Obviating the need to vent a gas burning appliance provides a number of advantages for various applications. First and foremost, it obviates the need to route combustion gases to the outside of a building or room thereby greatly simplifying the installation of certain appliances. Additionally, elimination of a vent may enhance the efficiency of certain gas- fired appliances as it precludes the escape of heat through such vent.

Combustion of certain gases such as natural gas (methane) or liquified petroleum gas can yield very clean, substantially toxin-free exhaust gases, essentially limited to C02 and H20, if the supplied combustion air has an adequate oxygen content. However, as the oxygen content drops below a critical level, an increasing amount of undesirable pollutants are created during the combustion of the fuel. Additionally, as the oxygen is depleted in an enclosed space, less and less will be available for the occupants thereof to breathe as well.

It is therefore most desirable for combustion in a non-vented gas-fired appliance to immediately and automatically be discontinued should the oxygen content of the combustion air fall below a critical level.

5 A number of different approaches have been devised in an effort to address this problem. In its most fundamental form, a standard pilot light and thermocouple combination may serve as a crude oxygen depletion sensor in view of the fact that a substantial drop in the oxygen level will eventually cause a pilot flame to extinguish. This in turn will cause the voltage generated by the thermocouple to drop below the threshold level required for 10 maintaining the associated gas valve in its open position. However, substantially more sensitivity is desirable and indeed is required by currently applicable industry standards.

One approach that has been employed in an effort to enhance the sensitivity of the pilot light/thermocouple combination to low oxygen levels has exploited the fact that the pilot flame temperature decreases as the oxygen level drops off. By enhancing the sensitivity 15 of the gas valve to respond to the slightly reduced voltage generated by a thermocouple being subjected to a slightly cooler flame, the gas supply can be shut off long before a drop in oxygen level would actually cause the pilot flame to become extinguished. However, such devices nonetheless suffer from certain disadvantages. As an example, the heightened sensitivity of the valve to slightly reduced thermocouple voltages may give rise to unnecessary shut downs when a slight reduction in voltage is caused by something other than 5 an unacceptably low oxygen level. A shifting of the thermocouple relative to the pilot flame could cause the thermocouple to be subjected to a cooler region of the flame. Conversely, a shifting of the flame relative to the thermocouple, such as when it is distorted by a draft, could similarly cause the thermocouple to be subjected to a cooler region of the flame. In either event, a slightly reduced voltage is the result that may not be distinguishable from the 10 reduced voltage that results from an overall cooling of the flame caused by a decrease in the oxygen level. Additionally, such systems are vulnerable to the radiated heat generated by the main burner of a gas-fired device as the temperature actually sensed by the thermocouple may therefore be higher than that actually generated by the pilot flame and as a result, a cooling of the pilot flame caused by a drop in the oxygen level could go undetected.

15 The device described in USPN 5,674,065 to Grando et al. appears to be a representative example of heretofore known thermocouple and oxygen depletion sensor configurations and as such would suffer from the disadvantages described above. The

thermocouple is described as being conventionally installed as part of an oxygen depletion safety device wherein the thermocouple is positioned so as to be heated by the gas flame of the pilot. When the flame becomes unstable or diminished in size due to a reduction in the oxygen level, its temperature drops off causing a commensurate reduction in the voltage generated by the thermocouple. Any voltage reduction below the 14-22 open circuit millivolt range will cause the gas valve to close. As a consequence, a reduction of the output voltage to slightly below the nominal range for any reason would cause the shut down of the gas supply even if the oxygen level were not significantly depleted. Conversely, the sensitivity of such system could conceivably allow the radiant heat of the burner to maintain the output voltage within the nominal range even if the oxygen level were depleted to an unacceptable level. Either situation is undesirable.

Other disadvantages inherent in prior art devices are that they generally require a substantial reconfiguration in order to accommodate the use of different gaseous fuels. Different nozzles and different relative positions of the nozzle and thermocouple require a multiplication of the number of parts that must be manufactured and on hand for the various applications. As an example, in the apparatus described in Grando et al, the thermocouple must be repositioned from out in front of the pilot to an attachment point directly on the side of the pilot in order to accommodate a change from natural gas to liquified petroleum gas.

A simple, reliable oxygen depletion sensor is needed that overcomes the shortcomings of the heretofore known sensors. Such sensor should be easy to manufacture and should be 5 readily installable into existing appliance configurations.

SUMMARY OF THE INVENTION The present invention provides an oxygen depletion sensor that offers substantial advantages over previously known devices. The sensor is easily manufactured, very reliably responds to a drop in the oxygen content to below a preselected level, is substantially 10 unresponsive to slight shifts in pilot flame or thermocouple positions and additionally provides a visual indication of the oxygen level. In general terms, the present invention takes advantage of the effect that the oxygen content of combustion air has on flame velocity and causes the difference between the voltage generated by thermocouple during high and during low oxygen level conditions to be greatly exaggerated. In contrast to the previously known 15 devices wherein the thermocouple output gradually drops off as the oxygen level is depleted and the difference between the voltage that is generated during combustion at acceptable oxygen levels and at unacceptable oxygen level conditions is very slight, the device of the present invention needs only to recognize a substantial drop in the generated voltage.

The flame velocity of a particular gaseous fuel is indicative of how fast such gas burns 5 and depends to a large extent on the amount of oxygen that is available for combustion.

Flame velocity is therefore a direct function of the oxygen/fuel ratio in a mixture subject to combustion. By holding the air/fuel ratio constant, the flame velocity becomes a direct function of the oxygen content in the combustion air.

During combustion, the position of the flame, or more accurately, the periphery of the 10 base of such flame will stabilize where the flow velocity of the combustible mixture substantially equals the flame velocity. As the oxygen level decreases, the flame velocity drops off causing the base of the flame to move away from the nozzle emitting the combustible mixture. The unburned zone will tend to increase as the oxygen level continues to decrease until eventually, the flame is extinguished.

The velocity of a gas flowing from a nozzle is substantially a function of the volume of the gas flow and of the cross-sectional area of the nozzle through which such gas flow issues. The amount of combustion air that is drawn into a flow of gaseous fuel prior to its emission from a nozzle is substantially dependent upon the pressure drop generated by the 5 flow as it passes through a mixing chamber and upon the size of the primary air intake opening through which air is drawn into such mixing chamber in response to the pressure drop. The volume and velocity of the gaseous fuel flowing into and through such mixing chamber will determine the pressure drop relative to ambient conditions. The pressure at which the gaseous fuel is supplied and the size of the inlet orifice through which the fuel 10 flows into a mixing chamber will in turn determine the volume and velocity of the gaseous fuel as it passes through the mixing chamber. The combination of all these parameters will determine where the flame resulting from the combustion of the air/fuel mixture issuing from the nozzle will be positioned relative to such nozzle. As the oxygen content of the combustion air is decreased, the flame will move away from the nozzle.

15 The oxygen depletion sensor of the present invention exploits these fluid dynamic principles of gas flow and flame velocity. By advantageously positioning a thermocouple adjacent to an appropriately configured nozzle emitting an air/fuel mixture, such that the

thermocouple is enveloped by the flame during combustion of such mixture at oxygen levels above a preselected value and is wholly outside the flame at oxygen levels below such preselected level, a very substantial change in output voltage is thereby realized as a function of only a relatively minor change in the combustion air's oxygen content. An associated valve that controls the flow of gas to the appliance is configured such that only a voltage in the range generated by the thermocouple while enveloped by the flame is sufficient to hold the valve in its open position. The substantially lower voltage generated by the thermocouple when located in the unburned zone is substantially below the threshold voltage required for maintaining the valve in its open position. Such system is substantially impervious to slight shifts in the relative positions of the thermocouple and pilot flame as the voltage generated in even the coolest region of the flame is still well above the threshold temperature at which the valve shuts off. Conversely, the heat radiated by the main burner is incapable of raising the voltage generated by the thermocouple while positioned outside the flame to a level sufficient to maintain the valve in its open position. Only the substantial drop in output voltage commensurate with the temperature differential between a location inside and a location outside the flame is recognized as being indicative of an unacceptable drop in oxygen content.

The present invention provides an additional benefit in that the position of the flame relative to the thermocouple gives a visual indication of the oxygen content of the combustion air. The oxygen content will be at or above an acceptable level when the thermocouple is observed as being fully enveloped by the flame. Any shift of the periphery 5 of the flame toward the thermocouple that is apparent or any obvious instability or lifting of the flame is an indication of a depleted oxygen level. A gap between the flame and the thermocouple during for example manual override of an automatic shutoff valve is an indication of an unacceptably low oxygen level.

Finally, the present invention provides the additional advantage of being readily 10 adaptable for use with different gaseous fuels. Rather than requiring the use of a different nozzle configuration and/or the repositioning of the thermocouple relative to the nozzle in order to accommodate the use of a different gas, the same nozzle and the same relative placement of the nozzle and thermocouple may be employed by simply resizing the inlet orifice and air hole sizes.

15 These and other features and advantages of the present invention will become apparent from the following detailed description of a preferred embodiment which, taken in conjunction accompanyingdrawings,illustratesthe by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is semi-schematic view of an oxygen depletion sensor of the present invention as used in a gas-fired appliance; Fig. 2 is an enlarged cross-sectional view of the flame forming component of the sensor shown in Fig. 1; Fig. 3 is side view of the oxygen depletion sensor of the present invention with a pilot flame burning in combustion air having a high level of oxygen content; Fig. 4 is a side view of the oxygen depletion sensor of the present invention with a pilot flame burning in combustion air having a low level of oxygen content; and Fig. 5 is a graphic plot of the voltage versus oxygen level for a device constructed in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The Figures generally illustrate the present invention and more particularly, are directed to a preferred embodiment thereof. The oxygen depletion sensor of the present invention simultaneously serves as a pilot light for a gas-fired appliance and functions to shut off the flow of gas to such appliance should the oxygen content of the combustion air fall below a preselected level. The device is readily adaptable for use with different gaseous fuels. Additionally, the device provides a visual indication of the oxygen content that is present in the combustion air.

Fig. 1 is a semi-schematic representation of a preferred embodiment of the oxygen sensor device 12 of the present invention as used in a gas-fired appliance. A bracket 14 maintains a thermocouple 16 in position adjacent the mouth of nozzle 18 of a flame forming component 20. A gas valve 22 controls the flow of gas from a supply line 21 to both the flame forming component 20 via line 24 and to the main burner 26 of the appliance via line 28. The valve is configured such that the range of voltages generated by the thermocouple while subject to temperatures encountered within a pilot flame is sufficient to hold the valve in its open position while voltages generated by the thermocouple at temperatures below 5 those encountered within a pilot flame are insufficient to hold the valve open and therefore allow the valve to close and shut off the flow of gas to the appliance. The general configuration of such a valve may conform to that which is well known in the art wherein a spring loaded valve element is held in its open position by an electromagnet that is energized by the thermocouple. The particular performance parameters of such valve 10 configuration are selected such that the threshold voltage for holding the valve open is below the voltage generated by the thermocouple while positioned in a pilot flame and above the voltage generated by such thermocouple while positioned in the unburned zone adjacent a pilot flame.

The bracket may additionally hold an ignitor 30 in position so as to enable the gaseous 15 mixture issuing from the nozzle to be ignited to establish a pilot flame. Additionally, the bracket may hold a thermopile 32 in position above the flame. The voltage generated by the thermopile may be used by the valve 22 to regulate the flow of gas to the main burner 26.

Fig. 2 is a cross sectional view of the flame controlling component 20 of the oxygen depletion sensor and illustrates its essential features. Gas line 24 supplies the component with gaseous fuel which enters a mixing chamber 34 through inlet orifice 36. An air hole 38 is formed in the wall of the mixing chamber and the gaseous fuel flowing from inlet orifice as well as the air drawn in through the air hole exits through nozzle 18. The line pressure of the gas and the cross-sectional area of inlet orifice 36 dictate the velocity with which the gas emanates from the orifice. The velocity of the gas flow and size of the cross-sectional area of the air hole in turn determines the rate at which combustion air is drawn into the mixing chamber. The respective flow rates of the gaseous fuel and the combustion air in turn determine the air/fuel ratio of the resultant mixture. Finally, the diameter of the nozzle determines the velocity with which the air/fuel mixture issues from the nozzle.

By properly selecting the various parameters, i. e. type of gaseous fuel, line pressure, inlet orifice size, air hole size and nozzle size, a stable flame forms adjacent the mouth of the nozzle upon combustion of the mixture issuing from the nozzle. As the oxygen content of the combustion air that is drawn in through the air hole decreases, the position of the flame shifts away from the end of the nozzle. Thermocouple 16 is positioned so as to be located within the flame in the position the flame assumes when the combustion air contains what is considered to be an adequate oxygen content and outside of the periphery of the flame, i. e in the unburned zone, when the flame is in the position it assumes when the combustion air contains what is considered to be an inadequate oxygen content.

The following dimensions and relationships are representative of a configuration that 5 functions in accordance with the present invention: gas type: methane inlet pressure: 3.5 to 10. 5" water column inlet orifice: 0.013" diameter air hole: 0.166" diameter 10 nozzle: 0.125" diameter TC from nozzle: 0.35" TC output at 600-700 F: 13mV-16mV min. to hold valve open: 2.5mV 15 An oxygen depletion sensor constructed in accordance with the above specifications has been found to automatically shut off the flow of gas when the oxygen content in the combustion air drops from the normal 20.8% to between 18.2-19.4%. Further depletion of the oxygen content is thereby curtailed to prevent the formation of noxious products of combustion and to leave an adequate oxygen content for breathing.

Fig. 3 illustrates the oxygen depletion sensor 12 in operation when the combustion air 5 contains an adequate content of oxygen. The pilot flame 40 forms and stabilizes a short distance adjacent to the mouth of nozzle 18. The base of the flame, or more accurately, the proximal end of the periphery of the flame approximately conforms to the loci of where the flame velocity is substantially equal to the flow velocity of the combustible mixture of gaseous fuel and combustion air issuing from the nozzle. The thermocouple 16 is positioned 10 within the flame and generates a voltage commensurate with the 600-700 F temperature it is thereby subjected to. Such voltage is well above the threshold voltage selected to hold the valve 22 in its open position thus allowing it to supply gas to the flame forming component 20 as well as to the main burner 26 as needed.

The configuration of the device has the added benefit of maintaining a fairly high flow 15 velocity of the combustible mixture which allows the main burner 26 to be distanced further away from the nozzle 18 and thermocouple 16 than is possible when using a low velocity configuration. This serves to reduce the amount of radiated heat sensed by the thermocouple. Additionally, the higher velocity stabilizes the flame which is thereby less susceptible to distortion by drafts and less susceptible to buoyancy effects making it feasible to mount the device in inclined and even vertical orientations as well as the illustrated horizontal 5 orientation.

Fig. 4 illustrates the oxygen depletion sensor 12 in operation in an environment wherein the combustion air has a depleted oxygen content. The flame 40a forms at a position further removed from the mouth of the nozzle 18 which causes the thermocouple 16 to be positioned outside the flame in the unburned zone. The temperature outside the periphery 10 of the flame is substantially less than inside the flame, on the order of several hundred degrees Fahrenheit, resulting in a commensurately reduced voltage output. The lower voltage output is well below the threshold voltage required to maintain the gas valve 22 in its open position which causes it to shut off all gas flow. Additionally, a visual inspection of the pilot flame offers a clear indication of the oxygen content by virtue of the position of 15 the flame relative to the thermocouple. Any gap between the visible periphery of the pilot flame and the thermocouple or any instability or lifting of the pilot flame is an indication of an unacceptably low oxygen content and that shut off is imminent.

Fig. 5 is a representative plot of thermocouple voltage versus oxygen content of the combustion air for a oxygen sensor device of the present invention. As is apparent, the voltage generated by the thermocouple gradually decreases as the oxygen content of the combustion air becomes depleted. In the zone identified by reference numeral 50, the thermocouple is enveloped by the flame and therefore subject to very high temperatures, as is evidenced by the relatively high voltage output. As the oxygen content is depleted, the flame velocity of the mixture decreases which causes the flame temperature to drop off slightly as is evidenced by the slight slope in the curve. The depletion of the oxygen content also causes the flame to gradually move away from the nozzle and thermocouple until its periphery is substantially aligned with the thermocouple. At such point, the temperature sensed by the thermocouple is subject to a precipitous decline as is evidenced by the abrupt drop in the voltage in zone 52. Further depletion of the oxygen content would cause the voltage to continue to decrease (zone 54) as the flame moves further away from the thermocouple and its temperature continues to decrease. The flame forming component of the present invention is configured such that the abrupt transition zone 52 substantially includes the preselected limit 56 below which the oxygen content is considered unacceptable.

Additionally, the gas valve is configured such that the threshold voltage necessary to maintain the valve in its open position is selected to correspond to a voltage 58 encountered

in the transition zone 52. As a consequence, maximum sensitivity and repeatability is simultaneously achieved.

The oxygen depletion of the sensor of the present invention can be readily modified to accommodate the use other gaseous fuels by either altering the inlet orifice diameter, the air hole diameter, or both. The same thermocouple may be used, the same spacing between the thermocouple and the mouth of the nozzle may be maintained and the same overall flame forming component may be employed.

While a particular form of the invention has been illustrated and described, it will also be apparent to those skilled in the art that various modifications can be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited except by the appended claims.