TEMPERATURE CONTROL FOR A BED HEATER

Abstract: Electric heating device for beds, in particular water beds, incubators and other similar applications in which there are strict requirements as to temperature accuracy and to safety. The problem with such devices is to obtain a sufficiently accurate temperature control without making the design too expensive. The heating device preferably comprises a flat heating element (1) and a switch and control unit (2) with a thermostat (8 - 11). This unit is connected to the heating element through electrical leads (6, 7). Between the heating element (1) and the switch and control unit (2) there is provided a hollow, preferably somewhat flexible hose member (3) which in addition to accommodating the electrical leads (6, 7) also accommodates other components in the device, in particular a sensing element (9) for the thermostat. There is also a general description of a thermostat for electrical heating devices having strict requirements as to temperature accuracy, and comprising an accelaration element (14), in which the heating effect from the acceleration element (14) is adjustable, preferably by means of an adjustable series resistor (17) or a potentiometer in a current supply circuit (15, 16) for the acceleration element (14).

Description: "Temperature control for a bed heater". This invention relates to an electrical heating device for beds, in particular water beds, incubators and other similar applications in which there are strict requirements as to tempera ture accuracy and to safety. Such heating devices comprise a preferably flat heating element and a switch and control unit wit a thermostat. This unit is connected to the heating element thro electrical leads. The invention shall be described in the following with refe¬ rence to the water bed application, as the present heating device primarily has been developed with regard to the particular condi¬ tions found in connection with water beds. It is obvious, howeve that the invention may also with advantage be adapted for other and similar applications, for example incubators for babies, liquid baths and/or soft, possibly stiff heating panels which re¬ quire an accurate temperature control. The heating device with associatedtemperature control has essential * significance in order that a water bed shall be satis¬ factory for the user and with respect to safety. An important factor in this connection is the fact that for a user of a water bed the body will largely have a very close heat conducting con¬ tact with the water in the bed, which may contain as much as 800 liters of water. For most people the ideal skin temperature is about 32°C. At this temperature the heat condition of the body is in the best balance with the environment, and measurements both on newborn babies and adults show that the breathing fre¬ quency as well as the breathing depth are at the lowest at this temperature, i.e. the metabolism is at its lowest and the best resting conditions are obtained. For newborn babies it may be a direct life-threat if the skin temperature sinks only by 2° C even if the inner body temperature is normal. Experiments have also shown that only an increase or a decrease of 1 C in tempera ture, can be clearly felt and has influence on the comfort ex¬ perienced by the user of a water bed. Thus, it is quite decisive for the usefulness of water beds, incubators or the like that the temperature control is effected with high accuracy and with the least possible temperature fluctuations (difference between switching on and off by the thermostat) . As mentioned known electrical heating devices for water beds have a flat design with a heat surface which may for example have dimensions of 100 x 20 cm and a thickness of about 0,5 cm. The current supply takes place through a rubber cable which may be vulcanized to the heating element so that the structure will be water tight. The heating element itself is arranged underneat a safety sheet which in turn is lying underneath the actual water bag which contains the water in the water bed. Accordingly, the water is heated through two sheet layers, normally sheets of plastic material. A thermostat for the temperature control is usually mounted on the outer side of the water bed. The thermostat may be con¬ trollable in a temperature range from 20 to 36° C. and may for example be of the capillary tube thermostat type. The tempera¬ ture sensor must be positioned underneath the water bag and the safety sheet in order to sense the water temperature as correctly as possible. From the sensing element /the capillary tube is then extended to the actual thermostat in the control box at the outsi of the bed, in order to make possible comfortable operation of switches, control knobs, etc. The electrical cable from the heating element is either permanently connected or by means of a separatable plug connected to the control box and the thermosta From the control box there is provided a mains chord with a plug to a wall outlet. In addition to a control knob the control box may also be provided with a glow-lamp which shows whether or not the heating element is switched on. Common capillary tube thermostats have a switching differenc (temperature difference between switching the thermostat on and off) which is not below 1 - 2 C. Under practical conditions in connection with a water bed the temperature differential will as a rule be at least 2° C, which may be due to variations in the individual thermostats as well as the positioning of the tempera¬ ture sensing element of the thermostat in relation to the actual heating element and the mattress of the bed. This may inter alia have to do therewith that the user has not located these parts of the electrical heating device in the correct way, as for example stated in an instruction for use. It is obvious, however, that the accuracy of the temperature control by the thermostat depends upon how the heating element and the temperature sensor are posi tioned in relation to each other. Thus it is a substantial problem in connection with water beds, incubators or the like that the switching difference which is obtainable by means of capillary tube thermostats and other known thermostat types, is- not satisfactory. This has led to th development of electronic thermostats with a rated switching difference of 0,3 C for bed heaters. Such electronic thermosta however, are more complicated and involve far higher risks of failure than what is the case with for example the known capilla tube thermostats. When employing bed heaters in hospitals, electronic thermostats frequently cannot be used because of electrical interference inter alia on electronic monitoring equipment. Besides, the electronic thermostats are significantl more expensive so that the employment thereof is more or less ex cluded also for that reason. Another consideration of importanc is that there is here the question of electrical installations and equipment which must satisfy official regulations which auth rizing bodies in the different countries use as a basis for checking such equipment. The present invention has for an object to provide an elec¬ trical heating device of the kind stated in the introduction to this description, which eliminates the deficiencies and incon¬ veniences found in the known designs of such heating devices, at the same time as it will be possible to lower the manufacturi costs and obtain a better quality, including a higher accuracy in the temperature control and a better electrical safety. In an electrical heating device as stated in the introduc¬ tion to this description this is obtained by means of the invent in the first place thereby that between the heating element and the switch and control unit there is provided a hollow, continuo and somewhat flexible hose member which in addition to accommo¬ dating the electrical leads, also accommodate other components in the device, in particular a sensing element for the thermostat positioned in the hose member so close to the heating element that it senses the temperature in the region to be heated thereby With this solution there is obtained an electrical heating device having all the necessary parts and components which con¬ stitute a convenient assembled unit, but nevertheless designed so that the users have a certain flexibility with respect to the positioning of firstly the actual heating element and secondly the control box, without any practical problems or uncomfortable effects resulting from the necessary connection between these. It is a substantial advantage that the sensing element of the thermostat will have a well defined position with respect to the heating element, at one hand so that the sensing element will sen the temperature at a correct location with regard to an accurate control, and at the other hand so that the sensing element will not inadvertently or erroneously lie more or less outside the region to be heated by the heating element. This latter situa¬ tion could involve a risk of overheating and possible injury. Whenapplied to water beds, the thermostat sensor is positioned so as to sense the water temperature at the same time as it has a controlled heat influence from the heating element itself, and possibly from an acceleration element to be described more closer below. The result of this is that the switching differential of the thermostat will be as desired. Not the least the solution given here at the same time in¬ volves an advantageous mechanical protection and an outer electri cal insulation of the leads and components inside. In the following the invention shall be explained more close with reference to the drawing, which partly simplified and schematically shows an electrical heating device according to the invention. The main parts of the heating device shown are the actual heating element 1, a switch and control box 2 and a hose member 3 which serves for the mechanical and electrical connection be¬ tween the heating element 1 and the box 2. For the mechanical anchoring of the hose member to the heating element and the con¬ trol box respectively, there are shown screw clamps 5 and 4 whic can for example be similar to known chordanchorage devices. In order to obtain a water-tight design the heating element 1 may b enclosed by a plastic enclosure consisting for example of a flexible plastic sheet 19 which at a mouth part is heat sealed t the hose member 3 as indicated at 19, or the plastic sheet enclo sing the heating element can have a sealed channel which enclose the plastic hose as. far as beyond the edge of the bed. - The intermediate hose member is so long that the heating pa nel with the sensor may be located at a best suitable location under the water mattress and the regulator with thermostat at a suitable location outside the bed. A usual length is about 1 - 1,2 meters. In order that the user shall not be able to feel the heating element and the hose through the water mattress, for example when sitting in the bed, it has a diameter of not more that 12 - 15 mm and is made of a soft material which upon pressure from the water mattress may be somewhat flattened. For electrical current supply to the heating element 1 the are provided leads 6 and 7 which extend through the hose member 3 and which as shown purely schematically, are extended through the box 2 to outer supply leads 6a and 7a, which may for example terminate in a plug for cooperation with a socket outlet. The structure of the heating element-may be based upon known designs, and one way of construction comprises two metal plates having grooves for two electrical tube elements with a wound en¬ capsulation of for example teflon sheet. The plates are rolled together along their longitudinal edges and at the short edges thereof there are mounted two covers of insulating material fastened to the plates by screws. At one end the tube elements are interconnected through a fuse. At the other end and as indi¬ cated above, the elements are connected to a mains current supply through a thermostat as schematically shown in the box 2 and as shall be explained more closely below. The thermostat arrangement in this embodiment comprises a capillary tube thermostat having a sensing element 9, a capillary tube 10 and a bellows 8 which actuates a switch element 11. Thus the switch element of the thermostat is connected in series with the heating element in the current supply circuit 6a, 7a, 6, 7. The sensing element 9 is inserted into the interior cavity in the hose member 3 up to a point at a chosen distance from the heating element 1, the capillary tube 10 together with electrical leads being extended into the box 2, in which the capillary tube 10 is shown schematically extended to the bellows 8. If necessary, there may be employed some form of attachment means in order to fix the sensing element 9 at the desired location in the hose member 3. It is also possible to arrange other components in the cavit of the hose member. As an example there is indicated a component 12 which may be a diode being provided in a common manner in orde to control the heating effect, for example by adjustment to full and half power, respectively. Such- a diode 12 will develop heat and the locating thereof in the control box 2 may involve problem By locating the diode 12 in the hose member 3 it is possible to obtain a cooling effect as a consequence of the adjacent water in the water bed. Also other components may be thought of as parts of the electrical circuit for such a heating device, with corresponding undesired heating effects which may be eliminated by locating the components concerned in the hose member 3 in a way corresponding to what is explained with respect to the diode T2. In the design shown the plastic hose 3 will interconnect the heating element 1 and the control box 2 so that there is obtained a continuous or integrated mechanical unit in which there is taken advantage of the intermediate cavity within the hose member for accommodating essential parts and components in the assembled electric heating device. When the hose consists of a plastic * material, it will serve as an outer insulation of the electric leads and components so that the complete structure will have a double insulation. The hose member also gives a very important mechanical protection for both the sensing element 9 and the capillary tube 10. These are components which are usually sensi¬ tive to mechanical injury. Another significant component in addition to those mentioned above, is an acceleration element 14. Such acceleration elements are previously known per se, for example in thermostatically con¬ trolled heaters for room heating. In connection with the electri heating device shown here the switching differential of the capillary tube thermostat employed, may be improved by mounting the accerlation element 14 on the sensing element 9. This is'not possible unless the sensor for the thermostat together with the lead to the acceleration element are protected by an outer plasti hose. The accuracy and the lifetime- of a thermostat system are closely related to the condition that the temperature influence of the acceleration element on the sensing element is closely adapted to the switching differential of the thermostat. This has to do with the fact that the lifetime of a thermostat is fre quently stated in the number of switch operations. Therefore it is important that the switching frequency is not too high. Ther will then be an accurate temperature control, but the lifetime i reduced, whereas a low switching frequency gives large switching differences, but a longer lifetime. Besides, a thermostat when it becomes worn, will have a larger switching difference. Usual the acceleration element is experimentally adapted to each separ thermostat type being manufactured, and the adaption is performe in such a manner that at an average there will be satisfactory conditions, and this form of one time adaption is usually suffi¬ cient for most purposes. For reasons having to do with producti however, the switching differential of the thermostats will have a tolerance in practice, for example indicated by a differential range of 1,5 - 2° C. If these tolerances with respect to diffe¬ rential shall be reduced, the manufacturing costs will increase substantially. For obtaining a maximum of accuracy in practice of the tempe rature set in thermostats, when applied to water beds, incubators or the like, there is according to the invention proposed a simpl control method which is illustrated on the drawing. As shown, th acceleration element 14 by means of lead connections 15 and 16 is connected into the current supply circuit consisting of the above mentioned leads 6 and 7 as well as 6a and 7a , respectively. The lead 16 extends from the acceleration element 14 to an adjustab resistor 17 which makes it possible to control the effect of the acceleration element within certain limits. With this solution the resistor 17 may be so adjusted that the acceleration element in each individual case or under varying conditions of operation is adapted as well as possible first to the thermostat concerned and second to the particular conditions under which the same operates in the use or situation concerned. For adjusting the resistance value there is indicated a control element 17a, for example a rotary knob outside the box 2. It is obvious that this adjustability of the acceleration element by electrical means, may be carried out also in other ways, for example by means of a potentiometer. By introducing such an adjustment device, for example in the form of a variable series resistor, there may •» be employed thermostats having a larger differential, larger tolerances, a better lifetime and a lower price.

Claims: _, . C L A I M S: 1. Electric heating device for beds, in particular atfer b incubators and similar applications, in which there are fetri requirements as to temperature accuracy and to safety, compr a preferably flat heating element (1) and a switch and contr unit (2) located at some distance from the heating element, with a thermostat (8 - 11), said unit being connected to the heating element through electrical leads (6, 7), characteriz in that between the heating element (1) and the switch and c unit (2) there is provided a hollow, continuous and somewhat flexible hose member (3) which in addition to accommodating electrical leads (6, 7) also accomodates other components in device, in particular a sensing element (9) for the thermost positioned in the hose member so close to the heating elemen (1) that it senses the temperature in the region which is to heated thereby. 2. Device according to claim 1, characterized in that- " the t stat sensing element (9) is mounted so as to have a fixed and well defined position in the hose member (3) . ' ' . 3. Device according to claim 1 or 2, characterized in- that hose member (3) consists of an electrically insulating materi preferably a plastic material. 4. Device according to claim 1 , 2 or 3, characterized in th the hose member (3) together with the heating element (1-) con stitutes a watertight structure (18, 19) . 5. Device according to any one of the preceding claims;, characterized in that among said other components there are c ponents (12) which develop heat which is undesired per se, du operation of the heating device, and which are so positioned the hose member (3) that this heat is given off to substantia the same surroundings as the heat from the heating element (1 itself. 6. Device according to any one of the preceding claims, characterized in that associated with the sensing element (9) there is provided an acceleration element (14) as known per s the heating effect of which is adjustable (17) by means.of a control element (17a). _ , _-- .

Inventors: WESSELTOFT PER, WESSELTOFT, PER
Applicants: WESSELTOFT PER, WESSELTOFT, PER
Application year: 1984
Main classification: H05B1
IPC codes: G05D23, H05B1, H05B3