GB2072851A - Liquid level detector circuits - Google Patents

Abstract

In containers such as tanks for in place cleaning purposes, it is often desirable to terminate the flow of liquid into or from the container at a pre-determined level. Circuits are known which produce an operation signal when a polar liquid reaches (or falls below) a pre-determined level. Problems arise because of turbulence which may lead to a premature operation signal and thus premature cessation of the flow of liquid, and because of foam at the head of the liquid which may have a similar effect. A liquid level detector circuit is disclosed which discriminates between three conditions at a probe, namely polar liquid, foam or air and ensures that turbulence does not result in the generation of an operation signal by virtue of a delay means T in the circuit. <IMAGE>

Description

SPECIFICATION Liquid level detector This invention relates to a liquid level detector.

In containers, such as tanks for in place cleaning purposes or bulk storage of liquid, it is often desirable to terminate the flow of liquid into or from the container at a predetermined level.

Various detectors are known for use with polar liquids, which are employed to produce an operation signal which enables the flow of liquid to be terminated. For example, the detector may operate a relay in turn coupled to pumps or a solenoid.

One type of known detector employs a sensor probe in the container to detect a change in conductivity in the medium between the probe and another circuit part which usually is a wall of the container. A premature operation signal may be generated because of either one of two commonly occurring phenomena in the liquid passing to or from the container. The first phenomena is the problem of froth or foam.

Turbulence in the liquid may give rise to a head of froth or foam. This head will reach the predetermined level in a container being filled prior to the actual surface of the filling liquid. The conductivity of the foam or froth may be such as to cause the detector to generate an operation signal since the detector senses a conductivity change which is within its sensitivity range. This, of course, leads to an error since the flow of liquid will be terminated before the actual surface of the liquid reaches the predetermined level. If the container has a large surface area, then a small error in liquid level due to the head of foam or froth can lead to a significant error in the total volume of liquid in the container which error may be significant in the context of the process for which the container is employed.The second phenomena is the problem of surges or turbulence in the liquid setting up transient peaks and troughs in the actual surface level of the liquid entering or leaving the container. If, for example, a wave appears at the surface of the liquid filling the container the level of the liquid at the probe location may transiently reach the predetermined level. The same effect results from a trough at the probe used for detecting a predetermined level when emptying the container. Both results in the detector momentarily generating an operation signal. If there is considerable turbulence, as there may be when large volumes of liquid are being pumped, then the device responsive to the operation signal of the detector may start hunting.

.Therefore, it is desirable to prevent the detector generating an operation signal in response to transient fluctuations in the surface level of the liquid due to turbulence.

The present invention seeks to provide a liquid level detector circuit, which overcomes the problems resulting from the above phenomena.

According to the present invention there is provided a liquid level detector circuit for generating an operation signal in response to the presence of a predetermined level of polar liquid in a container and for use with a probe which is to be located at said predetermined level in the container, said liquid level detector circuit comprising generating means operable to generate an oscillating probe signal which is applied to an output line of said generating means, said output line including a first connection for said probe and a sensitivity level adjustment means, the resistance at said probe (when said probe is connected to said first connection) and said sensitivity level adjustment means being operable as a voltage divider whereby for a first condition at the probe corresponding to the absence of polar liquid the signal received an input to a threshold detector means coupled to said output line represents a first voltage level, and for a second condition at the probe corresponding to the presence of polar liquid the signal received at the input to the threshold detector means represents an attenuated, second voltage level and for an intermediate condition at the probe corresponding to the presence of foam or froth at the head of polar liquid the signal received at the input to the threshold detector means represents a third voltage level intermediate said first and second voltage levels, said threshold detector means being operable to generate an output signal at a first level when its input signal is above a threshold voltage level and an output signal at a second level when its input signal is below said threshold voltage level, said sensitivity level adjustment means comprising a variable resistance which enables, in use, the voltage of the input signal to the threshold detector means to be adjusted such that the voltage of this input signal is only below said threshold voltage level when polar liquid is present at the probe and is above said threshold voltage level when the probe detects foam, froth or the absence of polar liquid, and delay means operable to receive said threshold detector output signals and responsive to a change of the threshold detector output signal from said first level to said second level, said delay means only generating an operation signal in response to detection of a predetermined level of polar liquid when said second level output signal is maintained for a predetermined period, whereby, in use, no operation signal is generated when, owing to turbulence, the polar liquid level at the probe attains said predetermined level for less than said predetermined period.

In a preferred embodiment, the delay means includes variable time delay means for adjusting said predetermined time interval.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a block diagram of a liquid level detector according to an embodiment; Fig. 2 is a preferred circuit diagram for the detector of Fig. 1; and Fig. 3 is a partly schematic diagram of the integrated circuit component IC1 of Fig. 2; Fig. 4 is a partly schematic diagram of the integrated circuit component IC2 of Fig. 2; Fig. 5 is a schematic diagram of the voltage divider formed by the probe resistance and the reference resistance; and, Fig. 6 is a diagram showing the signals appearing at various points of the liquid level detector circuit during its operation.

Fig. 1 of the drawings shows a block diagram of a liquid level detector circuit for generating an operation signal OP in response to the presence of a predetermined level L of polar liquid in a container (not shown). A probe P is employed with the circuit and is to be located at said predetermined level L (shown schematically by broken line) in the container. The circuit comprises generating means G operable to generate an oscillating probe signal Ps which is applied to an output line 1 of said generating means G. Output line 1 is connected to a sensitivity adjustment means S and then by line 4 to line 2 which is a first connection for the probe P and line 4 which is a connection to the threshold detector TD.

The resistance at probe P (when said probe is connected to said first connection) and the sensitivity level adjustment means S are operable as a voltage divider in the sense that the voltage VS on line 4 is responsive to the ratio of the resistances Rr and Rp.

For a first condition at the probe P corresponding to the absence of polar liquid at level L, the signal VS received an input 4 to a threshold detector means TD represents a first voltage level VS1. For a second condition at the probe P corresponding to the presence of polar liquid at the level L, the signal VS received at the input 4 represents an attenuated, second voltage level VS2. For an intermediate condition at the probe P corresponding to the presence of foam or froth at level L and at the head of polar liquid, the signal received at the input 4 represents a third voltage level VS3 intermediate said first and second voltage levels VS1 and VS2.The threshold detector means TD is operable to generate an output signal OS at a first level OS1 when its input signal VS is above a threshold voltage level VT and an output signal at a second level OS2 when its input signal VS is below said threshold voltage level VT. The sensitivity level adjustment means S comprises a variable reference resistance RF which enables, in use, the voltage of the input signal VS to the threshold detector means TD to be adjusted such that the voltage of this input signal VS is only below said threshold voltage level VT when poiar liquid is present at level L and the input signal VS is above said threshold voltage level VT when the probe detects foam, froth or the absence of polar liquid.Delay means T is operable to rec;eive on line 5 the threshold detector output signals OS and responds to a change of the threshold detector output signal OS from the first level OS1 to said second level OS2. This delay means T only generates an operation signal OP in response to detection of a predetermined level L of polar liquid when said second level output signal OS2 is maintained for a predetermined period t.

When, owing to turbulence, the polar liquid level at the probe P attains said predetermined level L for less than said predetermined period t, the delay means T does not generate the operation signal Op. As shown in Fig. 1 the delay means T is coupled by line 6 to a variable time delay means V which is adjustable to vary the predetermined period t.

As above mentioned, the delay means T and variable time delay means V receive the signal OS from the threshold detector TD. When signal OS changes from level OS1 to level OS2 this causes a capacitor within the delay means T to charge and reach a threshold value after the time period t is determined by RC components of the delay means T and variable time delay means V (adjustment of the resistance of the variable time delay means V alters the time constant t for these RC components). After signal OS2 is maintained for the time period t, the capacitor of the delay means discharges at the internal threshold value and causes the delay means to output an operation signal Op. If the signal OS returns to its level OS within the time period t then the delay means will not generate operation signal Op as its internal threshold voltage is not reached.In this situation, when the signal OS returns to its level OS1, this causes the capacitor within the delay means to discharge to ground. The delay means T has its output line 7 connected to a relay RE with normally closed contacts. The operation signal Op is employed to de-energise the relay RE thereby opening its contacts which, in turn, terminate the flow of polar liquid, for example, by closing a valve. The relay RE has normally closed contacts, and the valve (not shown) which it controls is normally energised to allow liquid to flow into the container. In the event of a power failure affecting the liquid level detector circuit, the relay RE will de-energise, thereby opening its normally closed contacts, just as it will if the circuit generates an operation signal OP. Thus, the circuit is failsafe in that liquid flow is terminated in the event of a power failure.

Turning to Fig. 2 there is shown a preferred circuit diagram for the liquid level detector circuit of Fig. 1. A 30 volt DC supply is arranged between positive line L, and earthed line L2. A resistor RX taps a 12 volt supply from the line L3 for the line L4. In line L3 a capacitive impedance C7 (100 y farad) is arranged to protect the circuit components should the supply line L, exceed 35 volts with respect to line L2. Between lines L2 and L4 a zener diode Z is arranged likewise to protect the circuit components should supply line L4, exceed 12 volts with respect to line L2. Capacitor C, is arranged in parallel with Zener diode Z, for this purpose also. The main component of both the generating means G and threshold detector means TD of Fig. 1 in this circuit is the integrated circuit IC1 which will be described with reference to Fig. 3. The integrated circuit IC1 is connected at pin P 13 by line L6 carrying the probe signal PS generated by the oscillator in IC1. The sensitivity level adjustment means S of Fig. 1 is provided by the components C2 and R2 of Fig. 2. Resistor R2 which is a variable 1 OOk ohm resistor and capacitor C2 are arranged in line L6; capacitor C2 merely acts as a blocking capacitor for any DC signal present at the probe P. Line 6 is connected by line 5 to the probe P and input pin P10 which .receives the signal VS.Line L connected between pins P, and P7 of the integrated circuit 101 includes a timing capacitor C3 having a value of 1 n farad.

Similarly, line L8 between pin P9 of integrated circuit IC1 and line L2 includes capacitor C4 having a value of 22 y farad. Pin P14 is connected by line L21 to line L4, pin P,2 is connected to line L9 and pin P is connected to line L209 The output of the threshold detector TD (Fig. 1 ) at pin P,2 to the line L9 from integrated circuit IC, to pin P6 of integrated circuit IC2 includes resistor R4 which combined with variable resistor R3 and capacitor C5 provide the variable time delay means V of Fig. 1.Variable resistor R3 is arranged between lines L1 and Lg. Capacitor C5 is arranged in line L10 between pin P2 of integrated circuit IC2 and earthed line L2. Integrated circuit IC2 will be described with reference to Fig. 4 and comprises the time delay means T of Fig. 1 and has a pair of direct connections at pins P4 and P8 by lines L,1 and Lr2 to line L4, a direct connection at pin P, by line L,3 to line L2 and a connection at pin P5 by means of line L14 including capacitor Cg to line L2.

Capacitor Cs has a value of 0.1 y farad. Line 1 5 takes the output of the time delay means T at pin P3 of the integrated circuit IC2 via line L16 and transistor T1 to relay RE which de-energises a process control element such as a valve to terminate liquid flow in response to the threshold detector means TD generating a second level output signal VS2 for the predetermined period t.

Line L16 includes resistor R5 in order that the output of integrated circuit IC2 may be applied to the base of transistor T,. Transistor T, is connected by line L" to earthed line L2 and by line L.8 including relay means RE to positive line L.

A diode D, is arranged in line L,g for protection of relay RE. Relay RE is a two pole change over relay. TransistorT, responds to the signal from pin P3 of the integrated circuit 1C2 in order to switch ON or OFF the relay RE.

In the circuit of Fig. 2, the probe signal PS has a maximum value of 1.5 volts at a frequency of 6 KHz. The sensitivity means S enables a reference resistance to be selected in a range between 13K 1 3K ohm and 1 AK ohm; the integrated circuit IC1 has an internal reference resistance of 13K ohm and R2 is variable from 0 to 1 OOK ohm.

.The variable delay means V (associated with integrated circuit 102) permits the time delay t to be selected in a range of between 2 and 30 seconds.

Capacitor C3 with a value of 1 n farad determines an oscillator frequency and thus a probe signal frequency of 6 KHz.

A current limiting resistor R6 (1 K ohm) is inserted in series with resistor R3 to prevent error signals arising in the even that R3 is adjusted to zero.

In Fig. 3 there is shown a diagram of the integrated circuit IC, of Fig. 2. The external connections of integrated circuit IC, of Fig. 3 have the same references as in Fig. 2. Thus, line L21 at pin P14, is the connection from the positive 30 volt line L, to the internal voltage regulator VR which supplies a regulated voltage to the oscillator OS.

Line L7 at pins P and P7 includes the external capacitor C3 of Fig. 3 which has the value 1 n farad and determines an oscillator frequency of 6 KHz. Line L6 at pin P,3 includes the variable resistor R2 (0--100K ohms) of Fig. 2 which increases the sensitivity range of the detector and acts in series with the internal resistor R6 having a value of 13K ohms. Capacitor C2 of Fig. 2 which is also included in line L6 merely ensures that probe P does not carry D.C. voltage and thereby set up a plating effect in the container. Line L8 at pin P9 includes the filter capacitor C4 of Fig. 2 which prevents stray AC voltage appearing on the signal to internal diode D2.Internal diode D2 is connected to the base of internal transistor T2 which has its emitter earthed by line 20 at pin P" and provides the input for integrated circuit IC2 on line L9 at pin P,2. If external resistance PR at the probe P is the same order as the internal resistance R6 (13K ohms), then the probe signal PS may be taken directly from the connection marked NC.

However, it is to give the detector an adjustable sensitivity that line L6 is used to send the probe signal PS via the resistance R2 (of the sensitivity adjustment means comprising R2 and R6 as the reference resistance) to pin P,O.

Internally, the integrated circuit IC, has an oscillator OS (corresponding to the generator means G of Fig. 1) which generates the probe signal PS at pin P13 and a threshold detectorTD which receives an input signal VS at pin P,O. The integrated circuit IC, also has a voltage regulator VR which regulates the voltage to the oscillator OS. There is also diode D2 which is connected between the internal threshold detector TD and output transistor T2 itself connected internally to pin P,2, from which output signal OS is taken on line Lg. Integrated circuit 101 is available from National Semi-conductor as their integrated circuit Lem 1830.

The integrated circuit IC2 will now be described with reference to Fig. 4 in which the pin connections are connected externally as shown in Fig. 2. This integrated circuit IC2 is available from SIGNETICS as their linear integrated circuit NE555. The output signal OP of this integrated circuit IC2 appears at pin P3. The signal VO appears at pin P6. External resistance R3, R4 and capacitor C5 determine the period of the delay period t. This period is varied by adjustment of resistance R3. When the output signal OS from integrated circuit IC, is at its first level OS1 (indicating the absence of polar liquid at the probe P); then the flip-flop FF is in a first state and the output stage connected to output pin P3 generates a high signal. When the signal VO changes to its second level OS2 (indicating the presence of polar liquid at the probe P), then capacitor C5 begins to charge via the output at pin P2. Internal resistances R7, R8 and comparators 1 and 2 enable the integrated circuit IC, to respond to a change of output signal VO from first level V01 to second level V02 when capacitor C5 discharges after delay period t. When capacitor C5 discharges at the internal threshold voltage of the integrated circuit IC2, the flip-flop FF is caused to change state rendering the output stage non-conductive and a "zero" operating signal Op appears at pin P3.

If the output signal OS (from pin 12 of integrated circuit IC,) returns to its first level OS within the delay period t, then capacitor C8 does not reach the internal threshold voltage level of integrated circuit IC2 and the flip-flop FF does not change state. Consequently, the output stage of integrated circuit IC2 remains conductive and no "zero" operation signal Op appears at pin P3.

However, in this case, the timing capacitor C5 discharges externally of integrated circuit IC2 when output signal OS returns to its first level OS1. Integrated circuit transistor T3 does not have any function in the operation of the integrated circuit IC2 when it is used in this mode. The capacitor C5 discharges externally of IC2 via transistor T2 of IC,. This open collector transistor T2 is thus employed as part of the delay mechanism.

A brief description will now be given of the operation of the liquid level detector circuit of Fig. 2 with reference to Figs. 5 and 6. Turning to Fig. 5 there is shown the voltage divider formed by the resistance Rp at the probe P and the reference resistance RF. In Fig. 2, the reference resistance comprises the variable resistance R2 (0--1 OOK ohms) and the fixed internal resistance R6 (13K ohms) of integrated circuit IC1. The connections 2, 3, 4 from the output 1 of generator G are shown in Fig. 2 as lines L5, L6. The resistance Rp will be one of three values, namely RA when air is present, R1 when liquid is present and RT when foam is present.The ratio of reference resistance Rr to the probe resistance R will determine the voltage of the signal VS. It will be readily apparent to those skilled in the art that when air is present at level L the voltage VS will be at a high level Us 1, whereas when liquid is present at level L the voltage VS will be at a low level VS2. If foam is present at level L, the voltage will be at level VS2 intermediate voltage levels VS 1 and VS2. The integrated circuit IC1 is responsive to the voltage at pin P,O rising above a threshold level VT.By adjusting the variable resistance R2 of the reference resistance RF the user can ensure that the only voltage VS2 is below the threshold level VT and that the integrated circuit VC1 does not respond to a change of voltage at pin P,O from voltage VS1 to voltage VS3, i.e. this ensures that the presence of foam does not result in the integrated circuit IC1 responding to the presence of foam, as opposed to the polar liquid, at the levelL(Fig. 1).

Turning to Fig. 6, values are shown for the voltage VS at pin P10 of IC1, the voltage OS at pin P12 of IC1, pin P6 pin P6 of IC2 and the voltage at pin P3 of IC2.

When the probe resistance is at RA or RT, signal VS has either value VS 1 or VS3 and consequently signal OS has value OS1 and pin P3 of IC2 is high.

When the probe resistance is at RL, signal VS has value VS2 and signal OS has value OS2.

In consequence, provided that signal OS2 is maintained for the predetermined time period=t, then after period t the output at pin P3 of IC2 changes from high to the zero signal Op which is the operation signal. If signal OS2 is not maintained for the predetermined time period t, then capacitor C5 discharges externally of the integrated circuit IC2 and the output at pin P3 of IC2 is unchanged.

In Fig. 6 at time T,, air is present at the probe P.

At time T, foam is present at the probe P. At time T2 liquid is present at probe P, but after an interval less than the predetermined period t, the signal VS returns to value VS3 at time T3. At time T4 liquid is again present at the probe P and this is maintained for the period t such that at time T5 the voltage at pin P3 of IC2 changes from high to the operation signal (zero) value Op.

As relay R6 has normally closed contacts, transistor T, is operative in response to the signal Op to change the voltage on line L18 from zero to an energising voltage VRE, thereby opening the relay contacts.

If the relay RE is desired to control a falling level of liquid, then the relay RE (controlling the flow of liquid from a container) could be arranged to be in its operative state when liquid is present and change to its inoperative state when air (or gaseous medium) is detected.

Claims (4)

1. A liquid level detector circuit for generating an operation signal in response to the presence of a predetermined level of polar liquid in a container and for use with a probe which is to be located at said predetermined level in the container, said liquid level detector circuit comprising generating means operable to generate an oscillating probe signal which is applied to an output line of said generating means, said output line including a first connection for said probe and a sensitivity level adjustment means, the resistance at said probe (when said probe is connected to said first condition) and said sensitivity level adjustment means being operable as a voltage divider whereby for a first connection at the probe corresponding to the absence of polar liquid the signal received an input to a threshold detector means coupled to said output line represents a first voltage level, and for a second condition at the probe corresponding to the presence of polar liquid the signal received at the input to the threshold detector means represents an attenuated, second voltage level, and for an intermediate condition at the probe corresponding to the presence of foam or froth at the head of polar liquid the signal received at the input to the threshold detector means represents a third voltage level intermediate said first and second voltage levels, said threshold detector means being operable to generate an output signal at a first level when its input signal is above a threshold voltage level and an output signal at a second level when its input signal is below said threshold voltage level, said sensitivity level adjustment means comprising a variable resistance which enables, in use, the voltage of the input signal to the threshold detector means to be adjusted such that the voltage of this input signal is only below said threshold voltage level when polar liquid is present at the probe and is above said threshold voltage level when the probe detects foam, froth or the absence of polar liquid, and delay means operable to receive said threshold detector output signals and responsive to a change of the threshold detector output signal from said first level to said second level, said delay means only generating an operation signal in response to detection of a predetermined level of polar liquid when said second level output signal is maintained for a predetermined period, whereby, in use, no operation signal is generated when, owing to turbulence, the polar liquid level at the probe attains said predetermined level for less than said predetermined period.

2. A liquid level detector circuit as claimed in Claim 1, wherein the delay means includes variable time delay means for adjusting said predetermined time interval.

3. A liquid level detector circuit for generating an operation signal in response to the presence of a predetermined level of polar liquid in a container and for use with a probe which is to be located at said predetermined level in the container, arranged, constructed and adapted to operate substantially as hereinbefore described with reference to the accompanying drawings.

4. A container including a probe located at a predetermined level in the container and a liquid level detector circuit associated with that probe for generating an operation signal in response to the presence of a predetermined level of polar liquid in the container, said circuit comprising a circuit as claimed in any one of Claims 1 to 3.