Microwave Oven Food Temperature-Sensing Probe

1. Field of the Invention

The present invention relates to a microwave oven food temperature-sensing probe assembly which prevents over-cooking of the food in a region immediately adjacent the probe. The present invention is an improvement of the probe assembly disclosed and claimed in copending application Ser. No. 643,092, filed Dec. 22, 1975, concurrently herewith, by Flora L. Meek and Richard E. Hornung, entitled "Microwave Oven Food Temperature-Sensing Probe Assembly Including a Reflector", and assigned to the same assignee as the present invention.

2. Description of the Prior Art

Electric thermometer probes which monitor the internal temperature of the food while it is being cooked have been developed for use in microwave ovens. Examples of such probes, adapted for use in microwave ovens, are disclosed and claimed in copending application Ser. No. 616,049, filed Sept. 23, 1975, by David Y. Chen and Louis H. Fitzmayer, and copending application Ser. No. 616,050, filed Sept. 23, 1975, by Louis H. Fitzmayer, each entitled "Food Thermometer for Microwave Oven", and assigned to the same assignee as the present invention. Such probes comprise a needle-like probe adapted to be driven into the food and a temperature-sensing element, such as a thermistor, positioned internally of the probe housing near the distal end thereof. A flexible shielded cable connects the thermistor to circuitry which is responsive to thermally-induced changes in its resistance. The cable shield is electrically connected at one end to the probe housing and at the other end to a wall of the oven cooking cavity. In the embodiments disclosed in the above-mentioned Chen and Fitzmayer application Ser. No. 616,049, the total effective electrical length of the probe and cable, measured along the cable and probe from the cooking cavity wall to the distal end of the probe, is selected to be approximately equal to n .lambda./2, where n is any integer and .lambda. is the wavelength of the microwaves being used to cook the food in the oven. Such a probe and cable assembly may successfully be operated in a microwave oven with minimal direct heating of the proble and cable by the microwave energy.

In general, probes of the above-described type operate satisfactorily and do minimize problems caused by direct heating of the probe and cable by the microwave energy. However, when they are used to monitor the internal temperature of certain foods, including meats and baked potatoes, overcooking of the food in a cone-shaped region immediately adjacent the probe (generally along the length of the probe) sometimes occurs. This is considered undesirable from the standpoint of appearance of the food after it has been cooked. Further, the temperture-sensing probe necessarily responds to the temperature of the food in the region immediately adjacent the probe. When the food in this immediately adjacent region cooks faster than the other regions of the food, there is an indication of doneness before the bulk of the food (all the other regions within the food) is fully cooked to the desired degree of doneness. The condition which results in overcooking of the food in the region immediately adjacent the probe has been termed "feed in" because the cable and probe seemingly pick up microwave energy present within the cooking cavity and feed or direct the energy into the food along the length of the probe.

The above-described feed in effect is to be distinguished from a premature indication of doneness due to direct heating of the probe by the microwave energy, one of the problems to which the inventions comprising the subject matter of the above-mentioned copending Chen and Fitzmayer application Ser. No. 616,049 and the above-mentioned Fitzmayer application Ser. No. 616,050 are directed. When a feed in problem occurs, the probe may be accurately sensing temperature; the temperature of the food which the probe is sensing is simply not the temperature of the food as a whole.

The above-mentioned Meek and Hornung application Ser. No. 643,092 discloses and claims a microwave oven food temperature-sensing probe assembly which, in order to eliminate the feed in problem, includes a disc-shaped reflector positioned along the probe housing. If the probe and reflector assembly is inserted so far into the food or if the position of the reflector is adjusted so that the reflector touches the surface of the food, or is within 1/8 inch of the surface of the food, then the shielding effect is so complete that the food in the region immediately surrounding the probe housing is not sufficiently cooked. By the present invention, there is provided a means for preventing the reflector from being pushed completely against the food.

It is an object of the invention to provide a microwave oven food temperature-sensing probe assembly which includes a reflector to minimize feed in and which is convenient to use.

It is another object of the invention to provide such a probe assembly which includes means to positively prevent the reflector from being pushed so near the food that excessive shielding occurs.

These and other objects are accomplished by the present invention in which there is provided a needle-like probe adapted for insertion into the food to be cooked, a temperature-sensing element being positioned internally of the probe housing near the tip thereof. A flexible shielded cable is provided for connecting the temperature-sensing element to suitable circuitry. The cable shield is electrically connected at one end to the probe housing and is adapted at the other end for connection to a wall of the microwave oven cooking cavity. An electrically-conductive reflector is mounted on the probe housing at a predetermined distance from the tip of the probe housing.

The probe assembly also includes a spacer mounted adjacent the reflector and extending toward the tip of the probe housing. Preferably, the spacer has a thickness ranging from 1/8 inch to 1/4 inch and a diameter ranging from 3/16 inch to 1/2 inch. The spacer serves to space the reflector away from the food at least a predetermined distance equal to the spacer thickness. Preferably, the spacer is formed as an integral part of the reflector.

The term "reflector" is used in reference to the structure described herein on the assumed theory of operation that the structure operates to reflect microwave energy propagating along the cable towards the probe (and the food) back away from the probe (and the food) towards the oven wall end of the cable. While the explanation of the operation of the invention herein is believed to be correct, it is realized that other, presently unappreciated, principles of operation may be applicable and that, in such event, the reflector, as described herein, may not acutally be functioning as a reflector in the usual sense of the word. If such is ultimately deemed to be the case, then the term reflector is intended, nevertheless, to mean the structure as described herein and its equivalents.

While the novel features of the invention are set forth with particularity in the appended claims, the invention, as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings, in which:

FIG. 1 is a front perspective view of a countertop microwave oven shown with the oven door open and illustrating a portion of a food temperature-sensing probe assembly embodying the principles of the present invention.

FIG. 2 is a perspective view of a probe assembly including one embodiment of the present invention.

FIG. 3 is a side elevational view of a handle and reflector assembly incuding a second embodiment of the invention, with a portion of the assembly cut away.

FIG. 4 is a side elevational view of a handle and reflector assembly, similar to FIG. 3, including a third embodiment of the invention, with a portion of the assembly cut away.

Referring first to FIG. 1, there is illustrated a countertop microwave oven, generally designated by the numeral 10, including a food temperature-sensing probe assembly 12 constructed in accordance with and embodying features of the present invention. The oven 10 includes a cooking cavity generally designated at 14, defined by side walls 16 and 18, a top wall (not shown), a bottom 20, and a door 22. The door 22 is shown in its open position and is hingedly supported at the left side by hinges 24 and 26. The door 22 also includes a conventional microwave energy seal, shown generally at 28, to prevent the escape of microwave energy from the cooking cavity 14. The oven 10 further includes a control panel 30 which includes a temperature-setting control 32.

As is conventional, the oven 10 is also provided with a magnetron (not shown) for generating microwaves of a predetermined frequency, preferably at a nominal frequency of 2450 MH.sub.z. The oven 10 further includes a waveguide (not shown) foor transmitting the microwave energy from the output of the magnetron to the interior of the cooking cavity 14.

Within the cooking cavity 14, there is shown a shallow container 34 constructed of a material transparent to microwaves, such as a Pyrex dish, containing therein a piece of food 36, shown for purposes of illustration as a half of a ham. The ham 36 is to be cooked to an internal temperature of approximately 160.degree. F. A portion of the probe assembly 12 is inserted into the ham 36 and is connected to control circuitry (not shown) contained within the oven 10 by means of a flexible shielded cable 38 and a connector 40 mounted on the oven side wall 18. Details of suitable circuitry which may be used in conjunction with the probe assembly 12 is disclosed in copending application Ser. No. 634,336, filed Oct. 21, 1975 by Richard E. Hornung, entitled "Food Temperature Responsive Control Apparatus", and assigned to the same assignee as the present invention. Preferably, the circuitry serves to turn off the magnetron and operate an audible indicator when the internal temperature of the ham 36 reaches the temperature set on the temperature-setting control 32.

Referring now to FIG. 2, there are shown additional details of the probe assembly 12, which is only partially shown in FIG. 1. The probe assembly 12 includes a tubular electrically-conductive housing 42 surrounding a temperature-sensing element such as a thermistor 44 (shown in broken lines) located internally of the housing 42 near the distal end or tip 46 thereof. While a tubular probe housing having a circular cross section is shown, it will be apparent that other cross sections, such as square or hexagonal, may be employed without departing from the scope of the invention. One thermistor lead 48 is electrically and thermally connected to the probe housing 42 adjacent the tip 46. As is conventional, heat conduction to the thermistor 44 is primarily along the lead 48.

The flexible shielded cable 38 serves to electrically connect the thermistor 44 to the circuitry (not shown) and is preferably a coaxial cable. As is described in greater detail in the above-mentioned copending Chen and Fitzmayer application Ser. No. 616,049, alternative forms for the flexible shielded cable 38 are possible. Examples are a two-conductor shielded cable having two conductors and an outer shield, or, a flexible hollow tublar cable shield and a single inner conductor passing therethrough, but not necessarily centered axially.

The other thermistor lead 50 is electrically connected to the inner conductor 52 (shown in broken lines) of the cable 38 at one end thereof. At the same end of the cable 38, the cable shield 54 (broken lines) is electrically connected to a connection end 56 (broken lines) of the probe housing 42. The probe housing 42 and the cable shield 54 thereby cooperate to form a continuous conductive sheath from the tip 46 of the probe housing 42 to the other end of the cable 38. At the other end of the cable 38, the cable shield 54 is electrically connected through the connector 40 to the side wall 18.

For a temperature-sensing probe assembly which is to be used in a microwave oven, it is preferable to make the total effective electrical length of the probe and the cable, measured along the cable 38 and the probe housing 42 from the cooking cavity wall 18 to the distal end or tip 46 approximately equal to n .lambda./2, where n is any integer and .lambda. is the wavelength of microwave energy being used to cook the food, as it taught by the above-mentioned Chen and Fitzmayer application Ser. No. 616,049. As is more fully explained in the Chen and Fitzmayer application, when the total effective electrical length is so selected, heating of the probe and cable by direct action of the microwave energy is minimized. Therefore, for best results, the length should be so selected. It is believed, however, that substantial benefits of the present invention may still be obtained when the total effective electrical length of the probe and cable is other than that which is taught in the Chen and Fitzmayer application.

The probe assembly 12 additionally includes an electrically-conductive reflector 58 mounted at a fixed position on the probe housing 42. The reflector 58 is located a predetermined fixed distance from the tip 46 and preferably in the general form of a disc, as shown. A handle 60 made of insulating material is attached to the reflector 58. As is described in the copending Meek and Hornung application Ser. No. 643,092, a minimum effective reflector diameter has been found to be one inch, but for best results the diameter should be at least 1.25 inches. A practical reflector diameter has been found to be 1.6 inches.

The shielding effect produced by the reflector 58 varies, both with reflector diameter and with reflector-to-food spacing. The shielding effect increases with increased reflector diameter and with decreased reflector-to-food spacing. Since, for a particular probe, the reflector diameter is fixed, there is a point at which the reflector-to-food spacing becomes too great for the reflector to function effectively, and excessive feed in occurs. For best results, greater reflector-to-food spacing is not recommended. This point of maximum recommended reflector-to-food spacing may also be expressed as a minimum recommended insertion depth because, as will be apparent, as the probe housing 42 is inserted farther into the food 36, the reflector-to-food spacing decreases.

In order to show the minimum recommended insertion depth of the probe housing 42 into the food 36, a minimum insertion depth indicator 62 is included on the housing 42. In the illustrated embodiment, the indicator 62 comprises an enlargement in diameter of the housing 42, the large diameter portion of the housing 42 being from the indicator 62 to the reflector 58. The indicator 62, however, forms no part of the present invention, but, rather, is a part of the subject matter of copending application Ser. No. 643,109, filed Dec. 22, 1975, concurrently herewith, by Richard E. Hornung, and also entitled "Microwave Oven Food Temperature Sensing Probe Assembly".

As described in the copending Hornung application, it has been found that the reflector diameter and the distance between the reflector 58 and the recommended minimum insertion depth indicator 62 are approximately related as expressed by the formula:

where .lambda. is the wavelength of the microwave energy used to cook the food 36 in the oven 10.

For a microwave frequency of 2450 MH.sub.z, .lambda. is approximately equal to 4.8 inches, and 3/4 .lambda. then equals approximately 3.6 inches. In accordance with the above formula, for a convenient reflector diameter of 1.6 inches, the distance between the reflector 58 and the minimum insertion depth indicator 62 should be approximately 2 inches.

For the same microwave frequency, it has been found satisfactory to make the distance between the reflector 58 and the probe tip 46 approximately equal to 7/8 .lambda.. At 2450 MH.sub.z, 7/8 .lambda. is approximately equal to 4.4 inches.

In addition to a maximum recommended reflector-to-food spacing, or, expressed in another way, a minimum recommended insertion depth, there is a minimum recommended reflector-to-food spacing. If the reflector 58 is positioned too close to the surface of the food 36, then the shielding effect produced by the reflector 58 is too complete. Not only is feed in eliminated, but normal cooking in the region of food immediately adjacent the probe housing is eliminated as well.

In accordance with the present invention the probe assembly 12 additionally includes a spacer 64 mounted adjacent the reflector 58 and extending toward the probe housing tip 46, the spacer 64 serving to space the reflector 58 away from the food 36 at least a predetermined minimum distance. In the preferred form of the invention illustrated in FIG. 2, the spacer is formed as a part of the reflector 58 and is made of the same conductive material.

Referring now additionally to FIGS. 3 and 4, there are shown alternative forms of handle and reflector assemblies embodying the invention. In both FIGS. 3 and 4, the reflector 58 is fitted into a recess in the handle 54. The spacer 64 shown in FIG. 3 is a separate element adjacent the reflector 58, and may be made of either dielectric or conductive material. The spacer 64 shown in FIG. 4 is formed as a part of the reflector 58, similar to the construction shown in FIG. 2. When the spacer 64 is metal, a configuration such as is shown in FIGS. 2 and 4 is preferred to that shown in FIG. 3 becasue it insures that a good electrical connection is maintained between the spacer 64 and the reflector 58.

For best results, the thickness t of the spacer 64 should be within the approximate range of from 1/8 inch to 1/4 inch. The spacer diameter D may be within the approximate range of from 3/16 inch to 1/2 inch. If the diameter D is too small, the spacer will not be effective to keep the reflector 58 away from the surface of the food 36. If the spacer diameter D is too large, the spacer 64 will begin to function as a shield.

It will be apparent that the spacer 64 of the present invention and the minimum insertion depth indicator 62 which, as mentioned above, is a part of the subject matter of the copending Hornung application Ser. No. 643,109, cooperate to define a range of insertion depths of the probe housing 42 into the food 36. Within this range, the best cooking results are obtained.

Having described preferred embodiments of the invention, the operation thereof will now be described. In preparation for cooking, the probe housing 42 is inserted into the food 36 to a position such that the outer boundary of the food 36 is between the minimum insertion depth indicator 62 and the maximum insertion depth indicator or spacer 64.

It is believed that, without the reflector 58, microwave energy is picked up by the cable 38 and the probe housing 42 and follows along the cable 38 and probe housing 42 to be directed and concentrated into the food 36 in the region immediately adjacent the probe housing 42. The reflector serves to reflect a controlled amount of microwave energy back up the cable 38 toward the connector 40, thereby preventing excessive microwave energy from reaching the food 36.

When a spacer according to the present invention is employed, excessive shielding caused by pushing the reflector all the way against the food and the resultant under-cooking is avoided.

While specific embodiments of the invention have been illustrated and described herein, it is realized that numerous modifications and changes will occur to those skilled in the art. 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.