GB2029722A - Pressure cookers - Google Patents

Abstract

An automatic domestic pressure cooker, which can be adjustably preset to cook for a period of which the start may be adjustably preset and of which the termination is automatic, includes a valve 28 which can close to seal a cooking chamber and a thermal sensor 26 which is disposed to sense a vapour temperature between 2 DEG and 5 DEG C below the boiling point of water, and to cause consequentially the closure of the valve. The cooker includes an electrical heating element 3 disposed in the base of the bowl which sits in a recess in a plinth carrying the electrical controls. The element is controlled by a regulator 19 which maintains the chamber during cooking at a predetermined temperature above the boiling point of water. The cooker includes a timer 20 which, at the end of the period of cooking, causes the venting of the cooking chamber if desired, and either the cessation or reduction of heating of the cooker by the heater. <IMAGE>

Description

SPECIFICATION Pressure cookers This invention relates to automatic domestic pressure cookers and in particular to an automatic domestic pressure cooker which can be adjustably preset to cook for a period of which the start may be adjustably preset and of which the termination is automatic.

Although the general purpose of the present invention is to facilitate the provision of a reliable automatic domestic pressure cooker which cooks palatable food, one of the significant features of the invention is the provision of a valve which can close to seal a cooking chamber and a thermal sensor which is disposed to sense the occurrence within the cooking chamber of a temperature which connotes the dispiacement of air from the chamber by steam and to cause consequentially the closure of the valve. Such a provision enables a reliable determination of the commencement of the period of cooking. The aforesaid temperature is preferably selected to be in the range from 5 to 2 degrees below the boiling point of water.

The cooker preferably incorporates an electrical heating element, for example an electrical heating element disposed in the base of a vessel constituting part of the cooking chamber. The element may be controlled by a regulator which maintains the chamber, during a period of cooking, at a predetermined temperature above the boiling point of water. The said predetermined temperature may be detected by a second thermal sensor. It is preferred that the cooker includes a timer which, at the end of the period of cooking, causes the venting of the cooking chamber and, according to the setting of an adjustable control, either the cessation or reduction of heating of the cooker by the heater.

There follows a description of an exemplary embodiment of the invention with reference to the accompanying drawings, of which: Figure 1 is a diagrammatic drawing of a domestic pressure cooker; and Figure 2 is a schematic drawing of a control circuit disposed in accordance with the present invention.

Figure 1 of the drawings illustrates, for the sake of completeness, the principal components of the automatic pressure cooker. It is well known that a pressure cooker in its usual form comprises a bowl, which in use contains the food for cooking, and a removable cover which usually has peripheral flanges which cooperate with flanges on the rim of the bowl in order that the cooker may constitute a closed chamber when the bowl and cover are mutually secured. The cover usually includes a valve which is operable to seal and vent the chamber; in ordinary pressure cookers the valve is controlled by an adjustable weight.

The cooker which is illustrated in Figure 1 comprises a bowl 1 which can be closed at the top by a removable cover 2 which has a flanged rim cooperating with the rim of the bowl to ensure that when the bowl and cover are in predetermined relative positions, the cover is secure on the bowl.

The bowl 1 incorporates in its base an electrical heating element 3 which has terminals 4. The bowl 1 fits within a recess 5 in a plinth 6 which on its side surface carries a module 7 that contains most of the components that will be described with reference to Figure 2 and which serves as a mount for the various adjustable controls that will be described later. The terminals of the heater cooperate with terminals in the plinth in a manner which ensures that current cannot be supplied to the heater unless the bowl is properly fitted and locked into the recess; the plinth includes terminals for the reception of a mains supply of electric current.

The cover includes a solenoid operated valve (not shown in Figure 1) to which electrical connection extends from the module 7 through the plinth and a shield 8 that is disposed on and forms part of the cover 2. At the interface between the shield 8 and the plinth 6 there is preferably an arrangement of mutually engageable sets of terminals which ensure that current can be supplied to the aforementioned valve only when the cover is correctly positioned. At the base of the module 8 is a lever 9 which is operable to lock the bowl in the plinth and to release the bowl from the plinth.

The foregoing brief description with reference to Figure 1 is provided merely for completeness. It is not intended that the invention be limited to use in a cooker of the particular construction that has been described.

The cooker may have various manually operable controls constituted by push-buttons. For example, five of them enable the selection of the particular heating required by the cooker for browning, cooking at 5, 10 and 15 pounds per square inch and warming after cooking respectively. Further controls may set a fast running of an internal clock for the rapid setting or resetting of a display, a slow running of the clock for the final setting of-the display to the desired cooking time, and there may be a control for the selection of fast or slow depressurisation of the cooker after the cooking period, according as the push-button is partly or fully depressed, and a control to start the operation of the cooker and, when released, to cut off the electrical supply to the heater. There are, preferably, three safety interlocks which will not be described in detail.

One of them prevents the closure of the aforementioned valve and thereby prevents the increase of pressure in the cooking chamber if the first heat, used for browning, is selected when the lid is fitted. A second interlock prevents the supply of electricity to the heating element if the cooker is lifted. A third interlock requires the correct fitting of the lid (unless browning is selected) for the supply of electric current to the heating element Reference will now be made to Figure 2, which illustrates the principal parts of a control circuit which exemplifies a particular embodiment of the invention. It is preferred that those parts which are disposed within the region denoted by the chain line 10 in Figure 2 are located within the module 7; the other parts are disposed elsewhere.

However, Figure 2 is schematic only and such devices as the terminals of the heating element have deliberately been omitted for convenience.

A mains supply for the control circuit shown in Figure 2 is connected to a pair of input terminals 11, 12 in orderto supply electric current to the heating element 3, which is preferably in the base part of the bowl of the cooker, and to a transformer and rectifier network 13 which provides a 12-volt power supply to the electronic switching and control circuits within the module.

The electric power for the heating element 3 is switched on and off by a triac 14 which is under the control of a switching circuit 1 5. This circuit comprises any suitable circuit which turns on the triac for greater or lesser fractions of time according to which of five external resistors 6 is connected to it. A first resistor (HEAT 1) is selected when warming or browning is desired.

Three (HEAT 2 to HEAT 4) may be selected by a switch 17 7 operated by the respective push- buttons and determine the heater's current for cooking at 5, 10 and 15 pounds per square inch respectively. A fifth (HEAT 5) is selected when keeping the cooker's contents warm after the cooking period is desired. The switching circuit 1 5 has a control constituted by an electrical latch 1 8 of which the operation will be described later. It has a further control constituted by a thermal sensor 1 9, preferably a thermistor, of which the primary function is to regulate the temperature of and within the cooking chamber during the period of cooking.This thermal sensor would normally be set to detect the attainment of a temperature, such as 122 C, at which pressure cooking of comestibles is normally performed.

The latch is also under the control of a timer 20 which is adjustable and which determines the length of the cooking period. The timer is started by a signal from the thermal sensor 1 9. It may be adjusted to provide a desired cooking period with the aid of a switch 21 and it drives a numerical display 22 which displays the time that remains of the cooking period.

In this embodiment of the invention, there is provision for an adjustable delay before cooking commences. The duration of the adjustable delay is controlled by a timer 23 which may be adjusted by a switch 24 and which drives a display 25 that displays the time which is yet to elapse before cooking commences.

An important feature of the present invention is a thermal sensor 26, constituting the aforementioned first thermal sensor. This may comprise a thermistor or other sensor of temperature. It is set to detect a temperature, such as a temperature in the range 95 to 980C, which is associated with the production of steam. The purpose of this temperature sensor is to cause the sealing of the cooking chamber at a time when the air in the cooking chamber has been at least partly displaced by steam. For efficient cooking, the cooking chamber should be as full as possible with steam.

The temperature sensor 26 may be located in or near the outlet which is controlled by the aforementioned valve, but it might be placed at some other suitable place provided that it could reliably detect the temperature within the cooking chamber.

The sensor 26 signals a driver circuit 27 when it detects the requisite temperature; the driver circuit closes a switch which feeds electric current to the valve 28.

The valve's driver circuit 27 has a control terminal governed by the latch 18 through a delay 29 so that the driver circuit is switched off and the valve is opened thirty seconds after the end of the cooking period. This control is inhibited if slow depressurising of the cooker is selected. The latch 18 is also coupled to a tone generator 30 and a loudspeaker 31 to produce audible signals at, or after, the end of the cooking period.

The electrical parts of the interlocks comprise a switch 32 in the line coupling the circuit 27 to the valve 28. This switch 32 is ganged with a (manually-operable) switch 33 in the line to the resistorfor"HEAT1", a switch 34 in series with the switch 1 7 and a switch 35 which is connected between a zero-volt line and an inhibit line 36 from the timer 20 to the circuit 15.

Another electrical latch 37 is connected by a starting switch 38 to the latch 18 and by a manual override switch 39 to the resistor for "HEAT 5".

The following is a typical cooking cycle for the cooker. After the cooker has been loaded and the cover interlocked with the base, the delay time and the heating level (out of HEATS 2, 3 and 4) are selected. Fast or slow depressurisation is selected; full depression of the respective pushbutton closes a switch 40 which by-passes the delay 29. If the contents of the cooker are to be kept warm after the cooking cycle is ended, the switch 39 is operated by the respective pushbutton.

Heating of the cooker proceeds until the thermal sensor 26 detects 950C and causes the closure of the valve 28. The temperature within the cooking chamber continues to rise until the selected heat has been reached. At this point the cooking cycle commences and the timer will count down to zero. During cooking the electrical supply to the element is alternately cut-off and restored according to the control exerted by the second thermal sensor and the particular heat (and thereby cooking pressure) which is selected.

When the cooking period ends, electrical supply to the element is (by means of a signal on the inhibit line 36) either cut-off completely or, if desired, reduced to a rate (HEAT 5) which is sufficient to keep the contents warm. An audible warning, provided by the loudspeaker, is given to indicate that cooking is completed. After that time the pressure valve opens. If "slow" depressurisation has been selected, the valve' is not opened.

If the contents are to be kept warm after the cooking period, the audible warning may readily be caused to operate at intervals of one minute, for three seconds at a time, to remind the user that the cooker is still operating. A visible warning may also be provided.

Claims (5)

1. An automatic domestic pressure cooker which includes a cooking chamber, a valve which can close to seal the chamber, and a thermal sensor, which is disposed to sense the occurrence within the chamber of a temperature which connotes the displacement of air from the chamber by steam and to cause consequentially the closure of the valve.

2. An automatic pressure cooker according to claim 1, in which the said temperature is in the range from 50 to 2"C less than the boiling point of water.

3. An automatic pressure cooker according to claim 1 or claim 2, in which an electrical heating element incorporated in the cooker is under the control of a regulator which maintains the chamber, during a period of cooking, at a predetermined temperature above the boiling point of water.

4. An automatic pressure cooker according to claim 3, in which there are adjustable controls for setting the said period and a delay before the period commences.

5. An automatic pressure cooker according to claim 3 or claim 4, including a timer, which at the end of a period of cooking, causes the venting of the cooking chamber and either the cessation or the reduction of the heating of the cooker by the heater.