DE102015115761A1 - Method for on-site calibration of a thermal flow measuring device, method for carrying out a temperature-compensated flow measurement and thermal flow meter - Google Patents

Abstract

A method for on-site calibration of a thermal flow meter in a measuring medium, wherein the thermal flow meter at least a first sensor element, which is designed as a heater, and at least a second sensor element, which is designed as a temperature sensor, characterized by the following steps: I Entering at least two different flow rates and / or flow rates for at least two measured different power coefficients for the measuring medium, II. Determining Reynolds numbers for the respective data pairs from a power coefficient and a flow rate and / or flow rate associated therewith; III. Determination of temperature-compensated Reynolds numbers from the at least two different power coefficients using mathematical models to describe an internal and an external heat transfer of the flowmeter in the medium under consideration of the thermal material properties of the medium; and IV. Creation of a calibration curve by assigning the determined Reynolds numbers from step II to the determined temperature-compensated Reynolds numbers from step III for a respective power coefficient, and a method for performing a temperature-compensated flow measurement and a thermal flow meter.

Description

The present invention relates to a method for on-site calibration of a thermal flow meter according to the preamble of claim 1; a method for performing a temperature compensated flow measurement and a thermal flow meter.

Oils have different thermal properties compared to water. This results in a flow measurement of oils within a flow measuring range and a temperature measuring range by means of a thermal flow meter, inter alia, other Prandtl and Reynolds numbers than for water at comparable measuring ranges (see 2 ).

In the case of an on-site calibration, a determination of a power coefficient takes place and the user assigns to this power coefficient a flow value which he has preset, for example. This will create a calibration curve. (Please refer 6 ). The curve is usually the same medium, at approximately the same pressures and temperatures a good way to convert a measured flow into a real flow, which takes into account different measurement conditions at the on-site measuring point (eg turbulence, contamination, etc.).

After a successful on-site calibration, however, the thermal properties of the medium can change as the temperature changes. This change may cause the created on-site calibration to be inaccurate for flow measurement. It is therefore an object of the present invention to provide a temperature-compensated measurement and a correspondingly designed on-site calibration.

Based on the aforementioned preliminary consideration, it is now the object of the present invention to perform an on-site calibration which takes into account the temperature effects and to determine a flow rate of a measuring medium which is temperature-compensated and thereby e.g. taking into account changing thermal material properties.

The present invention solves the present objects by a method having the features of claim 1 and a method having the features of claim 2.

Typically, a thermal flow meter on a first and a second sensor element, wherein one of the sensor elements is designed as a heater and the other sensor element as a temperature sensor.

• I. Eingabe von zumindest zwei unterschiedlichen Durchflussgeschwindigkeiten und/oder Durchflüssen für zumindest zwei gemessene unterschiedliche Leistungskoeffizienten für das Messmedium,

An inventive method for on-site calibration of the aforementioned thermal flow meter in a measuring medium is characterized by the following steps:

• I. input of at least two different flow rates and / or flow rates for at least two measured different power coefficients for the measured medium,

• II. Ermittlung von Reynoldszahlen für die jeweiligen Datenpaare aus einem Leistungskoeffizient und einem ihm zugeordneten Durchfluss und/oder Durchflussgeschwindigkeit;

It is particularly advantageous if more than two measuring points are determined in the form of power coefficients. These can advantageously be assigned by a user in each case to a flow rate and / or a flow rate, in particular a mass flow rate. Alternatively, the flow and / or the flow rate can also be measured by a second flow meter, for example by a so-called clamp-on flowmeter and be transmitted directly, for example by a wireless connection to the evaluation of the thermal flow meter.

• II. Determining Reynolds numbers for the respective data pairs from a power coefficient and a flow and / or flow rate associated therewith;

• III. Ermittlung von temperaturkompensierten Reynoldszahlen aus den zumindest zwei unterschiedlichen Leistungskoeffizienten anhand von mathematischen Modellen zur Beschreibung eines internen und eines externen Wärmeübergangs des Durchflussmessgerät in dem Messmedium unter Einbeziehung der thermischen Stoffeigenschaften des Messmediums;

The Reynolds numbers can be determined directly from the power coefficient and the flow rate and / or the flow with known pipe diameter.

• III. Determination of temperature-compensated Reynolds numbers from the at least two different power coefficients using mathematical models to describe an internal and an external heat transfer of the flowmeter in the medium under consideration of the thermal material properties of the measured medium;

From the power coefficients determined by measurement, a temperature-compensated Reynolds number can now be determined. These Reynolds numbers are determined by mathematical models. A corresponding mathematical model may preferably be a mathematical polynomial obtained by computer simulation, e.g. with the help of Ansys.

Finally, in step IV, a creation of a calibration curve can be carried out by assigning the determined Reynolds numbers from step II to the determined temperature-compensated Reynolds numbers from step III for a respective power coefficient. One of the two Reynolds numbers thus forms the x-axis and the other Reynolds number the y-axis. This curve can also be a straight line or a line consisting of linear sections.

The calibration curve can then be stored on a memory unit of the evaluation unit and contracted for measuring signal processing during measuring operation.

Furthermore, according to the invention is a method for performing a temperature-compensated flow measurement with a thermal flow meter, characterized by the following steps

• A determination of a power coefficient based on measurement signals which were tapped at the sensor elements,

• B Ermittlung einer temperaturkompensierten Reynoldszahl aus dem Leistungskoeffizient anhand von mathematischen Modellen zur Beschreibung eines internen und eines externen Wärmeübergangs des Durchflussmessgerät in dem Messmedium unter Einbeziehung der thermischen Stoffeigenschaften des Messmediums;

The power coefficient is also called power coefficient. It can be determined from a measurement signal, in particular on one of the sensor elements, preferably the heater.

• B determination of a temperature-compensated Reynolds number from the power coefficient using mathematical models to describe an internal and an external heat transfer of the flowmeter in the medium under consideration of the thermal properties of the medium to be measured;

• C Ermittlung einer zur temperaturkompensierten Reynoldszahl korrespondierenden Reynoldszahl anhand einer Kalibrationskurve; welche durch ein Verfahren zur Vor-Ort Kalibration gemäß Anspruch 1 erstellt wurde;

The determination of the temperature-compensated Reynolds number can preferably be carried out analogously to step III of claim 1.

• C determination of a Reynolds number corresponding to the temperature-compensated Reynolds number using a calibration curve; which was prepared by an on-site calibration method according to claim 1;

This Reynolds number corresponds to the flow velocity and / or flow rate determined by the user during on-site calibration and / or the flow rate determined by measurement. Since, however, temperature changes or material properties changed by temperature change have already been taken into account in the temperature-compensated Reynolds number, this temperature compensation is also transferred to the corrosponding Reynolds number.

Finally, in step D, the calculation of the temperature-compensated flow takes place in a manner known per se on the basis of the corresponding Reynolds number.

Advantageous embodiments of the invention are the subject of the dependent claims.

m

(Punkt) der Massedurchfluss in kg/m³ ist;

u

die Durchflussgeschwindigkeit in m/s;

A

der Rohrquerschnitt in m²

ρ

die Dichte des Mediums in kg/m³ ist;

und Re = ρ·u·l/μ , wobei

Re

die Reynoldszahl ist

µ

die dynamische Viskosität in Pa*s; und

l

die charakteristische Länge in m, also der Sensordurchmesser, ist.

The determination of the Reynolds numbers can advantageously be carried out in accordance with step II by calculation and / or the calculation of the temperature-compensated flow rate in accordance with step D using the following formulas: ṁ = ρ · u · A , in which

m

(Dot) is the mass flow in kg / m ³ ;

u

the flow rate in m / s;

A

the pipe cross-section in m ²

ρ

the density of the medium is in kg / m ³ ;

and Re = ρ · u · l / μ , in which

re

the Reynolds number is

μ

the dynamic viscosity in Pa * s; and

l

the characteristic length in m, so the sensor diameter is.

These formulas and calculations are known per se and can be used in the present case for the calculation of target values

• a. Bereitstellen eines ersten mathematischen Modells für die Ermittlung einer Nusseltzahl das zur Beschreibung des internen Wärmeübergangs in dem thermischen Durchflussmessgerät dient;
• b. Ermittlung eines Leistungskoeffizienten bei einer Messung des Messmediums;
• c. Ermittlung einer Nusseltzahl anhand des Leistungskoeffizienten unter Zuhilfenahme des ersten mathematischen Modells, wobei das erste mathematische Modell vorzugsweise durch eine Computersimulation ermittelt wurde;
• d. Ermittlung einer Reynoldszahl für das Messmedium anhand der Nusseltzahl unter Zuhilfenahne eines zweiten mathematischen Modells das zur Beschreibung des externen Wärmeübergangs vom thermischen Durchflussmessgerät in das Messmedium dient; wobei das zweite mathematische Modell durch eine Computersimulation erstellt wurde und

Durch Aufnahme der Nusseltzahl in die Berechnung können auch Abweichungen von Wärmeübergangen an der Sensoroberfläche berücksichtigt werden. Die Nusseltzahl für den Übergang an der Grenzfläche vom Sensor in das Messmedium wurde bislang nicht bei der Ermittlung Durchflusses Basis auf eines mathematischen Modells ermittelt.The determination of temperature-compensated Reynolds numbers can advantageously be carried out from the at least two different power coefficients according to step III and / or the determination of a temperature-compensated Reynolds number according to step B by the following steps:

• a. Providing a first mathematical model for the determination of a Nusselt number used to describe the internal heat transfer in the thermal flow meter;
• b. Determination of a power coefficient during a measurement of the medium to be measured;
• c. Determining a Nusseltzahl on the basis of the power coefficient with the aid of the first mathematical model, wherein the first mathematical model was preferably determined by a computer simulation;
• d. Determining a Reynolds number for the measuring medium on the basis of the Nusselt number with the aid of a second mathematical model which serves to describe the external heat transfer from the thermal flow meter into the measuring medium; wherein the second mathematical model was created by a computer simulation and

By including the Nusselt number in the calculation, deviations from heat transitions at the sensor surface can also be taken into account. The Nusselt number for the transition from the sensor to the measuring medium at the interface has not been determined in the determination of the flow base on a mathematical model.

The provision of the first mathematical model can be done during a calibration in take a different calibration medium from the measuring medium. However, this is not absolutely necessary. The calibration medium can also be of the same type as the measuring medium.

By using the aforementioned mathematical model, a real Nusselt number can be determined. An estimate is not needed in this case. Furthermore, the second mathematical model can be determined by means of a flow simulation.

To determine the Nusselt number, a heat transfer coefficient can be determined from the power coefficient with the aid of the first mathematical model for describing the internal heat transfer.

Before the Reynolds number is determined, a second mathematical model can advantageously be provided for describing the external heat transfer, wherein an evaluation unit determines a function of a Prandtl and Reynolds number by means of the mathematical model and information provided on the thermal properties of the measured medium and the Nusselt number determined ,

From the function of the Prandtl and Reynolds number a Reynolds number can advantageously be determined on the basis of the thermal properties of the measuring medium.

The first mathematical model can likewise preferably be determined on the basis of a computer simulation.

For providing the first mathematical model, preferably a calibration with a calibration medium can be carried out.

It is advantageous if different coefficients for the first mathematical model for different Prandtl number ranges are stored.

Alternatively or additionally, different coefficients for the second mathematical model for different Prandtl number ranges can be advantageously stored.

The measuring medium may preferably be a hydrocarbon and more preferably an organic oil.

It is advantageous if the calibration medium is water.

A thermal flow meter comprises at least one sensor and an evaluation unit, wherein the sensor has at least one sensor element with a heating element and a sensor element with a temperature sensor, wherein the evaluation unit is designed for carrying out a method according to claim 1 and / or claim 2.

For this purpose, for example, the first and the second mathematical model can be stored on a data memory, as well as possibly thermal properties of the measuring medium. Furthermore, the evaluation unit can have a computing unit, which serves for the calculation of the heat transfer coefficient and the Nusselt number, as well as the Prandtl number, the Reynolds number and the flow rate.

The invention will be explained in more detail with reference to several figures. Show it:

1 schematic representation of a thermal flow meter in a sectional view;

2 Representation of the behavior of different media;

3 Representation of an internal and external heat transfer;

4 schematic representation of the sequence of steps in a variant of a method according to the invention;

5 schematic representation of several Prandtlzahlbereiche;

6 schematic representation of a known on-site calibration curve; and

7 Calibration curve created by the method according to the invention.

Conventional thermal flow measuring devices usually use two heat resisting thermometers configured as identically as possible, which are arranged in, usually pin-shaped, metal sleeves, so-called stingers or in cylindrical metal sleeves and which are in thermal contact with the medium flowing through a measuring tube or through the pipeline. For industrial application, both resistance thermometers are usually installed in a measuring tube; The resistance thermometers can also be mounted directly in the pipeline. One of the two resistance thermometers is a so-called active sensor element, which is heated by means of a heating unit. As a heating unit, either an additional resistance heating is provided, or the resistance thermometer itself is a resistance element, for. B. an RTD (Resistance Temperature Device) sensor, by implementing a electrical power, z. B. is heated by a corresponding variation of the measuring current. The second resistance thermometer is a so-called passive sensor element: it measures the temperature of the medium.

Usually, in a thermal flow meter, a heatable resistance thermometer is heated so that sets a fixed temperature difference between the two resistance thermometer. Alternatively, it has also become known to feed a constant heat output via a control / control unit.

If no flow occurs in the measuring tube, then a temporally constant amount of heat is required to maintain the predetermined temperature difference. If, on the other hand, the medium to be measured is in motion, the cooling of the heated resistance thermometer depends essentially on the mass flow rate of the flowing medium. Since the medium is colder than the heated resistance thermometer, heat is removed from the heated resistance thermometer by the passing medium. Thus, in order to maintain the fixed temperature difference between the two resistance thermometers in a flowing medium, an increased heating power for the heated resistance thermometer is required. The increased heating power is a measure of the mass flow or the mass flow of the medium through the pipeline. The heating power can be described by a so-called power coefficient PC.

If, however, a constant heating power is fed in, the temperature difference between the two resistance thermometers decreases as a result of the flow of the medium. The respective temperature difference is then a measure of the mass flow of the medium through the pipe or through the measuring tube.

Thus, there is a functional relationship between the heating energy necessary for heating the resistance thermometer and the mass flow through a pipeline or through a measuring tube. The dependence of the heat transfer coefficient on the mass flow of the medium through the measuring tube or through the pipeline is used in thermal flowmeters for determining the mass flow. Devices based on this principle are offered and distributed by the applicant under the name 't-switch', 't-trend' or 't-mass'.

When measuring the mass flow, a thermal flow meter can reach a maximum performance limit when measuring liquids. Since liquids have a much higher coefficient of thermal conductivity than gases, a larger heat energy is removed from the surface of the active temperature sensor at higher speeds. As the velocity of the medium increases, saturation of the sensor characteristic curve or the power upper limit of the measuring electronics is achieved quickly, so that the measuring range is limited to liquids with low flow velocities. This disadvantage is due to a sensor of a thermal flow meter, as it is in 1 is pictured, fixed.

1 shows in a schematic way the construction of a thermal flow meter 1 , as in the DE 10 2013 108 099 A1 is described in detail ..

This has a sensor cap 2 with a longitudinal axis X on which sensor cap 2 partially or completely in contact with a measuring medium. The sensor cap 2 has a wall area 3 with a medium-contacting face on. This end face is divided into at least three sub-segments, with a central sub-segment 3b , the upstream and downstream two opposite the central sub-segment 3b angled subsegments 3a and 3c having. These subsegments serve to form boundary layers and recirculation areas on the surface of the wall area 3 , Furthermore, the wall area 3 two lateral inclined surfaces 6 on.

On the side of the wall area facing away from the medium 3 is at the subsegments 3a and 3c in each case a heatable temperature sensor 4 arranged. One of the temperature sensors serves as a heater or active sensor element and the second temperature sensor measures the medium temperature and serves as a passive sensor element. From the temperature sensors go signal and / or power supply cable 5 from which to an evaluation unit 7 to lead.

The thermal flow meter, as it is in 1 is shown by way of example, will now be calibrated for an unknown, preferably liquid, medium. However, most calibration systems are designed for a special medium. As a rule, this is water.

Based on this preliminary consideration, the present invention allows to transfer from one medium to another medium when a calibration has been carried out. This applies in particular to the conversion from a successful calibration with the calibration medium "water" to the measuring medium "oil".

2 shows in a comparison of different media on the underlying problem, when trying to transfer a calibration with a calibration medium to a measuring medium, which is different to the calibration. In 2 are measuring ranges for Prandtl numbers (μ / k) shown, which were plotted on Reynolds numbers. In addition, the transition from laminar flow to turbulent flow is shown. The measurement in water is represented by measuring range with the reference symbol I. The measurement in oils is represented by the measuring ranges with the reference symbols II and III. As you can see, the measuring ranges are not congruent.

It has been shown that to compensate for a medium change, the heat transfer and distribution between the sensor, respectively the sensor surface, and the sensor environment, respectively the respective media must be considered.

In the context of the present invention, a distinction is therefore made for a better understanding between an internal heat transfer and an external heat transfer. This heat transfer detects e.g. the heat transfer of the temperature sensor, e.g. a Pt100 sensor. The term heat transfer covers e.g. also the heat transfer of the bonding material to the sensor cap, this can e.g. a thermal bridge, e.g. made of copper.

The internal heat transfer defines the entirety of the heat transfer between the heated temperature sensor 4 to the sensor cap 2 ,

The internal heat transfer is essentially characterized by the thermal heat conductivity and the size of the materials.

The external heat transfer describes the entirety of the heat transfers and heat distributions between the medium-contacting surface and the measuring medium.

It should be noted that the properties of some measuring media, in particular of oils, are strongly dependent on the respective temperature range in which the measurement is carried out. This must also be taken into account when transferring a water calibration to other media.

This task can be enabled by a method which i.a. Computer simulations for the determination of mathematical models includes. Such computer simulations are known per se and may be accomplished by computer software, e.g. through the commercial software Ansys. The following describes a corresponding method, wherein the individual method steps do not necessarily have to be performed in the predetermined order.

In a first step A of the method according to the invention, provision is made of a first mathematical model of the sensor of the thermal flowmeter for simulating the internal heat transfer. This provision includes i.a. the creation of the mathematical model for the respective real existing sensor, respectively its dimensioning, its structure and the materials used. It is particularly important to the structural design in the sensor cap. From the input data, a polynomial for the dependence of the heat transfer coefficient h and the power coefficient PC (power coefficient) can then be determined.

One or more calibration coefficients of the mathematical model are created by an iterative method in the calibration of the flowmeter. The calibration can be carried out in a different calibration medium from the measuring medium. Typical and preferred calibration medium is water. Typical and preferred measuring medium is a hydrocarbon.

The heat transfer coefficient h describes the intensity of the heat transfer at the interface of the sensor of the thermal flow meter. Due to the uneven heat transfer of the sensor, the thermal coefficient is difficult to detect by measurement and must therefore be determined on the basis of the first mathematical model.

Then, the power coefficient of the sensor is determined in a method step B. This can be done, for example, by calculating or measuring the medium to be measured, in particular oil.

Subsequently, in a method step C, the heat transfer coefficient h of the sensor can be determined on the basis of the abovementioned first mathematical model, in particular of the polynomial provided, by means of the determined power coefficient. 3 shows in the figure above a typical dependence of the heat transfer coefficient on the power coefficient, as determined by the mathematical model for the special sensor of the 1 was determined.

3 above shows in curves IV and V correlations at different temperatures and correspondingly different Prandt numbers.

Curves IV and VI correspond to the data obtained from the simulation, and according to V and VII are the first mathematical models created from these data.

After the heat transfer coefficient has been determined, the Nusselt number can be calculated in a manner known per se in a method step D with known dimensions of the measuring sensor and known thermal properties of the measuring medium, in particular if the thermal conductivity is known.

The Nusselt number describes the convective heat transfer at the interface between the medium-contacting surface of the thermal flowmeter and the measuring medium.

The Nusselt number can be described as a function of the Reynolds number and the Prandtl number. For this purpose, in a method step E, a corresponding second mathematical model is obtained, which is determined by computer simulation, e.g. created by computer program Ansys, is determined, used. The mathematical model describes the external heat transfer from the sensor into the measuring medium.

In a method step F, it is now possible to determine the Prandtl number for the measuring medium. To determine the Prandtl number, the type of the measuring medium and / or the thermal properties of the measuring medium users can be specified. The thermal properties can be taken from a database when specifying the measuring medium or can be determined by a previous measurement. 3 below shows a typical dependency of the Reynolds number on the Nusselt number as correlation curves VIII and X at different temperatures and Prandtl numbers. Curves VIII and X correspond to the data obtained from the simulation, and correspondingly IX and XI are the first mathematical models generated from these data. These are in 5 shown as single correlation curves for parts of the Prandtl Reynolds numbers again. In particular, different coefficients for different Prandtl number ranges can be used for the first mathematical model.

In this context, it has proved favorable to use different coefficients for different Prandt-Reynolds number ranges in a method step F because of the broad Prandtl-Reynolds number range, which should advantageously cover the flowmeter.

5 represents several correlation curves for Nusselt numbers as a function of the Prandtl-Reynolds number. Depending on the change in the temperature of the measured medium or the Prandtl number, one of the correlation curves or the associated coefficients is selected, with which the second mathematical model determines the Reynolds number. The coefficients are fixed values over a given Prandtl number range.

In a method step G, the determination of the Reynolds number for the respective measuring medium now takes place by using the abovementioned second mathematical model.

µ

die dynamische Viskosität in Pa*s;

u

die Durchflussgeschwindigkeit in m/s;

l

die charakteristische Länge in m, also der Sensordurchmesser;

ρ

die Dichte des Mediums in kg/m³;

in Verfahrensschritt H nunmehr ein Durchfluss für das Messmedium ermittelt werden.Finally, by means of the formula: Re = ρ · u · l / μ where the expression ρ * u is proportional to the mass flow; and, being

μ

the dynamic viscosity in Pa * s;

u

the flow rate in m / s;

l

the characteristic length in m, so the sensor diameter;

ρ

the density of the medium in kg / m ³ ;

in method step H, a flow for the measuring medium is now determined.

This is now issued to the user.

7 shows a calibration curve according to the invention of two types of Reynolds numbers. The temperature-compensated Reynolds number is on the x-axis. The temperature-compensated Reynolds number is based on mathematical models or formulas, which are based on computer simulation programs of numerical fluid mechanics. Although this Reynolds number does not reflect the exact Reynolds number of the measuring medium in a measurement, but it takes into account the thermal material properties such as the thermal conductivity and the like, the medium to be measured. The thermal properties of the material can be determined, taken from a database or measured separately. From the thermal material properties, preferably the Prandtl number can be determined.

The corresponding Reynolds number on the y-axis corresponds to the Reynolds number, which was estimated and entered by the customer or measured by a meter. However, this Reynolds number can not account for temperature changes and the changing thermal properties of the material. However, by calculating the temperature-compensated Reynolds numbers and then determining the actual Reynolds number by means of the calibration curve, a temperature-compensated flow can be calculated.

By means of this method, it is possible to use a calibration medium, which is a liquid and to use a measuring medium, which is also a liquid, wherein the calibration medium and the measuring medium are different from each other or different substances, such as water and oil.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102013108099 A1 [0050]

Claims (13)

Method for on-site calibration of a thermal flow meter in a measuring medium, wherein the thermal flow meter has at least one first sensor element, which is designed as a heater, and at least one second sensor element, which is designed as a temperature sensor, characterized by the following steps: I. input of at least two different flow rates and / or flow rates for at least two measured different power coefficients for the measuring medium, II. Determination of Reynolds numbers for the respective data pairs from a power coefficient and a flow associated therewith and / or a flow rate associated therewith; III. Determination of temperature-compensated Reynolds numbers from the at least two different power coefficients using mathematical models to describe an internal heat transfer in the flowmeter and an external heat transfer from the flowmeter into the medium under consideration of the thermal material properties of the medium; and IV. Creation of a calibration curve by assignment of the determined Reynolds numbers from step II to the determined temperature-compensated Reynolds numbers from step III for a respective power coefficient. Method for carrying out a temperature-compensated flow measurement with a thermal flow meter, wherein the thermal flow meter at least a first sensor element, which is designed as a heater, and at least a second sensor element, which is designed as a temperature sensor, characterized by the following steps; A he averaging a power coefficient based on a measurement signal which has been tapped on at least one of the sensor elements, B Determination of a temperature-compensated Reynolds number from the coefficient of performance using mathematical models describing an internal heat transfer in the flowmeter and an external heat transfer from the flowmeter into the medium under consideration of the thermal properties of the medium to be measured; C determination of a Reynolds number corresponding to the temperature-compensated Reynolds number using a calibration curve; which was prepared by an on-site calibration method according to claim 1; and D Calculation of a temperature-compensated flow and / or a flow velocity based on the corresponding Reynolds number. Method according to one of the preceding claims 1 or 2, characterized in that the input of the at least two different flows and / or flow rates according to step I by a user or by means of a data transfer from another flow meter takes place. Method according to one of the preceding claims, characterized in that the determination of the Reynolds numbers according to step II and / or the calculation of the temperature-compensated flow in step D is carried out using the following formulas: ṁ = ρ · u · A where m (dot) is the mass flow in kg / m ³ ; u the flow rate in m / s; A is the pipe cross-section in m ² ρ the density of the medium in kg / m ³ ; and Re = ρ · u · l / μ where Re is the Reynolds number μ is the dynamic viscosity in Pa * s; and l is the characteristic length in m. Method according to one of the preceding claims, characterized in that the determination of temperature-compensated Reynolds numbers from the at least two different power coefficients according to step III and / or the determination of a temperature-compensated Reynolds number in step B by the following steps: a. Providing a first mathematical model for the determination of a Nusselt number used to describe the internal heat transfer in the thermal flow meter; b. Determination of a power coefficient during a measurement of the medium to be measured; c. Determination of a Nusselt number based on the Performance coefficients using the first mathematical model; and d. Determination of the temperature-compensated Reynolds number for the measuring medium on the basis of the Nusselt number with the aid of a second mathematical model which serves to describe the external heat transfer from the thermal flowmeter into the measuring medium; wherein the second mathematical model was created by a computer simulation. A method according to claim 5, characterized in that for determining the Nusseltzahl a heat transfer coefficient from the power coefficient with the aid of the first mathematical model for the description of the internal heat transfer is determined. Method according to one of claims 5 or 6, characterized in that prior to the determination of the temperature-compensated Reynolds number providing a second mathematical model for the description of the external heat transfer takes place, wherein an evaluation unit by means of the mathematical model and based on information provided on thermal properties of the measuring medium and the determined Nusseltzahl determined a function of a Prandtl and Reynolds number. Method according to one of the preceding claims 5-7, characterized in that from the function of the Prandtl and Reynolds number based on the thermal properties of the medium to be measured a Reynolds number is determined. Method according to one of the preceding claims, characterized in that the first and / or the second mathematical model was determined by means of a computer simulation. Method according to one of the preceding claims, characterized in that for providing the first mathematical model, a calibration with a calibration medium, preferably a liquid, more preferably water, is performed. Method according to one of the preceding claims, characterized in that the measuring medium is compared to the calibration medium-different liquid, preferably a hydrocarbon, particularly preferably an oil. Method according to one of the preceding claims, characterized in that different coefficients for the first and the second mathematical model for different Prandtl number ranges are deposited, which have been determined with the aid of computer simulations. Thermal flowmeter ( 1 ) comprising at least one sensor and an evaluation unit ( 7 ), wherein the sensor has at least the first and the second sensor element, characterized in that the evaluation unit ( 7 ) is configured for carrying out a method according to claim 1 and / or 2.

Images