Patent Publication Number: US-6670585-B2

Title: Humidity cabinet with automatic fluid fill system

Description:
The present application is a continuation-in-part of U.S. application Ser. No. 09/671,700, entitled “Holding Cabinet with Closed-Loop Humidity Control System and Method for Controlling Humidity in a Holding Cabinet,” filed Sep. 28, 2000, now U.S. Pat. No. 6,454,176, which claims priority from U.S. Provisional Patent Application No. 60/217,707, entitled “Holding Cabinet with Closed-Loop Humidity Control System and Method for Controlling Humidity in a Holding Cabinet,” filed Jul. 12, 2000, and U.S. Provisional Patent Application No. 60/156,449, entitled “Holding Cabinet with Closed-Loop Humidity Control System and Method for Controlling Humidity in a Holding Cabinet,” filed Sep. 28, 1999, which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a holding cabinet, which provides a more consistent and accurate holding environment for food products. In particular, the invention relates to a holding cabinet, which provides a more consistent and accurate holding environment for food products by maintaining a desired fluid level in a fluid reservoir. 
     2. Description of Related Art 
     With the increasing popularity of “fast food” establishments where food is precooked for later sale, there is a demand for food holding devices that maintain food at a substantially uniform temperature for selected periods of time while preserving the taste, moisture content, texture and quality of the food. Further, in other applications, it is desirable to be able to restore food, particularly baked goods, to acceptable quality after long storage periods. 
     In many instances, storage of “fast foods” is particularly difficult because heat loss, bacteria growth and moisture loss experience by the food at storage conditions provided by prior art devices, particularly where the food is to be stored warm, contribute to rapid deterioration of the food. 
     More particularly, it has been found that air circulation characteristics and improper storage temperature contribute significantly to bacteria growth and excessive loss of moisture which leads to food shrinkage, so that in improper storage atmosphere the food deteriorates after only a short period of time and loses its tenderness, appetizing taste, and appearance. 
     It has also been found that even where food is stored under favorable conditions in an enclosure, the food deteriorates at a rate dependent on the time the door to the enclosure is opened so the storage chamber is exposed to the ambient atmosphere. 
     Additionally, it is known that in storage of some foods, such as fried chicken or fish, where a crust is provided, it is particularly desirable to maintain the crispness of the crust while minimizing the moisture loss from the underlying meat. Storage of such foods tends to involve the satisfaction of seemingly mutually exclusive conditions, to hold the crispness of the crust by maintaining low moisture content in the crust while minimizing moisture loss from the food. In such foods, excessive moisture-loss results in shrinkage and loss of tenderness and adversely affects the texture of the meat. This may be prevented by controlling the temperature and humidity of the storage atmosphere. The problem is to prevent moisture flow from the underlying food to the crust while holding the crust in low moisture content. 
     There are presently numerous cabinets for holding food products or other items in a temperature and humidity-controlled state. These cabinets, however, suffer from a common shortcoming. When the cabinets are opened to insert additional food products or other items or to remove such products or items from the cabinets, heat and humidity are lost. Unless the lost heat and humidity is restored, the items stored in the cabinets may cool or dry out, or both. 
     Proofing and holding are distinct food preparation processes. Proofing is a process generally applied to yeast bread products, in which the yeast grows and the bread rises due to yeast growth by products. Holding, however, is a process during which food characteristics are maintained, e.g., the temperature, moisture content, texture, and color of the food remain unchanged. Thus, in proofing, food product characteristics change, while in holding, those characteristics remain the same. 
     In terms of process parameters, proofing may be distinguished from holding mainly by lower process temperatures. Humidity may be greater than about 80% RH, but the selected humidity may vary widely depending on the particular bread product to be proofed. Nevertheless, proofing temperatures are generally lower than holding temperatures. High proofing temperatures might inhibit yeast growth. However, high holding temperatures are desirable because such temperatures may suppress the growth of bacteria, molds, and the like and may increase the holding time for food products. 
     Previously, various methods and devices have been developed to attempt to maintain heat and humidity. For example, pans of water have been placed in the cabinets and allowed to evaporate naturally in an attempt to maintain humidity. Despite its simplicity, this method has not been completely successful. Natural evaporation does not quickly compensate for humidity losses. Further, while humidity naturally increases, items stored in the cabinets are subject to the drying effect of heat. Moreover, because natural evaporation is effected by the temperature within the cabinet, the rate of humidity adjustment may fluctuate with temperature changes, but humidity adjustments will probably lag behind such temperature changes. 
     Systems have been developed by which the heat and humidity levels of air within a cabinet are more closely controlled. Air may be heated by passing it over, across, or through various types of heating elements. Air may also be passed over, across, or through water in order to raise the humidity of the air. Despite these improvements, known systems remain unable to precisely adjust for losses of heat or humidity due to disruptions to the cabinet environment, such as opening and closing the cabinet access, and adding or removing food products or other items. 
     Further, the addition of heating elements and humidity generating means create additional problems. If heat or humidity rise too quickly, the air within the cabinets could become overheated or too moist. Such uncontrolled fluctuations in heat and humidity may be detrimental to food product or other items stored within the cabinets. 
     Cabinets commonly are equipped with thermostats in an attempt to control the heat of the air circulating within the cabinets. By controlling the air temperature, however, the humidity of the air also may be affected. Nevertheless, such controls alone do not provide adequate control of the humidity within the cabinet. Moreover, a thermostat or manual potentiometer may not maintain temperature and humidity within predetermined parameters. Generally, such devices only cause the heating elements to heat the air when the air temperature falls below a set value. 
     The effectiveness of humidity generating systems depends, in part, on the ability of those systems to maintain fluid in a fluid reservoir, so that the fluid in the reservoir may be heated, e.g., heated to a boiling point, to produce a fluid vapor. The fluid vapor may be circulated through holding or proofing cabinets to create and maintain a desired level of humidity. A fluid supply mechanism may supply fluid to the fluid reservoir. A problem may arise, however, if fluid is not maintained in the reservoir, so that holding or proofing cabinets may be humidified. For example, a fluid level sensor may not accurately measure a fluid level in a reservoir. The fluid supply mechanism or the fluid level sensor may become inoperative. As a result, the reservoir may become empty, and the humidity generating system may not humidify the holding cabinets. 
     A further problem may arise when fluid is added to a reservoir. As fluid is added to the reservoir, fluid levels within the reservoir may fluctuate due to fluid turbulence. As a result, a fluid level sensor may detect relatively rapidly changing fluid levels within the reservoir. As the level of fluid in reservoir approaches a level at which the reservoir is considered to be full, the fluid level sensor may detect fluid levels that indicate alternately that the reservoir is full and that the reservoir is not yet full, depending upon the level of turbulence in the reservoir. If an automatic fill system controls the fluid supply mechanism to supply fluid to the reservoir based on fluid levels detected by fluid level sensor, the alternating signals may result in damage to various components of the fluid supply mechanism, e.g., fluid pumps, fluid motors, valves, or the like, as the components cycle on and off in response to the alternating signals of the fluid level sensor. Moreover, when the components cycle on and off in response to alternating signals of fluid level sensor, the cycling of the components may create additional turbulence and additional fluctuations in fluid level sensor. 
     SUMMARY OF THE INVENTION 
     A need has arisen for holding cabinets for attaining closed-loop humidity control by means of an effective humidity transducer. A further need has arisen for a cabinet that may be used for both proofing and holding. It is a feature of such a cabinet that its control system defaults to a generally higher temperature associated with a holding mode of operation. It is an advantage of this default setting that the cabinet may inhibit the growth of bacteria in food products. 
     A still further need has arisen for a fluid fill system and method that measure a fluid level in a reservoir accurately and that maintain the fluid level within a reservoir, so that a humidity generating system may humidify one or more holding or proofing cabinets. In particular, a need has arisen for a system and method of filling a fluid reservoir, so that fluid in the reservoir may be maintained at a desired level and so that damage to components of a fluid supply mechanism may be reduced or avoided. 
     In an embodiment of the invention, a holding cabinet with a closed loop humidity control system and a method of controlling humidity in a holding cabinet are disclosed. The method comprises determining relative humidity set points; activating a heater in a fluid pan; determining if a fluid is present in the fluid pan; measuring the relative humidity in the cabinet; and maintaining the relative humidity within a predetermined range. 
     According to another embodiment of the invention, a holding cabinet with a closed-loop humidity control system includes a holding cabinet; an air temperature probe for measuring an air temperature within the holding cabinet; a humidity sensor for measuring a humidity within the holding cabinet; a heater for heating air within the holding cabinet to a predetermined temperature; a fan for circulating the air and for introducing air from outside the holding cabinet; a slide vent and motor; and a water pan for holding water within the holding cabinet. 
     According to still another embodiment of the invention, a system for humidity measurement includes a humidity sensor; an oscillator circuit; and a microprocessor to measure oscillator frequency. According to another embodiment of the invention, a system for maintaining a relative humidity level in a cabinet includes means for determining a relative humidity set point; means for activating a heater in a fluid pan; means for determining if a fluid is present in the fluid pan; means for measuring the relative humidity in said cabinet; and means for maintaining the relative humidity within a predetermined range. 
     In yet a further embodiment of the invention, a fluid fill system comprises at least one sensor for detecting whether a fluid level in a reservoir is equal to or greater than a predetermined fluid level, and a process control, wherein the process control obtains a reading from the at least one sensor at a predetermined interval and increments a counter if the reading indicates that the fluid level is equal to or greater than the predetermined level and decrements the counter if the reading indicates that the reservoir fluid level is less than the predetermined fluid level, and wherein the process control operates a fluid supply mechanism when a value of the counter moves beyond a predetermined value. 
     In a still further embodiment of the invention, a method of filling a reservoir with a fluid comprises the steps of obtaining a fluid level reading at a predetermined interval to determine whether a fluid level in a reservoir is equal to or greater than a predetermined level, incrementing a counter if the fluid level reading is equal to or greater than the predetermined level, decrementing the counter if the fluid level reading is less than the predetermined level, and operating a fluid supply mechanism when a value of the counter moves beyond a predetermined value. 
     Other objects, features, and advantages will be understood by persons skilled in the art from the following detailed description of preferred embodiments of the present invention, in view of the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention are described below with reference to the accompanying figures, which are provided by way of example only, and are not intended to limit the present invention. 
     FIG. 1 depicts a front view of the holding cabinet according to an embodiment of the present invention. 
     FIG. 2 depicts a side view of the holding cabinet according to an embodiment of the present invention. 
     FIG. 3 depicts a cross-sectional view of the holding cabinet of the present invention, along line III—III of FIG.  1 . 
     FIG. 4 depicts a cross-sectional view of the holding cabinet of the present invention, along line IV—IV of FIG.  2 . 
     FIG. 5 is a schematic depiction of the air and humid air circulation within the holding cabinet according to an embodiment of the present invention. 
     FIG. 6 is a perspective view of a water pan cover and ring assembly according to an embodiment of the present invention; 
     FIG. 7 is a schematic depiction of the humidity generating pan and the control and monitoring interconnections of the holding cabinet according to an embodiment of the present invention. 
     FIG. 8 depicts the circuitry of the humidity detection transducer according to an embodiment of the present invention. 
     FIGS. 9A and 9B are side and top views of a slide vent according to an embodiment of the present invention. 
     FIGS. 10A and 10B are schematic depictions of the slide vent and cabinet openings according to an embodiment of the present invention. 
     FIG. 11 is a flowchart of the process for vent operation according to an embodiment of the present invention. 
     FIG. 12 is a flowchart of the calibration process for the slide vent motor according to an embodiment of the present invention. 
     FIG. 13 is a depiction of the period of the slide vent according to an embodiment of the present invention. 
     FIG. 14A depicts a humidity regulation state diagram according to an embodiment of the present invention. 
     FIG. 14B is a graphical representation of the humidity control process according to an embodiment of the present invention. 
     FIG. 15 is a flowchart of the process for increasing humidity according to an embodiment of the present invention. 
     FIG. 16 is a flowchart depicting the operation of the closed-loop humidity control system. 
     FIG. 17 shows a fluid fill system according to an embodiment of the invention. 
     FIG. 18 is a flowchart depicting the operation of a fluid fill system according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Embodiments of the present invention and their technical advantages may be better understood by referring to FIGS. 1 though  13 , like numerals referring to like and corresponding parts of the various drawings. 
     Referring to FIGS. 1 and 2, a front view of the holding cabinet and a side view of the holding cabinet according to an embodiment of the present invention are provided. Holding cabinet  100  has a front  102 , back  104 , and sides  106  and  108 . Front  102  and back  104  may both have at least one door with a corresponding locking mechanism  110 . In the embodiment depicted in FIGS. 1 and 2, front  102  and back  104  each have two doors. 
     Module  114  is provided to house equipment used to control the relative humidity in cabinet  100 . In an embodiment, holding cabinet  100  may be provided with a plurality of wheels  112 . 
     Referring to FIG. 3, a cross-sectional view of the holding cabinet of the present invention, along line III—III of FIG. 1 is provided. Referring to FIG. 4, a cross-sectional view of the holding cabinet of the present invention, along line IV—IV of FIG. 2 is provided. 
     Referring to FIG. 5, a schematic depiction of the air and humid air circulation within the holding cabinet according to one embodiment of the present invention is provided. Blower motor  708  is provided, as are heaters  706 . In the embodiment shown, two heaters  706  are provided; other numbers and locations of heater  706  may also be used. 
     Water pan  316  is provided with water pan cover and ring assembly  502 , which is shown in detail in FIG.  6 . Water pan cover and ring assembly  502  includes inner ring  520 , outer ring  522 , and cover  524 . Steam exhaust ports  526  may be provided. In one embodiment, two exhaust ports  526  are provided, at opposite sides of the rings. 
     Referring again to FIG. 5, water pan in water pan  316  is heated by a water pan heater  506 , which causes the water in water pan  316  to vaporize into steam  504 . Inner and outer rings  520  and  522  of assembly  502  concentrate heat generated by water pan heater  506 , assisting in the vaporization. 
     FIG. 7 depicts a block diagram of system  700  according to an embodiment of the present invention. System  700  includes air temperature probe  702 , which measures the temperature of the air in the holding cabinet. Air temperature probe  702  may also be used to provide temperature compensation for humidity sensor  704  In one embodiment, air temperature probe  702  may be part number DC32006A-3-18, manufactured by Durex Industries, Cary, Ill. 
     Humidity sensor  704  measures the relative humidity of the air in the cabinet (H 1 ). In an embodiment, humidity sensor  704  may be E&amp;E Electronik Part No. EE00-FR3, manufactured by JLC International, Warminster, Pa. Air heater  706  heats the air in the cabinet to the set point specified by the user. In one embodiment, air heater  606  may be part number U3-32-764-34, 500 W, 1000 W, or 1500 W, manufactured by Watlow, Hannibal, Mo. Air fan  708  circulates heated air through the cabinet so that the entire cabinet volume is at the same temperature. In one embodiment, air fan  708  may be part number SX-19695 (240V) or SX-20441 (208V), manufactured by Jakel, Highland, Ill. 
     Water pan  716  holds water to be boiled to create humidity. In one embodiment, water pan heater  722  may be #-8-MSM22866-xxx, manufactured by Minco, Minneapolis, Minn. In another embodiment, heating elements may be screened onto water pan  716 . Float switch  720  is provided to determine the water level in water pan  716 . In an embodiment, float switch  720  may control water flow into water pan  716  when the water level is below a desired level. An water pan heater (RTD) temperature sensor  723  is affixed to water pan heater  722 . Alternatively, sensor  723  may be integral with heater  722 . Sensor  723  may measure the temperature of heater  722  and input such measured temperature values to System  700 . 
     Water pan heater temperature sensor  723  is linked to control system  700  to ensure that water pan heater  722  remains off when either of at least two conditions occurs: first, when no water is in water pan  716  or second, when float switch  720  fails. In normal operation, float switch  720  signals control system  700  that water pan  716  is empty, so control system  700  does not activate water pan heater  722 . Nevertheless, line build-up, debris, or abuse may cause float switch  720  to fail in the “full water pan” position. Water pan  716  and water pan heater  722  may be quickly damaged if water pan heater  722  is activated while water pan  716  is empty. Water pan heater temperature sensor  723  performs as a backup to float switch  720  to reduce or eliminate the risk of such damage to water pan  716  or water pan heater  722 , or both. 
     Slide vent motor  730  controls the movement of the slide vent, which, in turn opens and closes the cabinet vent. Slide vent position switch  732  is provided to provide an indication of the status of the vent. In one embodiment, side vent position switch  732  may be part number KWABQACC, manufactured by Cherry Electrical Products, Pleasant Prairie, Wis. Switch  732  may also be an switch, optical proximity switch. 
     Process inputs and outputs connect to the process control as shown. Temperature sensor  798  is built into heater  722  and measures the water pan temperature. 
     The cabinet air temperature is regulated with air temp sensor  702 , air heater  706  and air fan  708 . The air temp regulation is obvious to those skilled in the art, and consists simply of regulating the air temperature to the programmed set point. This may be a simple thermostatic (on/off) control with hysteresis, or may be a more sophisticated PID (proportional/integral/derivative) control algorithm. 
     Humidity may be regulated by 1) adding humidity when the cabinet humidity is below the humidity set point, and 2) decreasing humidity by introducing outside ambient air to the cabinet, when the cabinet humidity is above the programmed set point. Thus, there are two separate systems to regulate the humidity: a humidity generation system, and a “venting” system. 
     Referring to FIG. 8, humidity transducer circuit  800  according to one embodiment of the present invention is provided. Timer U 1  forms an astable oscillator with output frequency, F O , set by capacitors C x , C 1 , and resistor R 1 . Capacitors C 2  and C 3  bypass power supply. Capacitor C 1  blocks DC voltage to transducer C x , which is damaged by DC voltage. Resistor R 1  sets the frequency, F O . Resistor R 2  drains charge from capacitor C 1  during power-down. Transducer C x  capacitance varies with humidity. Microprocessor μP measures F O  period by counting pulses (n 2 ) for {fraction (1/16)} second. 
     Example values for the elements in FIG. 8 are provided below: 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Element 
                 Value 
               
               
                   
                   
               
             
            
               
                   
                 U1 
                 LMC 555 C Timer 
               
               
                   
                 R 1   
                 24.9 K 
               
               
                   
                 R 2   
                 5 M 
               
               
                   
                 C 1   
                 .039 μF, 50 V, 1%, 100 PAM 
               
               
                   
                 C 2   
                 0.1 μF ceramic disk 
               
               
                   
                 C 3   
                 10 μF Tantalum 
               
               
                   
                 C X   
                 Humidity Transducer, E &amp; E 
               
               
                   
                   
                 Electronik EE00-F123 
               
               
                   
                   
               
            
           
         
       
     
     The relative humidity percentage (% RH) may be determined by the following equation:          %                 R                 H     =     419.734                   (       4343.287       n   2     +   360       -   1     )                       
     Capacitance C x  also is affected by temperature, therefore, % RH is compensated for temperature with this equation: 
     
       
         % RH   C =[( T   F −140)(0.0016667)+1](% RH ) 
       
     
     where: 
     T F  is air temperature in ° F. % RH C  is used to display and regulate humidity. 
     The systems of the present invention may implement a proofing mode of operation. As noted above, this invention may combine the proofing and holding functions in a single cabinet. For example, on initiation of any power-up condition, a user interface, e.g., a display, for the control system may offer the user the opportunity to initiate a “Proof” option. The user may have a limited time window, e.g., ten (10) seconds, within which to accept this option. The user may accept the option by activating a particular switch, e.g., a TEMP switch, or a combination of switches. If the option is not accepted during the time window, the control system initiates the hold (higher temperature) mode. However, if the option is accepted, the control system initiates the proof (lower temperature) mode. 
     The bold and proof modes are distinguished by the maximum allowable air temperature set point. For example, in the proof mode, the maximum allowable air temperature set point may be the minimum hold temperature. Thus, if the minimum hold temperature were 150° F., the maximum proof temperature setpoint would be 150° F. Similarly, if the minimum hold temperature were 150° F., the maximum allowable hold mode air temperature set point might be 220° F., and the hold mode temperature range might be 150° F. to 220° F. 
     Referring to FIGS. 9A and 9B, side and top views of a slide vent according to an embodiment of the present invention are provided. In general, cabinet panel  902  is provided with slide panel  904 . Both cabinet panel  902  and slide panel  904  have at least one opening  906  In one embodiment, openings  906  in cabinet panel  902  are fixed, while openings  902  in slide panel  904  slide relative to openings  906  in cabinet panel  902 . Gear motor  908  drives slide panel  904  linearly to open or close openings  906  via lever arm  912  and slide pin  914 . In one embodiment, motor  908  is model number EB-5206, manufactured by Custom Products, Inc., New Haven, Conn., or part number AB, manufactured by Hurst Manufacturing Corporation, Princeton, Ind. 
     As slide panel  904  slides relative to cabinet panel  902 , openings  906  on slide panel  904  line up with openings  906  on cabinet panel  902 , in effect opening a passage to the blower inlet and outlet (not shown). When slide panel  904  slides its full distance, openings  906  in cabinet panel  902  are fully uncovered. At this point, slide panel  904  begins sliding in the opposite direction, and openings  906  in cabinet panel  902  are covered, blocking access to the blower inlet and outlet (not shown). 
     Switch  916  is provided to indicate when vents  906  are fully closed. In another embodiment, switch  916  may be provided to indicate when vents  906  are fully opened. This variance may depend on the position of switch  916  with respect to slide  904 . Other arrangements may be provided as desired. Switch  916  may be used during calibration to determine the period of slide vent  904 . This is discussed in greater detail, below. 
     Referring to FIGS. 10A and 10B, depictions of the slide vent in its closed and open positions are provided, respectively. In FIG. 10A, slide vent  904  is positioned such that air does not flow from the exterior of the cabinet into blower inlet  1010 , and out of blower exhaust  1012 . When motor  908  is activated, however, slide vent  904  is moved, shown in FIG. 10B, opens blower inlet  1010  and blower exhaust  1012 . 
     Referring to FIG. 11, a flowchart of the general operation of the cabinet is provided. In step  1102 , the cabinet is powered up. This may involved initializing cabinet components, which is known to one of ordinary skill in the art. 
     In step  1104 , the vent motor is calibrated. This process is described in greater detail in FIGS. 12 and 13, below. 
     Referring to FIG. 12, a flowchart of the slide vent motor calibration process according to one embodiment of the present invention is provided. The purpose of the calibration is to account for variations in the actual time required to move the vent from one position to another. Even though a synchronous AC motor may be used, the time for one revolution may vary because 1) the line frequency may be 50 Hz or 60 Hz, and 2) friction and debris in the mechanism may slow the vent movement. 
     In general, the control software needs to know the time for one complete revolution to be able to move the vent from the fully-opened to the fully-closed position. The control knows when the vent is fully-closed, because a vent switch actuates at that position. Thus, if the actual period for the vent movement is T VENT , then the vent is fully open at time T VENT /2. Also, the control may move the vent to other positions, such as 50% open area, by actuating the motor for some time that is a fraction of T VENT . For example, to open the vent to about 50% open area, the control activates the motor for about T VENT /4, from either the fully-open or fully-closed position. 
     In one embodiment, although the vent open area is not a linear function of the vent motor actuation time, it provides a suitable approximation, permitting the vent motor actuation time to be used to position the slide vent. In another embodiment, different shapes for the vent holes may be used to provide a linear relationship between motor actuation time and vent open area. 
     FIG. 13 depicts the vent operation as far at the control is concerned. As the motor turns and the vent actuates the vent switch, the vent switch is really actuated for some period of time, which may be referred to as the “dwell time,” or T DWELL . The control may account for T DWELL  when calculating the time needed to actuate the motor to achieve a given vent position. 
     Referring again to FIG. 12, in one embodiment, the vent calibration routine uses a timer that is always running, so there is no need to start or stop the timer, just a need to reset it to find the dwell time and the period. In step  1202 , there is a predetermined delay, during which timers and interrupts are synchronized. In one embodiment, this may be a one second delay; other delays may be used, as required. In another embodiment, this delay may be omitted. 
     In step  1204 , after the timers and interrupts are synchronized, the vent motor is activated, causing the slide vent to move. The timer is cleared in step  1206 , and, in step  1208 , the control waits for a first transition signal from vent switch. This signal indicates that the vent switch is being activated. If there is no switch signal within a predetermined time, an error message is presented to the user in step  1210 . This may be by a visual or audible signal, such as a CRT, a LED, a bell, a chime, and the like. In an embodiment, a suitable message, such as “Vent Stuck” is displayed for the user. 
     In one embodiment, the predetermined amount of time may be 48 seconds. Other suitable lengths of time may be used as desired. This time may be selected based on, inter alia, the known general period of the vent. The time may also be selected to prevent damage to the motor. After the predetermined time is elapsed, the motor may be shut off. 
     If a signal is received from the vent switch, in step  1208 , the timer is cleared, and in step  1214 , the control waits for a second transition signal from the vent switch, indicating that the vent switch is no longer actuated. Similar to above, if a predetermined time passes without a signal from the vent switch, the user may be notified in step  1210 . Once the second transition signal is received, in step  1216 , the timer is read, indicating the dwell time, or T DWELL . In step  1220 , similar to steps  1208  and  1214 , the control waits for a transition signal from the vent switch. Once a transition signal is received, indicating that the vent has completed its cycle, in step  1222 , the timer is read. This is T VENT . 
     In step  1224 , the vent is moved to the fully-closed position. As discussed above, this may be achieved by activating the motor for T VENT /2. 
     The control may use the time required to move the vent to detect faults in the vent system. If it takes longer than a predetermined time for one complete revolution, the control assumes that the vent is stuck, or the motor has failed, and displays a fault message. 
     Referring again to FIG. 11, in step  1106 , the control determines if the vent position is within a predetermined tolerance of its requested position. In an embodiment, the vent position may be expressed as an opening percentage—from 100% open, to 0% open. In this step, it is determined if the actual position is within a predetermined window of the desired position. This may be about 10%, 5%, 2%, and the like. In one embodiment, it is about 1%. If the vent is within this window, no adjustments are made. 
     If, in step  1108 , it is determined that the vent is not within the predetermined window, the vent motor is activated for a determined amount of time to move the vent to its desired position. 
     In step  1110 , the device may be powered down. When this occurs, it is possible that humidity may condense on the humidity sensor as the air temperature within the cabinet drops. This may 1) damage the humidity sensor, or 2) cause false humidity readings during operation. In order to compensate for this problem, in one embodiment, the device enters “purge” mode that is activated when the control switch is changed from “operate” to “standby” or “off.” In this mode, the air heater and the water heater are turned off, and the fan is activated if the humidity is greater than a predetermined level. The predetermined humidity level may be selected as a compromise between low humidity (much lower than 100%) and high ambient humidity that exists within restaurants or other operating environments. In one embodiment, this percentage may be 80%. 
     When the fan is activated, air from outside the cabinet is injected into the cabinet, for the most part, preventing the humidity in the cabinet from exceeding the predetermined level. In general, controlling the humidity in the cabinet involves regulating the water heat output and the vent motor output. The water heat output is usually turned on to increase humidity within the cabinet, while the vent is usually opened to reduce humidity within the cabinet. 
     According to an embodiment of the present invention, the humidity control method consists of three states: Idle, Increase Humidity, and Decrease Humidity. Referring to FIG. 14A, a humidity regulation state diagram is provided. In the decrease humidity state, the vent is either open 50% or 100%, depending on how far the actual humidity is above the set point. Other opening percentages may be used as desired. FIG. 14B provides a graphical representation of the humidity regulation. 
     In addition, the control levels of SP+9% RH and SP+7% just amount to a hysteresis band that switches between about 50% and about 100% vent opening. 
     In the Increase Humidity state, the net result of the flow chart logic is to determine a duty cycle setting for the water heat output. The duty cycle is the number of {fraction (1/16)} second intervals, out of a period of 2 seconds that the water heat is on. For example, in a duty cycle of 25%, the heat is on for 0.5 seconds, which is 8 intervals of {fraction (1/16)} second. Referring to FIG. 15, a flowchart depicting the Increase Humidity logic according to one embodiment of the present invention is provided. 
     The humidity control is similar to PID control, but the derivative information is only used to update the integral term. 
     Blocks  1502  to  1508  set the water heat duty cycle when the actual humidity is the same as the set point. If the temperature is below 125° F., the duty cycle is set to 25%. If the temperature is above 125° F., the duty cycle is set to 31%. These cycles act to maintain the humidity near the set point. A higher duty cycle is needed at higher temperatures. Blocks  1510  and  1512  set the duty cycle to 100% (full on) if the actual humidity is more than 3% RH below the humidity set point. This acts to bring the humidity back to the set point. Block  1514  calculates the humidity error (humidity set point-actual humidity) and saves it in a variable called hum_temp_byte. 
     Blocks  1516 - 1526  adjust the integral correction term I.E.L (which stands for the code variable integral_error_level). The test in block  1516  limits I.E.L. to values of 20 and 200. Block  1518  adds the humidity error to I.E.L. Blocks  1520 - to  1526  add 5 to I.E.L. if the humidity is decreasing, and subtract 20 from I.E.L. if the humidity is increasing. 
     The initialization of I.E.L. is not shown, but I.E.L. is set to zero whenever the Increase Humidity state is entered, or whenever the measured humidity equals the set point. 
     The blocks in  1528  set a new variable, E.O. (for error_offset) from the value of I.E.L. just found. Note that a larger value of I.E.L. results in a larger value of E.O. 
     The blocks in  1530  find the duty-cycle on-time, called t(on). t(on) is a function of E.O. and the air temperature Ta. t(on) is just the sum of a constant that depends on the air temperature and the value of E.O. 
     Finally, block  1532  show that the actual duty cycle is calculated from t(on)/31. The divisor is “31” because a 16 Hz clock is used for the water heat output. The duty-cycle period is 2 seconds, but the clock actually counts from 0 to 31. 
     Referring to FIG. 16, a flow chart of the operation of a closed-loop humidity control system is depicted. In this chart, T H  is the water pan heater temperature measured by water pan heater temperature sensor  723 , and T UM  is the maximum allowable water pan temperature. A Float-Switch-Fault is true when float switch  720  has failed. Float switch  720  has failed when it fails to accurately detect significant changes in the water level in water pan  716 . 
     Various operational conditions are detailed with respect to FIG.  16 . If water pan  716  is found empty during normal operations, float switch  720  will indicate allow water level (Step B) and a “low water level” message is displayed (Step F). Water pan heater  722  then will be disabled (Step I), and control system  700  will complete its operation (Step L). 
     Similarly, if water pan  716  is incorrectly found empty during normal operations, float switch  720  again will indicate a low water level (Step B). However, control system  700  will inquire whether T H&gt; T LIM  (Step C). If T H ≦T LIM , the Float-Switch-Fault is true (Step D), and water pan heater  722  is enabled (Step J). Control system  700  then again completes its operation (Step L). 
     If a Float-Switch-Fault is detected, a low water level is again detected (Step B) and control system  700  again will inquire whether T H&gt; T LIM  (Step C). If T H &gt;T LIM , then water pan  716  is empty or low on water and Float-Switch-Fault is true (Step E). The display may then indicate “Float Switch Failed” and “Out of Water” or “Pan Empty” (Step G). Water pan heater  722  will be disabled (Step I), and control system  700  will complete its operation (Step L). 
     While waiting for a Float-Switch-Fault to clear, Float switch  720  will initially indicate that the water level in water pan  716  is low (Step B). Control system  700  then will inquire whether T H&gt; T LIM  (Step C). If T H ≦T LIM  then Float-Switch-Fault is true (Step D), and if whether T H&gt; (T LIM −100° F.) or the reset delay timer is not set to zero (Step H), water pan heater  722  is disabled (Step I). Control system  7800  then will complete its operation (Step L). 
     Once the Float-Switch-Fault has cleared, if Float switch  720  indicates that the water level in water pan  716  is low (Step B), control system  700  inquires whether T H&gt; T LIM  (Step C). If T H ≦T LIM , then Float-Switch-Fault is true (Step D), and control system  700  inquires whether T H&gt; (T LIM −100° F.) and whether the reset delay timer is set to zero. (Step H). If both these conditions exist, the Float-Switch-Fault is false (Step K), and water pan heater  722  is enabled (Step J). Control system  700  then will complete its operation (Step L). 
     FIG. 17 shows a fluid fill system  1700  according to an embodiment of the invention. Fluid fill system  1700  includes a reservoir  1701 , a fluid supply line  1702 , a fluid supply mechanism  1703 , a fluid level sensor  1704 , and a process control  1705 . 
     Fluid reservoir  1701  is capable of holding a fluid, such as water, for the humidity control system. Fluid reservoir  1701  may include a pan, container, or the like, that is capable of receiving and holding a fluid, such as water. Fluid may be supplied to reservoir  1701  by a fluid supply line  1702 . Fluid supply line  1702  may include a fluid supply mechanism  1703 , e.g., a fluid supply valve  1703 . Fluid supply mechanism  1703  may be positioned in fluid supply line  1702  to open fluid supply line  1702 , so that fluid may be supplied to reservoir  1701 , or to close fluid supply line  1702 , so that fluid may be prevented from flowing to reservoir  1701 . Fluid supply mechanism  1703  may include a solenoid valve that may be actuated to open and close fluid supply line  1702 . In another embodiment of the invention (not shown), fluid supply mechanism may include a pump that supplies fluid to reservoir  1701 , via fluid supply line  1702 . Fluid supply line  1702  may include one or more fluid supply lines  1702  that supply fluid to reservoir  1701 . 
     A fluid level sensor  1704 , e.g., a float switch or the like, may be positioned in reservoir  1701  to detect, e.g., to measure, or the like, the level of fluid in reservoir  1701 . In an embodiment of the invention, fluid level sensor  1704  may detect a level of fluid in reservoir  1701  and indicate whether the fluid level within reservoir  1701  is equal to or greater than a predetermined level. The predetermined level may correspond to a fluid level at which reservoir  1701  is considered to be full. In this embodiment, fluid level sensor  1704  has two states: a first state indicating that a fluid level in reservoir  1701  is equal to or greater than a predetermined level and a second state indicating that a fluid level in reservoir  1701  is less than the predetermined level. When the fluid level in reservoir  1701  is equal to or greater than the predetermined level, fluid level sensor  1704  will so indicate. When the fluid level in reservoir  1701  is less than the predetermined level, fluid level sensor  1704  will so indicate. Fluid level sensor  1704  may be adjusted to detect different predetermined fluid levels, as necessary. A float switch or the like may be used as a fluid level sensor  1704  to detect fluid levels in reservoir  1701 . Moreover, more than one fluid level sensor  1704  may be positioned in reservoir  1701 . 
     Process control  1705  may include a processor, a microprocessor, or the like. Process control  1705  may communicate with fluid supply mechanism  1703  to operate fluid supply mechanism  1703 , e.g., to activate or deactivate fluid supply mechanism  1703 . Process control  1705  may operate fluid supply mechanism  1703  to supply fluid to reservoir  1701 , or process control may operate fluid supply mechanism  1703  to stop a supply of fluid to reservoir  1701 . For example, process control  1705  may operate fluid supply mechanism  1703  to supply fluid to reservoir  1701  by activating a fluid supply valve  1703  to open fluid supply line  1702 , so that fluid may be supplied to reservoir  1701 . Process control  1705  may operate fluid supply mechanism  1703  to stop supplying fluid to reservoir  1701  by deactivating fluid supply valve  1703  to close fluid supply line  1702 . By selectively activating and deactivating fluid supply mechanism  1703 , process control  1705  may adjust a level of fluid in reservoir  1701 . 
     Fluid level sensor  1704  may communicate with process control  1705 , so that process control  1705  may monitor a level of fluid in reservoir  1701 . Process control  1705  and fluid level sensor  1704  may communicate in a variety of ways. For example, process control  1705  may query fluid level sensor  1704  at a predetermined interval to obtain readings of a fluid level in reservoir  1701 , so that process control  1705  may monitor a level of fluid in reservoir  1701 . Each reading of fluid level sensor  1704  may indicate whether the level of fluid in reservoir  1701  is equal to or greater than a predetermined level, or whether the fluid level in reservoir  1701  is less than the predetermined level. In another embodiment of the invention, fluid level sensor  1704  may detect a fluid level in reservoir  1701  at a predetermined interval and transmit each reading to process control  1705 , so that process control  1705  may monitor a fluid level in reservoir  1701 . Each reading from fluid level sensor  1704  may indicate whether the level of fluid in reservoir  1701  is equal to or greater than a predetermined level, or whether the fluid level in reservoir  1701  is less than the predetermined level. Moreover, the predetermined interval may be selected from a range of predetermined intervals, so that process control  1705  may obtain readings from fluid level sensor  1704  at different frequencies. 
     In either embodiment, fluid level sensor  1704  may communicate with process control  1705  at a predetermined interval, so that process control  1705  may obtain fluid level readings that correspond to a level of fluid in reservoir  1701 , thereby enabling process control  1705  to monitor a level of fluid in reservoir  1701 . Process control  1705  may increment or decrement a counter (not shown) based on each reading. For example, if a reading of fluid level sensor  1704  indicates that a fluid level in reservoir  1701  is equal to or greater than a predetermined level, process control  1705  may increment counter by one. Process control  1705  may increment the counter by one each time that process control  1705  obtains a fluid level reading from fluid level sensor  1704  indicating that the fluid level in reservoir  1701  is equal to or greater than the predetermined level. If a reading of fluid level sensor  1704  indicates that the fluid level in reservoir  1701  is less than the predetermined level, process control  1705  may decrement counter by one. Process control  1705  may decrement counter by one each time that process control  1705  obtains a fluid level reading from fluid level sensor  1704  indicating that the fluid level in reservoir  1701  is less than the predetermined level. 
     Process control  1705  also monitors a value of the counter each time that process control  1705  increments or decrements the counter. In one embodiment of the invention, process control  1705  compares a value of the counter to one or more predetermined counter values. The predetermined counter values may correspond to fluid levels in reservoir  1701 , to fluid fill operations, or the like. For example, the predetermined counter values may include a first counter value, which corresponds to a fluid level in reservoir  1701  at which fluid is added to reservoir  1701 . The predetermined counter values may include a second counter value, which corresponds to a fluid level in reservoir  1701  at which fluid is not added to reservoir  1701 . 
     In operation, fluid level sensor  1704  detects a level of fluid in reservoir  1701  at a predetermined interval and transmits each reading to process control  1705 . When a fluid level in reservoir  1701  decreases below a predetermined level, fluid level sensor  1704  detects the fluid level at a predetermined interval and transmits each reading to process control  1705 . Process control  1705  decrements a counter each time that process control  1705  obtains readings from fluid level sensor  1704  indicating that the fluid level in reservoir  1701  is below a predetermined level. When process control  1705  has decremented the counter such that a value of the counter is less than a first counter value, process control  1705  may operate fluid supply mechanism  1703  to supply fluid to reservoir  1701 . As the fluid level in reservoir  1701  increases above a predetermined level, fluid level sensor  1704  detects the fluid level at predetermined intervals and transmits each reading to process control  1705 . Process control  1705  increments the counter for each reading from fluid level sensor  1704  indicating that the fluid level in reservoir  1701  is equal to or greater than a predetermined level. When process control  1705  has incremented the counter such that a value of the counter exceeds a second counter value, process control  1705  may operate fluid supply mechanism  1703  to stop supplying fluid to reservoir  1701 . 
     In another embodiment of the invention, process control  1705  may query fluid level sensor  1704  at a predetermined interval to obtain readings of a level of fluid level in reservoir  1701 . When a level of fluid in reservoir  1701  decreases below a predetermined level, fluid level sensor  1704  detects the fluid level. Process control  1705  may obtain readings of a fluid level in reservoir  1701  from fluid level sensor  1704  when process control  1705  queries fluid level sensor  1704  at a predetermined interval. Process control  1705  decrements a counter each time that process control  1705  obtains a reading indicating that the fluid level in reservoir  1701  is below a predetermined level. When process control  1705  has decremented the counter such that a value of the counter is less than a first counter value, process control  1705  may operate fluid supply mechanism  1703  to supply fluid to reservoir  1701 . As the fluid level in reservoir  1701  increases above a predetermined level, fluid level sensor  1704  detects the fluid level at predetermined intervals. Process control  1705  obtains fluid level readings from fluid level sensor  1704  at a predetermined interval. Process control  1705  increments the counter each time that process control  1705  reads fluid level sensor  1704  and obtains a reading indicating that the fluid level in reservoir  1701  is equal to or greater than a predetermined level. When process control  1705  has incremented the counter such that a value of the counter exceeds a second counter value, process control  1705  may operate fluid supply mechanism  1703  to stop a supply of fluid to reservoir  1701 . 
     According to these embodiments of the invention, process control  1705  may not operate fluid supply mechanism  1703  to supply fluid to reservoir  1701  until the counter has been decremented below a first counter value. Thus, selection of a first counter value and a predetermined interval at which process control  1705  obtains fluid level readings from fluid level sensor  1704  enables process control  1705  to precisely initiate a fluid fill operation. Moreover, during a fluid fill operation of reservoir  1701 , process control  1705  may not operate fluid supply mechanism  1703  to stop a supply of fluid to reservoir  1701  until the counter has been incremented above a second counter value. Thus, selection of a second counter value and a predetermined interval at which process control  1705  obtains fluid level readings from fluid level sensor  1704  enables process control  1705  to precisely terminate a fluid fill operation. Moreover, by enabling process control  1705  to precisely terminate a fill operation, the present invention eliminates or reduces the cycling of fluid fill mechanism components on and off in rapid succession, thereby damaging the components in known systems and methods. 
     The invention will be further clarified by a consideration of the following examples, which are intended to be purely exemplary of the use of the invention. In one embodiment of the invention, a first counter value was set at sixteen, and a second counter value was set at sixty-four. Moreover, process control obtained fluid level readings from a fluid level sensor sixteen times per second, i.e., every {fraction (1/16)}th of a second. When the fluid level in the reservoir was less than a predetermined level, process control obtained readings from the fluid level sensor every {fraction (1/16)}th of a second, and process control decremented a counter for each reading indicating that a fluid level in reservoir was less than the predetermined level. Once process control decremented the counter to a value of fifteen, i.e., to a value less than the first counter value of sixteen, process control operated, i.e., activated, a fluid supply mechanism to fill reservoir with fluid. Process control continued to obtain readings from the fluid level sensor every {fraction (1/16)}th of a second. While the fluid level in reservoir remained less than the predetermined level, process control continued to obtain readings from the fluid level sensor every {fraction (1/16)}th of a second and to decrement the counter for each reading. Process control continued to decrement the counter until the counter value reached zero. 
     Process control continued to obtain readings from the fluid level sensor while operating the fluid supply mechanism to the fill reservoir with fluid. When the fluid level in the reservoir became equal to or greater than the predetermined level and process control obtained readings from the fluid level sensor indicating that the fluid level was equal to or greater than the predetermined level, process control incremented the counter for each such reading. Once process control incremented the counter to a value of sixty-five, i.e., to a value greater than the second counter value of sixty-four, process control operated, i.e., deactivated, fluid supply mechanism to stop supplying fluid to the reservoir. Process control continued to obtain readings from fluid level sensor and to increment or decrement the counter accordingly. While the fluid level in the reservoir remained above the predetermined level, process control continued to obtain readings from the fluid level sensor and to increment the counter every {fraction (1/16)}th of a second. Process control continued to increment the counter for each such reading until the counter value reached eighty. 
     According to this embodiment of the invention, process control did not operate the fluid supply mechanism to initiate a fluid fill operation until process control decremented the counter value to fifteen. Moreover, once process control had completed a fluid fill operation, process control did not initiate a subsequent fluid fill operation for at least three (3) seconds after the termination of a fill operation, because process control had incremented the counter to a value of sixty-five before terminating the fluid fill operation. In fact, process control did not initiate a fluid fill operation for at least three (3) seconds after process control began to obtain readings from the fluid level sensor indicating that the fluid level in the reservoir was less than the predetermined level. When the fluid level remained equal to or greater than the predetermined level after termination of a fluid fill operation, process control continued to increment the counter until the counter reached a value of eighty, so that process control did not initiate a subsequent fluid fill operation for at least four (4) seconds after process control began to obtain readings from the fluid level sensor indicating that the fluid level in the reservoir was less than the predetermined level. Thus, according to this embodiment of the invention, process control did not cycle components of the fluid supply mechanism on and off, even in those instances in which the fluid in reservoir experienced some turbulence during a fill operation, or during heating of the fluid to produce a vapor for humidification. Of course, different values could be selected for the first counter value and the second counter value to provide a larger or smaller interval. In addition, process control could obtain fluid level readings at different predetermined intervals. 
     A heater  1706  may be positioned at reservoir  1701  to heat fluid within reservoir  1701 . For example, heater  1706  may be positioned beneath reservoir  1701 , as shown in FIG. 17, to heat fluid contained in reservoir  1701 . Process control  1705  may regulate heater  1706  to heat fluid in reservoir  1701 , e.g., to heat fluid in reservoir  1701  to a boiling point, to produce vapor for humidifying holding cabinets. 
     A temperature sensor (at  1707 ) may be positioned at reservoir  1701  to measure a temperature of reservoir  1701  and to prevent damage due to overheating of reservoir  1701  by heater  1706 . Temperature sensor  1707  may be integral with reservoir  1701 , as shown in FIG. 17, or temperature sensor  1707  may be positioned on an inner surface or an outer surface of reservoir  1701  to measure a temperature of reservoir  1701 . Temperature sensor  1707  may include a plurality of sensors positioned on a surface of reservoir  1701  or positioned integral with reservoir  1701 . If reservoir  1701  contains fluid, the temperature of reservoir  1701  should remain below a threshold temperature. If reservoir  1701  becomes empty, however, the temperature of reservoir  1701  may exceed a threshold temperature. Temperature sensor  1707  detects these temperature changes. 
     Process control  1705  communicates with temperature sensor  1707 , so that process control  1705  may monitor the temperature of reservoir  1701 . When temperature sensor  1707  indicates that the temperature of reservoir  1701  exceeds a threshold temperature, process control  1705  may shut off reservoir heater  1706  to avoid or reduce damage to reservoir  1701  and other components of the fluid fill system. In one embodiment of the invention, a threshold temperature of 450° F. may be used. 
     FIG. 18 discloses a method of operating a fluid fill system according to an embodiment of the invention. Process control  1701  obtains fluid level readings from fluid level sensor  1704  at a predetermined interval (step S1). Based on each fluid level reading obtained from fluid level sensor  1704 , process control  1701  determines whether a fluid level in reservoir  1701  is equal to or greater than a first predetermined level (step S2). Process control  1701  increments or decrements a counter based on each fluid level reading obtained from fluid level sensor  1704 . If a fluid level reading indicates that the fluid level in reservoir  1701  is equal to or greater than a predetermined level, process control increments counter (step S3). If a fluid level reading indicates that the fluid level in reservoir  1701  is below a predetermined level, process control  1701  decrements counter (step S4). 
     Process control  1701  also reads a value of the counter each time that process control  1701  increments or decrements the counter (steps S5, S6). If a value of the counter is less than a first counter value (step S5), process control  1701  may operate fluid supply mechanism  1703  to supply fluid to reservoir  1701  (step S7). If a value of the counter is equal to or greater than a first counter value (step S5), process control  1705  obtains a fluid level reading from fluid level sensor  1704  (step S1). If a value of counter is greater than a second counter value (step S6), process control  1705  may operate fluid supply mechanism  1703  to close fluid supply line  1702  and stop a supply of fluid to reservoir  1701  (step S8). If a value of the counter is equal to or less than a second counter value (step S6), process control  1705  obtains a fluid level reading from fluid level sensor  1704  (step S1). 
     Process control  1701  may monitor the duration of a fill operation to ensure that fluid supply mechanism  1703  and other components of fluid fill system are operational. For example, process control  1701  may start a timer during a fill operation of reservoir  1701  and monitor the time that elapses during the fill operation to determine whether the fill operation is completed within a predetermined time limit (step S9). If the fill time of reservoir  1701  is less than or equal to the predetermined time limit, process control  1705  maintains the fill operation to supply fluid to reservoir  1701  (step S11), and process control  1705  obtains a fluid level reading from fluid level sensor  1704  at a predetermined interval (step S1). If the fill time exceeds a predetermined time limit, process control  1701  may issue a signal (step S10), e.g., an alarm to an equipment operator. The signal may be visual or audible, or both. The signal may advise the operator that a fluid level in reservoir has not reached a predetermined fill level and that the operator should inspect the fluid fill system, e.g., the fluid supply line, the fluid supply mechanism, the fluid level sensor, or the like, to ensure that the system components are operational. For example, process control  1705  may issue a message “water pan not filling, check water supply.” The signal may continue until process control  1705  obtains a reading from fluid level sensor  1704  indicating that the fluid level in reservoir  1701  is equal to or greater than a predetermined level. In an embodiment of the invention, the timer may be reset when process control  1705  is shut down or restarted. 
     Process control  1705  may communicate with temperature sensor  1707  (step S12) to determine whether the temperature of reservoir  1701  exceeds a threshold temperature. If process control  1705  obtains readings from temperature sensor  1707  indicating that the temperature of reservoir  1701  is equal to or less than a threshold temperature, process control  1705  continues to obtain fluid level readings from fluid level sensor  1704  (step S1). If the temperature of reservoir  1701  is greater than a threshold temperature, process control  1705  may issue a signal (step S13), e.g., an alarm, or the like, to an operator. The signal may be visual or audible, or both. The signal may advise an operator that reservoir  1701  may be empty. For example, process control may issue a message “no water, float switch failed.” Process control  1705  also turns off reservoir heater  1706  (step S14). Process control  1705  may turn off reservoir heater  1706  irrespective of a status of fluid level sensor  1704  and a value of the counter. This procedure reduces or prevents damage to system components in the event that fluid level sensor  1704  becomes inoperative or otherwise fails to indicate that reservoir  1701  is empty. 
     Other embodiments of the invention will be apparent to those skilled in the art from a consideration of this specification or a practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims.