DE2507176A1 - FOOD THERMOMETER FOR MICROWAVE OVEN - Google Patents

Description

Food thermometer for microwave oven

The invention relates to a temperature monitoring device and more particularly to a device for monitoring the temperature of meat or other food products during the process be cooked in a microwave oven.

Determining the temperature inside the food or the meal while it is being cooked in the stove provides a good indication of how well the food is cooked (for example "half cooked", • medium "," medium cooked "or" well done "for cooking or cooking Roasting meat). This can be achieved in that a

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suitable temperature sensing probe placed in the food will. A simple type of probe would have a temperature indicating scale to read the temperature from.

The present invention provides a temperature monitoring device for use in a microwave oven. In particular, a temperature sensing device is proposed, which prevents the unintentional leakage or loss of microwave energy from the boiler room and the calibration or Calibration of the temperature sensing element maintains.

According to the present invention there is provided a temperature monitoring device proposed for use in a microwave oven that has a temperature sensing device in a probe housing for insertion into the feed and is connected by a coaxial cable to a display device, wherein the temperature sensing device from the microwave energy completely is shielded while preventing resonant currents in the device.

It is a primary object of the present invention to provide an apparatus to monitor the internal temperature of a low-impedance food in a microwave oven is cooked with microwaves of a predetermined wavelength. the Apparatus according to the invention comprises a coaxial transmission line with an outer conductor which is grounded at one end around its entire circumference to the associated hearth, and an inner conductor, temperature sensing means between the inner and outer conductor of the coaxial transmission line on its other Connected at the end, a conductive housing that can be inserted into the food to be cooked and with the outer conductor connected to the transmission line near its other end to cooperate with and surround the temperature sensing means, and finally a display device coupled to the coaxial transmission line at one end thereof

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Scanning
the / means responds to generate a signal which is a measure of the internal temperature of the associated food, the effective electrical distance from one end of the transmission line to the distal end of the housing is approximately equal to ηA / 2, where η is an integer and / Λ is the predetermined wavelength. As a result, the transmission line and the housing are not in resonance at the predetermined wavelength during operation and the temperature sensing means are completely shielded from the microwave energy.

The invention will now be based on further features and advantages the following description and the drawing of an exemplary embodiment explained in more detail.

Figure 1 is a front view of one shown with the door open Microwave oven having a constructed according to the invention Represents temperature monitoring device.

Figure 2 is an enlarged side view of the temperature sensing probe and the cable assembly according to the invention with a portion of the cable removed.

FIG. 3 is a vertical sectional view of that shown in FIG Probe and cable structure.

Figure 4 is a schematic diagram of a preferred one Embodiment of the control and display circuit of the temperature monitoring device according to the present Invention.

In Figure 1, a generally designated 10 microwave oven with a food thermometer system is shown, which is a total of Reference number 50 is provided and is constructed according to a first embodiment of the invention. The oven 10 is a conventional microwave oven, and also if a built-in stove for illustration purposes

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is shown, any desired type of microwave oven can be used be used. The oven 10 is of standard construction with a cooking or baking space, which is through a box-like stove cover 14 and a front opening Access door 16 is formed, this door being shown in Figure 1 in its horizontal, fully open position. The door Ib has a door handle 18 running along its upper edge is arranged, and hinge straps 20 at the lower corners to pivot the door on the stove body. The stove cover 14 includes a bottom wall 22 and opposing side walls 24, which are provided with brackets or paragraphs 26 for an insert. Between the supports 26 is the food supporting sliding tray 28 suspended. Might upon request a roasting or baking element (not shown) can be provided either next to the top wall or the bottom wall of the stove cladding, as is customary with electric cookers. Such heating elements are usually jacketed, electrical resistance heating elements of quite small diameter and great length, shaped into a large loop or serpentine shape.

The oven 10 is further provided with a magnetron for generating microwaves at a predetermined frequency, which is preferably 2450 MHz, and provided with a waveguide for transmission the microwave energy from the exit of the magnetron to the boiler room within the oven shroud 14. A control panel 34 can be connected to the front of the stove 10 may be provided above the boiler room.

The oven drawer 28 carries a flat container 36 from

a material that is permeable to microwaves and is attached to the shell

for example as Pyrex). In the container 36 there is a piece of a foodstuff 38, which (F ⁰ 150) is shown to be baked ham or thigh half for purposes of illustration as at an internal temperature of about 66 ° C. Needless to say, other types of food or larger or smaller pieces of meat for a longer or shorter period of time

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be heated. This variable cooking time creates an inconvenience as it requires close attention to the hearth. The dining thermometer system 50 according to the present invention serves to avoid this difficulty.

The feed thermometer system 50 includes a coaxial transmission line or a cable which is provided with the reference number 60 and an inner conductor 6l and one of these through a Insulating layer 62 has separate external conductors 63. Preferably, the outer conductor 63 is also through a layer of an electrical and thermal insulation 64, which is preferably consists of a material such as polytetrafluoroethylene, which is commonly sold under the trade name Teflon. That Coaxial cable 60 is preferably very flexible and has the characteristics of a small diameter conductor wire. The outer Insulation 64 is detached from the ends of the cable 60 and the Ends of the outer conductor 63 are widened outwardly by coupling sections 69 and 71 with a large diameter in a corresponding manner near the opposite ends of the cable 60, each having an outside diameter that is substantially is equal to the outer diameter of the insulating layer 64.

One end of the cable 60, which is shown in FIGS. 2 and 3 as the right end, can be connected to a control and display device, which is designated as a whole by the reference number 80 (see FIG. 4), through a coaxial plug 65 and a plug receiver 98 can be coupled, which is mounted in the one side wall 24 of the oven casing 14. The plug 65 has a central probe 66 which is connected to the inner conductor 6l of the cable by suitable means such as solder and which is held by a circular insulating spacer 67 in a cylindrical sleeve section 68 of the plug 65. The sleeve portion 68 is provided with an integral, inwardly extending neck portion 68 of reduced diameter, which the cable insulating layer 64 on the adjacent

. with

Barten end of the cable 60 surrounds and / the connecting portion

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of the outer conductor 63 around its entire circumference is in a contact of 36O ^0th The sleeve 68 and the central probe 66 of the plug 65 are correspondingly able to be received by corresponding contact parts of the plug receptacle 98, which in turn are connected to conductor wires 88 and 8 9, as is well known.

The large-diameter connecting portion

recorded

at the other end of the cable 60 is / and in contact with an elongated cylindrical section 72 of a probe housing 70 /

which is also provided with a generally conical tip portion 73 which is integral with the cylindrical Part 72 is formed at the distal end of the probe housing 70 for insertion into the food or the piece of food 38 to facilitate. A generally frustoconical non-metallic handling part 74 has a neck portion 7 ^ a, around the cylindrical part 72 next to the the other end of it is flanged. The handling part 74 can be provided with a side opening to allow the Take up cable 60 (see FIG. 1). Preferably, the cylindrical portion 72 of the probe housing 70 has a sufficient one Diameter to snugly fit therein the outer insulation 64 and the large diameter connecting portion of the cable 60, as best shown in FIG. A heat sensing element is located in the probe housing 60 arranged, which is preferably a thermistor, whose inner connection 77 is electrically connected to the inner conductor oil of the cable 60 and its outer conductor 78 is electrically connected to the inner Surface of the tip portion 73 of the probe 70 is in communication. Preferably, the thermistor 75 is in a shrinkable one Teflon bushing 76 supported, which also surrounds the adjacent end of the insulating layer 62 of the cable 60. In the preferred embodiment According to the invention, the probe housing 70 is about 7.5 cm (3 inches) in length from its distal end to handling part 74.

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It is an important feature of the present invention is that the bushing or sleeve 68 of the plug 65 and therefore, the outer conductor 63 of the cable 60 to the wall 2 4 of the oven panel 14 around the entire circumference of 36O ⁰ of the bush 68 around electrically connected to earth or Ground is connected so that the probe housing 70 is also grounded and the temperature sensing element 75 is completely shielded from microwave energy. The handle part 7 ¹ * of the probe housing 70 may serve as a stop to limit the depth to which the probe housing can be inserted into the Pleischstück.

The circuit arrangement includes input supply terminals 93 and 9 ² J for connection to a low voltage AC power source, such as a suitable step-down transformer 91 with a primary winding 95 and a secondary winding 96. The primary winding 95 can be connected to a normal power source of 115 or 220V and 60 or 60 respectively. 50 Hz, as it is usually used in the household, must be connected via suitable lines L and a center conductor N. The step-down transformer can be a separate transformer for this electrical thermometer system, or the power can be taken from the transformer provided for the stove control.

A four-legged impedance bridge circuit is provided with first and second common connections, which are connected via the input side Supply terminals 93 and 94 are connected, such as the Wheatstone bridge, which has a first leg with a plague resistor 97, a second leg with a plague resistor 98, a third leg with a plague resistor 99 and a fourth leg with a plague resistor 100. The first and second legs and the third and fourth legs form voltage divider circuits with comparison connections 101 and 102 in a corresponding manner. The temperature sensing thermistor 75 of probe 70 is electrically so connected to the first leg so that it is parallel to the plague resistor 97 lies. A variable resistor 104 is electrically connected

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so in the circuit arrangement of the first leg that he with the parallel connection of the thermistor 75 and the Pest resistor 97 is connected in series. A variable resistor 105 is in series with the fixed resistor 98 in the second leg switched.

A programmable unijunction transistor (PUT) 10ό possesses a cathode 107, which is connected to the first common network connection and the input terminal 93, an anode IO8, which is connected to the comparison terminal 101 is connected between the first and second legs of the bridge, and a gate or a control electrode 109 connected to the comparison terminal 102 between the third and fourth legs of the bridge is in connection.

An indicating device, for example, an audible signal emitting device such as an alarm buzzer with a coil 112 is connected to a Zener diode 113 between the anode 108 of the PUT 106 and the second common network connection or Input terminal 93 connected in series.

A programmable unijunction transistor (PUT) is a PNPN device having characteristics such that when the diode (anode to gate) is forward biased (i.e. when the anode is more positive than the gate), the device is conductive, the regeneration inherent in such a device causes this to switch on and a negative resistance characteristic is generated from the anode to the cathode. Therefore, the PUT 10ό works in the manner described above as a detector for the bridge circuit.

The thermistor as used here in the first leg of the bridge has a negative temperature coefficient. As the temperature of the thermistor rises, its resistance falls away. During the positive half-cycle of the alternating supply voltage, this voltage at terminal 101 or the anode voltage becomes more positive. When the anode voltage is slightly more positive than the gate voltage

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then the PUT switches on, with a voltage drop of about 1 volt from the anode to the cathode. Most notably then the voltage is determined by the cooperation of the impedances from the thermistor 75, the resistor 97 and the variable resistor 104 lowered in the first leg. If the positive voltage at terminal 93 is greater than the Zener breakdown voltage of the Zener diode 113, current flows through the diode and coil 112, which activates the alarm buzzer. Since the coil 112 is connected in series with the Zener diode 113 and the anode-cathode path via the input terminals, the current is through the coil 112 therethrough is limited only by the coil impedance. Preferably, a capacitor 116 bridges the anode 108 and the Gate 109 of the PUT 10ö, while a diode 117 in parallel with the variable Resistor 105 is connected.

As already mentioned, it is done by adjusting the control knob 55 to a given temperature of the scale 58 a setting des Reostat 104 to such a resistance that the Igniting or switching through the PUT 10b coincides with the reaching of the temperature set in advance in the food, wherein this temperature is sensed by the thermistor 75 in the tip of the food probe 70. When you reach the im preset temperature switches the PUT 10b through, and it flows a current to actuate the buzzer coil 112 and thereby an audible signal is given that the predetermined temperature has been reached.

This electrical food thermometer system is a "compensation system", which sounds the buzzer 112 when the internal temperature of the food reaches the preset temperature, however, the existing temperature is not displayed directly. When the operator knows the internal temperature of the Would like to know food during the cooking cycle, they can determine this by moving the pointer 59 along the temperature scale moved down until the buzzer sounds, after which the temperature displayed on the scale 58 is read. White

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Further details of the structure and operation of the circuit arrangement are shown in U.S. Patent 3,778,798.

It should be noted that the non-resonance of the cable 60 serves to control the currents at a wavelength / A corresponding Keep microwave frequency to a minimum. The length of the probe housing 70 from its tip to the handling part 74 is preferably about / Ά / 4, and the length D ^ from the earth connection is on the hearth wall 24 up to the point tip of the probe housing 70 preferably equal to η / Λ / 2, where η is any integer. The length of the cable 60 from the ground connection on the hearth wall 24 to the probe housing 70 is selected so that the cable 60 does not is in resonance, the exact value of this length being dependent on the impedance of the food which the cable 60 at its probe end concludes. The cable is grounded at one end and is at the other end with a small impedance of the food completed, resulting in a mismatching of the impedances at the opposite ends of the cable 60, the desired non-matched or non-resonant state to create.

In the embodiment according to Figures 2 and 3 extends the insulation 64 of the cable 60 is only between the extended connection ends 69 and 71 of the outer conductor 63, this insulation prevents accidental flashover if the cable 60 comes into contact with the cooker shroud 12. Since it is assumed that the probe housing 70 is embedded in the food 38, no insulation is required. In these embodiments For example, the probe housing 70 is preferably made of stainless steel. As an alternative, both the cable as well as the probe housing in a coherent insulation jacket be included. In addition, to ensure complete insulation of the cable and the probe housing, this alternative arrangement also permits the use of less expensive aluminum or brass to construct the probe housing instead of stainless steel, with aluminum also having the added benefit of being a much better conductor

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for electricity and heat is called stainless steel.

From the above description it can be seen that an improved Food thermometer system is proposed, which is particularly suitable for use in a microwave oven is. In particular, a dining thermometer system with a simple and created economical structure that works accurately to monitor the internal temperature of the food to be cooked, while preventing interference with the operation of the system by the microwave energy.

In particular, there is an improved temperature monitoring device which includes a temperature sensing element which is in a probe for insertion into the food encapsulated and connected by a coaxial cable to a display device, the temperature sensing element is completely shielded from the microwave energy and the length of the connecting cable is chosen so that no resonance occurs at the predetermined microwave frequency used, so that electrical currents "in the cable at the microwave energy are reduced to a minimum.

In addition, alternative embodiments of the probe housing and the cable proposed, whereby only the cable or ^ alternatively, both the cable and the probe housing with a jacket made of thermally and electrically insulating material can be provided.

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Claims (1)

Expectations 1.) Device for monitoring the internal temperature of a food having a low impedance, which is cooked in a microwave oven with microwaves of a predetermined wavelength, characterized by, a coaxial transmission line (60) with an outer conductor (63), which at its one end around is grounded the entire circumference to the associated hearth, and with an inner conductor (61), temperature sensing means (75) connected between the inner and outer conductors (61, 63) of the coaxial transmission line (6 0) at its other end are connected, a conductive housing (70) which can be inserted into the food to be cooked and is connected to the outer conductor (63) of the transmission line (60) near its other end and surrounds the temperature sensing means (75), and by a Display device coupled to said coaxial transmission line (00) at one end thereof and responsive to temperature sensing means (75) for generating a signal, which is a measure of the internal temperature of the associated food (38), the effective electrical path from one end of the transmission line (60) to the distal end of the housing (70) being approximately equal to ηλ / 2, where η is an integer and ^ - is the predetermined wavelength so that the transmission line (60) and the housing are not in resonance at the predetermined wavelength and the temperature sensing means (75) are completely shielded from the microwave energy. Device according to claim 1, characterized in that the effective electrical path of one end of the transmission line (60) to the conductive housing (70) is selected such that the transmission line (60) is not in resonance. 509836/0885 3. Apparatus according to claim 1, characterized in that the conductive housing (70) has an effective electrical length of approximately η Λ · / 4. 4. Apparatus according to claim 1, characterized in that a non-metallic handling part (7 ¹ O is provided which is connected to the housing (70) to facilitate the insertion of the housing (70) in the food (38). 5. The device according to claim 1, characterized in that the temperature sensing device (75) has a first terminal connected to the inner conductor (6l) of the
transmission line (70) is connected, and has a second terminal connected to the conductive housing (70) near the distal end of the transmission line. 6. Apparatus according to claim 1, characterized in that the temperature sensing device (75) comprises a thermistor. 7. Device according to claim 1, characterized
characterized in that the display device
Having means for generating an output signal, if
the internal temperature of the associated food (38)
is equal to or greater than a predetermined temperature. 8. Apparatus according to claim 1, characterized in that the frequency of the microwave energy at the predetermined wavelength is 2450 MHz. 9- device according to claim 1, characterized that the outer conductor (63) of the coaxial transmission line with an insulating layer (64) of its one end is surrounded by the housing (70). 509836/0885 10. Apparatus according to claim 1, characterized that the conductive housing (70) is constructed from stainless steel. 509836/0885 Blank page