DE102015106187B4 - Device for measuring the liquid level - Google Patents

Abstract

Device for measuring a filling level (L) of a filling material (2) in a container (1) by means of microwave signals, taking into account temperature (T), pressure (p) and relative humidity (rH) as climatic influencing variables that are present at the location of the measurement, in particular in the container (1), the device comprising the following components: - An input unit (4) for entering the temperature (T) prevailing in or in the vicinity of the container (1), the pressure (p) and the relative humidity (rH), the input unit (4) for inputting the climatic influencing variables being connected to a measuring device (9) which has a sensor for recording the temperature (T) and/or a sensor for recording the pressure ( p) and/or a sensor for detecting the relative humidity (rH), - a signal generating unit (5) for generating the microwave signals, - a transmitter unit (6) for emitting the microwave signals in the direction of the filling material (2nd ),- an E receiving unit (7) for receiving the echo signals, - a control/evaluation unit (8) for determining the fill level (L).

Description

The invention relates to a device for measuring the fill level of a filling material located in a container using microwave signals, taking into account climatic factors that prevail at the location of the measurement, in particular in the container.

In automation technology, in particular in process automation technology, field devices are often used, which are used to record and/or influence process variables. Process variables are recorded by sensors that are integrated, for example, in level meters, flow meters, pressure and temperature meters, pH redox potential meters, conductivity meters, etc., which record the corresponding process variables level, flow rate, pressure, temperature, pH value, redox potential or Record conductivity. Actuators such as valves or pumps, which can be used to change the flow of a liquid in a pipeline section or the fill level in a container, are used to influence process variables. In principle, all devices that are used close to the process and that supply or process process-relevant information are referred to as field devices. In connection with the invention, field devices are also understood to mean remote I/Os, radio adapters or electronic components in general, which are arranged at field level. Endress+Hauser manufactures and sells a large number of such field devices.

Non-contact measuring methods have become established for measuring the fill level, as they are robust and low-maintenance. Another advantage is the ability to measure steplessly. Ultrasonic and radar-based measurement methods, which are based on the transit time principle, have become established here.

Above all, ultrasound is characterized by a high level of robustness and low manufacturing costs. However, due to the strong temperature dependence of the propagation speed of the sound, ultrasound-based methods cannot achieve such a high measurement accuracy as is the case with radar-based methods. As a rule, this limitation of the measurement accuracy is not critical for level measurement applications, since the accuracy of level measurements is often limited by the circumstances of the application, e.g. B. by foaming or wave formation in liquids, or jagged surfaces in solid bulk goods.

In certain applications, however, a high-precision determination of the filling level is desired, for example when measuring the filling level in tank systems in the oil and gas industry. A level difference in the millimeter range is already clearly noticeable here, since even small deviations are associated with a considerable difference in volume of the filling material. The standard to be followed in this area is the OIML R-85, which requires a maximum measuring error of the level of 1 mm within a measuring range of 30 meters. For this reason, radar-based methods are preferably used for applications that require such increased measurement accuracy. The devices used in this area can be based both on the pulse radar method and on the FMCW ("Frequency Modulated Continuous Wave") method. Furthermore, they can be designed as freely radiating radar or as guided radar.

Endress + Hauser sells free-radiating radar measuring devices under the "Micropilot" product line, for example, and guided radar under the "Levelflex" product line.

A typical circuit implementation of a filling level measuring device based on the pulse method is, inter alia, in the patent specification EP 2 795 268 B1 described. Here, increased measurement accuracy is achieved through statistical evaluation of the echo signals.

In the WO 2012/139 852 A1 shows a circuit arrangement used in the field of FMCW. With the arrangement described there, it is possible to calibrate the measuring device independently if required.

What the various radar measurement principles and embodiments have in common, however, is that as the width of the measurement range increases, it becomes difficult to comply with the specifications with regard to the maximum permissible measurement error. The main reason for this is the propagation speed (c) of the radar signals. Under real measurement conditions, this deviates from the vacuum speed of light (co), which means that the absolute measurement error increases with increasing measurement distance.

In the publication U.S. 2010/0 037 673 A1 a filling level measurement method based in particular on ultrasound is described, in which fluctuations in climatic influencing variables are compensated for. The compensation takes place by detecting a reference distance to a reference reflector under known conditions. The level value is corrected based on the known reference distance.

The PCT application translated into German DE 11 2006 002 691 T5 describes radar-based level measurement in containers in which the gas composition differs from the standard atmosphere, for example due to gas formation. In this context, it is proposed to compensate the resulting error in the level measurement by calculation, provided that the gas composition or its physical properties are known.

In the patent application DE 10 2012 104 926 A1 a calibration procedure for radar-based level gauges is described, in which the ambient temperature, the pressure and the humidity are recorded during the calibration. These variables are included in the calibration.

The object of the invention is to provide a radar-based method and a radar-based device for level measurement, in which the measurement error is reduced, particularly over large distances.

• - Die klimatischen Einflussgrößen werden ermittelt,
• - anhand der klimatischen Einflussgrößen und der Vakuum-Lichtgeschwindigkeit (c) wird eine korrigierte Ausbreitungsgeschwindigkeit (c) der Mikrowellen-Signale ermittelt,
• - ein Mikrowellen-Signal wird in Richtung des Füllgutes ausgesendet,
• - ein Echo-Signal, das durch Reflektion des ausgesendeten Mikrowellen-Signals an der Oberfläche des Füllgutes entsteht, wird empfangen,
• - eine Laufzeit (t) zwischen Aussenden des Mikrowellen-Signals und Empfangen des Echo-Signals wird ermittelt,
• - anhand der Laufzeit (t) und der korrigierten Ausbreitungsgeschwindigkeit (c) wird der Füllstand (L) bestimmt.

A method is described for measuring the fill level (L) of a filling material in a container using microwave signals, taking into account climatic factors that prevail at the location of the measurement, in particular in the container, the method comprising the following method steps:

• - The climatic factors are determined,
• - A corrected propagation speed (c) of the microwave signals is determined based on the climatic factors and the vacuum light speed (c),
• - a microwave signal is emitted in the direction of the product,
• - an echo signal, which is generated by the reflection of the emitted microwave signal on the surface of the medium, is received,
• - a transit time (t) between the transmission of the microwave signal and the reception of the echo signal is determined,
• - The fill level (L) is determined using the transit time (t) and the corrected propagation speed (c).

The advantage of the method described is that the propagation speed (c) of the radar signal can be corrected by detecting the climatic factors. This compensates for the measurement error that occurs due to the deviation of the corrected propagation speed (c) from the vacuum speed of light (co). This applies above all to the absolute measurement error at large distances. For level measurements in closed containers, it is advantageous if the climatic conditions are recorded directly in the container. A detection outside of the container only approximately reflects the climatic conditions that prevail in the container. For example, overpressure can prevail in the container. This is not relevant for level measurements on open containers, such as bulk material pits.

An advantageous embodiment provides that the fill level (L) is determined using the propagation time (t) and the corrected propagation speed (c) using at least one correction parameter (Ai) which is determined during the calibration of the device. Such a calibration is preferably performed individually for each individual device. With the help of this at least one correction _{parameter (A i} ), those measurement errors that are caused by influencing variables internal to the device are primarily compensated for. This can be, for example, signal delays caused by circuitry, or varying radiation characteristics of the antennas. Influencing variables external to the device, such as the climatic influencing variables, are not taken into account _{by the correction parameter(s) (A i ) unless further precautions are taken.}

• - Die klimatischen Einflussgrößen werden ermittelt,
• - anhand der klimatischen Einflussgrößen und der Vakuum-Lichtgeschwindigkeit (c) wird eine korrigierte Ausbreitungsgeschwindigkeit (c) der Mikrowellen-Signale ermittelt,
• - ein Mikrowellen-Signal wird in Richtung eines Testobjekts, das in einer vorbestimmten Referenz-Distanz (d) positioniert ist, ausgesendet,
• - ein Echo-Signal, das durch Reflektion des ausgesendeten Mikrowellen-Signals auf dem Testobjekt entsteht, wird empfangen,
• - eine Laufzeit (t) zwischen Aussenden des Mikrowellen-Signals und Empfangen des Echo-Signals wird gemessen,
• - anhand der Laufzeit (t) und der korrigierten Ausbreitungsgeschwindigkeit (c) wird eine Distanz (D) zum Messobjekt ermittelt,
• - durch Vergleich der Distanz (D) zur Referenz-Distanz (d) wird der Korrekturparameter (A) ermittelt.

In a further development of the last-mentioned embodiment, the at least one correction parameter (A) is determined using the following calibration steps:

• - The climatic factors are determined,
• - A corrected propagation speed (c) of the microwave signals is determined based on the climatic factors and the vacuum light speed (c),
• - a microwave signal is emitted in the direction of a test object positioned at a predetermined reference distance (d),
• - an echo signal, which is generated by the reflection of the transmitted microwave signal on the test object, is received,
• - a transit time (t) between the transmission of the microwave signal and the reception of the echo signal is measured,
• - a distance (D) to the measurement object is determined based on the transit time (t) and the corrected propagation speed (c),
• - The correction parameter (A) is determined by comparing the distance (D) to the reference distance (d).

The correction _{parameter(s) (A i} ) is/are then stored, for example, in a memory, which is then accessed for the level measurements. The calibration steps mentioned are generally repeated at a large number of different reference distances (d _i ), which extend over the entire filling level measuring range. This generates a corresponding number of correction parameters (A). By determining the climatic influencing variables and correcting the propagation speed (c) based on this, not only the device-internal error sources are compensated during calibration, but also the climatic influencing variables that prevail during the calibration of the device. This represents an important additional correction insofar as the calibration can extend over long periods of time, especially with a large number of different reference distances (d _i ), in which the climatic conditions can also change. The air conditioning of entire halls with the necessary precision is very complex or practically impossible. Repeating the calibration at a later point in time or at a different location under different climatic conditions is problematic. Accordingly, the calibration steps described above ensure that the climatic influencing variables are determined repeatedly during the entire calibration. As a result, the currently prevailing climatic influencing variables are always taken into account when creating many correction parameters (A _{i ).}

• - Eine Eingabe-Einheit zur Eingabe der im oder in der Umgebung des Behälters vorherrschenden Temperatur, des Druckes und der relativen Luftfeuchte, wobei die Eingabe-Einheit zur Eingabe der klimatischen Einflussgrößen mit einem Messgerät verbunden ist, welches einen Sensor zur Erfassung der Temperatur und/oder einen Sensor zur Erfassung des Drucks und/oder einen Sensor zur Erfassung der relativen Luftfeuchte umfasst,
• - Eine Signalerzeugungseinheit zur Erzeugung der Mikrowellen-Signale,
• - eine Sendeeinheit zum Aussenden der Mikrowellen-Signale in Richtung des Füllgutes,
• - eine Empfangseinheit zum Empfangen der Echo-Signale,
• - eine Regel/Auswerte-Einheit zur Bestimmung des Füllstands (L).

The object on which the invention is based is achieved by a device for measuring the fill level (L) of a filling material in a container using microwave signals, taking into account the temperature, pressure and relative humidity in or in the environment of the container prevail, the device comprising the following components:

• - An input unit for entering the temperature, pressure and relative humidity in or around the container, the input unit for entering the climatic influencing variables being connected to a measuring device which has a sensor for detecting the temperature and/or or comprises a sensor for detecting the pressure and/or a sensor for detecting the relative humidity,
• - A signal generation unit for generating the microwave signals,
• - a transmitter unit for emitting the microwave signals in the direction of the filling material,
• - a receiving unit for receiving the echo signals,
• - A control/evaluation unit for determining the fill level (L).

Compared to radar-based filling level measuring devices according to the prior art, the climatic influencing variables can be included in the measurement via the input unit and thus contribute to a reduction in measurement errors caused by the climatic influencing variables.

The influencing factors temperature (T), pressure (p) and relative humidity (rH) can be used to correct the propagation speed (c) as described by Rüeger. (Refractive Index Formulae for Radio Waves, JS28 Integration of Techniques and Corrections for Accurate Engineering, XXII International Congress, Washington DC USA, April 19-26, 2002). The three influencing factors affect the refractive index (n) of the transmission medium, which is linked to the speed of propagation (c) via the vacuum light speed (co): $$c=\frac{c_0}{n}$$

According to Rüeger, n depends on the pressure (p), the water vapor partial pressure (p _w ) and the temperature (T): $$n=\left[1+\mathrm{77,689}\frac{\left(p-p_w\right)}{T}+71.2952\frac{p_w}{T}+375463\frac{p_w}{T^2}\right]*{10}^6$$

The water vapor _{partial pressure (p w} ), in turn, can be described by the relative humidity (rH) in percent and the water vapor saturation pressure (p w_sat ): $$p_w=p_{w\_sat}\left(\frac{\mathrm{right}H}{100}\right)$$

Finally, to determine the water vapor saturation pressure (pw_sat), the Magnus formula can be used, which describes the relationship between (pw_sat) and the temperature (T): $$p_{w\_sat}=6.112*exp\left(\frac{17.62*T}{243.12+T}\right)$$

If the measurement does not relate to the gas composition typical of the atmosphere, this also has an effect on the propagation speed (c).

Other climatic influencing variables were not taken into account in Rüeger's considerations, or it was assumed that they are constant. For example the CO2 -Kon centration, which also affects the refractive index n. It goes without saying that if the corresponding models are available, other climatic factors, such as the CO2 concentration or other gases, can also be included in the correction.

In an advantageous embodiment of the device according to the invention, the microwave signals are periodically transmitted signal pulses. With this signal form, which is also known under the term pulse radar, the signal pulses, which have a transmission frequency in the microwave range, are emitted with a predetermined repetition frequency. The level (L) is determined via the transit time (t) of the signal pulse reflected on the surface of the level and the corrected propagation speed (c).

As an alternative to periodically transmitted signal pulses, the microwave signals are continuous signals with a periodically changing frequency. This form is known under the term FMCW (Frequency Modulated Continuos Wave). Here the propagation time (t) of the microwave signal can be determined via the frequency information of the reflected signal.

An alternative embodiment with regard to the input unit of the device according to the invention provides that the climatic influencing variables are entered manually into the input unit. In this case, the input unit includes a keypad or an equivalent input option with which the climatic influencing variables can be entered manually. In this case, it is advantageous if the input unit also includes a display or a graphic display, which at least temporarily shows the entered values.

In a development of the device according to the invention, the measuring device is an integral part of the device. There is thus no need for a separate arrangement of the measuring device, as a result of which a compact design of the device is achieved. This also ensures that the sensors for pressure (p), temperature (T) and relative humidity (rH) are automatically installed in the immediate vicinity of the container.

• 1 : Eine Vorrichtung 3 zur Messung des Füllstands L eines in einem Behälter 1 befindlichen Füllgutes 2 mittels Mikrowellen-Signalen.

The invention is explained below with reference to the figure below. It shows:

• 1 : A device 3 for measuring the filling level L of a filling material 2 located in a container 1 by means of microwave signals.

In 1 a device 3 for measuring the fill level L of a filling material 2 located in a container 1 is shown by means of microwave signals. For this purpose, the device 3 is attached to the top of the container 1 in such a way that the transmission unit 6 emits microwave signals in the direction of the surface of the filling material 2 . In this case, the microwave signals are generated in the signal generation unit 5 . In the exemplary embodiment shown, the device is shown as a free-radiating radar. However, it could also be designed as a guided radar, in which the microwave signal is guided through the container 1 via a waveguide.

The reflection of the microwave signals on the surface of the filling material 2 generates echo signals which are detected by a receiving unit 7 . In the present embodiment, the receiving unit 7 and the transmitting unit 6 are integrated in a single antenna. The filling level L is determined on the basis of the transit time differences t between the transmission of the microwave signals and the reception of the echo signals in a control/evaluation unit 8 . The determination is based on a corrected propagation speed c and taking into account the total height H of container 1: $$L=H-\left(\frac{c*t}{2}\right)$$

In order to determine the corrected propagation speed c based on the climatic influencing variables, the control/evaluation unit 8 is connected to an input unit 4 via which the climatic influencing variables pressure p, temperature T and relative humidity rH are provided. At the in 1 In the exemplary embodiment shown, these measured values are recorded by a measuring device 9 , the measuring device 9 being arranged in such a way that the pressure p, the temperature T and the relative humidity rH are recorded directly in the container 1 . The climatic influencing variables are thus recorded directly at the location of the measurement. As a result, the measurement errors in the level measurement, which are caused by the climatic influencing variables, are reduced.

Reference List

1

container

2

contents

3

contraption

4

input unit

5

signal generation unit

6

sending unit

7

receiving unit

8th

control/evaluation unit

9

gauge

Hey

correction parameters

c

Corrected speed of propagation

c0

vacuum speed of light

clk2

sampling rate

Tue

distance

you

reference distance

H

total height of the container

L

level

p

pressure

rH

relative humidity

T

temperature

t

duration

Claims (4)

Device for measuring a filling level (L) of a filling material (2) in a container (1) by means of microwave signals, taking into account temperature (T), pressure (p) and relative humidity (rH) as climatic influencing variables that occur on site of the measurement, in particular in the container (1), the device comprising the following components: - An input unit (4) for inputting the temperature (T), the pressure (p) and the relative humidity (rH) in or around the container (1), the input unit (4) for inputting the climatic influencing variables is connected to a measuring device (9) which comprises a sensor for detecting the temperature (T) and/or a sensor for detecting the pressure (p) and/or a sensor for detecting the relative humidity (rH), - A signal generation unit (5) for generating the microwave signals, - a transmitter unit (6) for emitting the microwave signals in the direction of the filling material (2), - a receiving unit (7) for receiving the echo signals, - A control/evaluation unit (8) for determining the fill level (L). device after claim 1 , whereby the microwave signals are periodically emitted signal pulses. device after claim 1 , where the microwave signals are continuous signals with a periodically changing frequency. Device according to at least one of Claims 1 to wherein the measuring device (9) is an integral part of the device (3).

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