GB2164151A - Acoustic liquid level measuring apparatus - Google Patents

Abstract

A method and apparatus for acoustically determining the level of liquid 3 in e.g. a tank or flume section 4, whereby a piezo transducer 5 is mounted on a tube 1 above the liquid, the other end 2 of the tube being immersed in the liquid. The section of tube between the transducer and the highest possible liquid level contains two or more target elements 8 the distance d between which is known and constant. The transducer produces source pulses which are partially reflected from the reference targets, the remaining pulse energy being reflected from the liquid surface, the echoes producing an electrical signal from the transducer. The apparatus is controlled by a microprocessor 7 which controls the transducer 5 and measures the time delay between the first and second reference echoes and the echo from the surface, computing from these intervals the liquid level or a parameter thereof. Other target elements 14 are used for setup and temperature gradient checking. <IMAGE>

Description

SPECIFICATION Measuring apparatus This invention related to liquid level or flow rate measuring apparatus, and in particular to apparatus for measuring the liquid level in a tank or other container or in a conduit, particularly for computing the flow rate of liquid passing through a flume section of a channel or for computing the volume of liquid in a tank.

A variety of devices for determining such levels are in wide-spread use. Many of these involve floats or the like which in turn operate other means to determine and display the level. In another type of device the level is determined by directing ultrasonic or other radiation from a transducer at a fixed location above the surface to the liquid surface and measuring the time taken for the echo to return from the surface.-to the transducer or to another pickup device The patents of WEL KOWITZ US Pat. No. 3,100,885 and ROD et al. 2,755,748 are typical of these types of devices. One problem with this type of echo detecting apparatus is that the time it takes the signal to be sent out and to return is a function of environmental conditions including the ambient temperature, humidity and pressure of the gaseous medium above the liquid.

There are many methods of compensation for these variations in the known art including methods using a reflector situated at a known distance from the transducer as in Austin et al. US Pat. No. 4,170,765. Where very precise indication is required, i.e. of the order of .01% of the measuring span the known art is insufficient due to difficulty of determining the exact acoustic position of the transducer and of determining and controlling the delay between the electrical input to the transducer and the emission of the acoustic pulse. The object of the present invention is to overcome these and other problems which cause inaccuracy in these systems. Using the method of the present invention a resolution of .1 mm is obtained in a 3 metre span.

According to the invention, there is provided apparatus for measuring the level of a liquid, comprising a tube having a first end submersible in a liquid, means disposed at the other end of the tube for transmitting an acoustic signal through the vapour medium in the space defined by the tube to a liquid surface located within the tube and for receiving reflected acoustic signals, reference means associated with the tube for producing at least two reflected reference signals from each acoustic signal transmitted by said transmitting and receiving means, and means associated with the acoustic pressure wave transmitting and receiving means for controlling said transmitting and receiving means and for computing an output parameter of the apparatus from reflected signals received by said transmitting and receiving means.

The means for producing reflected signals may comprise at least two reference means disposed within the tube and spaced apart from each other along the length of the tube.

The tube may be a length of small bore pipe, and the reference means may each comprise an annular ring disposed within the pipe. Alternatively a pin or plug in the wall of the pipe may be used as the reference means. Preferably the transmitting and receiving means comprise a transducer, and a single transducer may serve both for trnnsmiseion and receiving.

Suitably a piezo-transducer is used. The apparatus may comprise a third reference means disposed within the pipe between the abovementioned two reference means and the submersible end of the pipe and preferably in the region of the submersible end but above the highest liquid level. The pipe is suitably of a hard material such as metal and may be lagged to minimise temperature gradients along the length of the pipe. The means associated with the transducer most suitably consists of a micro-processor.

An embodiment of the invention will now be described by way of example with reference to the drawings, df which: Figure 1 is a schematic diagram of apparatus according to the invention arranged for use in measuring the liquid level in a flume or tank; Figure 2 shows a sequence of acoustic pulses as perceived at a point in the pipe in the region of the transducer of the apparatus of Fig. 1.

As shown in Fig. 1, liquid level measuring apparatus according to the invention includes a pipe, 1, one end of which, 2, is arranged to be submerged in a liquid, 3, contained in a confined space, 4, which may be a tank or flume section of a channel, 11. A small orifice, 15, is provided in the wall of the pipe 1, near its transducer end, to allow air to escape from or enter the pipe and establish a balance between the air and liquid pressures in the pipe. A transducer, 5, is disposed at the other end of the pipe, 1, and preferably the pipe is of sufficient length for the transducer to be mounted remote from the tank or flume in an enclosure, 6. The transducer may suitably be of a simple low-cost piezo type.Means 7, preferably electronic, are mounted in the enclosure together with the transducer for controlling the transducer in a known manner and for computing a required output parameter of the apparatus from signals transmitted and received by the transducer.

A sound pressure pulse transmitted by the trasndcer is guided to the liquid surface by the pipe 1. Two annular rings, 8, or alternatively two pins, are disposed in the pipe intermediate to its ends and are spaced apart along the pipe by a reference distance. As a further al ternative, radially extending plate portions extending inwardly from the wall of the pipe 1 over a portion of its periphery may be employed instead of the annular rings or pins.

Thus in use of the apparatus, when a pulse is transmitted by the transducer 5, three reflected pulses are received by it, the first two being reflections from the first and second annular rings, 8, respectively, and the third pulse being that reflected from the liquid surface. This is shown in Fig. 2. Pulse 9 is the initial transmitted pulse, pulse 10 is that reflected by the first ring, 8, and pulse 11 the reflection from the second ring, 8, while pulse 12 is the reflection from the liquid surface.

It will be seen that the time differences between the reception of the first and second reflected pulses, 10 and 11, (Td in Fig. 2) represents the reference distance "d", while the time differences between the first reflected pulse and the third reflected pulse (T1 in Fig.

2) correspond to the distance "1" between the first annular ring, 8, and the liquid surface in the pipe. Accordingly this latter time difference can be related to that between the first and second reflected pulses so that an accurate reading of the liquid level in the pipe can be computed. It will be appreciated that in the manufacture and installation of the apparatus, the distance between the first annular ring and the end of the pipe and between the end of the pipe and the floor of the tank or flume will be established so that from the precise determination of the distance between the first annual ring and the liquid surface, an equally precise reading of the liquid level in the tank or flume can be calculated.

The distance between the first annular ring and the end of the pipe may be determined in the case where the end of the pipe does not extend to the floor of the tank or flume by putting a cap over the end of the pipe and transmitting pulses from the transducer 5 in the same manner as in measuring a liquid level. In this instance, the third reflected pulse comes from the cap on the end of the pipe, and in this way the distance between the first annular ring and the pipe may be acurately established. It will be understood however that in this instance the distance between the end of the pipe and the floor of the tank or flume must also be established by other means, and that this distance must be held fixed within close limits during use of the apparatus according to the invention.

In another construction of the apparatus according to the invention (not illustrated) the pipe may extend to the floor of the tank or flume, a number of cut-outs or openings being provided around the periphery of the end of the pipe in order to-admit the liquid in the tank or flume to the pipe. In this instance, the distance between the first annular ring and the floor of the tank or flume may be determined directly by the transmission of a signal before the tank is filled or the liquid is admitted to the flume.In either method of calibrating or initially setting-up the apparatus according to the invention, it is preferred that the electronic means 7 be provided with a calibration mode, in which the means 7 is adapted to measure and store a value for the distance between the first annular ring and the capped pipe end or the floor of the tank or flume, as the case may be, for use in providing a reading for the liquid level in the tank or flume from the liquid level within the pipe, 1, as measured in normal use of the apparatus.

As is well known, the rate of flow of a liquid can be established by measuring the height of the liquid surface in a flume of known cross-sectional shape carrying the flowing liquid, so that the flow rate may also be derived from basic level reading produced by the apparatus according to the invention.

The microprocessor, 7, may be adapted to produce a flow rate reading from the liquid level recorded in the pipe, 1, or in the case of a tank with knowledge of its dimensions, compute the volume of the liquid.

A suitable size of pipe for the apparatus of the invention is 18 mm (3/4") but other sizes of relatively small bore may also be employed.

It is preferred that the pipe be of hard material, such as steel or another metal, so that there will not be exessive attenuation of the sound signal, and to avoid the need for excessively high amplification of the signal. The transducer is preferably a single unit of the transmitter/receiver type, but a separate transmitter and receiver may also be used.

The means 7 associated with the transducer is most suitably a microprocessor, which accomplishes the necessary filtering, averaging and calculation of the measured quantity in digital manner to produce the required output parameter or reading.

Other techniques which may be employed by the microprocessor associated with the transducer include the use of a time "window" so that the signals received by the transducer are only processed if they arrive in or about the expected time. In this way the effects of noise or other interference can be minimised. In addition, the impact of stray noises may be reduced by averaging out a series of readings. An amplifier may be included to augment the received signal in proportion to the distance it has travelled, in known manner. Any errors in liquid level due to dynamic effects of flow or other intereference may be reduced by placing the submerged end of the pipe in a stilling chamber connected to the main tank or the flume by a standpipe. The wall of such an optional stilling chamber is denoted by 13 in Fig. 1. It is also preferred that the pipe should be lagged to eliminate second order effects due to temperature gradients along its length. It is also preferred that the annular rings, 8, serving as the reference points, should be disposed so that they are nearer to the liquid surface than they are to the transducer, if possible, but this is not essential.

A third reference point, 14, again in the form of an annular ring or pin, may also be provided in the pipe, between the rings, 8, and the liquid surface, and in this way the system can be made self-calibrating and monitoring. Self-calibration is achieved by regular monitoring of the reflected signal received from the reference point 14, to detect any changes in the distance measured between it and the first annular ring. Such a change would indicate a change in the effective length of the pipe, possibly as a result of damage to the insulating lagging. Reference point 14 should be placed close to the liquid surface, but it will be appreciated that whether the end of the pipe is above the tank or flume floor or at the floor, this monitoring will not include that-section of the pipe below the reference ring 14.However it will also be appreciated that any damage or disturbance of any portion of the pipe is unlikely to go undetected by the continuous monitoring or self-calibration carried out in relation to the third reference point 14.

The apparatus according to the invention provides an accurate reading of liquid level, whether in a tank or for flowing fluid in a flume. It allows the sensitive transducing and microprocessing elements to be located remote from the liquid, and thus protected from the often harsh environment to be found in the vicinity of industrial or waste liquids.

It is of particular value for measuring level or flow or channels carrying hazardous liquids, or in tanks, e.g. petrol tanks. It also eliminates the need to position the reference rings 8 and 9 accurately with respect to the transducer, but their spacing from each other must of course be accurately controlled. This may be achieved by fitting the rings into a pipe section for insertion into the pipe in an appropriate position. The guiding of the ultrasonic wave through the interior space defined by the tube also allows good sensitivity to be achieved with a single transducer without requiring excessively large transmitted power.

It will be appreciated that the example of the apparatus according to the invention described herein is given by way of example only, and that the apparatus encompasses other equivalent constructions within the scope of the invention herein disclosed.

Claims (13)

1. An apparatus for producing an output related to the level of a liquid comprising: a tube having a first end submersible in a liquid, means disposed at the other end of the tube for transmitting an acoustic signal through the vapour medium in the space defined by the tube to a liquid surface located within the tube and for receiving reflected acoustic signals, reference means associated with the tube for producing at least two reflected reference signals from each acoustic signal transmitted by said trånsmitting and receiving means, and means associated with said transmitting and receiving means for controlling said transmitting and receiving means and for computing an output parameter of the apparatus from reflected signals received by said transmitting and receiving means.

2. An apparatus as in claim 1 wherein the reference means comprise at least two annular rings disposed above the highest permitted liquid level within the tube and spaced a fixed distance apart from each other along the length of the tube.

3. An apparatus as in claim 1 wherein the reference means comprise at least two radially extended plate portions disposed within the tube above the highest liquid level and extending inwardly from the wall of the pipe over a portion of its periphery and spaced apart a fixed distance along the length of the tube.

4. An apparatus as in claim 1 wherein the reference means comprises at least two pins or pegs disposed within the tube above the highest liquid level and extending inwardly radially from the wall of the tube and spaced apart a fixed distance along the length of the tube.

5. An apparatus according to claims 1, 2, 3, or 4, wherein the transducer is of the piezo-electric type.

6. An apparatus according to claims 1 to 5 wherein the tube is enclosed by thermal insulating means.

7. An apparatus according to any of the preceding claims wherein the control and computing means is a microprocessor.

8. A method for determining the level of a liquid comprising the steps of: directing from above the liquid surface through a tube an acoustical pulse towards the surface of said liquid and towards at least two reference targets the targets being disposed within the tube to partially reflect the pulse energy; detecting the echo signals received successively from said targets and the surface of said liquid; determining a first time between receipt of said echo signal from said first target and from said second target; determining a second time from receipt of said echo signal from first target and from said liquid surface; computing the distance from the first target to the liquid surface by dividing the second time by the first time and multiplying the quotient by the distance between the first two reference targets;; computing the liquid level by subtracting the result from the distance between said first target and the bottom of the liquid.

9. A method as in claim 8 further including the step of: completing the detecting path only in or about the time when an echo is expected to be received.

10. A method as in claim 8 further includ ing the step of: including in the detection path a variable gain amplifier controlled so as to maintain all echo pulses at constant amplitude.

11. A method as in claim 8 further includ ing the step of: rejecting only said first or second times that lie outside determined limits.

12. A method as in claim 8 further includ ing the step of: taking the average of a number of determi nations of said first and second times that lie inside determined limits and using the average values in the level computation.

13. A method as in claim 8 further include ing the step of: adding a third reference target disposed within the tube and situated just above the highest level and reguarly checking that the system observes its position to be constant.