NO154648B - SENSOR ELEMENT FOR CAPACITY LEVEL MEASUREMENT SYSTEM. - Google Patents

Description

The invention relates to improving, sensor elements for use in

a. kaps.sit.lvt measuring system for the level of an electrically conductive liquid's interface with an overlying non-conductive fluid. Examples of conductive fluids include water, especially seawater, and non-conductive fluids can be, for example, air or oil.

In most known capacitive level measurement systems, the difference in dielectric constant between the liquid and the medium above is used. Measuring probes can be carried out in different ways. Ln known probe has e.g. form as two long, electrically charged tubes, arranged concentrically, each of the tubes being a "plate" in a capacitor.

i-.n other type has shape similar to. an antenna fiat cable!,

so cross section shown in fig. 1. O' an electric field between the two "capacitor plates", which here are the two chargers in the cable, extends into the surrounding medium, and the capacitance thus depends on the dielectric constant of the medium.

The problem with this measuring principle, however, is that the dielectric constant in the liquid is rarely or never constant. Moreover, the change in capacity measured with the known probes of this type is very small, and difficult to pinpoint precisely. For example gives two concentric tubes a capacity change of the order of magnitude IDO pF/m. Furthermore, there are problems with zero-point operation, due to that a significant pressure is also present when the probe is not surrounded by liquid. Finally, it has proved difficult to make such probes which are mechanically stable with regard to temperature expansion.

Another principle is described in British patent document 1.31B.512. This principle is based on the fact that one of the media, primarily the sink at the bottom, is electrically conductive and ensures contact from an external metal electrode to a thin insulating coating that is attached to an internal, elongated "condenser plate", which is usually cylindrical in shape. thus the second "capacitor plate" is made up of the electrically conductive liquid itself, while the insulating coating is the capacitor's dielectric. dåisd33 it is therefore mainly the real (i.e. longitudinal) of the "condensator piates" which is the variable size. V od

□ rak 11 s k b r u k of this k j o n t o m o i e s o n d an o p p s t f. r .imi. ei .1 a r ii d problems with a lack of mechanical stability during temperature fluctuations. Uette gives low measurement accuracy. In addition, there is a real problem with overgrowth of the probe. Bath biofouling and inorganic deposits can cause incorrect measurements and result in an unreliable system.

["led sensor element according to the invention, which is based on the latter measurement principle, it is achieved that the problem with capacitance measurement errors due to temperature fluctuations is eliminated, as well as.

that the lifespan of the element is significantly increased. Furthermore, growth of sensor elements is prevented to a significant extent.

the element can also be used in fire and explosive environments and is extremely resistant to chemical influences. In relation to the conventional systems mentioned above, a sharp increase in capacitance change per meter of liquid, typically over 500 pF'/m.

This is achieved according to the invention by the metal electrode

in the condenser is made up of the screen on a self-regulating heating cable.

A more detailed description of the invention follows, with reference to the figures, where fig. 1 shows a previously known capacitive sensor cable in cross-section, fig. 2 shows a sensor cable according to the invention, also in cross section, while fig. 5 shows a simple electrical equivalent core fer the cable and the liquid.

A typical, commonly used sensor cable for capacitive level measurement in the liquid, based on the principle of different dielectric constants, is shown in cross-section in fig. i. The capacitance is measured between the two copper conductors 1, which are enclosed along their entire length by a plastic insulation 2, and depends on the dielectric constant of the surrounding medium 3, 'let the electric field as the figure also indicated. permeates the medium j>. In practice, with the solitaire stretched down into a reservoir or a tank, the level for the transition between the two relevant media 1, e.g. water and air, determine the total capacitance of the cable.

In fig. 2 shows a cross-section of a sensor element according to the invention, which is also essentially a solitaire that is lowered into a reservoir or a tank...

sr an electrically conductive wash, for example water, evaniuoit st non-conductive fluid, for example oil, depending on the height at which the cut is placed. in earth ingsoi ektrode r of bare metal is brought down together with the cable, and is in the example in fig. 2 shown with cylindrical shape. However, the shape is not decisive, as the purpose of the electrodes 5 is only to provide electrical contact to the charging liquid. The affective capacitor is formed where the conductive liquid is found, with this liquid itself as the "capacitor plate" next to a dielectric which is made up of a heating cable's outer insulation. Egg 6.

On the inside of the dielectric 6 is the capacitor's second "plate", namely the heating cable's metal shield 7. The capacitor's capacitance is thus measured between the heating cable's shield 7 and the conductive liquid, with which one makes electrical contact via the grounding electrode 5.

The outer insulation seal 6 on the heating cable is made

of a special fluoropolymer with a very smooth surface, which counteracts fouling of both chemical and biological types. The preferred material is a modified fluoropolymer under the trademark "Tefzei" manufactured by Du Pont de Nemours & Co., USmI The material is also highly resistant to chemical attack.

The heating cable is provided within the screen 7 with an insulation layer 8, preferably of fluoropolymer material.

Within layer 3 is the heating element itself, which is a semiconductor material 9, which fills up the core of the cable. Two metal chargers 10 ar embedded in the core. In use, the two metal sensors 10 are connected to a voltage source, e.g. 220 volts.

The semiconductor material's 9 properties (dv/s . temperature-dependent resistivity) mean that a certain temperature is kept constant along the entire length of the cable. Firstly, the problem of capacitance measurement errors associated with temperature variations is thereby removed, and secondly, a constant cable temperature of e.g. 80 degrees Celsius that biological growth is reduced. This will also prevent icing and any oil/wax coating on the cable.

The shape and relative dimensions in fig. 2 are random. The cable does not necessarily have a circular cylindrical shape,

but can e.g. similar to a flat solitaire.

A standard self-regulating heating cable which has been shown to function excellently as a central element in the condenser device according to the invention is manufactured by Raychem Corporation, USA under the designation "Chemelex Auto-Trace QTV2-CT". The cable proves to be very stable

both mechanically, chemically and with regard to temperature.

In fig. 3 shows an electrical equivalent circuit for further illumination of the sensor element's operation. The point 14 represents the grounding electrode 5 in fig. 2. In reality, one must assume that the electrically conductive liquid has a certain resistance Rv. This also includes a contact resistance in the interface between the grounding electrode and the liquid.

Incidentally, the conducting liquid nearer the cable is represented as a capacitor plate 15. The dielectric 16 in the capacitor corresponds to the insulating coating 6, and the capacitor side 17 represents the metal screen 7. It thus appears that the resistance Ru, which must be assumed to be invariable with the current being measured and also over time, will influence the measurements. The measuring bridge that is constructed for the capacitance measurement can, however, be arranged so that variations in Ru mean little for the measurement result.

Claims (5)

1. Sensor element in a capacitive level measurement system for an electrically conductive liquid's interface with an overlying non-conductive fluid, comprising an elongated capacitor device in which one capacitor side (15) is made up of the conductive liquid itself, which in turn is in direct electrical contact with an elongated grounding electrode (5), and the capacitor device's dielectric (16) consists of a thin insulating coating (6) placed around a metal electrode which forms the capacitor device's other capacitor side (17), characterized by that the metal electrode is made up of the shield (7) on a self-regulating heating cable. 2. Sensor element according to claim 1, characterized by that the self-regulating heating cable's screen (7) is tightly enclosed by a thin coating of a fluoropolymer with a particularly smooth surface and high thermal, chemical and mechanical stability, which thin coating (6) constitutes the capacitor's dielectric. 3. Sensor element according to claim 1 or 2 characterized by that the self-regulating heating cable, in a manner known per se, has centrally arranged two metal conductors (10) which are embedded in a semi-conducting material (9), which in turn is tightly enclosed by an insulating fluoropolymer layer (8) located immediately inside the screen (7) . 4. Sensor element according to claim 3, characterized by that all elements of the self-regulating heating cable, i.e. metal conductors (10), semi-conducting material (9), insulating layer (8), screen (7) and thin coating (6), are packed firmly and compactly on known way, and that all elements (6-10) are of materials with particularly low compressibility and high tensile strength. 5. Application of a self-regulating heating cable consisting of a centrally arranged electrical semi-conducting material (9) with two embedded, longitudinal electrical conductors (10) made of metal, which material (9) is tightly enclosed by an electrically insulating fluoropolymer layer (8) which in turn is enclosed by a shield layer (7) of metal and an outer, electrically insulating coating (6) of fluoropolymer material with a particularly smooth surface, as part of an elongated capacitor device which is a sensor element in a capacitive level measurement system for the interface of an electrically conductive liquid against an overlying non-conducting fluid, e.g. the interface water/air or water/oil, the capacitor device further including at least an additional volume of the conductive liquid, which is again in direct contact with an elongated grounding electrode (5).