CAPACITIVE MEASURING DEVICE

Abstract: A device for measuring the level of admission in a recipient for flowing substances, for example, fluids, com­ prising a capacitive Teflon-coated probe connected to an electric processing circuit (18) and inserted into the re­ cipient (22) said probe consisting of at least two relatively isolated measuring pipe-sections (1, 2) in line with one another and one of which serves to fix a reference value regarding the properties and nature of the fluid, whereas the other section concerns the measurement of the fluid level, said probe being enveloped by a grounded tube (27) connected with the wall of the recipient, whereas the top of said probe is mounted in a housing by means of a sealing being formed by at least one annular filling body (12) be­ tween a widened part of a bore in said housing and the coating of the probe, whilst pressing means impose an axial load on the or each annular body, and wherein the electric processing circuitry (18) includes communicating means for simultaneously energizing one measuring sec­ tion of the probe and grounding the other measuring sec­ tion and conversely.

Description: Capacitive measuring device The invention relates to a device for measuring the level of admission in a recipient for flowing substances, for example, fluids, comprising a capacitive probe connected to an electric processing circuit and inserted into the recipient. The invention has for its object to construct the probe so that apart from providing a very reliable measurement the probe permits of being used in recipients for any kind of substance, that is to say, chemically aggressive substances, explosive and poisonous substances. According to the invention the accuracy of the measurement is improved by using a probe consisting of at least two relatively isolated measuring sections. One section serves to fix a reference value regarding the properties and nature of the fluid, whereas the other section concerns the measurement of the level of admission in the recipient. Thanks to this division of the probe it is possible to use the device universally, since the disturbing influence of the changing properties of the filling substance is automatically levelled out. In order to raise the accuracy the two sections according to the invention are formed by pipe-shaped parts which are in line with one another, the lower pipe par being connected with the electric processing circuit through the upper pipe part. Preferably, said conductor is centered in the upper pipe part by means of foam plugs. Owing to the centrally arranged conductor a disturbing measuring error due to an additional capacity measurement between said conductor and, for example, the wall of the recipient is reduced by the upper pipe part as a result of its screening effect. Since the lower pipe part is used for the reference measurement on account of its low position, an accurate reference measuring value is obtained. In order to provide an independence of the shape of the recipient a pipe part which is open at the lower end and which has a shape similiar to the probe is arranged at a distance around the measuring probe and connected with the wall of the recipient so that the grounded side of the capacity is ensured. The flowing substance can flow through the open bottom end of the enveloping pipe part into the space between the probe and said pipe part so that the dielectric constant of the medium between probe and enveloping pipe part changes, which enables the measurement of the level of admission. In order to render the probe resistant to the attack of the flowing substance, the two pipe parts of the probe are covered by an insulating syntheric resin coating, preferably of Teflon. In order to avoid hazards of leakage and explosion the probe according to the invention is inserted at the top end into a bore of a housing to be connected with the recipient, the seal between housing and probe being formed by at least one annular filling body between a widened part of the bore and the coating of the probe, whilst pressing means impose an axial load on the or each annular body. The or each annular body preferably has a wedge-shaped longitudinal sectional area, the apex of the wedge being smaller than 30". It is thus ensured that the Teflon coating on the probe is firmly retained so that the probe remains free of leakage and corrosion respectively. According to the invention the electric processing circuitry includes communicating means for simultaneously energizing one measuring section of the probe and grounding the other measuring section and conversely. In this way a continual measurement of the level of admission can be carried into effect. In a preferred embodiment the communicating means are formed by an oscillator of a frequency of, for example, 20.000 Hz. In order to obtain a simple read the processing circuitry is constructed so that the signal values obtained from the two measuring sections are divided one by the other after each change-over in order to obtain a clean output value to be applied to a read unit, that is to say, a value directly related to the level of admission corrected for the property of the flowing substance. The invention will be described more fully in the following specification of the Figures illustrating two embodiments. The drawing shows in Fig. 1 a vertical sectional view of a first embodiment of the probe in accordance with the invention, Fig. 2 a diagram of the electric processing circuitry and, Fig. 3 a vertical sectional view like Fig. 1 of part of the probe in a second embodiment. The probe illustrated in the Figures mainly comprises two pipe-shaped parts 1, 2 registering with one another. Between the pipe-shaped parts an insulating body 3 is fastened by a screw joint. The lower side of the pipe-shaped part 2 is sealed by a plug 4 welded therein, to the lower side of which is tightly screwed a synthetic resin end stub 5 by means of a bolt 6. The synthetic resin stub 5 may be made from PFA. To the overall length of the pipe-shaped parts 1 and 2 and of the end stub 5 is applied a synthetic resin coating 7, for example of PTFA. On the top side the upper pipe part 1 is fastened to a sleeve 8, which is provided on the outer side with an annular shoulder 9. The bore of the sleeve 8 accomodates an insulating body 10 of synthetic resin. The probe thus constructed is inserted from the top into the contral bore of a housing 11 and an annular filling body 12 is disposed between a widened part of the bore of the housing and the probe. The filling body 12 has a wedge-shaped longitudinal sectional area, the apex of said wedge being. smaller than 30". The length of the annular body 12 is such that the body extends above a stop face 13 of the housing 11 so that the shoulder 9 bears on the filling body 12. On the side of the shoulder 9 remote from the filling body 12 a guide ring 14 is arranged for supporting pressing means formed by a set of cup springs 15. These cup springs can be subjected to stress by a pressing nut 16 screwed into a screwthreaded part of the bore of the housing 11. It will be obvious that when the nut 16 is sufficiently tightened, the compression springs urge the filling body 12 downwards via the shoulder 9, said body exerting by its wedge-shaped structure a radially directed force on the coating 7 of the probe so that a leakage- and explosion-free seal is established between the probe and the housing. Moreover the wedge-shaped filling body 12 provides a firm retaining force so that the synthetic resin coating 7 remains in place and will not shift down due to creep, which may otherwise occur in the case of a Teflon coating. The envelope 8 is connected through a conductor 17 to the electric processing circuitry 18 arranged on top of the housing 11 so that there is established a direct electric connection to the upper pipe part 1 of the probe and the electric processing unit 18. The lower pipe part 2 is connected through a conductor 19, which is directly inserted via a central bore into the insulator 3 and 10 respectively. The bores are such that the conductor 19 exactly passes through the centre of the pipe-shaped part 1. In the case of long probes centering means 20 may be provided at equal intervals, preferably in the form of foam material arranged via openings 21 in the upper pipe part 1 prior to the application of the synthetic resin coating 7. To the wall of the vessel designated by the line 2 is welded a flange 23, on which have to be disposed a flange 24 and the flange 25 of the housing 11, said flanges being clamped together by means of bolts (not shown) with the interposition of sealing rings 26. The intermediate flange 24 is welded to a pipe-shaped sheath portion 27, which surrounds the probe 1, 2 at a given distance. On the bottom side the interior of the sheath 27 communicates through the opening 28 with the space of the vessel 22. The pipe-shaped jacket portion 27 constitutes, in fact, the mass for the capacity formed on the one hand by the pipe-shaped part 1 of the probe and the jacket 27, which section serves for measuring the level of admission and on the other hand between the pipe-shaped part 2 of the probe and the jacket 27, which section serves for the reference measurement. Fig. 2 shows a diagram of a potential connection of the reference capacity R and the level measuring capacity N with a voltage source 30 and an electronic arithmetic unit 31. For this purpose each capacity has to be connected alternately through an oscillator 32 to the voltage source 30 and to earth so that each time either one or the other capacity will be alive or be discharged respectively. The oscillator 32 may have a frequency of 20.000 Hz. Dependent upon the dielectric constant of the capacity it will require a given time for obtaining the maximum charge, said time being a parameter which is applied to the electronic arithmetic unit 31. By dividing each time the value of the level measuring section by the measuring value of the reference measuring section, an output signal is obtained which can be applied -to the read unit 33 for optical display, which is determinative of the virtual level of admission in the vessel or recipient 22 irrespective of the nature and properties of the flowing substance in said vessel. Fig. 3 shows a seal in a different embodiment between the probe 1, 2 the surrounding coating 7 and the housing 11. Also in this case the probe is fastened on the top side to an envelope 8 having an annular shoulder 9 bearing on two annular filling bodies 40 and 41 of synthetic resin. Between the filling bodies 40 and 41 is arranged a third filling body 42 and the wedge-shape of the filling bodies is such that the ring composed by them has a cilindrical inner and outer face which bears on the one hand on the cilindrical inner wall of the widened bore of the housing 11 and on the other hand on the cilindrical outer wall of the probe 1, 2. Through a ring 43 the shoulder is loaded by a first set of helical springs 22 subjected to stress by a nut 45 screwed into an inner screwthread of the housing 11. A second seal is obtained in a similar manner by annular filling bodies 46, 47! and 48 which bear on the outer wall of the envelope 8 and the inner wall of a flexible sleeve 49 arranged in the bore. The filling bodies 46, 47, and 48 are pressed home through compression spring 50 by means of a nut 51 screwed into a second screwthreaded part of the housing 11. Such a seal is particularly suitable for storing heavy chemical substances in the -recipient at high pressures up to 250 bar, at high temperatures up to 220C C, whilst the length of the electrodes may be 4 metres in stirred vessels and 5 metres in non-stirred vessels, the probe having a diameter of 24 mm. The invention is not limited to the embodiments depicted above.

Claims: WHAT IS CLAIMED IS: 1. A device for measuring the level of admission in a recipient for flowing substances, for example, fluids, comprising a capacitive probe connected to an electric processing circuit and inserted into the recipient characterized in that the probe consists of two relatively insulated measuring sections. 2. A device as claimed in claim 1, characterized in that the two sections are formed by registering, pipe-shaped parts, the lower one being connected to the electric processing circuit through a conductor passed through the upper pipe part. 3. A device as claimed in claims 1 and 2, characterized in that the conductor is centered by means of a plug of a foam substance in the upper pipe part. 4. A device as claimed in claims 1 to 3, characterized in that at a distance around the probe a sheath portion which is open at the bottom and has a shape similar to that of the probe is arranged so as to be in connection with the wall of the recipient. 5. A device as claimed in claims 1 to 4, characterized in that the two pipe-shaped parts of the probe are intimately enveloped in an insulating synthetic resin coating, for example, of Teflon (PTFA). 6. A device as claimed in claims 1 to 5, characterized in that the probe is inserted at the top end into a bore of a housing to be connected with the recipient, the seal between housing and probe being formed by at least one annular filling body between a widened part of the bore and the coating of the probe, whilst pressing means impose an axial load on the or each annular body. 7. A device as claimed in claims 1 to 6, characterized in that the or each annular body has a wedge-shaped longitudinal sectional area, the apex being smaller than 30C. 8. A device as claimed in claims 1 to 7, characterized in that the or each annular body is made from a synthetic resin, preferably, Teflon (PTFA). 9. A device as claimed in claims 1 to 8, characterized in that the electric processing circuitry includes communication means for simultaneously energizing one measuring section. and grounding of the other measuring section of the probe and conversely. 10, A device as claimed in claim 9, characterized in that the communication means are formed by an oscillator having a frequency of, for example 20.000 Hz. 11. A device as claimed in claims 9 and 10, characterized in that the processing circuit divides the signal values from the two measuring sections one by the other after every change-over in order to obtain an output value to be applied to a read unit.

Inventors: MAASKANT, PIETER, STOLK, GERRIT
Applicants: P. MAASKANT & ZONEN B.V.
Application year: 1981
Main classification: G01F23
IPC codes: G01F23