SENSOR FOR MEASUREMENT BY ELECTRICAL HEATING

Abstract: A sensor for measurement by electrical heating has a core rod (2) covered with an electrically insu­ lating member (3), a thin metal wire (4) wound about the core (2), and an electrically insulating member (5) co­ vering the thin metal wire (4).

Description: BACKGROUND OF THE INVENTION This invention relates to a sensor used for a so-called electrical heating measurement in which a metal member is inserted in various liquid materials or semi-solid materials, the metal member is heated under the application of electrical current to result in higher temperature than those of said materials, and changes in a parameter of the metal member are continously measured in relation to time lapse to detect changes in state of the materials. The inventors of this application have already proposed, in Japanese Patent Application N° 58-92079 (EP-A-144443), a measuring method as below. According to this method, in a milk curdling process, a thin metal wire is inserted in milk stored in a tank, an electrical current is passed intermittently or continuously through the thin metal wire in relation to time lapse, and a curdy state of milk is judged from a measurement result. In this method, a sensor made of straight platinum wire of 0.1 mm diameter and 106 mm length is used. The sensor of platinum wire is however disadvantageous in that the platinum wire will be broken when intensively pressed by fingers, in that a tank whose interior surface is not applied with insulating treatment will possibly be eroded and perforated under the influence of electrolytic corrosion when the sensor is used for a long time, and in that even though the entire length of the sensor is relatively large, a large amount of electrical current is required for heating. Heating devices are known for use in gas-analysis, which comprise a metallic core surrounded by a glass insulating layer. A resistance wire is wound around the glass insulating layer and subsequently covered by a second glass layer such that either end of the resistance wire extend outside the second glass layer for connection to electrical lead posts. Such a device is described in DE-C-881 845. While such devices present the convenience of allowing the length of the wire to be increased as compared to the straight wire, the amount of generated heat per unit length of the sensor being increased proportionally, thereby making it possible to reduce the amount of current for obtaining the same sensitivity, the construction of these devices is still extremely fragile. Until present, no sensor capable of functioning in the type of application described while at the same time providing a good shock resistance has been proposed. SUMMARY OF THE INVENTION The present invention contemplates elimination of the above drawbacks of the conventional sensor, and thus has for its object to provide a shock-proof sensor which does not erode the tank under the influence of electrolytic corrosion and which can reduce the amount of current required for heating. According to this invention, the above object is realized by a sensor comprising a core rod, a first electrically insulating member covering said core rod, a thin metal wire wound about said first electrically insulating member and, a second electrically insulating member covering said thin metal wire. The sensor of the invention is characterized in that the first and second electrically insulating members are thermally contractible tubes. The thin metal wire wound about the core rod thus has considerably improved shock-proof properties and is therefore difficult to break. Also, the thin metal wire covered, interiorly and exteriorly thereof, with the electrically insulating members will not erode the tank under the influence of electrolytic corrosion. The invention also proposes a method for fabricating a sensor of the type defined above. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: Fig. 1 is a fragmental longitudinally sectional front view of a sensor according to an embodiment of the invention; Figs. 2A to 2G are diagrams sequentially illustrating manufacture steps of the embodiment; Fig. 3 is a schematic diagram of an automatic measurement and control system for a curd making process in which the sensor Is exemplarily used; Fig. 4 is a fragmentary sectional front view showing the state of the sensor used in the system of Fig. 3; and Fig. 5 is a graph showing a curd characteristic. DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 1 shows, in fragmentary sectional form, a sensor 1 embodying the invention. The sensor 1 has a core rod 2 such as a stainless steel rod, an electrically insulating member 3 made of, for example, teflon, a metal wire 4 made of, for example platinum and helically wound about the electrically insulating member 3, an electrically insulating member 5 made of the same material as the member 2 and covering the metal wire 4. Figs. 2A to 2G show the manufacture steps of the sensor. In Fig. 2A, a stainless steel rod 2 of 2 mm diameter is first covered with a thermally contractible tube 3 of teflon having an inner diameter of 3 mm, and as shown in Fig. 2B the tube 3 is heated and contracted by blowing hot air at about 400°C from a burner 20. Subsequently, as shown in Fig. 2C, the surface of the teflon cover is finished with a file 19, and a platinum wire 4 is helically wound about the finished surface and slightly bonded thereto with an adhesive, as shown in Fig. 2D. Thereafter, a thermally contractible tube 5 made of teflon and having an inner diameter of 3 mm is applied on the platinum wire coil as shown in Fig. 2E and heated for contraction by blowing hot air at about 400°C from the burner 20, as shown in Fig. 2F. Thus, a sensor 1 is completed as shown in Fig. 2G. For example, to carry out automatic measurement and control in curd making process with a system as shown in Fig. 3, two of the thus prepared sensors 1 are respectively fixed to flange plates 6 as shown in Fig. 4 to constitute sensor units 8 and 8', and sensor units are respectively mounted to whey discharge flanges 21 which extend from an end wall of a lateral type cylindrical cheese tank of about 20 m³ capacity and 4 m diameter at heights thereof corresponding to 5 m³ and 12 m³ levels. The sensor units 8 and 8' are supplied with an electrical current for measurement of voltage through multicore cables 18 shielded with a copper tape and composed of signal wires each having a crosssectional area of 1.25 mm², so that electrical resistance of each sensor can be measured on the basis of a four-wire method. As shown in Fig. 3, four current lead wires 9 connected to the signal wires connect to a DC power supply 10 which is controlled by a controller 17 comprised of a computer, and the other four voltage lead wires 11 connect to an A/D converter 13 and a relay card 14 of a data loader (data collecting controller) 12. The data loader 12 and the power supply 10 are connected to the controller 17 through GP-IB cables 15 and 16, respectively. All instructions necessary for measurement and sequence control are issued in accordance with a program pursuant to a standard curd characteristic curve which has been read in the controller. These instructions are fed via a sequence control system 18 to drive a motor, for example. The standard curd characteristic curve is representative of changes in temperature or electrical resistance of the thin metal wire of the sensor in relation to time lapse and illustrated in Fig. 5. In Fig. 5, B and B' denote temperatures of the thin metal wires when supplying milk, C a metal wire temperature when adding rennet, D a metal wire temperature at the termination of stirring, E a metal wire temperature at the commencement of curdling, F a metal wire temperature at the commencement of curd cutting, G changes in metal wire temperature when cooking, and H and H' temperatures of metal wires when discharging whey. For automatic control of cheese curd production by controlling the above parameters, the controller 17 first instructs the automatic power supply 10 to start supplying a constant DC current to the sensors 1 and the data loader 12 to start measuring voltages. Measured voltages are sent to the controller 17 so as to be converted into corresponding temperatures or electrical resistances which in turn are used to display a temperature versus time curve or an electrical resistance versus time curve on a cathode ray tube attached to the controller 17. When the commencement of curdling represented by E is detected after completion of supply of milk, a timer included in the controller is actuated for a predetermined time, followed by transmittal of a signal for curd cutting to the sequence control system 18 and consequent actuation of a motor (not shown) for rotating cutting blades. Finally, whey discharge is detected and measurement results are delivered out to a printer and a magnetic disc, ending an operation. Specifically, the sensor 1 has a stainless steel rod 2 of 2 mm diameter and 5 cm length, a teflon member 3 of 0.15 mm thickness covering the rod 2, a platinum wire 4 of 0.1 mm diameter wound about the teflon member 3 by ten terns per 1 cm of the stainless steel rod 2, and a teflon member of 0.15 mm thickness covering the coiled platinum wire 4. If the thickness of the outer teflon member of the sensor 1 is large, then response time will be increased. Therefore, practically, the outer teflon member preferably has a thickness which approximates the diameter of the platinum wire 4. The lower limit of this thickness may desirably be defined within a range over which mechanical strength and electrical insulation of the outer teflon member can be warranted and leakage of current can be prevented. When the sensor 1 is applied to foodstuffs such as cheese, this sensor must be cleaned for sterilization by using chemicals of strong alkali and strong acid. Therefore, the outer cover is preferably heat-proof and chemicals-proof and its plasticizer is desired to be insoluble from the standpoint of sanitation of foodstuffs. The thickness of the inner teflon member 3 of the sensor 1 may desirably be selected but is preferably defined within a range over which mechanical strength and electrical insulation of the inner teflon member can be warranted and heat dissipation can be prevented sufficiently. In addition, the pitch of the metal wire coil must be increased in proportion to an expected amount of heat generation. Further, if the metal wire is wound by a number of turns which is more than ten per 1 mm length of the stainless steel rod, the output will disadvantageously become unstable. Although a particular preferred embodiment of the invention has been disclosed in detail for illustrative purposes, it will be recognized that variations or modifications of the disclosed apparatus, including the rearrangement of parts, lie within the scope of the present invention as defined in the claims.

Claims: A sensor for measurement by electrical heating comprising: a core rod (2), a first electrically insulating member (3) covering said core rod (2), a thin metal wire (4) wound about said first electrically insulating member (3) and, a second electrically insulating member (5) covering said thin metal wire (4), characterised in that said first and second electrically insulating members are thermally contractible tubes. Method for fabricating a sensor for measurement by electrical heating, characterised in that it comprises the steps of: covering a core rod (2) with a thermally contractible first electrically insulating tube (3); applying heat to said first electrically insulating tube so as to contract same; winding a thin metal wire (4) about said first electrically insulating tube (3); covering said first electrically insulating tube (3) together with said thin metal wire (4) with a thermally contractible second electrically insulating tube (5); and applying heat to said second electrically insulating tube so as to contract same. Method according to claim 2, characterised in that it further comprises the Step of abrading the surface of said first electrically insulating tube (3) after the application of heat thereto and prior to the winding thereon of the thin metal wire (4). Method according to either of claims 2 or 3, characterised in that the thin metal wire (4) is slightly bonded to the first electrically insulating tube by means of an adhesive. Method according to any one of claims 2 through 4, characterised in that said thermally contractible first and second electrically insulating tubes consist of polytetrafluorethylene. Method according to any one of claims 2 through 5, characterised in that said thin metal wire is platinum.

Inventors: HORI, TOMOSHIGE, ITOH, KENSUKE
Applicants: SNOW BRAND MILK PRODUCTS & CO., LTD.
Application year: 1986
Main classification: G01N33
IPC codes: G01N33, G01N25, G01N27