Abstract:
Apparatus and methods are disclosed to measure airflow within a chassis-cooling pathway of an appliance. The rate of airflow is determined based on the differential heating among a pair of sensor devices, such as thermistors, transistors, diodes or resistive thermal devices operating at distinctly different power levels. The appliance utilizes the calculated airflow rate to perform safety-related tasks, such as de-energizing heating elements when low or no airflow is detected.

Description:
BACKGROUND OF THE DISCLOSURE 
     The present disclosure generally relates to appliances, and more particularly, to detecting airflow, and/or measuring airflow velocity, in a cooling pathway of an appliance. 
     Appliances such as cooking ranges are widely used. A cooking range typically includes an oven. The oven typically has a front-opening access door, and at least one heating element for heating up the inside of the oven cavity. As is known in the art, when energized, the heating element can heat up the inside of the oven cavity to a relatively high cooking temperature chosen by a user. Also as is known in the art, the cooking range often has a fan which is used to cool a component of the oven, such as the front-opening access door, or a heat sensitive component of the oven such as an electronic controller or display unit, to a temperature which is lower than the chosen cooking temperature. Some certification institutions, such as Underwriters Laboratories Inc. (UL), require that a Fan Apparency Device (FAD) be employed in the cooking range. The FAD is used to determine or detect whether the fan is working properly, that is, whether or not airflow is being created by the cooling fan. 
     As is known in the art, when a user selects or chooses a heating operation of the oven and turns on the oven, the turning-on supposedly activates the fan. The FAD then determines or detects whether the fan is working properly. If the fan is working properly, the FAD enables the selected heating operation of the oven to proceed. On the other hand, if the fan is not working properly, the FAD prevents the selected heating operation of the oven from proceeding. Various types of FADs are used to determine or detect whether the fan is working properly. 
     The most widely used FADs are thermal switches and sail switches. A thermal switch uses the heat from the oven to heat up a bimetal member of the switch to turn off the switch when the airflow from the fan is no longer present to cool off the bimetal member. Since the thermal switch usually is not disposed adjacent the intake end or the discharge end of the fan, it does not negatively affect the airflow passing through the fan. However, the thermal switch has a relatively slow reaction time. In addition, the thermal switch needs to be installed in an area of the oven where the temperature can raise quickly when the oven is turned on. Moreover, the thermal switch requires a significant amount of calibration and testing to prove that it will work as intended in all working conditions of the oven. 
     A typical sail switch uses the airflow generated by the fan to move a sail, typically a large, flat piece of sheet metal, to turn on or off the switch, typically containing a spring return. Compared with the thermal switch, the sail switch has a very fast reaction time, and requires significantly less testing time/effort to implement. However, the sail switch has its own limitations. In particular, when it is desirable to direct or point the exhaust end or discharge end of the fan toward the floor on which the appliance is placed, it can be difficult to satisfactorily employ the sail switch in this configuration. This is because in this configuration, the positive pressure side of the sail, which is positioned downstream of the fan, has to face up. As a result, the weight of the sail constantly urges the sail to move from its upper position where the sail opens the switch to its lower position where the sail closes the switch. To make matters worse, in this configuration the positive pressure side of the sail tends to collect a noticeable amount of dust particulates, grease, etc. All of these contribute to the problem that the sail may move to and stay in its lower position even when the fan is not activated, thus creating a false indication of the working condition of the fan. Sail switches also tend to be constructed of thin, flimsy metal and are easily damaged or knocked out of calibration during repair operations and abusive shipping disturbances (drops). Sail switches have also been prone to producing rattling/chattering noises, thus disturbing/annoying the operator of the appliance (aka the consumer). 
     A need is present to provide better methods and systems to measure air flow within appliances. 
     SUMMARY OF THE DISCLOSURE 
     In one aspect of the disclosure, an apparatus measures the airflow within the chassis-cooling pathway of an appliance. The apparatus is an air flow detecting device that provides a means by which the presence of airflow is determined and the approximate rate of airflow is measured. Based on the measured information the appliance control system can perform safety-related tasks, such as de-energizing the heating elements of the appliance, if there is too-low (or no) airflow. 
     In another aspect of the disclosure, an appliance comprises a heat source, a chassis and a cooling air passageway for circulating air around at least a part of the chassis is disclosed. An air flow measuring device that is supported and immersed within the cooling air passageway measures a rate of airflow through the cooling air passageway and includes a first device (e.g., a first thermistor or like device) that generates a self-heating effect and a second device (e.g., a second thermistor or like device) that does not self-heat and thus measures the local ambient temperature around both devices. The first device and the second device provide a voltage differential that corresponds to a difference in temperature between the first device and the second device that is inversely proportional to the rate of airflow. The two devices may be substantially identical (i.e., the same part number) but, they do not need to be if their differences are properly considered in the circuit design. The first device and second device are operated in such a manner that the first device experiences significantly more power dissipation than the second device, therefore, the first device self-heats significantly more than the second device. This can be accomplished by Pulse Width Modulating (PWM&#39;ing) the two devices at different duty cycles. For instance, the first device can be operated at 99% while the second device can be operated at 1% (i.e. the same PWM control signal can be used to drive both circuits, with one circuit using an inverted representation of the control signal), thus the first device would dissipate 99× the power of the second device. Alternatively, the first device could be operated continuously (100%) while the second device is activated infrequently (e.g., at 1 ms once every second). In an analog implementation the difference in temperature-dependent voltages or difference in temperatures generated by the two devices is compared against a reference voltage to determine if the airflow rate is above/below a predetermined threshold. This implementation uses an analog means to determine the differences, where the analog means includes at least one of an operational amplifier, instrumentation amplifier, or comparator. In a digital (microprocessor) implementation the two device voltages would be converted to a temperature indication by a variety of means (calculation, look-up table, etc.), and the difference in temperature would be calculated and compared against a predetermined threshold. A digital means may be used to measure the differences in temperature-dependent voltages or differences in temperature, which includes a microprocessor with analog/digital ports to sample the two voltages and calculating the voltage differences mathematically 
     In yet another embodiment, a method is disclosed for measuring air flow within an airflow passageway of an appliance. The method includes immersing a first device and a second device into the airflow of the airflow passageway. These devices could be diodes or bipolar transistors (i.e. devices having a PN junction), and the like. In one example, the devices are cycled through three processing phases. In the first phase the first device is activated in such a manner to produce self-heating; a relatively large amount of current is forced to flow through the first device, however, the second device does not experience this current. The primary purpose of the first phase is to cause a difference in self-heating between the two devices. In the second phase both devices are activated in such a manner that a very small amount of current is flowed through them, so that a baseline voltage measurement can be measured. In the third phase both devices are activated in such a manner that a larger amount of current (e.g., about 5× to 50× that of second phase, but significantly less than that of the first phase) is flowed through them, so that a second voltage measurement can be measured. The voltage difference between the phase 3 measurement and the phase 2 measurement indicates the temperature of the device, according to the following relationship typically used to measure the temperature of computer CPU cores, and upon which several commercially-available ICs (Integrated Circuits) have been developed/sold: 
               Δ   ⁢           ⁢     V   BE       =         k   ·   T     q     ·     ln   ⁡     (       I     C   ⁢           ⁢   1         I     C   ⁢           ⁢   2         )               
which can be rewritten as:
 
             T   =         q   ·   Δ     ⁢           ⁢     V   BE         k   ·     ln   ⁡     (       I     C   ⁢           ⁢   1         I     C   ⁢           ⁢   2         )                 
Where:
         T=Temperature of the measurement device (diode, bipolar transistor, etc.)   k=Boltzmann&#39;s Constant (1.38*10 −23 *J/K)   q=Electronic Charge (1.69*10 −19 ° C.) [note: C=Coulomb&#39;s, not ° C.]   ΔV BE =Change in Base-Emitter (or Anode-Cathode) voltage at 2 currents   I C1 =Larger of 2 measurement currents   I c2 =Smaller of 2 measurement currents
 
Thus, knowing the ratio of the phase 3 to phase 2 currents, the difference voltage can be used to determine/calculate the device temperature. This device temperature measurement is performed for both first and second devices, and the temperature difference between the two devices is used to determine the airflow rate of the system. Phase 1 is required to cause the first device to self-heat in a consistent manner so that its temperature rise above ambient (second device) is consistently inversely proportional to airflow rate. Phase 1 is typically much longer in duration than phases 2 and 3. During phase 1 the second device experiences substantially no current, however, if the phase 2 or 3 currents are sufficiently smaller than the phase 1 current, the second device can operate at either of those levels during phase 1. An amount of airflow is determined with respect to time within the passageway based upon the voltage differential measured, wherein an increase in the voltage differential indicates a lower airflow over the time measured and a decrease in the voltage differential indicates a higher airflow over the time measured.
       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference is now made briefly to the accompanying drawings, in which: 
         FIG. 1  is a side view of a cooking appliance such as a dual oven in which is implemented an exemplary embodiment of an airflow detection device; 
         FIGS. 2   a  and  2   b  are top views of an electronics bay of the oven of  FIG. 1 ; 
         FIG. 3  is a schematic diagram of an exemplary aspect of the present disclosure; 
         FIG. 4  is a block diagram of another exemplary aspect of the present disclosure; 
         FIG. 5  is a block diagram of another exemplary aspect of the present disclosure; 
         FIG. 6  is a schematic diagram of another exemplary aspect of the present disclosure; 
         FIG. 7  is a flow diagram for detecting an airflow rate in an airflow passageway of an appliance according to an exemplary aspect of the present disclosure. 
     
    
    
     Like reference characters designate identical or corresponding components and units throughout the several views, which are not to scale unless otherwise indicated. 
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , illustrated is an exemplary appliance according to at least one aspect of the present disclosure, such as a double wall oven  10 . The double wall oven  10  includes an outer housing  12  defining an interior space in which food or other items to be heated are placed, and upper and lower doors  14  and  16  for providing access to said interior space inside of which one or more heating elements are located. An electronics bay  18  is located on an upper side  24  of the oven  10  and contains various electronic controls for operation of the oven  10 . Although an oven appliance  10  is illustrated, the present disclosure is not limited to any one type of appliance. Accordingly, basic features are described in the oven  10  briefly as an exemplary aspect of some embodiments herein. 
     For example, the oven  10  includes one or more cooling air-flow passageways  22  for circulating air around the oven chassis and the electronics bay  18 . The passageway  22  defines a flow path F for the flow of air from an inlet I near a lower side  26 , up the front and through the electronics bay  18  where one or more blowers  30  ventilate air there-through. The air travels further down the rear of the oven  10  and forward to an outlet O. 
     The electronics bay  18  supports one or more of the blowers  30 , which are activated during operation of the oven  10  to circulate air through the flow passageway  22  for cooling the oven chassis and/or electronics bay  18 . The blowers  30  are configured to draw air across the electronics of the bay  18  and then exhaust the air via outlet O. The electronics bay  18  further includes an air flow measuring device  32  for detecting an air flow and a rate of the air flow in the flow passageway  22 . Although the air flow measuring device  32  is illustrated within the electronics bay  18 , other locations within the air flow and along the flow passageway  22  are also envisioned. For example, the air flow detection device  32  can reside at the rear of the oven  10  or anywhere else along the passageway  22 . 
     In one embodiment, the air flow detection device  32  measures airflow through the passageway  22  and provides data related to the air flow to a main controller board residing within the electronics bay  18 . In turn, the main controller board is operable to control a heating element  28  of the upper side  24  of the oven  10  and a heating element  29  of the lower side  26  of the oven  10 . For example, if no air flow or substantially low air flow is detected, main controller board could control the heating elements  28  and  29  to provide less heat into the cavity, including complete deactivation of the heating elements. Other appliances having ventilation passageways and/or airflow passageways are also envisioned as within the scope of this disclosure. It is also conceivable and practicable for a double oven to contain two cooling fans  30  (one for upper, one for lower) and thus two airflow detection devices  32 . 
     Referring to  FIGS. 2   a  and  2   b , illustrated are exemplary aspects of an airflow detector for an appliance  100 . The appliance  100  includes any, appliance having an airflow passageway, such as for ovens, microwave ovens, clothes dryers, and the like. 
     The appliance  100  includes a heat element  128  for heating items within the appliance and at least one blower  130  for drawing air through a flow passageway  122 . An airflow detector  132  detects an air flow  124  through the passageway  122  and the rate in which air is moving. The air flow detector  132  includes at least two sensor devices, a first device  140  and a second device  142  located within the airflow  124  for detecting a temperature differential, which is then used for determining the air flow and rate of air flow. For example, the devices  140  and  142  are immersed within the air flow  124  of the flow passageway  122 . The control circuit  144  can include one or more processors adapted to calculate airflow velocity and/or volumetric flow rate based on the sensed air flow at the air flow detector  132  and/or an analog comparator circuit for performing the same functions. In the illustrated embodiments ( FIGS. 2   a  and  2   b ), a main PCB  146  is connected to the control circuit  144  and is configured to deactivate the heating element  128  when insufficient air flow is detected. Different configurations can be envisioned in which the control circuit  144  located between the main PCB  146  and heating element  128  to directly disable the heating element as in  FIG. 2   a , or in which the control circuit  144  supplies a signal to the main PCB  146  so that the main PCB controls the heating element based on the state of this signal as in  FIG. 2   b . This disclosure is not limited to any one example. Other switching mechanisms can also be associated with the circuit board  144  for switching on or off various components of the oven based on detected airflow or the lack thereof. In some embodiments control circuit  144  is a stand-alone board/module electrically connected to a stand-alone sensor module  132  and connected to the main controller  146 . In some embodiments the functionality of control circuit  144  is co-located within the main controller PCB  146 . In some embodiments the control circuit  144  is co-located inside the sensor module  132 . In some embodiments it is conceivable to co-locate all components ( 132 ,  144 ) of the airflow detection/measurement system within the main controller PCB  146 . It is also conceivable to implement an embodiment, such as shown in  FIG. 2   a , in which the control circuit  144  (either separate from sensor module  132 , or co-located with it) is interspersed in the path between the main controller  146  and the heat element  128 , such that the airflow detection system ( 132 + 144 ) is in final control of the heat element, thus over-riding the commands from the main controller  146 . 
     In one embodiment, the first device  140  and the second device  142  include thermistors having a variable electrical resistance according to a surrounding temperature. Although the first device and second device include thermistors in one exemplary embodiment, other solid state devices are also envisioned, such as diodes, transistors and resistance temperature detectors (RTDs). Power is applied to the first thermistor  140 , which causes it to self-heat; the extent to which the thermistor self-heats (i.e. the amount of temperature rise which the thermistor experiences) is inversely proportional to the rate of airflow past the thermistor. In certain embodiments, two thermistors are employed as the first device  140  and the second device  142 . The second thermistor  142  dissipates little power, and thus, does not appreciably self-heat, and is used to measure the “ambient” air temperature. The first thermistor  140  dissipates greater power compared to the second thermistor  142 , and thus, it does appreciably self-heat, the extent of which is measured or detected by circuitry at the airflow detector  132 . The two thermistors  140  and  142  are biased so that if no self-heating is present, thermistor circuits produce the substantially same voltage (corresponding to substantially equal temperatures). When the first thermistor  140  self-heats, due to low/no airflow, the imbalance (difference) in outputted voltage directly indicates the rate of airflow past the thermistors. This difference voltage is used to measure the airflow rate of the airstream in which the thermistors are immersed. 
     In one aspect of the embodiment, the two thermistors are chosen so that the impedance of the self-heating thermistor or the first device  140  is much lower than the impedance of the second device  142  or non-self-heating thermistor acting as a reference. For example, impedance difference is a factor of 5× or greater. This impedance imbalance causes the first device that operates as a sensing thermistor to dissipate considerably more power than the second device  142  that operates as a reference thermistor. As a result, in still air, the first device  140  becomes noticeable hotter than the second device due to its self-heating. In another example, the impedance disparity between thermistor  140  and  142  was 10:1, and thus, 10× more power is dissipated in the first  140  than the second thermistor  142 . With still air (or very low airflow) the first device  140  heats-up considerably more than the second, and therefore, the voltage measured across the thermistors differs considerably. When there is high airflow the first thermistor  140  is cooled by convective means, and the voltage produced more nearly matches that of the second device  142  operating as a reference thermistor. 
     In another embodiment, the two thermistors are identical models, having the same impedance. The disparity in dissipated power is achieved by “pulsing” (pulse width modulating or PWM&#39;ing) the Reference thermistor at a very small duty cycle (say 10% or less). Thus, although the Reference thermistor momentarily dissipates the same power as the Sensing thermistor, its overall (average) power dissipation is much less than the Measure device, and therefore it does not heat-up significantly. The voltage of both thermistors is sampled at a point in time when both thermistors are active, and the voltage difference is once again used to determine the rate of airflow around both thermistors. 
     Referring now to  FIG. 3 , illustrated are exemplary aspects of a pair of thermistor circuits for the first and second device  140  and  142  of  FIG. 2 , which sense airflow rates in an airflow passageway. A first thermistor circuit  300  operates as a Measure device and is activated by an enable/disable switch circuit having a first transistor Q 1  coupled to a resistor R 5  and controlled by a microprocessor (or other means). The enable/disable switch controls the amount of power dissipated by the Measure device, thermistor TH 1 , according to a signal CTRL_M. A “Hi” state at CTRL_M enables the switch transistor Q 1 , and thus forces Measure device TH 1  to dissipate power; it is during this state that output voltage VT_M is sampled by the microprocessor (or other circuitry). CTRL_M is configured to represent a large duty cycle (i.e. is mostly “On” with very little “Off” time), so that the power dissipated by TH 1  is relatively large. A second thermistor circuit  301  operates as a Reference device and is activated by a second enable/disable switch having a second transistor Q 2  coupled to a resistor R 10  and controlled by a microprocessor (or other means). The enable/disable switch of circuit  301  is activated briefly when the control circuit  144  of  FIG. 2  provides a signal CTRL_R to measure ambient air temperature. A “Hi” state at CTRL_R enables the switch transistor Q 2 , and thus activates Reference device TH 2  so that the output voltage VT_R represents the temperature of the Reference device, which is sample by the microprocessor (or other circuitry). 
     The circuits  300  and  301  include a thermistor TH 1  and TH 2  respectively, in which a resistance therein varies according to changes in temperature. While thermistors are discussed herein, the disclosure is not limited to thermistors and also envisions using diodes, transistors and RTDs as also being within the scope of this disclosure for measuring a temperature differential therebetween. The thermistors TH 1  and TH 2  are connected in series respectively to resistors R 1  and R 6 , which have fixed resistances, and to a voltage Vcc thereat. Resistors R 2  and R 7  are configured in parallel to the thermistors of each circuit and resistors R 1  and R 6 , wherein a linearization network is formed with resistors R 3  and R 8  of each circuit  300  and  301 . The resistors R 1 , R 2 , and R 3 , surrounding the first device TH 1  (and similarly R 6 , R 7 , and R 8 , surrounding the second device TH 2 ) are provided to linearize the voltage with respect to temperature, and any series/parallel combination of one or more resistors can be used to perform this linearization function. Other linearization schemes are also envisioned, such as single or two resistor schemes. Resistors R 4  and R 9  form a low pass filter with capacitors C 1  and C 2  respectively to provide the signals Vt_M and Vt_R which indicate their respective thermistor&#39;s temperature, and which are sampled by a microprocessor (or other circuitry). 
     Transistors Q 1  and Q 2  are illustrated as MOSFET devices, but are not limited to any particular solid state device and may include BJT, FET devices, and the like; they could also be relays. The transistors Q 1  and Q 2  are each coupled to resistors R 3  and R 8  at a respective terminal and to resistors R 5  and R 10  at another terminal respectively. The terminals of the transistors Q 1  and Q 2  provide for control signals Ctrl_M and Ctrl_R for controlling the measurement of the voltage differential therebetween. 
     Referring to  FIGS. 4 and 5 , illustrates digital and analog implementations of the present disclosure respectively. 
     In one embodiment,  FIGS. 4 and 5  illustrated a single control port where circuits  300  and  301  are controlled. For example, control to a switch for measure circuit  300  has a high duty cycle (Hi % DC) to dissipate considerable power and self heat. Reference circuit is operate with a low duty cycle (Lo % DC) to maintain low power dissipation in the thermistor of the circuit. A single control port is used to operate both circuits where CTRL_M (from  FIG. 3 ), for example, is the inverse of CTRL_R as indicated by the inverter (INV). Therefore, a single very low duty % DC when inverted creates a very high % DC signal and only one clocking signal is implemented. Different signals may further be envisioned in other implementations and the present disclosure is not limited to only one clocking signal. 
       FIG. 4  illustrates a digital implementation where the outputs of each device  300  and  301  are provided to an analog/digital converter (A/D) for conversion to a digital representation and those digital representations are subsequently subtracted to obtain a voltage difference. The differential corresponds to a difference in temperature between the measure device and the reference device that is inversely proportional to the rate of airflow. The measurements are calculated at a periodic rate (e.g., once per second or the like) and the temperature difference is used by calculation formula or a look-up table in order to determine the amount of airflow in the system. The look-up table and formula (both not shown) may be provided in a memory of the control circuit  144 , for example, and used for determining an airflow rate of an appliance. A threshold or predetermined level may be compared to the airflow rate to determine whether it is sufficient or not according to desired levels. Because substantially identical thermistors are used, the microprocessor could alternately use each thermistor as the “Reference” (low power dissipation) and the “Measure Sensor” (high power dissipation), thus balancing-out (sharing) the aging effects on the thermistors, making sure that both devices&#39; characteristics shift identically throughout the life of the product. 
     The measure device  300  and the reference device  301  provide a voltage differential. The measure device  300  and reference device  301  are operated in such a manner that the measure device  300  experiences significantly more power dissipation than the reference device  301 , therefore, the measure device  300  self-heats significantly more than the reference device  301 . This is accomplished by Pulse Width Modulating (PWM&#39;ing) the two devices at different duty cycles. For instance, the measure device can be operated at 99% while the reference device can be operated at 1% (i.e. the same PWM control signal can be used to drive both circuits, with one circuit using an inverted representation of the control signal), thus the measure device would dissipate 99× the power of the reference device. Alternatively, the measure device could be operated continuously (100%) while the reference device is activated infrequently (e.g., at 1 ms once every second).  FIG. 4  illustrates a single processor timer port to control the switches, however, addition timer ports could also be used, and an external clock could also be used in the clocking signal provided into the microprocessor to synchronize the A/D converter readings of the thermistor signals from the measure device  300  and the reference device  301 . 
       FIG. 5  illustrates an analog implementation where the difference in voltages generated by the two devices is compared against a reference voltage to determine if the airflow rate is above/below a predetermined threshold. An oscillator coupled to a divide-by-N-pulse generator provides the duty cycle signal to the circuits  300  and  301 , where only one signal is used here for example to generate very high and very low % Duty Cycle control signals to the respective circuits. 
     Referring now to  FIG. 6 , illustrated is another exemplary aspect of a dual transistor (or diode) circuit  400  for measuring a temperature differential within an airflow for an airflow detector in an appliance. The circuit  400  includes diodes D 1  and D 2  that operate as Reference and Measure devices, respectively. Alternatively, other devices can be used, such as diode-connected transistors (where collector and base terminals are connected). A transistor Q 4  is coupled at a terminal to an operational amplifier OA 1  via a resistor in order to form a dual-level constant current source (DLCCS) with a voltage reference signal CCS_VREF provided at a positive terminal of the amplifier OA 1 . The DLCCS is provided through the transistor Q 4  from a different terminal to supply the diodes D 1  and D 2  (or transistors Q 1  and Q 2 ). A signal Ctrl_M is provided to transistor Q 3  in order to control the DLCCS in order to generate two distinct current levels at a 10:1 ratio, for example, or any fixed ratio. The current is then passed through both diodes D 1  and D 2  (or transistors Q 1  and Q 2 ) in series so they receive substantially the same amount of current thereat. The ratio of 10:1 or other like ratios enables the control circuit  144  of  FIG. 2  to calculate the temperature of the two transistor devices as a first device  140  and a second device  142  discussed above. The changes in voltage across the diodes&#39; anode and cathode (or transistors&#39; base and emitter) terminals allows for this calculation of temperature from the measurements of Vsen_M and Vsen_R. During most times Ctrl_RM commands a low current so that the Reference device D 1 /Q 1  does not significantly self-heat and is essentially off. A Ctrl_M signal commands a second voltage reference at the switchable constant current source (SCCS) to force more current into the measure transistor device D 2  (or Q 2 ) so that it heats up accordingly. Current from the SCCS is injected into the circuit at a point where it only passes-through the Measure device D 2 /Q 2 . As was discussed earlier, there are 3 phases to controlling these current sources. Phase 1 (heat-up of the Measure device) controls the SCCS via signal Ctrl_M. Phases 2 and 3 (measure the temperature of both devices) controls the DLCCS via signal Ctrl_RM. 
     Example methodology  700  of a flow diagram for detecting an airflow rate in an airflow passageway of an appliance is illustrated in  FIG. 7  and is described with reference to  FIGS. 1-6 . While the method is illustrated and described below as a series of acts or events, it will be appreciated that the illustrated ordering of such acts or events are not to be interpreted in a limiting sense. For example, some acts may occur in different orders and/or concurrently with other acts or events apart from those illustrated and/or described herein. In addition, not all illustrated acts may be required to implement one or more aspects or embodiments of the description herein. Further, one or more of the acts depicted herein may be carried out in one or more separate acts and/or phases. 
     At  702 , the method  700  begins and is referenced above upon immersing a first device  140  and a second device  142  into an airflow passageway of an appliance  100 , for example. The devices can include thermistors, diodes, transistors and/or RTDs for measuring a differential between the two devices, which are substantially thermally insulated from one another. The impedance within each solid state device for measure airflow rate is the same in certain embodiments, but can vary in other embodiments. 
     At  704 , power is provided to each device. A constant current source is maintained to power the devices. When a measurement is made by a processor, for example, a signal is provided, such as a pulse width modulated signal to power one of the devices with additional current and causes the device to increase in temperature. Alternatively, where impedances within the solid state devices vary at a given ratio, no additional current source signal is provided. 
     At  706 , a first voltage differential is measured over a period of time between the first device and the second device. At  708 , this differential is utilized to determine an amount of airflow over time. For example, an increase in the voltage differential indicates a lower airflow over the time measured and a decrease in the voltage differential indicates a higher airflow over the time measured. 
     The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations.