Note: Descriptions are shown in the official language in which they were submitted.
~24538 DRUG 16129
BACKGROUND OF THE INVENTION
The present invention relates generally to improvements in a
microwave oven and more particularly to an arrangement for anticipating
the occurrence of combustion in the cooking cavity of a microwave oven.
When heating a food load in a microwave oven, if the food load
is allowed to heat substantially longer than required for proper cooking,
the food itself or the container for the food may become heated to the
combustion point, resulting in a fire in the oven cavity. The overcook-
in of whole potatoes, fats raised to the combustion temperature due to
excessive overcooking ox Yeats, and oven shelf hot spots due to impel-
fictions in the pyroceramic shelf when heated over a prolonged period,
are three primary sources of such cavity fires. The potato is particularly
vulnerable to fire since it dries out non-linearly with the core of the
potato getting hot before the outer portions. The heat from the core is
not readily dissipated. As a result the core of the potato can be heated
to its ignition point relatively quickly.
One commonly used approach to the cavity fire detection problem
is to provide a thermal cutout switch on the oven cavity ceiling which
cuts off the-magnetron when the oven cavity ceiling temperature exceeds
a predetermined level. This arrangement works satisfactorily but is
relatively costly and has a relatively slow response time. An alternative
approach is disclosed in US. Patent 4,133,995 to Buck. In the Buck
arrangement a humidity sensor and a temperature sensor are employed to
monitor the relative humidity and temperature in the cavity. A MicroPro
censor is programmed to detect a fire condition when both the temperature
rises and the humidity falls by predetermined amounts within a prescribed
time period, on the order of 10-30 seconds, and to de-energize the oven
upon detecting such a condition.
The present invention improves over both of the foregoing
approaches to fire detection in the microwave oven by providing an
S31!3
arrangement which anticipates fire conditions developing in the cooking
cavity before actual ignition employing a single sensor normally used
for cooking control which is responsive to the concentration of gases in
the circulating air exiting the cooking cavity. In addition to providing
an anticipatory warning of an imminent fire condition, this arrangement
eliminates the need for the relatively costly thermal cutout switch and
also eliminates the need for the "in situ" temperature sensor of the
Buck method.
US To I
lo The present invention provides a method and apparatus for
anticipating the occurrence of a fire in the cooking cavity of a microwave
oven. The oven is provided with means for continuously circulating
outside air through the cooking cavity to carry away gases emitted by
food loads being heated therein. A sensor responsive to the concentra-
lion of gases in the circulating air monitors the gas concentration
level and generates an output signal representative thereof. Sensor
monitoring means responsive to the sensor output signal is operative to
determine the rate of change of the gas concentration level, and detect
a characteristic rate of change indicative of the load being heated
closely approaching its combustion point. Upon detection of the kirk-
touristic rate of change, means responsive to the sensor monitoring means
de-energiz~s the oven and provides an indication to the user that a
combustion condition in the oven cavity is imminent.
In accordance with one form of the invention, the sensor
monitoring means detects the characteristic rate of change by first
detecting a relatively slow rate of change indicative of the food load
drying out and thereafter detecting a relatively rapid rate of change
indicative of the load being heated to a temperature closely approaching
its combustion temperature.
-~Z453~ -
Brief Description of the Draws
. While the novel features of the invention are set forth with
particularity in the appended claims, the invention both as to organization
and content will be better understood and appreciated from the following
detailed description taken in conjunction with the drawings, in which:
FIG. 1 is a perspective view of a microwave oven embodying the
prevent invention;
FUGUE is a schematic sectional view of the oven of FIG. 1
taken along lines 2-2;
FIG. 3 depicts a curve representative of the gas emission
characteristics of a baked potato heated in the microwave oven ox FIG. l;
FIG. 4 is simplified schematic circuit diagram of that
portion of the microprocessor-based control system of the oven of FIG. 1
illustratively embodying the present invention; and
FIG. 5 is a flow diagram illustratively embodying a fire
detection control algorithm implemented by the microprocessor in the
circuit of FIG. 4 in accordance with the present invention.
DETAILED DESCRIPTION
Referring now to FIGS. 1 and 2, there is shown a microwave
oven designated generally 10. The outer cabinet comprises six cabinet
walls including upper and lower walls 12 and 14, a rear wall snot shown),
two side walls 18 and 20, and a front wall partly formed by a hingedly
supported door 22 and partly by control panel 23. The space inside the
outer cabinet is divided generally into a cooking cavity 24 and a controls
compartment 26. Cooking cavity 24 is formed by a top wall 28, a bottom
wall 30, side walls 32 and 34, the rear cavity wall being a cabinet wall
and the front cavity wall being defined by the inner face 36 of door 22.
A shelf 38 of pyroceramic material or other suitable material previous
to microwave energy is provided in the lower region of cavity 24 to
support food loads to be heated in cavity 24.
~4S38
A magnetron 40 adapted to produce microwave energy is mounted
in controls compartment 26. Means for circulating air through the
cavity 24 includes a blower designated generally 42 which draws cooling
air into the controls compartment 26 through louvered openings (not
shown) formed in the buck cabinet wall. This air passes over cooling
fins 44 of magnetron 40. A portion of the air drawn in by blower 42
enters cavity 24 via perforations 46 formed in cavity side wall 32.
This air circulates past the food load (not shown) owing heated in
cavity 24 and exits cavity 24 through perforations 48 formed in upper
cavity wall 28 near the upper left corner of cavity 24. Vent holes 50
formed in side cabinet wall 20 near the upper left corner of the oven
cabinet permit air exiting cavity 24 via perforations 48 to exit the
oven cabinet to the exterior.
Foods heated in the microwave oven tend to emit water vapor
and other gases as the food heats. Such gases mix with the air circulating
in the cavity and are removed from the cavity by the circulating air.
Means for sensing the concentration level of such gases in the air
leaving the cavity is provided in the form of a gas sensor 52 disposed
in the air flow path between perforations 48 in cavity wall 28 and vent
openings 50 in cabinet wall 20. Sensor 52 is supported proximate perform-
lions 48 by mounting bracket 54 which is suitably secured to cavity wall
28. Sensor 52 in the illustrative embodiment of FIG. 2 is a gas sensor
readily commercially available from Figaro Engineering, Inc., identifiable
as Model TUGS No 186. This sensor is responsive to the cumulative
concentration of water vapor and various organic gases. Use of such a
sensor in a microwave oven for automatic cooking control is known in the
art. One example of such a control arrangement is described in US.
Patent 4,311,895.
In accordance with the present invention, information regarding
the concentration level of gases in the circulating air exiting cavity
2~9L~3~3
24 provided by sensor 52 is used to anticipate the occurrence of combustion
conditions in the cavity.
It has been empirically determined that for foods likely to
ignite in the oven cavity when overcooked, the concentration of gases
emitted by the food load into the air circulating through the cavity
changes during heating in a predictable fashion. At the beginning of
the cooking cycle, the food load is normally not sufficiently heated to
cause significant gaseous emissions from the food. Consequently, the
sensed gas concentration level remains relatively low and constant. As
the food continues to heat, it reaches sufficient temperature to begin
emitting water vapor and organic gases. There ensues a period of relatively
rapidly increasing gas concentration level which continues until the
food load is heated to a point of near maximum gaseous emissions. The
recommended cooking time for most foods is such that the cooking cycle is
normally terminated prior to reaching this point. However, if the food
continues to be heated beyond this point such as may inadvertently occur
due to the miscalculation of appropriate cooking time or simply forgetful-
news, the temperature of the food continues to rise but the gas concentr lion
level increases relatively slowly during what can be a relatively long
20 period, hereinafter referred to as the "drying out" period. Eventually,
the temperature of the food closely approaches its combustion temperature.
However, before actually reaching the ignition point, the food begins to
char, smoke or smolder, resulting in a second period of relatively
rapidly increasing gas concentration in the circulating air, comparable
25 to that rate of increase which characterizes the normal cooking period.
This second period of rapidly increasing concentration level, referred
to hereinafter as the "pre-combustion" period, typically begins a matter
of minutes before actual combustion in the cavity Thus, detection of
the transition from the drying out phase to the pre-co~bustion phase
provides an advance yarning that a combustion condition in the cavity is
4S~8
imminent. Hence, according to the present invention, a fire condition
in the Cavity of a microwave oven may be anticipated by monitoring the
gas concentration level of the air circulated through the cavity during
the cooking cycle, periodically determining the rate of change of the
sensed concentration level during heating, and detecting a rate of
change characteristic of the food load closely approaching its combustion
t~perature.
The sensor resistance versus time curve of FIG. 3 shows the
change in resistance of the sensing element of sensor 52 during the
I cooking cycle for a whole potato heated in cooking cavity Z4 of oven 10
in response to the changing gas concentration level in the air exiting
the cooking cavity as the potato is heated to its combustion point. The
potato was chosen for purposes of illustration because, as described
briefly in the Aground discussion, it is one of the foods most easily
overcooked to the point of combustion. Also, the basic shape of the
response curve is fairly representative of all food loads of the type
conducive to ignition when overcooked.
The resistance Us of the TUGS 186 sensor varies with gas con-
cent ration level in the air to which it is exposed in accordance with
the lo nmul a
Us = A x C where and Ox are constants and C is the gas concentration. Hence, the changing
resistance as the potato cooks reflects the aforementioned predictable
variation in concentration level.
I Referring particularly to FIG. 3, as the cooking cycle begins,
the resistance is relatively high and constant indicative of the relatively
low and essentially constant concentration level at this point. There
next ensues a period in which the resistance decreases relatively rapidly
indicative of the concentration level increase, characteristic of the
normal cooking phase for the potato. The change from a relatively
rapidly decreasing resistance to a relatively slowly decreasing resistance
--6--
I
is indicative of the change from a relatively rapidly increasing concern-
traction level to d period of relatively Wylie increasing concentration
level which marks the transition from the normal cooking phase to the
drying out phase for the potato. Continued heating of the potato continues
to rise its temperature but the resistance continues to decrease at a
relatively slow rate indicating that during the relatively long drying
out phase for the potato, which is roughly twice the duration of the
normal cooking period, the gas concentration level is increasing at a
relatively slow rate. As the temperature of the potato sufficiently
closely approaches its combustion point, the resistance begins to decrease
at d relatively rapid rate indicative of a relatively rapidly increasing
gas concentration level. The transition from relatively slow to relatively
rapid increase in concentration level marks the transition from the
drying out phase to the pre-combustion phase. For a potato cooked in
oven 10, this transition typically occurs at least 2-3 minutes prior to
reaching the combustion temperature of the potato. Thus, by detecting
the rate of change of the gas concentration level characteristic of the
pre-combustion phase early in this phase, it is possible to anticipate a
combustion condition in the cavity at least 2 to 3 minutes in advance.
20 By turning off the power to the magnetron and blower at this point, fire
in the cavity may be prevented.
Referring now to the simplified schematic circuit diagram of
FIG. 4, gas sensor 52 is incorporated in a microprocessor based control
arrangement which illustratively embodies the apparatus and performs the
25 method of the present invention. The circuit of FIG. 4 includes a gas
sensing circuit 60 for sensing the concentration level of gases in the
circulating air as it exits cooking cavity 24, and generating a voltage
signal having a magnitude representative of the sensed gas concentration
level; a sensor heater circuit 62 for maintaining the sensing element of
gas tensor 52 do its proper operating temperature; d microprocessor 54
--7--
453l3
for monitoring the voltage signal from sensing circuit 60; a power
circuit 65 for oven 10 including a magnetron power circuit 66, an air
circulating blower motor 67 and a power control relay 68 for controlling
energization of magnetron power circuit 66 and air circulating blower 67
in response to a control signal from microprocessor 64; and an alarm
circuit 69 responsive to a control signal from microprocessor 64 for
providing an audible signal to the user of an imminent fire condition in
the cooking cavity. Power for the circuit ox FIG. 4 is provided by a
conventional DC power supply circuit 70 which converts a standard 120
volt, 60 Ho AC signal input on lines Lo and N to do output power signals
of -9 volts, -9 volts and -16 volts on lines 72, 74, and 76, respectively.
Gas sensor circuit 60 includes a gas sensing element By. 'foment
80 is the gas sensing element of sensor 52 (FIG. 2)9 which is exposed to
the circulating air exiting cooking cavity 24. The resistance Us of
sensor element 80 varies with gas concentration level in the circulating
air exiting cavity 24 (FIG. 2) as herein before described with reference
to FIG. 3. Sensing element 80 is connected between the output terminal
82 and the inverting input terminal 84 of conventional operational Apple-
lien 860 Capacitor 88 connected in parallel with sensor element 80 serves
as a high pass filter for noise reduction. Input resistor 90 connects
inverting input terminal 84 of amplifier 86 to DC supply line 74. Refer-
once voltage divider 92 comprising resistors go and 96, serially connected
between do input line 74 and said system ground, provides the reference
input signal for amplifier 86 at terminal 98. This signal is coupled to
amplifier 80 by reference input resistor 100 connected between divider
terminal 98 and the non-inverting input terminal 102 of amplifier 86.
The output voltage Ye appearing at amplifier output terminal
82 is proportional to the ratio of the resistance of sensor element 80
to the resistance of input resistor 90. Hence, voltage VOW varies linearly
with the resistance of sensor element 80. Output terminal 82 of amplifier
86 75 connected to input port Jl/Al of microprocessor 64 to couple the
ill 4~3~3
sensor output voltage Ye to the microprocessor. As shown in FIG. 3, the
approximate dynamic range for sensor element resistance 25 of the thus-
trative embodiment during the cooking cycle is from 75K ohms at the
beginning of the cooking cycle to less than OK ohms at the combustion
point. The magnitude of input resistor 90 and the magnitude of reference
voltage from divider 92 are selected to scale and position the output
voltage YOU to fall within the desired input range for the internal
analog to digital convertor of microprocessor 64 over the dynamic range
of sensor element 80.
TUGS 186 gas sensor 52 includes heater element 104 as an integral
component of the sensor package to maintain sensing element 80 at an
appropriate temperature for satisfactory operation. For stable sensor
operation, the voltage applied to sensor heater element 104 should be
closely regulated. To this end, heater circuit 62 includes, in addition
to sensor heater element 104, a voltage regulator integrated circuit
106~ Hater element 104 is serially connected between the output of
voltage regulator 106 and system ground. Voltage regulator 106 regulates
the -volt do signal from line 72 of power supply 70 to provide a regulated
-5 volt signal to heater 104. Regulator circuit 106 in the circuit is d
standard 7905 integrated circuit readily commercially available from
National Semiconductor. Bypass resistor 108 connected in parallel with
integrated circuit 106 and capacitor 10 in parallel with heater 98 reduce
power dissipation in voltage regulator 106.
Sensor monitoring means responsive to the voltage signal VOW
generated by the sensor circuit 60 at ten~inal 82 is provided in the
circuit of FIG. 4 by microprocessor 64 which is appropriately programmed
to provide means for detecting a magnitude of VOW indicative of a gas
concentration level greater than a reference level, means for determining
the rate of change of the magnitude of V0, and means for first detecting
a relatively small rate of change of magnitude indicative of the drying
out phase for the food being heated following detection of concentration
I
~4~;38
level greater than said reference level and thereafter detecting a
relatively large rate of change of magnitude indicative of the transition
to the pre-combustion phase. Microprocessor 64 is further programmed to
generate a signal at output ports R11 and R12 upon detection of the
large rate of change. The signal at R11 is operative to energize alarm
oscillator circuit 69. Oscillator circuit 69 may comprise any one of d
number of conventional multi vibrator circuits known in the art to drive
load speaker 110 at a frequency discernible by the user to indicate that
a fire condition in cavity is imminent. The signal at R12 is operative
to open power control relay 68, de-energizing power circuit 65, thereby
de-energizing magnetron 40 and blower motor 67.
Microprocessor 64 is a standard TAMS 2670 series OK microprocessor
of the type readily commercially available from Texas Instruments. The
ROM of microprocessor 64 has been customized to perform the desired
control functions for microwave oven 10.
Power circuit 65 is connected between power input lines Lo and
N to provide power to magnetron 40 and blower motor 6g of blower 42
(FIG. 2). The power circuit 66 for magnetron 40 includes power transformer
112 having a high voltage secondary 114 connected to energize magnetron
40 through a half-wave voltage doubler comprising series capacitor 116
and rectifying diode 118 connected across the magnetron anode and cathode
terminals 120 and 122, respectively, and oppositely poled with respect
thereto. Secondary winding portion 124 of transformer 112 is connected
as a filament winding to heat the cathode of magnetron 40. Power line
Lo is coupled to terminal 126 of primary winding 128 of transformer 112
through a fuse 130 and main power relay switch 68. Main power relay 68
is controlled by a signal from microprocessor 64 coupled to the relay
coil 134 of relay 68 from output port R12 via conventional relay driver
circuit 136. The other terminal 138 of primary winding 128 it coupled
to power line # via trial 140 having a gate terminal 142. Trial 140 is
operated in a duty cycle control mode by suitable control signals from
- 1 0-
53~3
output port R13 of microprocessor 64 which are coupled to trial gate
terminal 142 via conventional opto-coupler circuit 143.
nergization of fan motor 67 is controlled by trial 144
serially connected to motor 67 across power lines Lo and N. Trial 144
is controlled by signals from output port R10 of microprocessor 64 con-
netted to trial gate terminal 146 of trial 144 via conventional opt-
coupler 145.
The following component values have been found suitable for
use in the circuit of FIG. 4. These values are exemplary only and are not
10 intended to limit the scope of the claimed invention.
FIXED RESISTORS
90107K ohms
94732 ohms
g6$11 ohms
10022K ohms
10856 ohms
CAPACITORS
88.01 of
10810 of
116.91 of
Operation of the circuit of FIG. 4 will now be described with
reference to the flow diagram of FIG. 5. This flow diagram illustrates
an algorithm which is implemented in the ROM of microprocessor 64. From
I this diagram, one of ordinary skill in the programming art can readily
. prepare a set of instructions for permanent storage in the ROM of micro-
processor 64. It is of course to be understood that other portions of
the microprocessor ROM may be utilized to implement additional oven
control algorithms. Since the details of such additional algorithms add
-11-
4538
nothing to the understanding of the present invention such details have
been omitted for brevity and simplicity.
The method of anticipating a fire condition in the cooking
cavity implemented by the algorithm illustrated in the flow diagram of
: 5 FIG. 5 includes the following steps. First, in order to distinguish the
initial period of relatively constant gas concentration which normally
occurs at the beginning of the normal cooking phase (FIG. 3), from the
later occurring "drying out" phase, the sensor output voltage VOW is
monitored to detect a voltage magnitude corresponding to a concentration
level indicative of having progressed in the cooking cycle to a point
beyond the initial plateau. Having detected a concentration level in
excess of the reference level, the rate of change of VOW corresponding to
the rate of change of sensed concentration level is periodically deter-
mined. The rate of change of voltage magnitude is then monitored, first
to detect a rate of change corresponding to a rate of change of concern-
traction level less than a predetermined rate signifying transition from
the normal cooking phase to the drying out phase. Having detected such
a lesser rate, the rate of change of YOU is thereafter monitored to
detect a rate of change corresponding to a rate of change of concentration
level greater than a predetermined rate, signifying the transition from
the drying out phase to the pre-combustion phase. Upon detection of the
greater rate, the magnetron and the blower motor are de-energized and an
indication is provided to the user such as an audible signal signifying
that a combustion condition in the cavity is imminent.
Referring now to FIG. 5, the FIRE DETECTION routine is periodic
gaily entered from the Easter or executive program for microprocessor 64
throughout the cooking cycle. Such periodic entry may be satisfactorily
made with a time between entries on the order of 2-5 seconds. However,
time between entries need not be limited to this range for satisfactory
performance. Upon entering the routine, the sensor output voltage I at
-12-
4,538
input port Jl/Al is sampled (Block 152). Inquiry 154 compares the
sampled voltage VOW wit to a predetermi nod reference value OR represent-
live of the food being heated having reached that portion of the normal
cooking phase characterized by a relatively rapidly increasing concentra-
lion level. In the illustrative example, a reference voltage level OR
corresponding to the sensor resistance at point A (FIG. 3) was somewhat
arbitrarily selected as being about the midpoint of this portion of the
normal cooking phase for a potato. The precise reference value is not
critical provided it represents a point between the initial constant portion
of the normal cooking phase and the transition into the drying out phase. A
No response to Inquiry 154 results in a return (lock 156) to the master
program. As the food continues to heat, at some point prior to the end of
the normal cooking phase the sensor output voltage VOW will drop below the
reference value OR, resulting in a YES to Inquiry 154. On this and each
subsequent pass through the FIRE DETECTION routine during the same
cooking cycle, the response to Inquiry 154 will be Yes and locks 158-164
and Inquiry 166 will determine the rate of change of the magnitude of
output voltage VOW As shown by the resistance curve in FIG. 3, the
change in resistance as the cooking cycle progresses is not a smooth
curve but rather, is characterized by irregular small oscillations.
For proper operation the system must respond to the average rate of
change computed over a sufficiently long period to smooth out the oscil-
lotions. The average rate of change is periodically determined as
follows. First, the current incremental change V in the output
voltage YOU is computed by subtracting VOW from the previously sampled volt
tare signal stored as VOW (Block 158). VOW is then placed in memory as VOW
(Block 160). The cumulative or total change VT in voltage over the
averaging period To is updated by adding the just computed TV to the old
I\ VT (Block 162). Next, the timer T is incremented (Block 164). The
duration of the averaging period is controlled by Inquiry 166 which detects
-13- -
~4538
T greater than To where To is the desired duration of the averaging period.
The duration of the averaging period is no critical and may fall in a wide
range of values provided only that it be of sufficient duration for adequate
smoothing and yet sufficiently short to provide a satisfactory response time
for the system. A period on the order of 30 seconds duration is thought to
provide satisfactory smoothing. Until completion of the period To, a No to
Inquiry 166 causes a return (Block 156) to the main program. At the completion
ox each averaging period signified by a YES to Inquiry 166, the timer is reset
(Block 168). The average rate of change (OF) for the just completed averaging
lo period is set equal to the cumulative change for that period \ VT (Bleakly).This value is directly proportional to the true average since the time period
To is constant. VT is then reset for the next averaging period (Block
172). Having updated the average rate, Inquiry 174 checks the status of a
Drying Out Flag, which is set upon detection of an average rate of change
characteristic of the cooking operation having progressed to the drying out
phase. Thus, initially, this Flag is not set and the response to Inquiry 174
is No. Inquiry 176 then compares (en-) with a predetermined reference I
~^\ S corresponds to an average rate of change sufficiently small as to be
characteristic of the cooking cycle having entered the drying out phase.
pence, Inquiry 176 detects the small rate of change characteristic of the
drying out phase. During the balance of the normal cooking phase, the
answer to Inquiry 176 is No and the program returns to the master program
at this point (Block 156). As the food continues to be heated beyond the
normal cooking phase, it enters its drying out phase During this phase,
as herein before described, the rate of change of the concentration level
drops off significantly. When a dot less than US (Yes to Inquiry 176)
is first detected indicative that the transition into the drying out phase
has occurred, the Drying Out Flag is set (Block 178) and the program returns
(Block 156). Thereafter, for the balance of the cooking cycle, Inquiry 174
I
45;~3
indicates that the Drying Out Flag is set and the program will proceed
to Inquiry 180 to detect a rate of change greater than a predetermined
reference rate of change A L characteristic of entering the pre-combustion
phase. For the balance of the drying out phase, Inquiry 180 will be No
and the program will return to the master program at this point (Block
156). Upon detection of a rate of change greater than reference Lo
(Yes to Inquiry 180) a signal is provided at output port R12 causing
power control relay 68 to open, thereby de-energizing magnetron 40 and
the blower motor 67 (Block 182). It will be appreciated that fan motor
67 and magnetron 40 could alternatively be de-energized by inhibiting
trigger signals to trial gate terminals 146 and 142, respectively,
rather than by opening relay 68. The de-energizating of both the magnetron
and the blower motor using either technique in effect turns off power to
the oven. Also following a Yes to Inquiry 180, a signal is provided at
R11 triggering the alarm oscillator circuit 69 into oscillation to
provide an audible signal to the user that a fire condition in the
cavity is imminent (Block 184).
By detecting the occurrence of a rapid increase in the rate of
change of concentration level by the herein before described method
magnetron 40 is de-energized prior to the load being heated in the oven
reaching its actual combustion temperature, thereby anticipating a
potential fire condition and preventing the load from reaching its
combustion point.
While a specific embodiment of the method and apparatus of the
present invention has been illustrated and described herein it is
realized that modifications and changes will occur to those skilled in
the art to which the invention pertains. For example, to distinguish
the drying out phase from the initial relatively constant portion of the
normal cooking phase, rather than detecting a concentration level greater
than a predetermined reference level to initiate looking for the relatively
I
iota
small rate of change marking the transition into the drying out phase,
the reference level may be computed as a percentage of the initial value
for each cycle. Alternatively, the elapsed time since initiation of the
cooking cycle might be monitored to determine a starting point for
detecting the small rate of change. It is therefore to be understood
that the appended claims are intended to cover all such modifications
and changes as fall within the true spirit and scope of the invention.
-16-
