TEMPERATURE SENSING APPARATUS

Abstract: The invention provides a temperature sensing element which can be connected to a control element by means of a cable. The temperature sensing element is provided with a thermally properly conducting casing which is connected in a moisture-tight manner to the sheath of the cable. The casing of the temperature sensing element consists of a plastics material containing 50-80% by weight of the follow­ ing fillers; magnesium oxide, silicon carbide, beryllium oxide, aluminium oxide. The main constituent of the plastics material and the main constituent of the cable sheath are derived from the same monomer.

Description: "Temperature sensing element." The invention relates to a temperature sensing element having a casing made of a first polymeric plastics material, this element being connected to a cable having a sheath meade of a second polymeric plastics material. Temperature sensing elements of this type are, for example, suitable for use in arrangements which utilize an automatically operating temperature control. This includes cooling apparatus as well as heating arrangements. For temperature sensing elements of this type a casing is often desirable to protect the element from moisture, c-orrosion and such like. Such a casing must, however, be adequately heat-conductive to enable a sufficiently rapid measurement of the temperature. Furthermore it is a requirement that it must be possible to manufacture such a casing rapidly and in a simple way and to connect it in a properly watertight manner to the sheath of the cable connecting the temperature sensing element to a control element. Casings which satisfy the above-mentioned requirements, particularly casings which are properly moisture-tight at temperature up to approximately minus 30 C have so far been unknown. The invention provides a casing which is properly moisture-tight up to a temperature of approximately minus 300C. The casing according to the invention is therefore perfectly suitable for use as a casing for temperature sensing elements in refrigerators, freezers and such like. According to the invention, the temperature sensing element is characterized in that the main constituent of the first plastics material and of the second plastics material is derive d from the same monomer and the casing of the temperature sensing element contains 5O-8OÇo by weight of a filter material chosen from magnesium oxide, silicon carbide, beryllium oxide and an aluminium oxide, calculated on the basis of the total quantity of plastics material plus filler material. In addition to the above-mentioned constituents, the casing may contain still further conventional con stituents such as plasticizers, stabilisers, catalysts and such like. If the cable sheath contains polyvinyl chloride as its main constituent, the casing of the temperature sensing element must also contain polyvinyl chloride as its main constituent. In that case, 50-80/ó by weight of magnesium oxide is preferably used as the filler material. It should be noted that main constituent must be understood to mean the polymeric plastics material present in the material of the casing in the largest quantity. In such sense a casing consisting of 600loo by weightof magnesium oxide and 400 by weight of polyvinyl chloride contains polyvinyl chloride as the main constituent. The casing of the temperature sensing element may be formed by, for example, casting or injection moulding. The invention will now be described in detail by way of example with reference to a Figure. The Figure shows a temperature sensing element (2). A suitable temperature sensing element consists, for example, of a ceramic material having a negative temperature coefficient. Suitable materials ar e described in, for example, Philips Product Data Catalog "Non linear resistor" December 1970, page C5. Via a cable (1) the sensing element (2) is connected to a control element, not shorn. The cable is provided with a sheath of a polymeric material, for example on the basis of polyvinyl chloride. The sensing element (2) is provided with a solid casing (3). This casing is moisture-tight by nature and is connected in a moisture-tight manner to the sheath of the cable. By way of example there now follows some methods of producing the above-described casing. Example 1: A castable mixture having the following compositions was prepared: 1. polyvinyl chloride having a molecular weight of 87.000 50 parts by weight 2. polyvinyl chloride having a molecular weight of 83.000 50 parts by weight 2. dioctylphthalate 78 parts by weight 4. dibutyltindilaurate 2 parts by weight r. epoxidized soybean oil 2 parts by weight 6. titanium dioxide 2 parts by weight 7. magnesium oxide 432 parts by weight. A ceramic element having a negative temperature coefficient was placed in a mould. The element was connected with a cable by means of soldering. The cable sheath consisted of polyvinyl chloride. The above-defined mixture was poured in the mould and kept for 4 minutes at 1500C. The combination obtained was removed from the mould. From tests it appeared that at a temperature from minus 300C to plus 1000C the casing was moisture-tight to a sufficient extent and offers a good protection from corrosion. The dioctylphthalate mentioned sub (3) may be replaced by dibutylphthalate or butylbenzylphthalate. Dibutyltindilaurate (4) may be replaced by dibutyltin or an alkyltin mercaptide; instead of epoxidized soybean oil (5) an alkyl or aryl phosphite may be used and calcium oxide or silicon oxide may be used instead of titanium dioxide (6). Adding polyvinyl acetate to the above-mentioned mixture may still further improve the adhesion to the cable sheath. Example 2: By means of injection moulding (1900C, pressure 900 kg/cm2) a temperature sensing element similar to that as described in Example 1 was provided with a casing consisting of: 1. polyvinyl chloride molecular weight 41.800 100 parts by weight 2. dioctylphthalate 75 parts by weight 3. epoxidized soybean oil 3 parts by weight 4. barium/cadmium stabilizer 1.5 parts by weight 5. titanium dioxide (rutile) 2 parts by weight 6. magnesium oxide 425 parts by weight. The bond between the cable sheath and the temperature sensing element may be further improved by first treating the cable with cyclohexanol or tetrahydrofurane. This also resulted in a casing which is suitable for use at minus 30 to plus 1000C.

Claims: 1. A temperature sensing element having a casing made of a first polymeric plastics material, this element being connected to a cable having a sheath made of a second polymeric plastics material, characterized in that the main constituent of the first plastics material and that of the second plastics material are derived from the same monomer and the casing of the temperature sensing element contains 50-80 by weight of a filler material chosen fromnagnesium oxide, silicon carbide, beryllium oxide and an aluminium oxide, calculated on the basis of the total quantity of plastics material plus filler material. 2. A temperature sensing element as claimed in Claim 1, characterized in that the main constituent of the first and the second plastics materials consists of polyvinyl chloride and that the casing of the temperature sensing element contains 50-80 /ó by weight of magnesium oxide.

Inventors: VAN BOKESTAL, ANDRE MARCEL ALFRED, Van Bokestal, André Marcel Alfred
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1982
Main classification: G01K1
IPC codes: H01B3, G01K1