SENSING ELEMENT FOR A CAPACITIVE LEVEL MEASURING SYSTEM

Abstract: A self regulation heatingcable is used as sensing element in a capacitive level measuring system for an electrically conducting liquid, the metal shield of the heatingcable constituting the one side of the capacitor, while the conducting liquid itself is the other side of the capacitor. The outer isolating layer (6) is the dielectric of the capacitor as the conducting liquid is in engagement with an earth electrode (5). Increased lifetime and reduced biological and chemical growth are attained. Capacitive measuring errors on account of temperature extensions are eliminated. As the heatingcable is given a high temperature and maintained at the temperature, layers of ice, oil/wax or biological materials will not deposit on the sensing element.

Description: Sensing Element for a capacitive Level Measuring System. The invention comprises an improved sensing element for use in a capacitiv measuring system sensing the position of the boundary layer between an electrically conducting liquid and an above lying non-conducting fluid. Water and especially seawater, can be cited as excample of an electrically conducting liqiud, while non-conducting fluid by excample may consist of air or oil- Most of the prior art capacitive level measuring systems take advantage of the relative permittivity caractericing the liquid and the above lying fluid. The measuring probe can be given a number of features, one of which is two concentrical, conducting long tubes constituting each of the "plates" of a capacitor. Another known probe has the shape of antenna cables, see the cross section given on figur 1. The electric field between the plates of the capacitor, here being the two conductors of the cable, propagates out in the surrounding fluid and the capacitance is in this waygiven by the dielectric constant of the fluid. One inherent problem of this measuring principle is that the dielectric constant of the liquid very seldom , if ever, ceeps a constant value. In addition, the capacitive changes measured by the known probes of this type are very small, and difficult to give with great accuracy. Two concentric tubes give for instance a change in capacitance in the order of 100 pF/m. A substantial capacitance is present when the probe is not surrounded by liquid, this causing difficulties known as zero point error. At last there shall be mentioned difficulties conected with making such probes mecanically stable with regard to temperature expansions. British patent no. 1.318.512 describes a different principle. This prin¬ ciple is based on that one of the fluids, primarily the lower liquid, is electrically conducting and causes contact from an outside lying metal electrode to a thin isolating layer fixed on a long inner capacitor "plate", primarily given sylindrical shape.This way the electrically conducting liquid constitutes the second "plate" of the capacitor, while the isolating layer makes up for the dielectric of the capacitor. Thus, it is mainly the area (or the length) of the * capacitor "plates" that is the variable quantity. In use, this known measuring probe shows lack of mecanical stability to temperature changes. This gives a low measuring accuracy. Besides, there is a real problem concerning growth on the probe. Both biological growth and inorganic deposits can cause malfunction and give an unreliable system. With a sensing system according to this invention, hich is based on the last measuring prinsiple, elimination of the problem regarding uncorrect capacitances due to temperature changes is achieved, while the lifetime of the element is substantially increased. Further, growth on the sensing element is substantially redused, and the element can be employed in areas subjected to danger from fire or explosion, and is extremly with¬ standing against chemical influences. In opposition to the previously cited conventionel systems, there is achieved a strong increase in capaci¬ tance changes per meter liquid, typically more than 500 pF/ . This is achieved according to this invention by letting the shield of a self regulating heatingcable constitute the metal electrode of the capacitor. A closer description of the invention is given in the following, refer¬ ences given to the drawings where fig. 1 shows a prior art capacitive sensing cable in cross section, where fig. 2 shows a sensing cable according to the invention (in cross section) and where fig. 3 shows a simple electrical equivalent diagram giving the cable and the liquid. Fig. 1 shows in cross section a typical, often used sensing cable for capacitivly measuring levels ina liquid, the measurement being based on the principle of different relative permittivity. The capacitance is mea¬ sured between the two copper wires 10 shielded in the entire length of - a plas ic isolation 2, and is given by the relative permittivity of the surrounding liquid 3 as the electric field shown on the figure penetrates the Tiqiud 3. In use with the cable inserted in a reservoir or tank, the level of the boundary between the two actual fluids, for instance water and air, will give the totale capacitance of the cable. Fig. 2 shows a cross sectional view of a sensing element according to the invention, where the sensing element mainly in this case too is a cable inserted in a reservoir or a tank. The surrounding medium 4 is an electri¬ cally conducting liquid, for instance water, eventually a non-conducting fluid, for instance oil, in dependence of where on the cable the cross section is done. An earth electrode 5 of unshielded metal is inserted together with the cable and is, in the excample of fig. 2, shown with a cylindrical shape. The actual shape is not important, as the purpose of the electrode 5 is to give electric contact with the conducting liquid. The working capacitor is created where the conducting liquid is situated, and the very liquid acting as a capacitor "plate" next to a dielectric made up of the outer isolating layer 6 of the heatingcable. Inside the dielectric 6, the metal shield 7 of the heatingcable- consti¬ tutes the second "plate" of the capacitor. The capacitance of the capa¬ citor is thus measured between the shield 7 of the heatingcable and the conducting liquid, which is given electric contact via the earth elec¬ trode 5. The outer isolating layer 6 of the heatingcable is made from a special fluoric polymer with a very smooth surface, which counteracts growth from both chemical and biological reasons. The preferred material is a modified fluoric polymer produced by DuPont de Nemours & Co., USA under the trade mark "Tefzel". This material is also very resistant to chemical attacks. Inside the shield 7 the heatingcable is given an isolating layer 8, preferentially of a fluoric polymer material. The heating element itself is situated inside the layer 8 and consists of a semiconducting material 9 which fills up the core of the cable. Two metal wires 10 are incorpor¬ ated in the core. In use the two metal wires 10 are connected to a voltage of for instance 220 volts AC. The characteristics of the semiconducting material 9 (temperature dependent resistivity) give as a result tempera¬ ture being kept constant over the full length of the cable. This way the problems arising from capacitance instabilities due to temperature vari¬ ations are eliminated. Further, a constant cable temperature of for instance 80 °C, involves biological growth being reduced, gives a deicing effect, and prevents oil - or wax-deposits on the cable. The shape and relative dimensions given on fig. 2 are occasional. The cable do not need to have a sircular cylindrical shape, it may resemble the shape of a flat antenna cable. A standard self regulating heatingcable functioning excellent in use as central element of the capacitor device according to the invention, is pro¬ duced by the Raychem Corporation, USA, under the name ''Chemelex Auto-Trace QTV2-CT". This cable shows high stability in mechanical as well as chemical and temperature relations. Fig. 3 shows an electric equivalent diagram for further understanding of the functioning of the sensing element. The point 14 represents the earth electrode 5 given, on fig. 2. In practical life the electrically conducting liquid has to be given a certain resistance R . Included is a contact resistance in the boundary between the earth electrode and the liquid. The figure represents the part of the conducting fluid nearest to the cable as a capacitor "plate" 15. The dielectric 16 of the capacitor corresponds to the isolating layer 6, and the side 17 of the capacitor represents the metal shield 7. From this it is evident that the resistance R , expected to be dependent of both measured level and time as well, will have influence on the measure ments. This can be overcome by letting the measuring bridge constructed for the capacitance measurements,be arranged ina way giving. aria ions in R practically no influence on the measured level.

Claims: Claims. 1. Sensing element for a capacitive level measuring system positioning the boundary between an electrically conducting liquid and an above lying non-conducting fluid, comprising an extended capacitor device of which, the conducting liquid itself constitutes the first capacitor side (15) and is in conducting engagement with a long earth electrode (5) , the isolating layer (6) constitutes the dielectric medium (16) of the capacitor and is surrounding a metal electrode which constitutes the seco side (17) of the capacitor, c h a r a c t e r i s e d i n t h a t the metal electrode is the shield (7) of a self regulating heatingcable. 2. Sensing element according to claim 1, c h a r a c t e r i s e d i n t h a t the shield (7) of the self regulating heatingcable is closely surrounded with a thin layer of a fluoric polymer whith an especial smooth surface and high thermal and mecanical stability, which thin layer (6) constitutes the dielectric of the capacitor. .4. Sensing element according to claim 3 . , c h a r a c t e r i s e d i n t h a t all the elements of the heatingcable, that is metal wires (10), semiconducting material (9), isolating layer (8), shield (7) and thin layer (6), are compactly and firmly packed, and that all the elements (6-10) are made up of materials showing very low compressibility and capable of sustaining high tensions. 3. Sensing element according to claim 1 or 2, c h a r a c t e.r i s e d i n t h a t two metal wires (10) are situated in the centre of the self regulating heatingcable and embedded in a semiconductin material (9), which also is closely surrounded whith an isolating layer of fluoric polymer (8) situated directlyinside the shield (7) . 5. Method for a capacitive level measuring system positioning the boundary between an electrically conducting liquid and an above lying non-conductin fluid by use of a sensing element according to claim 1 , the capacitance being measured between the conducting liquid itself and the metal electrod of the sensing element which electrode is constituted by the shield (7) of the self regulating heatingcable, c h a r a c t e r i s e d i n. h a the cable is heated, by supplying the two centrally distributed metal wires (10) with current, to a temperature sufficient high for obtaini an effective reduction of possible layers, e.g. of ice, oil/wax or layers of biological origin, for instance a temperature of 80 C, and being maintained at that temperature. 6. The use of a self regulating heatingcable, compricing a centrally distributed electrically semi-conducting material (9), having two embedded long electrical metal -wires(10) , which material (9) is closely surrounded by an electrically isolating layer of a fluoric polymer (8), which in turn is surrounded by a metal shield (7) , and outermost an electrically isolating layer (6) of a fluoric polymer material with an especial smooth surface, as part of an extendedcapacitor device acting as sensing element in a capacitive level measuring system for determining the boundary between an electrically conducting liquid and an above lying n conducting fluid, e.g. the boundary between water/air .or water/oil, the capacitor device further compricing at leastan adjacent volum of the con¬ ducting liquid, which is in conducting engagement with a long earth electrode (5) .

Inventors: GOTAAS EINAR, GOTAAS, EINAR
Applicants: SKAND INSTR AS, SKAND INSTRUMENT AS
Application year: 1985
Main classification: G01F23
IPC codes: G01F23