GB2025623A - Liquid level sensing device - Google Patents

Abstract

A liquid level sensor is in the form of a unit comprising a coaxial capacitor open to the liquid and electronic circuitry which detects the change from resonance of a parallel resonant L. C. circuit 45 formed by the capacitor and an associated inductor. The resonant circuit 45 is excited at its resonant frequency under quiescent conditions by an oscillator 50 and the output detected by a detector 46 and discriminated with respect to a reference level by a comparator 47. This operates a change-over relay 49 via a time delay circuit 48 which serves to desensitize the device to slopping of the liquid. <IMAGE>

Description

SPECIFICATION Liquid level sensing device The present invention relates to liquid level sensors.

In all industrial equipment and plants of any type where handling of liquids takes place, the informa tion that certain level has been reached is of special importance. The devices offering information on the level are called level indicators. In most cases the level indicators indicate the moment a sensing part element held at a particular height has touched the material by means of relay contacts which are the operative part of the device. The information so obtained is used either for audible and light warning or for level control by means of suitable actuators.

In view of the wide range of different types of liquid materials, there exist many different methods for level indication, and the devices are based on various methods to offer adequate accuracy, reliabil ity and price.

The present device which operates on the princi ple of resonance in electrical circuits can be applied for level indication in all types of liquid materials.

According to the present invention, there is pro vided a resonant, liquid level sensing device for liquids, the device having a liquid sensing element in the form of a coaxial capacitor open to the liquid and forming a unitary structure with the remainder of the device.

In view of its totally electronic construction, accur acy and reliability in operation, the embodiment of the device described below with reference to the drawings is primarily intended for indication and control of oil derivative in oil processing industry, of fuel in the tanks of aircrafts, marine vessels and motor cars as well as level indication in tanks with different oil types.

For illustration and comparison it may be men tioned that the present device can be compared with prior capacitive type level sensors which have been found to be of high quality. Prior capacitive type level sensors have measured the capacitance formed between the sensing part and the liquid, the measurement usually being performed by means of a a measuring bridge. These prior level sensors have the following deficiencies which are the consequ ence of the method of measurement, namely: susceptibility to material deposits on tankwalls; susceptibility to change of dielectric constant E of the liquid which results in the dependence of accuracy on the type of material (with the level sensing of materials with lower dielectric constant, higher errors are observed); and susceptibility to dielectric constant changes resulting from temperature changes.

In the present device, the coaxial structure of the capacitor reduces the influence of factors such as deposits on the tank wall on the capacitance be tween its plates, while the fact that the capacitor and the associated circuitry form a unit means that connecting leads between the two can be kept short so as to reduce effects associated with long leads.

The operation of the illustrated device makes use of the phenomenon of resonance in electrical circuits for level detection.

The sensing element of the probe is constructed in the form of a coaxial capacitor i.e. one plate inside the other. This capacitance, with additional inductance provided by an associated inductor forms a resonant circuit which is excited by an oscillator giving a sine wave signal the frequency of which is tuned to the resonant frequency of the probe under quiescent conditions. The probe is thus brought into resonance and a maximum voltage appears at the output of an associated detector made up of semiconductor diodes.

By immersing the capacitor in liquid, its capacitance will be altered and its resonant mode disturbed, so the magnitude of the detected voltage decreases considerably. The change of the detected voltage is processed in associated circuitry and used to change the state of relay contacts.

The influence of material deposits and temperature changes has very small repercussion on the change of resonant frequency and thus on the change of the detected voltage so that the device is insensitive to such types of influence.

The change of the detected voltage is fairly large when the probe is immersed into measured material of any dielectric constant , and when immersed into electroconductive materials it is greatest, so that the application of the device is universal from the point of view of liquids with which it can be used.

A strong change of the detected voltage as the result of contact of the probe with the measured material, results in high accuracy of indication (typically t1 mm from the set level point is achievable.

All-electronic construction enables the circuitry and the probe to be housed in the same housing, so that overall dimensions are very small. The weight of the complete device can be 300 gram approx.

The invention will be further described with reference to the accompanying drawings, in which: Figure 1 illustrates the construction of one embodiment of resonant level sensing device according to the present invention with indication of individual important items: Figure 2 is the functional block diagram of the embodiment of Figure 1; and Figure 3 is the circuit diagram of the embodiment of Figure 1.

The illustrated resonant level indicator for liquid comprises three main parts: the sensing element or probe in the form of a cylindrical coaxial capacitor 18, electronic circuitry 44 and a housing 41, Figure 1.

The circuitry and sensing element are mounted in the housing which is located in a tank of liquid at the level at which liquid is to be detected.

Via the connector 43, Figure 1, the device receives a DC supply voltage of 18 to 28 V. The functional block 51, Figure 2, is a voltage regulator circuit giving the required supply voltages for operation.

The voltage regulator circuit comprises the integrated voltage regulator 5 and the capacitors 4 and 6 shown in Figure 3.

The functional block 50, Figure 2, represents a transistor oscillator which can be tuned to the resonant frequency of the probe. The oscillator 50 comprises the transistor 10, capacitors 9 and 14 and coil 12 shown in Figure 3, the frequency being adjustable in steps by charging capacitor 9 continuously using the inductances 12, 13.

The oscillator produces a sine wave signal having a frequency of several megahertz which is fed via the capacitors 16 and 17 to the functional block 45 (Figure 2) which is in fact the sensing element of the device and comprises the cylindrical capacitor 18 with an associated inductor 19 connected in parallel, which together form a parallel resonant LC circuit.

From the sensing element, the signal is taken via the capacitors 20 and 21 to a detector comprising semiconductor diodes 22 and 23, the resistor 24 and the capacitor 27.

The DC voltage obtained at the detector output i.e.

across capacitor 27 is proportional to the amplitude of the oscillation at the sensing element. This amplitude is greatest when the frequency of the exciting generator is equal to the resonant frequency of the sensing element. The voltage so detected is applied via the resistor 26, to the input of the functional block 47, (Figure 2) which is an operational amplifier 28 (Figure 3) connected as a comparator circuit.

In the comparator circuit the detected voltage is compared with the reference voltage which is fed via the potentiometer 15 and the resistor 25 to the other input of the comparator 28. The comparator output assumes the logic level "1" or "0", in dependence of the difference between the two voltages.

The presence of the liquid inside the cylindrical capacitor 18, causes a change in the capacitance between the coaxial plates of the capacitor. This in turn changes the resonant frequency of the combination of the sensing element and inductor 19 and so the output of the detector 46 changes, eventually, if the change in capacitance is great enough, to the extent of causing the output of the comparator 28 to change.

From the comparator circuit, the "0" or "1 " signal passes the functional block 48 which has the task of introducing a delay in comparator operation, and then reaches the functional block 49, Figure 2, which is an electromechanical relay with change-over contacts.

The circuit introducing the time delay comprises the transistor 33, potentiometer 30 and resistor 31 and capacitor 32, Figure 3. By means of the potentiometer 30 the delay can be adjusted finely within 30 to 90 secs. The delay renders the device insensitive to slopping of the liquid.

If it is sustained for long enough, the "1" signal from the comparator circuit, drives the base of the transistor 38 having the relay in its collector circuit 37, via resistor 35 and Zener diode 36. With the transistor 38 in saturation or cut-off, the relay 37, Figure 3, is energised and deenergised, respectively.

The relay contacts thus change state according to whether or not the probe is immersed in the liquid and so can be used to perform indicating or control functions.

The functional blocks 51, 50,46,47,48,49 and a part of the block 45 which represents the inductance 19, Figure 2, are arranged on two printed circuit boards and housed in the housing 44 in Figure 1. The complete device is installed in tanks or other equipment by means of a mounting flange 42, so that the sensing element 18 is in the tank at the side of the body of liquid.

Claims (6)

1. A resonant, liquid level sensing device for liquids, the device having a liquid sensing element in the form of a coaxial capacitor open to the liquid and forming a unitary structure with the remainder of the device.

2. A device according to claim 1 wherein the capacitor forms a resonant circuit with an associated inductor.

3. A device according to claim 2 and including a circuit for exciting the resonant circuit with a signal equal to its resonant frequency under quiescent conditions.

4. A device according to claim 3 wherein means are provided to detect the change in resonant frequency of the resonant circuit when it comes in contact with the liquid.

5. A device according to claim 4 wherein said means comprises a detector and a comparator circuit arranged to compare the output of the detector with a reference level.

6. A liquid level sensing device constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.