessarily to scale. 
         FIG. 1  shows a prior art flameproof display panel interface using a prior art curable sealing material. 
         FIG. 2  shows a vibratory flowmeter according to the invention. 
         FIG. 3  shows a transmitter including a flameproof housing according to an embodiment of the invention. 
         FIG. 4  is a cross-sectional view AA of the flameproof housing of the transmitter according to an embodiment of the invention. 
         FIG. 5  shows a transparent panel for closing off a display aperture of the flameproof housing according to an embodiment of the invention. 
         FIG. 6  is a cross-sectional view AA of the flameproof housing of the transmitter according to an embodiment of the invention. 
         FIG. 7  is a cross-sectional view AA of the flameproof housing according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 2-7  and the following description depict specific examples to teach those skilled in the art how to make and use the best mode of the invention. For the purpose of teaching inventive principles, some conventional aspects have been simplified or omitted. Those skilled in the art will appreciate variations from these examples that fall within the scope of the invention. Those skilled in the art will appreciate that the features described below can be combined in various ways to form multiple variations of the invention. As a result, the invention is not limited to the specific examples described below, but only by the claims and their equivalents. 
       FIG. 2  shows a vibratory flowmeter  5  according to the invention. The vibratory flowmeter  5  comprises a flowmeter assembly  10  and meter electronics  20 . The meter electronics  20  is connected to the meter assembly  10  via leads  100  and is configured to provide measurements of one or more of a density, mass flow rate, volume flow rate, totalized mass flow, temperature, or other measurements or information over a communication path  26 . It should be apparent to those skilled in the art that the vibratory flowmeter  5  can comprise any manner of vibratory flowmeter, regardless of the number of drivers, pick-off sensors, flow conduits, or the operating mode of vibration. In some embodiments, the vibratory flowmeter  5  can comprise a Coriolis mass flowmeter. In addition, it should be recognized that the vibratory flowmeter  5  can alternatively comprise a vibratory densitometer. 
     The flowmeter assembly  10  includes a pair of flanges  101   a  and  101   b , manifolds  102   a  and  102   b , a driver  104 , pick-off sensors  105   a  and  105   b , and flow conduits  103 A and  103 B. The driver  104  and the pick-off sensors  105   a  and  105   b  are connected to the flow conduits  103 A and  103 B. 
     The flanges  101   a  and  101   b  are affixed to the manifolds  102   a  and  102   b . The manifolds  102   a  and  102   b  can be affixed to opposite ends of a spacer  106  in some embodiments. The spacer  106  maintains the spacing between the manifolds  102   a  and  102   b  in order to prevent pipeline forces from being transmitted to flow conduits  103 A and  103 B. When the flowmeter assembly  10  is inserted into a pipeline (not shown) which carries the flow fluid being measured, the flow fluid enters the flowmeter assembly  10  through the flange  101   a , passes through the inlet manifold  102   a  where the total amount of flow fluid is directed to enter the flow conduits  103 A and  103 B, flows through the flow conduits  103 A and  103 B and back into the outlet manifold  102   b , where it exits the meter assembly  10  through the flange  101   b.    
     The flow fluid can comprise a liquid. The flow fluid can comprise a gas. The flow fluid can comprise a multi-phase fluid, such as a liquid including entrained gases and/or entrained solids. 
     The flow conduits  103 A and  103 B are selected and appropriately mounted to the inlet manifold  102   a  and to the outlet manifold  102   b  so as to have substantially the same mass distribution, moments of inertia, and elastic modules about the bending axes Wa-Wa and Wb-Wb respectively. The flow conduits  103 A and  103 B extend outwardly from the manifolds  102   a  and  102   b  in an essentially parallel fashion. 
     The flow conduits  103 A and  103 B are driven by the driver  104  in opposite directions about the respective bending axes Wa and Wb and at what is termed the first out of phase bending mode of the vibratory flowmeter  5 . The driver  104  may comprise one of many well known arrangements, such as a magnet mounted to the flow conduit  103 A and an opposing coil mounted to flow conduit  103 B. An alternating current is passed through the opposing coil to cause both conduits to oscillate. A suitable drive signal is applied by the meter electronics  20  to the driver  104  via the lead  110 . Other driver devices are contemplated and are within the scope of the description and claims. 
     The meter electronics  20  receives sensor signals on the leads  111   a  and  111   b , respectively. The meter electronics  20  produces a drive signal on the lead  110  which causes the driver  104  to oscillate the flow conduits  103 A and  103 B. Other sensor devices are contemplated and are within the scope of the description and claims. 
     The meter electronics  20  processes the left and right velocity signals from the pick-off sensors  105   a  and  105   b  in order to compute a flow rate, among other things. The communication path  26  provides an input and an output means that allows the meter electronics  20  to interface with an operator or with other electronic systems. The description of  FIG. 2  is provided merely as an example of the operation of a Coriolis flowmeter and is not intended to limit the teaching of the present invention. 
     The meter electronics  20  in one embodiment is configured to vibrate the flow conduits  103 A and  103 B. The vibration is performed by the driver  104 . The meter electronics  20  further receives resulting vibrational signals from the pickoff sensors  105   a  and  105   b , The vibrational signals comprise vibrational responses of the flow conduits  103 A and  103 B. The meter electronics  20  processes the vibrational responses and determines a response frequency and/or phase difference. The meter electronics  20  processes the vibrational response and determines one or more flow measurements, including a mass flow rate and/or density of the flow fluid. Other vibrational response characteristics and/or flow measurements are contemplated and are within the scope of the description and claims. 
     In one embodiment, the flow conduits  103 A and  103 B comprise substantially U-shaped flow conduits, as shown. Alternatively, in other embodiments, the flow conduits can comprise substantially straight flow conduits or can comprise one or more flow conduits of curved shapes other than U-shaped flow conduits. Additional flowmeter shapes and/or configurations can be used and are within the scope of the description and claims. 
       FIG. 3  shows a transmitter  200  including a flameproof housing  202  according to an embodiment of the invention. The flameproof transmitter  200  includes the flameproof housing  202  wherein the flameproof housing  202  can hold one or more transmitter components  240  (see dashed lines). The flameproof transmitter  200  may hold and include the meter electronics  20 , among other things. The one or more transmitter components  240  may comprise circuit boards, but may also comprise other devices or systems. In some embodiments, the flameproof transmitter  200  may include communication electronics for a vibratory flowmeter or flowmeters  5 . The flameproof transmitter  200  may include operation and control electronics for a vibratory flowmeter or flowmeters  5 . The flameproof transmitter  200  may include power electronics for a vibratory flowmeter or flowmeters  5 . 
     The flameproof transmitter  200  mainly comprises a flameproof housing  202  that is substantially hollow see  FIG. 4 ). The flameproof housing  202  may be formed of any desired material, including metals, but may be formed of other materials if desired. Although the flameproof housing  202  is shown as being substantially cylindrical, it should be understood that the flameproof housing  202  is not limited to any particular shape or size. The flameproof housing  202  is configured to be substantially sealed and is configured to prevent ignition or flame to pass either into or out of the flameproof housing  202 . 
     The flameproof housing  202  includes a display aperture  212 . The display aperture  212  may be substantially circular, as shown, or may comprise other shapes. The display aperture  212  may be of any desired size and may take up any desired amount of the exterior surface area of the flameproof housing  202 . A display panel  220  may be at least partially visible through the display aperture  212 . The display panel  220  may include one or more display elements  250 , including electronic displays, light-generating and/or light manipulating displays, mechanical displays, or electromechanical displays. 
     However, the display panel  220  is not open to the exterior of the flameproof housing  202 . The flameproof housing  202  includes a transparent panel  230  that substantially seals the display aperture  212  (see  FIG. 4 ). The transparent panel  230  may comprise any suitable transparent material, such as glass or tempered glass in some embodiments. 
     The flameproof housing  202  may be designed to conform to applicable flameproof standards, wherein a flame is not permitted to pass out of or into the flameproof housing  202 . The flameproof transmitter  200  may further include any manner of isolation electronics and/or physical barriers for preventing a flame or ignition to enter or leave the flameproof transmitter  200 , such as where the flameproof transmitter  200  is located in a hazardous or explosive environment. 
     The interface between the transparent panel  230  and the flameproof housing  202  may comprise a spigot joint. A spigot joint is generally characterized by tightly toleranced mating parts that prevent flame propagation through rapid energy dissipation of a flame. 
     Spigot joints have not been used in the prior art for glass panels, due to the need for extremely tight tolerances. A spigot joint is employed herein through the achievement of glass processing techniques that can achieve tolerances on the order of thousandths of an inch in some embodiments. 
     The flameproof housing  202  may include a stand-off  215  that extends from the flameproof housing  202 . More than one stand-off  215  may be included in the flameproof housing  202  in some embodiments. The stand-off  215  includes a stand-off passage  216 , wherein wires, cables, optical fibers, or other communication links may enter and exit the flameproof housing  202  through the stand-off passage  216 . The stand-off  215  may further include a coupling section  218 , such as threading, wherein the coupling section  218  may removably attach the flameproof housing  202  (and therefore the transmitter  200 ) to another device or structure, such as a vibratory flowmeter  5  or associated structure. However, other coupling features are contemplated and are within the scope of the description and claims. 
       FIG. 4  is a cross-sectional view AA of the flameproof housing  202  of the transmitter  200  according to an embodiment of the invention. It can be seen from this figure that the housing  202  comprises a substantially hollow chamber defined by an interior surface  203 . It can also be seen that the display aperture  212  passes through the wall of the housing  202 . It should be understood that the housing  202  may include more than one display aperture  212 . It should be understood that the display aperture  212  may be located on other positions on the flameproof housing  202 . 
     In the embodiment shown, the housing  202  may be substantially cylindrical and the display aperture  212  may be substantially circular. However, it should be understood that the housing  202  and the display aperture  212  may be of any desired shape and size. 
     The housing  202  in the embodiment shown includes a shoulder  207 . If the housing  202  is substantially cylindrical, then the shoulder  207  may be substantially annular in some embodiments. The shoulder  207  may be machined or otherwise formed to be substantially smooth and flat, and may be formed to meet a predetermined surface tolerance. The predetermined surface tolerance may include a surface planarity tolerance in some embodiments. The predetermined surface tolerance may include a surface roughness tolerance in some embodiments. 
     A seal groove  209  may be formed in the shoulder  207 . A seal  210  may be received in the seal groove  209 . The seal  210  is provided to keep moisture and other contaminants from entering the housing  202  at the display aperture  212 . In some embodiments, such as where the seal groove  209  is substantially annular, the seal  210  may comprise an O-ring  210 . The seal  210  may be substantially resilient in some embodiments. 
     The seal or seals may comprise a solid seal or seals, such as O-rings, gaskets, or other components that may be clamped between the components. Alternatively, the seal or seals may comprise a liquid, paste, grease, or other material that does not have a predetermined shape and that can be applied to one or more of the components of the flameproof feed-through  200 . The seal or seals may comprise a material that does not substantially change. Alternatively, the seal or seals may comprise a material that hardens, cures, or otherwise transforms or is transformed during or after the assembly process. 
     The transparent panel  230  may be assembled to the shoulder  207 , with an outer face  231  of the transparent panel  230  (see  FIG. 5 ) contacting the shoulder  207 . The outer face  231  of the transparent panel  230  may also contact the resilient seal  210 . 
     In some assembly method embodiments, the transparent panel  230  is assembled to the flameproof housing  202  in order to form a spigot-type joint. One or more of the outer face  231  and the perimeter  232  of the transparent panel  230  are polished, planed, milled, ground, etched, turned, or otherwise processed to where one or both of the perimeter interface region  264  and the face interface region  260  do not exceed a predetermined flameproof gap limit. Similarly, one or more of the shoulder  207  and the interior surface  203  of the flameproof housing  202  are polished, planed, milled, ground, etched, turned, or otherwise processed to where the flameproof housing  202  achieves a predetermined gap with the transparent panel  230 . In other words, the transparent panel  230  and the flameproof housing  202  are polished or otherwise processed to achieve a gap height that is less than the predetermined flameproof gap limit. 
     The outer face  231  of the transparent panel  230  may be formed to meet a predetermined surface tolerance. Although only one face of the transparent panel  230  is labeled as the outer face  231 , it should be understood that both sides of the transparent panel  230  may be polished or processed to the predetermined surface tolerance and may face outward from inside the housing  202 . The predetermined surface tolerance may include a predetermined surface planarity tolerance in some embodiments. The predetermined surface tolerance may include a predetermined surface roughness tolerance in some embodiments. 
     A perimeter  232  of the transparent panel  230  may contact the interior surface  203  of the housing  202 . The perimeter  232  of the transparent panel  230  may be formed to meet a predetermined size tolerance. The predetermined size tolerance may include a predetermined dimensional tolerance in some embodiments. For example, where the perimeter  232  is substantially circular, the perimeter  232  may meet a predetermined diameter tolerance, wherein a gap height of a gap between the interior surface  203  of the housing  202  and the perimeter  232  is less than a predetermined flameproof gap limit. The predetermined size tolerance may include a predetermined surface roughness tolerance in some embodiments. 
     When assembled, the transparent panel  230  is placed inside the housing  202  and the outer face  231  of the transparent panel  230  is brought substantially into contact with the shoulder  207 . The outer face  231  comprises a surface that has been formed to meet the predetermined surface tolerance. 
     The shoulder  207  in some embodiments defines a face interface region  260 . The face interface region  260  comprises a region where the outer face  231  of the transparent panel  230  is brought substantially into contact with the surface of the shoulder  207 . The face interface region  260  comprises a substantially planar interface. The size, shape, and area of the face interface region  260  are defined by the size and geometry of the shoulder  207 . Because the shoulder  207  and the outer face  231  are formed to exacting tolerances, the face interface region  260  will provide a close fit, with a gap height of a gap between the shoulder  207  and the outer face  231  being less than a predetermined flameproof gap limit. This may be achieved where a gap height tolerance for the outer face  231  and for the shoulder  207  are each about one-half of the predetermined flameproof gap limit. 
     The face interface region  260  forms a second flamepath span L 2 . Some or all of the face interface region  260  may comprise the second flamepath span L 2 . The second flamepath span L 2  in some embodiments comprises a shoulder width of the shoulder  207 . The second flamepath span L 2  in some embodiments comprises a shoulder width of the shoulder  207  minus a seal groove width of the seal groove  209 . Alternatively, in other embodiments the second flamepath span L 2  comprises a shoulder portion  208  that is located outward from the seal groove  209 . 
     The assembly of the transparent panel  230  to the shoulder  207  will also create a perimeter interface region  264 . The perimeter interface region  264  comprises a region where the perimeter  232  of the transparent panel  230  is adjacent to the interior surface  203  of the housing  202 . The perimeter interface region  264  may comprise a first flamepath length L 1 . The size, shape, and area of the perimeter interface region  264  are defined by the size and geometry of the perimeter  232  of the transparent panel  230 . Because the perimeter  232  of the transparent panel  230  is formed to exacting tolerances, the perimeter interface region  264  will provide a close fit to the interior surface  203  of the housing  202 , with a gap height of a gap between the interior surface  203  and the perimeter  232  being less than a predetermined flameproof gap limit. This may be achieved where a gap height tolerance for the transparent panel  230  and a gap height tolerance for the shoulder  207  and the interior surface  203  are each about one-half of the predetermined flameproof gap limit. 
     It is desired that gaps do not exist between the transparent panel  230  and the flameproof housing  202 , or at least are less than the predetermined flameproof gap limit. Gaps may allow gasses to leak through and therefore may allow possible ignition of the gas or gasses. Gaps may allow ignition products to propagate around the transparent panel  230  and escape from the flameproof housing  202 . Consequently, the transparent panel  230 , the interior surface  203 , and the shoulder  207  are substantially smooth and regular, i.e., to within a predetermined surface finish. 
     The first flamepath span L 1  and the second flamepath span L 2 , when combined, provide a resulting or total flamepath length (L 1 +L 2 ). The flamepath length is configured to exceed a predetermined minimum flame path length. The flamepath length may be designed to exceed a predetermined minimum flame path length given by an applicable flameproof standard. By exceeding the predetermined minimum flamepath length, the flamepath length (L 1 +L 2 ) ensures that a flame cannot successfully pass around the transparent panel  230 . The flamepath length may be selected so that a flame may not propagate from one side of the transparent panel  230  to the other side with sufficient heat or energy content to cause ignition, given a gap between the transparent panel  230  and the interior surface  203  of the flameproof housing  202 . Consequently, a flame within the housing  202  cannot escape to the exterior, and a flame outside the housing  202  cannot travel to the interior of the housing  202 . 
     A flame path between the transparent panel  230  and the interior surface  203  of the flameproof housing  202  can be defined as having both a gap height and a flame path length. Compliance with a flame proof standard may require maintaining a small gap height, a long flame path length, or both. 
     The flamepath length is by definition the length of a gap or interface that a flame will be required to travel in order to pass through the interface. Flameproof standards typically define a minimum flamepath length required in order to achieve a flameproof characteristic. The minimum flamepath length is defined so that a flame that passes through the interface will dissipate before successfully passing all the way through the interface. 
     In some embodiments of the flameproof housing  202 , the flamepath length comprises a transparent panel thickness plus a shoulder width. In some embodiments of the flameproof housing  202 , the flamepath length comprises the first flamepath span L 1 , plus the second flamepath span L 2 . In some embodiments of the flameproof housing  202 , the flamepath length comprises the first flamepath span L 1  plus the second flamepath span L 2 , wherein the second flamepath span L 2  comprises a shoulder width minus a seal groove width. In some embodiments of the flameproof housing  202 , the flamepath length comprises the first flamepath span L 1  plus the second flamepath span L 2 , wherein the second flamepath span L 2  comprises a shoulder width of a shoulder portion  208  that is located outward from the seal groove  209 . 
     In some embodiments, the flameproof housing  202  may be designed to conform to the section 5.2.4.3 of TEC 60079-1:2007, which permits a spigot joint wherein a cross-sectional gap height can be a maximum of about 0.0059 inch (five point nine thousandths of an inch) or a maximum of about 0.15 millimeter (“Electrical apparatus for explosive gas atmospheres—Part 1: Flame proof enclosures ‘d’,” issued by Commission Electrotechnique Internationale as IEC 60079-1:2007). For example, the outer face  231  may be processed so that the surface does not have variations greater than about 0.00295 inch in height. Where both the outer face  231  and the shoulder  207  do not have variations greater than about 0.00295 inch, then when mated, the gap between the two components cannot be greater than about 0.0059 inch in cross-sectional height. 
     After the transparent panel  230  is in position, the transparent panel  230  may be held in position by a fastener element  236 , for example. The fastener element  236  may clamp or otherwise hold the transparent panel  230  against the shoulder  207 . Consequently, the fastener element  236  may ensure that the gap between the transparent panel  230  and the shoulder  207  (i.e., the face interface region  260 ) does not exceed the predetermined flameproof gap limit. The fastener element  236  may ensure that the transparent panel  230  cannot move away from the shoulder  207 . 
     The fastener element  236  may include a perimeter fastener feature  237  that corresponds to an internal housing fastener feature  204  on the interior surface  203  of the housing  202 . In some embodiments, the fastener features  204  and  237  comprise threading, wherein the fastener element  236  may be rotated to bring the fastener element  236  into retaining contact with the transparent panel  230 . 
     In another embodiment, the fastener element  236  may comprise a wave washer in combination with a snap ring. The wave washer is positioned between the snap ring and the transparent panel  230 . The snap ring is configured to be fixed in position in a predetermined location on the interior surface  203 . For example, the snap ring may engage a groove, ridge, or other projection or depression, or multiple such features, on the interior surface  203 . The wave washer comprises an undulating washer that has regions displaced away from a central plane of the washer and is formed of a resilient, springy material. As a result, the wave washer will generate an expansion force when compressed. When the wave washer is at least partially compressed between the snap ring and the transparent panel  230 , the wave washer will place a force on the transparent panel  230  (or other intervening component). The force presses the transparent panel  230  against the shoulder  207 . However, it should be understood that the fastener features  204  and  237  may comprise any suitable fastener features. 
     The display panel  220  may be located after the fastener element  236 , as shown, wherein the display panel  220  may be visible through the fastener element  236 , the display aperture  212 , and the transparent panel  230 . It should be understood that the fastener element  236  is shown with an exaggerated depth, but may be smaller relative to the transparent panel  230  and/or the display panel  220 . 
     In an alternative embodiment, the display panel  220  can fit into the fastener element  236 . The display panel  220  may extend at least partially into the region inside the fastener element  236 . Alternatively, the display panel  220  may extend at least partially into the region inside the fastener element  236  and may be affixed to the fastener element  236 . Fasteners (not shown) may affix the display panel  220  to the fastener element  236 . In another alternative, the display panel  220  and the fastener element  236  may be combined into a single component, wherein the fastener element  236  may comprise a portion of the display panel  220  and the display panel  220  therefore engages the internal housing fastener feature  204 . In yet another alternative, the display panel  220  can be brought into contact with the transparent panel  230  and the fastener element  236  may be assembled to contact the display panel  220 , wherein the display panel presses the transparent panel  230  against the shoulder  207 . 
       FIG. 5  shows the transparent panel  230  for closing off the display aperture  212  of the flameproof housing  202  according to an embodiment of the invention. The transparent panel  230  may be substantially planar. The transparent panel  230  may be of a desired shape and thickness. 
     In some embodiments, the transparent panel  230  comprises glass. In some embodiments, the transparent panel  230  comprises tempered glass. Alternatively, the transparent panel  230  in other embodiments comprises plexiglas or other clear or transparent plastic materials. It should be understood that other transparent materials are contemplated for the transparent panel  230  and are within the scope of the description and claims. 
       FIG. 6  is a cross-sectional view AA of the flameproof housing  200  according to an embodiment of the invention. In this embodiment, the shoulder  207  does not include the seal groove  209  or the seal  210 . As a consequence, the entire shoulder width can comprise the second flamepath span L 2 , as depicted in the figure. 
     In addition, the transparent panel  230  may include a chamfered or beveled edge  274 . The beveled edge  274  may comprise a bevel of any size or angle. The beveled edge  274  may enable easier assembly of the transparent panel  230  to the shoulder  207 . It should be understood that the beveled edge  274  is an optional element and may be included in any embodiment of the flameproof housing  202 . 
       FIG. 7  is a cross-sectional view AA of the flameproof housing  200  according to an embodiment of the invention. In this embodiment, the shoulder  207  is minimal in width and does not contribute significantly to the flamepath length. The shoulder  207  serves merely to provide a holding surface for positioning the transparent panel  230 . As a consequence, the perimeter  232  of the transparent panel  230  comprises the first flamepath span L 1  and comprises essentially the total flamepath length, as depicted in the figure. 
     The flameproof housing according to any of the embodiments may provide advantages. The flameproof housing provides a transparent panel interface without the need for handling or using a curable seal material. The flameproof housing provides a transparent panel interface that does not require a step of applying a curable seal material. The flameproof housing provides a transparent panel interface that is flameproof through achievement of very close tolerances. The flameproof housing provides a transparent panel interface that does not rely on the bonding of a curable seal material to both a transparent panel and a housing. 
     The detailed descriptions of the above embodiments are not exhaustive descriptions of all embodiments contemplated by the inventors to be within the scope of the invention. Indeed, persons skilled in the art will recognize that certain elements of the above-described embodiments may variously be combined or eliminated to create further embodiments, and such further embodiments fall within the scope and teachings of the invention. It will also be apparent to those of ordinary skill in the art that the above-described embodiments may be combined in whole or in part to create additional embodiments within the scope and teachings of the invention. Accordingly, the scope of the invention should be determined from the following claims.