GB2128326A

GB2128326A - Non-intrusive liquid level switch

Info

Publication number: GB2128326A
Application number: GB08228460A
Authority: GB
Inventors: David Wilson
Original assignee: ITT Industries Ltd
Current assignee: ITT Industries Ltd
Priority date: 1982-10-05
Filing date: 1982-10-05
Publication date: 1984-04-26
Also published as: GB2128326B

Abstract

A liquid level switch comprises transmitter (1) and receiver (2) piezo- electric elements mounted on the wall (3) of the container and operating at an ultrasonic frequency. A comparator (12) detects a change in the received signal, which is predominantly a plate or shear wave signal, and provides a control signal to cause a relay (13) to operate. Plate or shear waves are found to provide a significantly greater response to detect e.g. a water/air interface than hitherto conventional ultrasonic level sensors. <IMAGE>

Description

SPECIFICATION Non-intrusive liquid level switch This invention relates to a non-intrusive liquid level switch.

Non-intrusive liquid level switches are difficult to make because of the minimal effect detectable by a non-intrusive device with, for example, a steel interface formed by the wall of the container of the liquid. There are however a few commercially available devices and they do have limitations. The ones we are aware of operate at ultrasonic frequencies. One such switch is mounted at the bottom of the tank and operates with pulse techniques to establish the height of the liquid. It is also possible to mount a transducer at the top of a tank to similarly measure the distance in air to the liquid surface. Such devices would be sensitive to wave motions on the surface of the liquid which would significantly reduce the returned echo and would also be sensitive to other reflectors within the liquid.These devices are relatively expensive to manufacture requiring pulse techniques and signal processing to measure the delay time of the desired echo.

Another device directs the signal across the tank containing the liquid and this propagates longitudinal waves which are transmitted more efficiently in liquid than in air and thus detection of the delayed echo indicates the presence of the liquid and can be employed to switch when the level is detected. This device suffers from the effects due to air and other reflectors in the liquid and may only operate satisfactorily with a clean echo. Once again it is a pulse technique which may tend to make signal processing expensive.

According to the present invention there is provided a liquid level sensing switch adapted to be mounted on the surface of a container for the liquid, comprising an ultrasonic transmitter and an ultrasonic receiver for mounting side by side on the container wall, the receiver being sensitive to receive predominantly plate or shear waves from the transmitter via the container, and control means arranged to provide a switching control signal when the liquid level approaches toward or recedes from the receiver, in accordance with a change in output from the receiver.

According to another aspect of the present invention there is provided a method of determining if a liquid is present at a predetermined location in a container, comprising applying ultrasonic vibration to the wall of the container at a first position of said predetermined location and sensing predominantly plate or shear waves vibrations at an adjacent position on the wall of said location, and detecting a change in the sensed vibration caused by the liquid approaching toward or receding away from said location, and using the change to indicate the event.

Preferably the transmitter generates predominantly shear or plate waves and preferably also these waves undergo more than one reflection from the tank/liquid or tank/air interface before reception by the receiver.

In order that the invention can be clearly understood reference will now be made to the accompanying drawings in which: Fig. 1 shows a block diagram of an ultrasonic liquid level switch according to an embodiment of the invention, Fig. 2 shows a piezo-electric transmitter/receiver suitable for use in the embodiment of Fig. 1, Fig. 3 shows the frequency response of the transducers as effected by different liquid levels, and Fig. 4 shows part of a second embodiment of the invention.

Referring to Fig. 1 the switch comprises a piezo-electric transmitter 1 and piezo-electric receiver 2 mounted on the steel plate wall 3 of a container for liquid. A variable RF oscillator 4 operating in the range 130 kHz to 320 kHz is coupled to an amplifier 5 to drive the piezoelectric transmitter 1. However a fixed frequency oscillator could be used and other frequencies outside this band might be required. For example a fixed frequency in the range 2 MHz to 5 MHz. The receiver piezo-electric transducer 2 is mounted nearby and produces a signal whose level depends on the placing of the receiver transducer relative to the transmitter and on the resonant frequency for that particular configuration and the level of the liquid, i.e. whether the level is at or above the level of the transducers or below the level.

In order to cope with variation of the spacing between the transmitter and receiver, a variable attenuator and amplifier 6 enables the received signal to be adjusted to match the input of the amplifier.

In experimental investigation the variations in amplitude due to resonances and the effect of the liquid level can be observed on a meter 8. A peak rectifier 7 produces a DC level and the time constant of the rectifier (approximately equal to one second) reduces the effect of small disturbances on the liquid surface (waves). The signal frequency is adjusted together with the meter scaling by means of the meter drive and FSD adjustment block 9 to produce a full scale deflection on the meter with no fluid present. The switch 10 is then returned to the "trip set" side where a trip set circuit 11, which is variable, sets a trip voltage. A comparator 1 2 compares the peak rectified signal with the trip set signal and switches a relay and associated drive circuit 13 when the level passes the receive transducer.

The switch can be used to maintain a desired liquid level in the container.

The piezo-electric transducers 1 and 2 have been specially prepared so that they launch and receive predominantly plate or shear waves and one example of such a transducer is shown in Figure 2. This comprises a 1 MHz thickness-mode PZT 5A device 21, 1 cm diameter cut in half.

Faces of each half 21 a, 21 b, with like electrodes and fixed together using a silver-loaded epoxy with a copper electrode 22. The square edge is ground flat and two such transducers are used as the transmitter 1 and receiver 2, respectively.

The frequency response of these transducers mounted 20 mm apart at a height of about 6 cm on 3 mm thick glass plate, corresponding to contain wall 3 in Figure 1, is shown in Figure 3.

The plate formed one side wall of a plastic container into which water was introduced. Figure 3a shows the response with a iow water level, Figure 3b with water of a height of 5.5 cm, Figure 3c with water at 6 cm and Figure 3d with water at 6.5 cm. The analysis was done using a Marconi Instrument Spectrum Analyser. Settings:- 100 uV/division, linear 0.05 MHz/division, 500 Hz bandwidth.

We believe the results are due to shear or plate waves in the main being transmitted.

Shear waves may be affected by the presence of fluid at a boundary due to viscous loss effects, which may then alter the reflected amplitude. It is generaiiy accepted that shear waves may not propagate in a fluid, and such a solid/fluid boundary would result in complete reflection. However, shear waves may be supported in a very small boundary layer of fluid, whose depth is dependent upon the fluid's viscosity. It may be expected that the shear wave losses at such a boundary would be dependent upon the fluid's viscosity and the relative acoustic properties.

Plate waves may be set up in the wall of a tank.

Plate waves have an associated distortion of the surface. It would be expected that variation from air/plate to a fluid/plate interface would damp such a wave. This would occur because of the effective mass variation at the boundary as well as due to viscous and acoustic losses increasing.

In another embodiment of the invention shown in Fig. 4 which works at a fixed frequency, there are two receivers 31 and 32 which are used in a differential mode of operation and receive shear/plate waves 36 (indicated schematically as a multiple reflection wave) through the wall 33 of the container from a single shear/plate wave source 34. The receiver 31 would always remain above the desired liquid level, indicated at 35.

Thus temperature and other effects may be compensated and a switch signal provided at the desired liquid level, through taking the difference signal from differential amplifer 37, rectifying it and comparing it with a set level in much the same way as described in Fig. 1.

Claims

1. A liquid level sensing switch adapted to be mounfed on the surface of a container for the liquid, comprising an ultrasonic transmitter and an ultrasonic receiver for mounting side by side on the container wall, the receiver being sensitive to receive predominantly plate or shear waves from the transmitter via the container, and control means arranged to provide a switching control signal when the liquid level approaches toward or recedes from the receiver, in accordance with a change in output from the receiver.

2. A switch as claimed in claim 1, wherein the transmitter and/or the receiver comprises a piezoelectric element.

3. A switch as claimed in claim 2, wherein the element comprises a thickness-mode transducer which has been cut through its major surface and the surface of the cut is used as the mounting surface.

4. A switch as claimed in any preceding claim, comprising a second receiver for receiving the waves from the transmitter via the container. The control means being arranged to derive a difference signal from the two receivers.

5. A switch as claimed in any preceding claim, wherein the control means comprises an amplifier for amplifying the received signal, means to derive a d.c. level from the amplified received signal, a d.c. reference level, a comparator for comparing the two d.c. levels, and a relay, the comparator being responsive to a change between the levels to operate the relay.

6. A switch as claimed in any preceding claim comprising an oscillator and amplifier for driving the transmitter.

7. A liquid level switch substantially as hereinbefore described with reference to the accompanying drawings.

8. A method of determining if a liquid is present at a predetermined location in a container, comprising applying ultrasonic and predominantly plate or shear wave vibrations to the wall of the container at a first position at said predetermined location and sensing vibrations at an adjacent position on the wall at said location, and detecting a change in the sensed vibration caused by the liquid approaching toward or receding away from said location, and using the change to indicate the event.

9. A method of liquid level detection substantially as hereinbefore described with reference to the accompanying drawings.