The present invention relates to a non-invasive measurement method to determine the liquid level in a metal tank. The invention also relates to a device for non- invasive measurement of the liquid level in a metal tank and an assembly of such a measurement device and a metal tank.

Metal containers and tanks are widely used for the storage and transport of all kinds of liquids. Such tanks may be found in for example the pharmaceutical- , food- and (petro)chemical industry. Due to the material properties of metal this material is often preferred when for example compared to plastics or other synthetic materials for several types of liquids and in several types of industry. Metal tanks may be relative solid and robust when compared to synthetic tanks and may have material properties that may suit the transportation of for instance aggressive and/or vulnerable liquids. Some of such desired properties may be chemical and/or impact resistance as well as fire resistance. This makes metal tanks for example applicable for storage and transport of inflammable or toxic liquids. Furthermore, metal tanks may be preferred to synthetic tanks in for example the food industry due to the fact that the metal may have no influence on the smell and/or taste of consumables.

The determination of the liquid level in tanks and containers may be a demand or desire in various storage and transport situations. The existing methods of liquid level measurement in a metal tank make use of intrusive measurement techniques. Besides the fact that mounting and maintenance of intrusive measurement systems is inconvenient, the exposure of the sensor to the liquid contained may be undesirable, unlawful and/or dangerous. For several applications the tank needs to be completely closed e.g. in order to maintain the required pressure, in order to prevent contamination or exposure of the content or to facilitate thorough cleaning. Therefore, there is a demand for a reliable non-invasive method to determine the liquid level in a metal tank.

The use of non-invasive measurement methods for synthetic tanks is known.

Determining the liquid level in a synthetic tank is normally done by detecting the liquid level from the top of the tank. The existing measurement techniques are normally limited to only indicating whether the liquid has reached a certain threshold level and not so accurate that they enable determining an continuous liquid level. More important, the biggest problem with the current available liquid level measurement techniques is that they are not applicable for metal tanks.

The present invention has the intention to provide a solution for non-invasive liquid level measurement in a metal tank.

The invention provides for this purpose a method for non-invasive measurement of a liquid level in a metal tank, comprising the method steps of mounting a transmitter and a receiver to the outside of a bottom wall of a metal tank; transmitting at least one ultrasonic signal from the ultrasonic transmitter through the bottom wall of the metal tank into the liquid contained by the tank; receiving at least a part of a reflected signal from the transmitted ultrasonic signal according the previous processing step with the receiver, which reflected signal is reflected by a liquid surface of the liquid contained by the metal tank; measuring the time period between the moment of transmitting a signal and the moment of receiving the corresponding reflected signal; and calculating the height of the liquid surface to the transmitter and/or receiver based on the measured time period according to the previous processing step.

The abovementioned method is applicable for liquid level determination in a closed metal tank but also in a temporary open tank of the closes type as well as an open metal tank. When referring to a metal tank, it is noted that at least part of the bottom wall of the tank, through which the ultrasonic signal is transmitted, consists of metal and that the other components of the tank may possibly at least partially consist of another material. The use of an ultrasonic signal coming from the bottom of the tank, thus directly transferred from the metal tank wall into the liquid (without the presence of an intermediate gas layer), enables to bring the signal with only limited loss of energy into the liquid. The ultrasonic signal then passes through the liquid until a fraction of the signal is reflected by the liquid surface or liquid level (also to be referred to as the liquid-gas interface) when reaching it. When the liquid level is substantially parallel to the transmitter, part of the signal transmitted by the transmitter is reflected back into the direction of the transmitter as an ultrasonic echo signal (thus in a direction reverse of the initial signal), towards the bottom of the tank. The reflected signal passes back through the liquid downwards where a fraction of the reflected signal enters the metal tank wall again so to be detected by the receiver connected to the metal tank wall. Although a substantial part of the signal sent by the transmitter will not reach the receiver as a first reflection enough of the signal will remain to be easily detected with of the shelf ultrasonic receivers. Also noted here is that a substantial part of the ultrasonic signal will be scattered, counteracted and/or further reflected but the fact remains that the reflected signal (normally the first reflected signal of a series of reflections) is easily to be detected. The response time is a useful indicator for determining the liquid level. When referring to the "bottom wall" of the tank, it is noted that the "lower part" of the tank is referred to, which part may also be referred to as the "bottom surface" or "lower wall". The bottom wall may for example be horizontal, concave or have any other geometry. With the "the outside of a bottom wall of a metal tank" for mounting a transmitter and a receiver is referred to the "outer surface" of the "bottom" of "lower wall" of the metal tank.

The ultrasonic signal send by the ultrasonic transmitter has preferably a high frequency to enable to practise a relatively small ultrasonic beam angle. A small ultrasonic beam angle has the advantage that less of the sent signal is lost and thus that a detected return signal is relative strong and therefore better detectable and readable. In a preferred method the transmitted ultrasonic signal from the ultrasonic transmitter may be higher than 0.8 MHz.

In a possible method the ultrasonic transmitter is a pulse generator. Each pulse may for example have a duration of 100-500 nanoseconds. Generating a longer and/or more complex signal enables to include more information in the signal and thus to further enhance the reliability of the method. The period between two subsequent signals may be [1 - 100] ms, preferably [5 - 20] ms, to ensure on one hand that a previous signal is substantially disappeared and on the other hand to enable to conduct constant measurements which is for instance desirable in situations where the liquid level is moving (fluctuating), e.g. due to movement of the tank, which enables to make detect a reflection just at a moment the liquid level is parallel to the transmitter and receiver. A coupling material may be applied between the transmitter and receiver and the surface wall of the tank. The coupling material ensures a connection between the tank and the transmitter/receiver that enables a good transfer (transfer with only limited losses) from the transmitter to the tank and back to the receiver. The coupling material may be a signal-conductive material like for instance a gel another flexible material that ensures a contact without an intermediate gas layer. The transmitter/receiver may also be located in a housing to protect the

transmitter/receiver and to enable easy mounting of the transmitter/receiver. The ultrasonic transmitter may for example be a piezoelectric element or a piezo transducer which are easily available at limited cost. A piezoelectric transmitter is also reliable, easy in use an simple to be replaced. Also the ultrasonic pulse generated by such a transmitter may be suited to travel through the metal wall and the liquid.

In a specific embodiment of the method according the present invention the transmitter and receiver are combined as a transceiver. Such a transceiver may be small in order to simplify the mounting to the tank wall. Furthermore could a large transceiver possibly cause signal dispersion or the requirement of the installation of a lens. The transceiver may for example have a diameter in the range of 10 - 30 mm. The transceiver is preferably placed facing up on the outer side of the bottom part of a metal tank. The combination of the transmitter and receiver in a single item (the transceiver) not only simplifies the construction but also makes a controlled relative positioning of two independent features (a transmitter and a receiver) superfluous and thus makes the method even more robust. The transmitter and receiver or the transceiver are preferably suitable for operating at temperatures between -40 and 230 degrees Celsius.

The invention also relates to a device for non-invasive measurement of the liquid level in a metal tank, comprising an ultrasonic transmitter and an ultrasonic receiver; at least one connector for connecting the transmitter and receiver to the outside of a bottom wall of a metal tank; a power supply connected to the transmitter and the receiver and an intelligent control unit connected with the transmitter and the receiver for measuring the time period between the moment of transmitting a signal and the moment of receiving the corresponding reflected signal and calculating the height of the liquid surface to the transmitter and/or receiver based on the on the measured time period. With such device the advantages as already mentioned in relation to the method according the present invention may be realised. As for the presence of a power supply this enables measurement of liquid levels for non-stationary tanks as well as that it also makes an external power feed for stationary tanks superfluous which may be

advantageous for a cost point of view, but may also be preferred for safety reasons.

In a specific embodiment the device may be provided with an adjustable voltage source, e.g. to strengthen the signal. A stronger signal may compensate for losses that appear as a consequence of increasing the frequency and is also beneficial in order to enable low liquid level readings. The intelligent control unit as part of the measurement device according the present invention may not only be used for processing the received data but may also (proactive) for instance modify the output voltage of the power supply based as well as other signal related

specification (time, frequency, strength and so on). For instance in case no reflection is received the intelligent control unit may provide an increase of the of the applied voltage and/or the signal frequency. A stronger signal may compensate for losses that appear as a consequence of increasing the frequency and is also beneficial in order to enable low liquid level readings. It is furthermore possible to connect a damping load to the transceiver in order to damp the reverberation before the echo returns as reverberation prohibits the detection of the reflecting signal. The intelligent control unit may also be provided with a display or any other type of output means to enable several types of result reading.

The ultrasonic transmitter may be arranged for transmitting an ultrasonic signal with a frequency higher than 0.8 MHz. As mentioned before a higher frequency provides a smaller beam angle which is beneficial for the reflection rate level to be detected. The transmitter may for instance be a transducer.

Also a load may be connected to the transmitter/transceiver/transducer in order to deaden the reverberation before any return signals arrive to be detected. Metal walls are known for causing heavy reverberation of acoustic signals making it more difficult to accurately detected any reflected signals. Furthermore, it may prevent distinguishing any echo signals and reverberation. It yet a further embodiment the intelligent control unit may comprise an AD- converter. The AD-converter may measure the ultrasonic response signal. When the response echoes have regular intermediate time intervals they are more likely to be identified as reflections of an originally send ultrasonic pulse. The echoes may be measured until a consecutive pulse is send. The sample rate of such an AD-converter may be at least two times the resonant frequency of the embedded ultrasonic transmitter (that is the resonant frequency of the transmitter in the situation wherein it is attached to or mounted to the tank), and preferably at least four times to provide sufficient resolution for accurate measurement.

As also mentioned before a coupling material may applied between the transmitter, the receiver and the surface wall of the tank to enhance easy transitions from the signals into and out of the metal tank. In a further embodiment the device may also comprise a track and trace device, like a quarter-band GPS. The track and trace device may be battery powered and the device may also include - in combination with the track and trace device or also excluding the a track an trace device - a mobile communication device. The implementation of a track and trace enables to provide even more information from a tank, which is especially desirable for mobile tanks. Use of a mobile

communication device enables to uncouple detection an reading of the results, like for instance a continuous measurement of liquid levels of highly dangerous or poisonous liquids. In another embodiment the device may also comprise a movement sensor. The movement sensor may for instance be useful to plan a measurement of a liquid level in a situation when a (mobile) tank is not moved. Ion a non-moving situation the liquid level measurements will be simpler and more reliable. The intelligent control may be programmed so that liquid-level measurement does not occur in case a certain threshold of movement is detected, for instance to reduce (battery) power consumption.

The device according the present invention may also comprise a temperature sensor. The liquid temperature has effect on the liquid density and the velocity of sound in the liquid and therefore also on the acoustic impedance of the liquid. The temperature is and therefore a useful parameter to include in the accurate determination of liquid levels. The temperature measurement may be a

measurement of tank temperature, as an indicator of liquid temperature but also a direct liquid temperature measurement is an option. The detected temperature may be sent to the intelligent control unit which processes the detected data into a liquid level. It must be clear that for accurate measurement also the composition of the measured liquid is to be included by the intelligent control as well as any other information that may influence the acoustic impedance of the liquid. As an alternative for temperature measurement it is also possible to calibrate the device by sending a signal over a (known) predetermined length through the liquid of which the liquid level has te be measured. This is for instance possible by sending a signal (e.g. an ultrasonic signal) horizontal through the tank at a position below the liquid level, and thus through the liquid. In case the distance between a calibrating pulse generator and a calibrating pulse receiver is known the time registered from sending a pulse by the calibrating pulse generator to the moment of receiving the pulse by the calibrating pulse receiver provides enough information to calculate the density of the liquid and may thus be used for calibrating the method for non-invasive measurement of a liquid level in a metal tank according the present invention. The invention also includes the device according the present invention comprising a calibrating pulse generator, a calibrating pulse receiver and an intelligent control for processing the calibration information. The intelligent control for processing the calibration information may be combined with the intelligent control unit connected to determine the height of the liquid surface which control may also comprise a central database to store the information. Furthermore a single calibration measurement system may be used to calibrate plural devices for non-invasive measurement of the liquid level in a metal tank according the present invention. The invention also provides the assembly of a metal tank and a device for the noninvasive measurement of the liquid level in the metal tank according to the present invention and as clarified above. The tank may be a stationary tank or a mobile tank. A mobile tank may for example be a tank rail wagon or a tank truck. The invention will be further elucidated herein below on the basis of the non- limitative exemplary embodiments shown in the following figures. Herein shows; figure 1 a schematic view of an assembly of a metal tank and a device for non-invasive measurement of a liquid level according to the present invention;

figure 2 a perspective view on a partially cut-away measurement device as shown in figure 1 ;

figure 3 a schematic view of a second embodiment of an assembly of a metal tank and a device for non-invasive measurement of a liquid level according to the present invention;

figure 4 a schematic cross section through the metal tank showing the noninvasive measurement of a liquid level according to the present invention; figure 5 a schematic cross section through the metal tank showing an alternative non-invasive measurement of a liquid level according to the present invention;

Figure 1 shows a perspective view of an assembly (1 ) of a metal tank (2) and a device (3) for non-invasive measurement of the liquid level in that tank (2). The measurement device (3) comprises an ultrasonic transmitter and an ultrasonic receiver (not visible in this figure but illustrated in figure 2) which are carried by a connector (4). The connector (4) is attached to a bottom wall (5) of the metal tank (2). The measurement device (3) also included a service unit (6) which holds (non- visible in this figure but also illustrated in figure 2) a power supply and an intelligent control unit. A wiring (7) connects the service unit (6) with the connector (4) holding the transmitter and receiver.

Figure 2 shows a perspective view on the measurement device (3) including the connector (4) holding an ultrasonic transmitter (8) and an ultrasonic receiver (9), e.g. embodied as piezo elements. In the situation wherein the connector (4) is attached to the metal tank (2), see figure 1 , a signal-conductive material like for instance a gel may be inserted between the transmitter (8) / receiver (9) and the metal tank (2). The measurement device (3) also includes service unit (6) holding an intelligent control unit (10) (here illustrated as a printed circuit board, PCB) and a battery (1 1 ). Wiring (7) connects the transmitter (8) and receiver (9) with the intelligent control unit (10) and the battery feeds both the transmitter (8) / receiver (9) and the intelligent control unit (10). Also incorporated in the service unit (6) is a telecommunication unit (12) for sending measured liquid level information to an external user. The telecommunication unit (12) enables a constant liquid level control of not only stationary tanks (2) but also of mobile tanks (2). Additionally the service unit (6) may also include for instance a GPS, a movement sensor, a thermometer, a modem, a leakage/pollution sensor, and/or any other detection or communication device. Figure 3 shows an alternative an assembly (20) of a metal tank (2) and a device (21 ) for non-invasive measurement of the liquid level in that tank (2) that

corresponds partially with the assembly (1 ) as shown in figure 1 (identical features are referred to with identical reference signs) but here the assembly (21 ) also includes a calibrating pulse generator (22) and a calibrating pulse receiver (23) both connected (see wiring 24) with the service unit (6).

In figure 4 is schematic cross section through the tank (2) showing an ultrasonic signal (30) send by a transceiver (31 ) - which transceiver (31 ) is a combined transmitter and an ultrasonic receiver - is sent through the bottom wall (5) of the metal tank (2) into liquid (32) held by the tank (2). When this sent ultrasonic signal (30) reaches the liquid surface (33) the signal is reflected (34) to travel back down to be received by the transceiver (31 ) again. The time period between the moment of transmitting the signal (30) and the moment the transceiver (31 ) receives the reflected signal (34) is used to calculate the height (H) of the liquid surface (33) in the tank (2). The intelligent control unit (10) as illustrated in figure 2 is connected via wiring (7) with the transceiver (31 ) but this communication may also be wireless.

Figure 5 shows a schematic cross section through the metal tank (2) showing an alternative non-invasive measurement of a liquid level using the assembly (20) as depicted in figure 3. For the measurement of a liquid level (33) in the tank (2) reference is made to figure 4. Here however also calibrating pulse generator (22) and calibrating pulse receiver (23) are attached to the tank (2). The calibrating pulse generator (22) sends a signal (40) that is received by the calibrating pulse receiver (23). The time period for the signal (40) to travel from the calibrating pulse generator (22) to the calibrating pulse receiver (23) is a measure for the density of the liquid (32) and is thus to be used for calibrating the measurement device (3). IN practise such calibration step makes it possible to have accurate liquid level measurements independent of liquid composition and liquid temperature as these variables are already included in the calibration measurement.