Patent Description:
Radar level gauge systems are in wide use for measuring filling levels in tanks. Radar level gauging is generally performed either by means of noncontact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a probe. The probe is generally arranged vertically in the tank. The electromagnetic signals are reflected at the surface of the product, and the reflected signals are received by a receiver or transceiver comprised in the radar level gauge system. Based on the transmitted and reflected signals, the distance to the surface of the product can be determined.

More particularly, the distance to the surface of the product is generally determined based on the time between transmission of an electromagnetic signal and receipt of the reflection thereof in the interface between the atmosphere in the tank and the product contained therein. In order to determine the actual filling level of the product, the distance from a reference position to the surface is determined based on the above-mentioned time (the so-called time-of-flight) and the propagation velocity along the probe of the electromagnetic signals.

In addition to the reflection at the interface between the atmosphere in the tank and the product (and at other material interfaces where applicable), there is typically also a reflection at the end of the probe close to the bottom of the tank. In most currently available GWR-type radar level gauge systems, this reflection at the end of the probe prevents accurate determination of filling levels close to the end of the probe. The filling level range for which accurate determination of filling levels is prevented may be referred to as the lower dead zone or blind zone.

In an effort to avoid or reduce the lower dead zone for a coaxial two conductor probe, <CIT> proposes to inductively connect the inner conductor and the outer conductor with a spiral spring at the end of the probe. The inductive connection between the inner conductor and the outer conductor delays the reflection (echo) from the probe end and, according to <CIT>, the lower dead zone can be reduced or even avoided by choosing the inductance of the connection between the inner conductor and the outer conductor.

A higher inductance, however, requires a longer and/or narrower electrical connection between the inner and outer probe conductor, which may be difficult to achieve without requiring a higher precision in the manufacturing and/or sacrificing some robustness of the probe.

In view of the above, it would be desirable to provide for an improved GWR-type radar level gauge system, in particular a more robust and/or production-friendly GWR-type radar level gauge system having a reduced lower dead zone.

According to a first aspect of the present invention, it is therefore provided a method as defined by claim <NUM>.

The "transceiver" may be one functional unit capable of transmitting and receiving electromagnetic signals, or may be a system comprising separate transmitter and receiver units.

The tank may be any container or vessel capable of containing a product, and may be metallic, or partly or completely non-metallic, open, semi-open, or closed.

The propagation parameter may be any parameter indicative of a position along the probe. For example, the propagation parameter may be any one of a time-of-flight of the reflection signal, a distance from a reference position at the first probe end, and a level in the tank, etc..

The present invention is based upon the realization that the lower dead zone can be reduced or even avoided without a highly inductive probe termination if the level of the surface of the product can be determined from a composite peak in the echo signal formed by a combination of the echo signal from reflection at the surface of the product and the echo signal from reflection at the second end of the probe.

The present inventors have further realized that this can be achieved by determining the level of the surface of the product, at least when the level is close to the second probe end, based on the position along the probe where the echo signal strength reaches a predetermined threshold value, and an offset distance from that position towards the second probe end.

Hereby, the probe termination arrangement can be made more robust, since the inductance can be lower without sacrificing the ability to reduce or avoid the lower dead zone. This may make the radar level gauge system less sensitive to damage and disturbances, and therefore suitable for a greater range of applications. Furthermore, the requirements on the manufacturing tolerances of the probe can be reduced, resulting in simpler and more cost-efficient manufacturing and/or installation at the tank.

The offset may advantageously be a predetermined value or may be selected among a set of predetermined values based on at least one measured property, such as a temperature, or a system specific property. Furthermore, the offset may depend on at least one material property of the second substance, such as the dielectric constant of the second substance. For example, the offset may be determined based on an estimated echo signal indicative of reflection of the transmit signal at the surface of the product only. Alternatively, or in combination, the offset may be determined based on one or several echo signals resulting from reflection of the transmit signal at the surface of the product when the surface of the product is sufficiently separated from the second probe end for the reflection at the surface of the product to result in an isolated peak in the echo signal. Such a measured isolated peak in the echo signal can be used to establish a mathematical model of the peak. The mathematical model, which may be simple (as will be described further below) or more complex can be used to determine the offset for a given threshold signal strength.

According to embodiments, furthermore, the transmit signal may comprise a first pulse train having a first pulse repetition frequency; and the method may further comprise the steps of: generating, by the transceiver, an electromagnetic reference signal in the form of a second pulse train having a second pulse repetition frequency controlled to differ from the first pulse repetition frequency by a frequency difference; and the echo signal may be determined based on the reflection signal, the reference signal, and the frequency difference.

The pulses in the first pulse train may advantageously be so-called DC-pulses.

It should be noted that the steps of methods according to embodiments of the present invention need not necessarily be carried out in any particular order, unless explicitly or implicitly required.

According to a second aspect of the present invention, it is provided a radar level gauge system as defined by claim <NUM>.

According to embodiments, the probe termination arrangement may provide an inductance between the first probe conductor and the second probe conductor being higher than about <NUM> nH and lower than about <NUM> nH.

By configuring the probe termination arrangement to provide an inductance in the above range, a favorable trade-off between robustness and reduction in the dead zone at the second probe end can be achieved.

Further embodiments of, and effects obtained through this second aspect of the present invention are largely analogous to those described above for the first aspect of the invention.

In summary, the present invention thus relates to a method of determining a level of a product in a tank, comprising generating and transmitting an electromagnetic transmit signal; guiding the transmit signal towards and into the product; returning an electromagnetic reflection signal resulting from reflection of the transmit signal; receiving, the reflection signal; determining, based on the reflection signal and a timing relation between the reflection signal and the transmit signal, an echo signal exhibiting an echo signal strength as a function of a propagation parameter indicative of position along the probe; and determining the level of the surface of the product based on a propagation parameter value indicative of a first threshold position along the probe for which the echo signal has reached a predetermined threshold signal strength, and an offset indicative of an offset distance along the probe from the first threshold position towards the second probe end.

<FIG> schematically shows a level measuring system <NUM> comprising a radar level gauge system <NUM> according to an example embodiment of the present invention, and a host system <NUM> illustrated as a control room.

The radar level gauge system <NUM>, which is of GWR (Guided Wave Radar) type, is arranged at a tank <NUM> having a tubular mounting structure <NUM> (often referred to as a "nozzle") extending substantially vertically from the roof of the tank <NUM>.

In the present exemplary measurement situation, the tank <NUM> contains a product <NUM> and a tank atmosphere <NUM> above the product <NUM>. The tank atmosphere <NUM> may be air or vapor, and the product <NUM> may, for example, be oil or another liquid through which electromagnetic signals can be guided by a probe.

The radar level gauge system <NUM> is installed to measure the level of the surface <NUM> of the product <NUM>. The radar level gauge system <NUM> comprises a measuring electronics unit <NUM> arranged outside the tank <NUM>, and a probe <NUM>, extending from a first probe end <NUM> coupled to the measuring electronics unit <NUM>, through the tubular mounting structure <NUM>, towards and into the product <NUM>, to a second probe end <NUM> at the bottom of the tank <NUM>. In the example measurement situation in <FIG>, the surface <NUM> of the product <NUM> is indicated as being close to the second probe end <NUM>, at a level that may be inside the so-called lower dead zone or blind zone for various existing radar level gauge systems.

As is schematically indicated in <FIG>, in particular in the enlarged schematic functional view from the second end <NUM> of the probe <NUM>, the probe <NUM> has a first probe conductor <NUM>, a second probe conductor <NUM>, and a probe termination arrangement <NUM> conductively coupling the first probe conductor <NUM> to the second probe conductor <NUM>.

In the example embodiment in <FIG>, the probe <NUM> is shown in the form of a large coaxial probe with the first probe conductor <NUM> being an inner conductor and the second probe conductor <NUM> being a coaxially arranged outer conductor. It should, however, be noted that the probe <NUM> may alternatively be any other kind of probe comprising first <NUM> and second <NUM> probe conductors, such as a twin line transmission line probe, with parallely extending wires or rods, or an "ordinary" coaxial probe with a smaller diameter of the outer conductor (and the inner conductor) than the large coaxial probe in <FIG>. Furthermore, while the probe termination arrangement <NUM> is conceptually indicated in <FIG>, the skilled person will realize that there are many possible ways of implementing the probe termination arrangement <NUM>. Some representative examples of probe termination arrangements that may be suitable for various embodiments of the radar level gauge system <NUM> will be described further below with reference to <FIG>.

In operation, an electromagnetic transmit signal ST is transmitted and guided by the probe <NUM> towards and into the product <NUM>. An electromagnetic reflection signal SR is returned, by the probe <NUM>. Based on the reflection signal and a timing relation between the reflection signal and the transmit signal, the measurement unit <NUM> can determine the level of the surface <NUM>. The radar level gauge system in <FIG> will now be described in more detail with reference to the schematic block diagram in <FIG>.

Referring to the schematic block diagram in <FIG>, the measurement unit <NUM> of the radar level gauge system <NUM> in <FIG> comprises a transceiver <NUM>, a measurement control unit (MCU) <NUM>, a wireless communication control unit (WCU) <NUM>, a communication antenna <NUM>, an energy store, such as a battery <NUM>, and a feed-through <NUM> between the outside and the inside of the tank <NUM>.

As is schematically illustrated in <FIG>, the MCU <NUM> controls the transceiver <NUM> to generate, transmit and receive electromagnetic signals. The transmitted signals pass through the feed-through <NUM> to the inner probe conductor <NUM> of the probe <NUM>, and the received signals pass from the probe <NUM> through the feed-through <NUM> to the transceiver <NUM>.

The MCU <NUM> may determine the level of the surface <NUM> of the product <NUM>, and provide a value indicative of the level to an external device, such as the control center <NUM> in <FIG>, from the MCU <NUM> via the WCU <NUM> through the communication antenna <NUM>. The radar level gauge system <NUM> may, for example, be configured according to the so-called WirelessHART communication protocol (IEC <NUM>).

Although the measurement unit <NUM> is shown to comprise an energy store <NUM> and to comprise devices (such as the WCU <NUM> and the communication antenna <NUM>) for allowing wireless communication, it should be understood that power supply and communication may be provided in a different way, such as through communication lines (for example <NUM>-<NUM> mA lines).

The local energy store need not (only) comprise a battery, but may alternatively, or in combination, comprise a capacitor or super-capacitor.

The radar level gauge system <NUM> in <FIG> will now be described in greater detail with reference to the schematic block diagram in <FIG>.

Referring now to <FIG>, there is shown a more detailed block diagram of the exemplary transceiver <NUM> in <FIG>.

As is schematically shown in <FIG>, the transceiver <NUM> comprises a transmitter branch for generating and transmitting the transmit signal ST, and a receiver branch for receiving and operating on the reflection signal SR. As is indicated in <FIG>, the transmitter branch and the receiver branch are both connected to a directional coupler <NUM> to direct signals from the transmitter branch to the probe <NUM> and to direct reflected signals being returned by the probe <NUM> to the receiver branch.

As is schematically indicated in <FIG>, the transceiver <NUM> comprises pulse generating circuitry, here in the form of a first pulse forming circuit <NUM>, a second pulse forming circuit <NUM>, and a timing control unit <NUM> for controlling the timing relationship between the transmit signal output by the first pulse forming circuit <NUM> and the frequency shifted reference signal SREF output by the second pulse forming circuit <NUM>.

The transmitter branch comprises the first pulse forming circuit <NUM>, and the receiver branch comprises the second pulse forming circuit <NUM> and measurement circuitry <NUM>.

As is schematically indicated in <FIG>, the measurement circuitry <NUM> comprises a time-correlator, here in the form of a mixer <NUM>, a sample-and-hold circuit <NUM> and amplifier circuitry <NUM>. In embodiments of the present invention, the measurement circuitry <NUM> may further comprise an integrator <NUM>.

Additionally, as was briefly described above with reference to <FIG>, the radar level gauge system <NUM> comprises processing circuitry <NUM> that is connected to the measurement circuitry <NUM> for determining the level of the surface <NUM> of the product <NUM> in the tank <NUM>.

When the radar level gauge system <NUM> is in operation to perform a filling level determination, a time correlation is performed in the mixer <NUM> between the reflection signal SR and the reference signal SREF that is output by the second pulse forming circuit <NUM>. The reference signal SREF is a pulse train with a pulse repetition frequency that controlled to differ from the pulse repetition frequency of the transmit signal ST, by a predetermined frequency difference Δf. When a measurement sweep starts, the reference signal SREF and the transmit signal ST are in phase, and then parameter values indicative of a time correlation between the reference signal and the reflected signal SR are determined to form an echo signal, together with the frequency difference Δf. Based on an analysis of the echo signal, level of the surface <NUM> of the product <NUM> in the tank <NUM> can be determined, as will be described further below.

The time-expansion technique that was briefly described in the previous paragraph is well known to the person skilled in the art, and is widely used in pulsed radar level gauge systems.

As is clear from the above discussion, the output from the mixer <NUM> will be a sequence of values, where each value represents a time correlation between a pulse of the reference signal SREF and the reflection signal SR. The values in this sequence of values are tied together to form a continuous signal using the sample-and-hold circuit <NUM>.

In this context it should be noted that the sample-and-hold circuit <NUM> is simply an illustrative example of a device capable of maintaining a voltage level over a given time, and that there are various other devices that can provide the desired functionality, as is well known to the person skilled in the art.

In the example embodiment of <FIG>, the time-correlated signal - the correlation signal SC - output from the sample-and-hold circuit <NUM> is provided to an integrator to form a measurement signal SM, which is amplified by the low noise amplifier LNA <NUM>. The above-mentioned echo signal can be formed, by echo signal forming circuitry <NUM>, based on the measurement signal SM and the frequency difference Δf. The filling level of the product <NUM> (the level of the surface <NUM>) can, according to embodiments of the present invention, be determined by the level determining circuity <NUM>. Along a segment of the probe <NUM> that is neither close to the first <NUM> nor the second <NUM> probe end, the filling level may be determined using conventional methods.

According to example embodiments of the present invention, the filling level close to the second probe end <NUM> may be determined in accordance with the method described below, with reference to the schematic flow-chart in <FIG> and further reference to other figures as indicated.

In step <NUM>, the transmit signal ST is generated as a pulse train of transmit pulses, and transmitted by the transceiver <NUM>.

In step <NUM>, taking place at the same time as step <NUM>, the reference signal SREF is generated as a pulse train of reference pulses.

In step <NUM>, the transmit signal ST is guided by the probe <NUM> towards and into the product <NUM> in the tank <NUM>.

In step <NUM>, the reflection signal SR resulting from reflection of the transmit signal ST at impedance transitions encountered thereby is returned by the probe <NUM> and received by the transceiver <NUM>. Notably, the impedance transitions encountered by the transmit signal ST include impedance transitions provided by the surface <NUM> of the product <NUM> and the probe termination arrangement <NUM> at the second probe end <NUM>. For further illustration of the above-described steps <NUM> to <NUM>, <FIG> are now referred to.

<FIG> is a simplified timing diagram schematically showing the relative timing of the transmit signal ST, the reflected signal SR, and the reference signal SREF according to an example embodiment of the invention.

As is schematically indicated in <FIG>, the transmit signal ST, formed by transmit pulses <NUM>, and the reference signal SREF, formed by reference pulses <NUM>, are controlled by the timing control unit <NUM> to be in phase at the start of the measurement sweep. A full measurement sweep may typically be defined by the difference frequency Δf, since the transmit signal ST and the reference signal SREF, in this particular example, need to be in phase at the start of a new measurement sweep. As is also schematically indicated in <FIG>, the reflection signal SR here comprises a first set of reflection pulses <NUM> resulting from reflection of the transmit pulses <NUM> at the surface <NUM> of the product <NUM>, and a second set of reflection pulses <NUM> resulting from reflection of the transmit pulses <NUM> by the impedance transition provided by the probe termination arrangement <NUM> at the second probe end <NUM>. Each of the first <NUM> and second <NUM> set of reflection pulses has the same pulse repetition frequency as the transmit signal ST, but lags behind the transmit signal ST with a time corresponding to the time-of-flight indicative of the electrical distance to the surface <NUM> of the product and the probe termination arrangement <NUM>, respectively.

The reference signal SREF is initially in phase with the transmit signal ST, but will, due to its lower pulse repetition frequency "run away from" the transmit signal ST and "catch up with" the surface reflection signal SR.

When the time-varying phase difference between the transmit signal ST and the reference signal SREF corresponds to the time-of-flights of the reflection pulses of the reflection signal SR, there will be a time-correlation between pulses of the reference signal SREF and pulses of the surface reflection signal SR. This time-correlation results in a time-expanded correlation signal SC, which can, in turn, be converted to a measurement signal SM.

Example waveforms of the transmit pulses <NUM> and the reference pulses <NUM> are provided in the schematic magnified view in <FIG>.

Returning to the flow-chart in <FIG>, the echo signal is determined, in step <NUM>, by the echo signal forming circuitry <NUM>, based on the reflection signal and a timing relation between the reflection signal and the transmit signal. For example, the echo signal may advantageously be determined based on the above-mentioned time-expanded measurement signal SM and the frequency difference Δf.

The above thorough explanation was provided for the case of a so-called pulsed measurement technique. The echo signal may alternatively be determined using other techniques, in which a frequency modulated transmit signal is used, as will be apparent to those of skilled in the art of radar level gauging.

An example of the echo signal, for an exemplary measurement situation where the surface <NUM> of the product <NUM> is close to the second probe end <NUM> of the probe <NUM>, will now be described with reference to <FIG>.

<FIG> schematically shows an echo signal <NUM> exhibiting an echo signal strength (or amplitude) as a function of a propagation parameter indicative of position along the probe <NUM>. In this case, the chosen propagation parameter is position z along the probe in relation to a reference position at the first probe end <NUM>. <FIG> is an enlarged view of a portion of the echo signal <NUM> indicating reflection by the impedance transitions provided by the surface <NUM> of the product <NUM> and the probe termination arrangement <NUM> at the second probe end <NUM> of the probe <NUM>.

As is schematically shown in <FIG>, the echo signal <NUM> indicates a reference echo <NUM> resulting from reflection of the transmit signal ST at a reference impedance transition (such as the feed-through <NUM>) at the first probe end <NUM>, and a composite peak <NUM> formed by a combination of the echo signals from reflection at the impedance transitions provided by the surface <NUM> of the product <NUM> and the probe termination arrangement <NUM> at the second probe end <NUM>.

As is schematically shown in <FIG>, the composite peak <NUM> is a broad and assymmetrical echo peak that only exhibits a single local extremum (maximum) <NUM>, so that the surface <NUM> of the product <NUM> and the second probe end <NUM> cannot be distinguished based on conventional peak detection. If conventional peak detection were used, the product level <NUM> in this example would be considered to be in the lower dead zone.

Returning to the flow-chart in <FIG>, the level of the surface <NUM> of the product <NUM> in the tank <NUM> is instead determined using the procedure described below.

In step <NUM>, a first threshold position zTH1 along the probe for which the echo signal <NUM> has reached a predetermined threshold signal strength TH is determined.

The first interface level is then determined, in step <NUM>, based on the first threshold position zTH1 and a predetermined offset distance Δz along the probe <NUM> from the first threshold position zTH1 towards the second probe end <NUM>.

The predetermined offset distance Δz is determined based on a model of the expected reflection of the transmit signal ST at the surface <NUM> of the product <NUM> only, and/or on previous test measurements. The echo pulse shape of the reflection at the surface <NUM> can be calculated based on known propagation properties of the probe <NUM> and the dielectric constants of the tank atmosphere <NUM> and the product <NUM> in the tank <NUM>.

For the case where the tank atmosphere <NUM> is air, the product <NUM> is oil, and the probe <NUM> is an exemplary coaxial probe, the shape of the echo pulse <NUM> from reflection at the surface <NUM> only can be approximated by the general curve shape expression: <MAT> where Q ≈ <NUM>.

It should be noted that the value of Q depends on the particular configuration of the radar level gauge system <NUM>, and that it may be temperature dependent. For an example configuration, the Q-value may be selected from values in the range <NUM>-<NUM>, depending on the temperature.

This means that the offset distance Δz can be determined according to the following relation: <MAT>.

The position along the probe <NUM> of the surface <NUM> of the product <NUM> in relation to the reference impedance transition (such as the feed-through <NUM>) then becomes: <MAT>.

The level of the surface <NUM> can be determined based on the position z<NUM> (distance along the probe <NUM> from the reference impedance transition), and the known position of the reference position impedance (such as the feed-through <NUM>).

<FIG> show example configurations of the probe termination arrangement <NUM> comprised in the radar level gauge system in <FIG>. A suitable probe termination arrangement <NUM> should be easy to mount to the probe <NUM> at the second probe end <NUM>, and it should be mechanically and electrically robust. In particular, it should maintain its electrical properties even if subjected to harsh environments and vibrations etc. Advantageous electrical properties for a substantial reduction of the lower dead zone may be that the probe termination arrangement <NUM> provides an inductance between the first probe conductor <NUM> and the second probe conductor <NUM> being higher than <NUM> nH. To keep the probe termination arrangement <NUM> as mechanically robust as desired, it may be beneficial to configure the probe termination arrangement <NUM> to provide an inductance below about <NUM> nH. The different exemplary probe termination arrangement configurations shown in <FIG> all provide an inductance of about <NUM>-<NUM> nH when installed in a "Large Coaxial Probe" with an outer diameter of the outer conductor <NUM> being <NUM>.

The first example configuration of the probe termination arrangement <NUM> shown in <FIG> comprises an electrically conductive member <NUM> that is attached to the first probe conductor <NUM> and to the second probe conductor <NUM>. In this first example configuration, the electrically conductive member is provided in the form of a metal sleeve that is conductively and mechanically connected to the first probe conductor <NUM> and the second probe conductor <NUM> by inserting a nut <NUM> in the first probe conductor <NUM>, passing a bolt <NUM> through a hole in the second probe conductor <NUM>, the metal sleeve, and a hole in the first probe conductor <NUM>, and joining the bolt <NUM> and the nut <NUM> to press the metal sleeve between the outer surface of the inner conductor <NUM> and the inner surface of the outer conductor <NUM>.

The second example configuration of the probe termination arrangement <NUM> shown in <FIG> comprises an electrically conductive member that is attached to the first probe conductor <NUM> and to the second probe conductor <NUM>. In this second example configuration, the electrically conductive member <NUM> is provided in the form of a metal sleeve accommodating the first probe conductor <NUM>. The metal sleeve is conductively and mechanically connected to the first probe conductor <NUM> by a fixing screw <NUM> (inside the hole in <FIG>) and to the second probe conductor <NUM> by a screw <NUM>. To allow for bigger tolerances in manufacturing and/or assembly, the longitudinal extension of the electrically conductive member <NUM> (metal sleeve) may be at least <NUM>.

In the third example configuration of the probe termination arrangement <NUM> shown in <FIG>, the electrically conductive member <NUM> is conductively and mechanically connected to the first probe conductor <NUM> by a bolt <NUM> and to the second probe conductor <NUM> by a rivet <NUM>.

In the third example configuration of the probe termination arrangement <NUM> shown in <FIG>, the electrically conductive member <NUM> is conductively and mechanically connected to the first probe conductor <NUM> by a first weld <NUM> and to the second probe conductor <NUM> by a second weld <NUM>.

Claim 1:
A method of determining a level of a product (<NUM>) in a tank (<NUM>), using a radar level gauge system (<NUM>) comprising:
a transceiver (<NUM>);
a probe (<NUM>) arranged to extend towards and into the product (<NUM>) from a first probe end (<NUM>) coupled to the transceiver (<NUM>) to a second probe end (<NUM>), the probe (<NUM>) comprising a first probe conductor (<NUM>) and a second probe conductor (<NUM>) being conductively coupled to each other by a probe termination arrangement (<NUM>) at the second probe end (<NUM>); and
processing circuitry (<NUM>),
the method comprising the steps of:
generating and transmitting (<NUM>), by the transceiver (<NUM>), an electromagnetic transmit signal (ST);
guiding (<NUM>), by the probe (<NUM>), the transmit signal (ST) towards and into the product (<NUM>);
returning (<NUM>), by the probe (<NUM>), an electromagnetic reflection signal (SR) resulting from reflection of the transmit signal (ST) at the surface (<NUM>) of the product (<NUM>) and at the second probe end (<NUM>);
receiving, by the transceiver (<NUM>), the reflection signal (SR);
determining (<NUM>), based on the reflection signal (SR) and a timing relation between the reflection signal (SR) and the transmit signal (ST), an echo signal exhibiting an echo signal strength as a function of a propagation parameter indicative of position along the probe (<NUM>), the echo signal comprising a composite peak (<NUM>) formed by a combination of an echo signal from reflection at the surface (<NUM>) of the product (<NUM>) and an echo signal from reflection at the second probe end (<NUM>); and
determining (<NUM>), by the processing circuitry (<NUM>), the level of the surface (<NUM>) of the product (<NUM>) based on a propagation parameter value indicative of a first threshold position (ZTH1) along the probe (<NUM>) for which the composite peak (<NUM>) of the echo signal has reached a predetermined threshold signal strength, and an offset indicative of an offset distance (ΔZ) along the probe (<NUM>) from the first threshold position (ZTH1) towards the second probe end (<NUM>).