WO2016096460A1 - Thermal flow meter having diagnostic function - Google Patents

Abstract

The invention relates to a thermal flow meter (1) for determining the mass flow rate and/or a flow velocity of a flowable medium (3) in a pipeline (2) and to a method for operating a thermal flow meter, said thermal flow meter comprising at least one sensor element (4, 7) and an electronics unit (9) having a sampling rate in the range of milliseconds or less, wherein the at least one sensor element (4, 7) is in thermal contact with the medium (3) at least partially and/or at times and comprises a heatable temperature sensor (5), wherein the electronics unit (9) is designed to heat the at least one sensor element (4, 7) by means of a heating power (P), to sense the temperature (T) of the at least one sensor element, to determine the mass flow rate and/or the flow velocity of the medium (3) from the heating power (P) and/or from the temperature (T) and/or from at least one quantity derived from at least one of these quantities, and wherein the heating power is abruptly changed (ΔΡ) at a definable time (tstart), and wherein the electronics unit is designed to generate and/or output a statement about the state of the at least one sensor element from a jump response of the at least one sensor element to an abrupt change in the supplied heating power (ΔΡ).

Description

 Thermal flowmeter with diagnostic function

The invention relates to a thermal flow meter, in particular a thermal flow meter for determining and / or monitoring the mass flow and / or the flow rate of a fluid through a conduit having at least one sensor element and an electronic unit, and a method for operating such a flow meter. Furthermore, a statement about the state of the at least one sensor element can be made. The

Flowmeter thus has a diagnostic function.

Thermal flowmeters are widely used in the field of

Process measurement. Corresponding field devices are manufactured and distributed by the applicant, for example, under the name t-switch, t-trend or t-mass. The underlying measurement principles are accordingly made up of a large number of

Publications known.

Typically, a generic flow meter comprises at least two sensor elements, each of which has a temperature sensor designed as similar as possible, and at least one of the sensor elements is designed to be heated. For this purpose, either an additional resistance heater can be integrated within the respective sensor element. Alternatively, however, the temperature sensor itself as

Resistance element, z. B. in the form of an RTD resistance element (Resistance

Temperature Detector), in particular in the form of a platinum element, as it is also commercially available under the names PT10, PT100, and PT1000, be formed. The resistance element, also referred to as a resistance thermometer, is then on the implementation of a supplied electrical power, for. B. due to an increased power supply, heated.

Often, the temperature sensor within a cylindrical sleeve, in particular a sleeve made of a metal, in particular made of stainless steel or Hastelloy arranged. The sleeve then has the function of a housing which protects the temperature sensor, for example against aggressive media. In each case at least one heatable

Temperature sensor must also be ensured that there is the best possible thermal contact between the heated temperature sensor and the sleeve.

In order to detect the mass flow rate and / or the flow velocity, the at least two sensor elements are introduced into a pipeline which is flowed through at least intermittently and partly by a flowable medium, such that they communicate with the Medium in thermal contact. They can either be introduced directly into the pipeline, or be integrated into a measuring tube, which can be installed in an existing pipeline. Both possibilities are the subject of the present invention, even if in the following always only one pipeline will be mentioned.

During operation, at least one of the at least two temperature sensors is heated (active temperature sensor) while the second temperature sensor remains unheated (passive temperature sensor). The passive temperature sensor is used to detect the temperature of the flowable medium. In this case, the temperature of the medium is understood to mean the temperature which the medium has without an additional heat input of a heating unit. The active sensor element is usually either heated so that sets a fixed temperature difference between the two temperature sensors, wherein the change in heating power is used as a measure of the mass flow and / or the flow rate. Alternatively, however, the fed heating power can be kept constant over time, so that the corresponding

Temperature change to determine the mass flow and / or the

Flow rate is used.

If there is no flow in the pipeline, the heat is dissipated from the active temperature sensor via heat conduction, thermal radiation and possibly also via free convection within the medium. To maintain a specific

Temperature difference is then required, for example, a time constant amount of heat. In the presence of a flow, however, there is an additional cooling of the active temperature sensor by the passing colder medium. There is an additional heat transfer due to forced convection. Accordingly, either an increased heating power must be fed as a result of a flow in order to maintain a fixed temperature difference can, or the

Temperature difference between the active and passive temperature sensor decreases. This functional relationship between the active temperature sensor

supplied heating power or the temperature difference and the mass flow and / or the flow rate of the medium through the pipe can be expressed by means of the so-called heat transfer coefficient. The dependence of the heat transfer coefficient on the mass flow of the medium through the pipeline is then utilized for its determination and / or the determination of the flow rate. In addition, the thermophysical properties of the medium and the pressure prevailing in the pipeline have an influence on the measured flow. In order to take into account the dependence of the flow of these sizes, are For example, the thermophysical properties in the form of characteristics or as components of functional determination equations within an electronic unit of the flowmeter deposited. By changes in the thermal resistance of a sensor element, it may possibly lead to significant Messwertverfälschungen, which can lead to a change of heat transfer from the heating unit in the medium with otherwise constant flow conditions. Such a change in thermal resistance is also referred to as sensor drift. Under certain circumstances, if the change in the effective thermal resistance remains below a certain predefinable limit, and if the change is detected, the sensor drift and the negative influence on the determination of the mass flow rate and / or the flow rate can be at least partially remedied by appropriate countermeasures. Otherwise, if necessary, the flowmeter must be at least partially replaced.

Basically, a distinction is made in terms of the thermal resistance of an inner and an outer thermal resistance. The internal thermal resistance depends i.a. of individual components within the sensor element, e.g. inside the pods, off. Thus, a sensor drift due to defects in Lötanbindungen due to tensile loads due to material expansion or the like come about. The outer thermal

 On the other hand, resistance is influenced by deposit formation, material removal or material conversion (for example corrosion) on the surfaces of the respective sensor element that touch the medium. A change in the external thermal resistance is thus relevant in particular in long-term operation and / or in contact with aggressive media. In the case of gaseous or vaporous media, the measurement of the mass flow or the

 Flow rate beyond also be affected by condensation on at least one of the temperature sensors.

Several flowmeters have become known from the prior art by means of which a diagnosis can be made via at least one of the respectively used sensor elements. Thus, it is possible to make a statement about the state of at least one of the respectively used sensor elements.

DE102005057687A1 describes a thermal flow meter with at least two heatable designed temperature sensors, wherein the first temperature sensor and the second temperature sensor alternately as a passive, unheated temperature sensor, which provides information about the current temperature of the medium during a first measurement interval, and as an active heated temperature sensor, the during a second measurement interval information about the mass flow of the medium through the pipeline provides, controllable. A control / evaluation unit issues a message and / or makes a correction as soon as the during the first measurement interval and the second

Measurement interval provided corresponding measured values of the two

Temperature sensors differ from each other. In this way, deposit deposits and condensate formation can be detected.

Similarly, DE102007023823A1 discloses a thermal flow meter with two phase-wise alternately or alternately heatable sensor elements and a method for its operation. The mass flow is then determined alternately on the basis of the respectively heated sensor element, wherein the respective non-heated sensor element is used to determine the medium temperature. From a comparison of the measured values obtained with the two sensor elements, a contamination of at least one of the two sensor elements can additionally be detected. In US8590360B2 is described to heat or cool a first heatable sensor element with a first heating power, and at the same time to heat or cool a second heatable sensor element with a second heating power. Typically, the two heating powers are chosen so that the temperatures of the two sensor elements differ. By comparing the medium temperature, and / or at least two independent variables characterizing the heat transfer coefficients, a diagnosis can then be made via the flow meter.

Finally, from WO / 2008 / 142075A1 a method has become known in which heatable with a medium in thermal contact standing temperature sensor is heated with an alternating current or voltage signal and emits the heat generated at least partially to the flowing medium. The course of a heating and / or cooling process taking place within the temperature sensor is measured and from this the condition of the temperature sensor, in particular a contamination and / or a coating, is diagnosed. At the same time, the mass flow can be determined.

In principle, with the described flowmeters with a diagnostic function, a change in the thermal resistance is detected. This is then concluded that there is a covering and / or condensate formation. This corresponds to a change in the external thermal resistance. However, as described above, sensor drift can also be caused by a change in internal thermal resistance. The internal thermal resistance is determined by the structure and the materials used within the respective sensor element, in particular by the various components, for example within the sleeves or housing, or by different Material connections, or transitions, such. B. solder joints. It would thus be desirable to have a diagnostic function available, by means of which, in the case of a sensor drift on at least one of the at least three sensor elements, a distinction could be made between a change in the external and internal thermal resistance.

In the case of resistance thermometers GB2140923A an in situ method and an apparatus for testing the properties of the respective thermometer has become known, in which a statement about the properties is possible on the basis of a model for heat transfer or for the corresponding transfer function, or defects can be detected on the resistance thermometer. For this purpose, the reaction of the resistance of the thermometer is recorded to changes in temperature during a determinable in the duration and supplied heating power heating period.

Based on the cited prior art, the present invention therefore has the object, a thermal flow meter and a method for

 Operation of a corresponding flow meter to provide by means of which a statement about the functionality of at least one sensor element can be made. This object is achieved by a thermal flow meter for determining the mass flow rate and / or a flow rate of a flowable medium in a pipeline, with at least

 a sensor element and an electronic unit with a sampling rate in the range of milliseconds or less,

 wherein the at least one sensor element

 is at least partially and / or temporarily in thermal contact with the medium, and

 comprises a heatable temperature sensor,

 wherein the electronics unit is configured to

 to heat the at least one sensor element with a heating power P, to detect its temperature T,

 from the heating power P and / or temperature T and / or at least one of at least one of these variables derived size

 Mass flow and / or to determine the flow rate of the medium, and

from a step response of the at least one sensor element to a sudden change in the supplied heating power ΔΡ a statement generate and / or output via the state of the at least one sensor element.

The supplied heating power can either be constant, ie correspond to a fixed value, or adjustable, such that during operation the supplied

Heating power can be changed, controlled and / or regulated. Advantageously, from the step response, a change in the internal thermal resistance of the at least one sensor element can be detected. This allows a statement about the functionality of a sensor element, or on the condition of individual components, for example, within the sleeves, or via Lötanbindungen. The diagnostic function can furthermore advantageously be carried out during the operation of the respective flowmeter. The measuring device does not need to be specially removed. Also during the execution of the diagnostic function medium can flow through the pipeline. The sudden change in the supplied heating power AP = PrP ₂ can be positive or negative. Accordingly, the step response of the at least one sensor element can be detected, for example, if a previously unheated sensor element is heated from a certain point in time, or conversely, if the heating power supplied to a sensor element is switched off. But even with a change in each supplied heating power ΔΡ from a first to a second value, the

 Step response can be recorded. It is important that the change in the supplied heating power ΔΡ occurs abruptly, and is not regulated continuously or steplessly. An electronic unit according to the invention must be designed accordingly to record a step response, that is, have a sufficiently high sampling rate f for detecting the measured variable whose step response is to be recorded and analyzed. Ideally, the sampling rate should be in the range of milliseconds or less. It is therefore a method which can be implemented in a simple manner in a measuring device. The procedure for carrying out an analysis of a step response of the at least one sensor element for diagnostic purposes is based on the following facts: The abrupt change in the supplied heating power ΔΡ also abruptly changes the heat transported by the heating unit within the sensor element to its surface, ie the heat propagation. This heat transfer depends in general on different factors and different physical, chemical and

material-specific parameters, in particular the thermophysical

Material properties, such as the density p, the thermal conductivity λ, the specific heat capacity c or the thermal diffusivity α of the particular used Material or the materials used, from. But also the geometry of the respective component as well as material transitions play a role. In reality, however, further effects caused by the aging of the sensor element come into play, which influence the heat transfer, such as thermal and / or mechanical loads.

Until the point in time at which the respective transported heat has reached the surface of the sensor element, the heat transfer is defined only by the variables mentioned. From the moment on which the heat transported reaches the surface of the sensor element, the heat transfer, however, by the on

 Sensor element overflowing medium dominates. The analysis of the step response, therefore, because it concentrates on the time scale in which the transported heat has not yet reached the surface of the sensor element, provides information about a change in the internal thermal resistance. An electronic unit with a sampling rate f> 1 HZ, preferably f> 10Hz, which is equivalent to a sampling interval in the range of

Milliseconds or less, ensures that during the relevant time scale, in which the transported heat has not yet reached the surface of the sensor element, a sufficient number of data points can be recorded. In a preferred embodiment, the at least one sensor element comprises a

 Housing, in particular of a metal, in particular stainless steel or Hastelloy, wherein in the interior of the housing at least the temperature sensor, in particular an RTD resistance element, is arranged such that the housing and the temperature sensor are in thermal contact. The housing protects the sensor element

Damage. Especially when used in aggressive media such protection is of great advantage.

It is advantageous if the electronic unit comprises a memory unit on which memory unit at least one reference for a reaction of the sensor element to a sudden change of the supplied power is stored in the functional state. This reference curve can be used at the time of manufacture or parameterization of the

Flowmeter are included. The analysis of the step response can then be made from a comparison of a measured curve with a reference curve stored on the memory unit, for example by comparing the respective function values at specific predefinable characteristic times.

Furthermore, it is advantageous if the electronics unit is designed so that it can record at least 100 measured values in a time interval of typically less than 100 ms. This requirement for a minimum sampling rate of the measured variable whose step response is analyzed ensures a sufficient number of measured values in that short time interval available for the detection of the step response. With respect to the method, the object of the invention by specifying a

 A method for operating a thermal flow meter for determining the mass flow and / or a flow rate of a fluid medium in a pipeline, comprising at least one sensor element and a

Electronic unit with a sampling rate in the range of milliseconds or less, wherein the at least one sensor element is heated with a heating power, and whose temperature is detected, wherein from the heating power and / or temperature and / or at least one of at least the heating power and / or temperature derived Size of the mass flow and / or the flow rate of the medium are determined, wherein at a definable time, the heating power is changed rapidly, and wherein from the step response of the sensor element to the change in heating power generates a statement about the state of the at least one sensor element and / or output becomes. Advantageously, a change in the internal thermal resistance of the at least one sensor element can be determined from the step response. This allows a statement about the functionality of the at least one sensor element, or about the state of individual components, e.g. within the sleeves, or also via solder connections. The sudden change in the supplied heating power can be positive or negative.

It is advantageous if the step response of a characteristic of the heating element dependent characteristic value of the sensor element, in particular the temperature or the electrical resistance, is evaluated. These sizes are particularly easy to capture. The temperature is recorded anyway in a thermal flow meter, for example.

A particularly preferred embodiment includes that the step response of the

Temperature T (t) and / or the resistance R (t), the at least one sensor element is recorded as a function of time and is, wherein by means of a comparison of

recorded step response of the temperature and / or resistance, the at least one sensor element with at least one reference jump response of the temperature and / or the resistance to a change in the thermal resistance of the at least one sensor element is closed, and wherein in the case of exceeding a

predeterminable limit value for the change of the thermal resistance, a message about a malfunction of the at least one sensor element is generated and output. Once the heat generated by the heating unit to the surface of the at least one Sensor element is reached, the curve of the temperature or the electrical resistance changes as a function of the external heat transfer coefficient and / or the external flow conditions. In a further particularly preferred embodiment, the gradient of the temperature and / or the resistance is determined, wherein by means of a comparison of the gradient of the step response of the temperature and / or the resistance and / or derived therefrom size of the at least one sensor element with the gradient of at least one Reference jump response of the temperature and / or resistance is closed to a change in the thermal resistance of the at least one sensor element, and

wherein, in the case of exceeding a predeterminable limit value for the change in the thermal resistance, a message about a malfunction of the at least one sensor element is generated and output. The changes between the

Under certain circumstances, the measured step response and the reference step response may be more clearly visible through the analysis.

In principle, it is advantageous if the time interval for recording the step response of the temperature T (t) and / or of the resistor R (t) is selected such that it is less than the time which is supplied by means of the abrupt change in the heating power

Heat is needed to get from the interior of the sensor element to its surface. Based on appropriate estimates, a maximum expected time period for the heat transfer within the at least one sensor element can be determined. This saves the storage of unnecessary measurements, which at a time

be absorbed, in which the heat transfer is already dominated by the flowable medium, and concomitantly the capacities of the thermal flow meter are saved in relation to the available computing power.

The invention as well as its advantages will be explained in more detail with reference to the following FIGS. 1 to 4. Show it

1 is a schematic drawing of a thermal flow measuring device according to the prior art,

Fig. 2 are schematic drawings of two typical sensor elements Fig. 3 (a) shows a temperature change as a function of time in response to a sudden change in heating power (b) is an electrical equivalent circuit diagram of a

Sensor element as shown in Fig. 2; and Fig. 4 are graphs showing different temperature gradients by varying the internal thermal resistance in response to a power jump on a sensor element.

In the figures, the same features are identified by the same reference numerals. The device according to the invention in its entirety has the reference numeral 1. Dashes at a reference number indicate different embodiments in each case.

In Fig. 1, a thermal flow meter 1 according to the prior art is shown. In one of a medium 3 through-flowed pipe 2, two sensor elements 4,7 are tightly integrated such that they are at least partially and at least temporarily in thermal contact with the medium 3. Each of the two sensor elements 4, 7 comprises a housing 6, 6 a, which in this case is cylindrical, in each case one

Temperature sensor 5,8 is arranged. In particular, the two temperature sensors 5, 8, each of the two sensor elements 4, 7 should be in thermal contact with the medium 3.

In this example, the first sensor element 4 is designed as an active sensor element that it has a heatable temperature sensor 5. It goes without saying that a sensor element 4 with external heating element, as mentioned above, also falls under the present invention. In operation, it can be done accordingly by supplying a

Heating power P1 are heated to a temperature T1. The temperature sensor 8 of the second sensor element 7, however, is not heated and serves to detect the

Medium temperature T _M. Finally, the thermal flow meter 1 also comprises an electronic unit 9, which serves for signal acquisition, evaluation and feeding. This electronic unit can optionally be assigned a memory unit 9a on which reference values or sensor-specific characteristics or characteristic quantities are stored. Both thermal flow measuring devices 1 with more than two sensor elements 4, 7 have become known over the course of time, as have very different geometrical configurations and arrangements of the respective sensor elements 4, 7. In Fig. 2 are schematic, perspective views of two sensor elements, as they can be used for example for the flowmeter shown in Flg.1 shown. Both are basically designed as active sensor elements 4 and can be heated if necessary. The two housings 6, 6 'each have the shape of a

cylindrical pin sleeve on. The end faces 10, 10 'protrude during operation at least partially and / or temporarily with the medium 3 in thermal contact. The materials used for the construction of the sensor elements 4, 4 'are usually those materials which are distinguished by a high thermal conductivity. For simplicity, for the two sensor elements 4,4 'each of the end faces

10.10 'opposite second end sides, which are fixed, for example, in a housing of the electronics unit or on a sensor holder, not shown. The same applies to the illustration of Figure 1. The sensor element 4 in Fig. 2a terminates at its end face 10 with a plug 1 1, which is usually welded to the housing 6. This plug and a subsequent spacer 12 in this example form a one-piece, monolithic component, which is in mechanical and thermal contact with the inner side 13 of the pin-shaped housing 6. However, there are also known two-part designs. On the

Spacer 12 is also a resistance element 14 soldered, so that a good thermal contact and, correspondingly, a good heat conduction is ensured. The second solder joint opposite surface 14a of the resistor element 14 is free in this example. A second embodiment of a typical sensor element is shown in FIG. 2b.

The spacer 12 'forms an interference fit with the housing 6' in the form of a pin sleeve. Usually, it is inserted during manufacture by means of the plug 1 1 'from the end face 10', starting in the housing 6 '. The plug 1 1 'is then welded to the housing 6', for example via a laser welding process. The spacer 12 'has the shape of a cylinder with a along the longitudinal axis extending groove 15', in which a resistance element 14 'is soldered. The second solder joint opposite surface 14a 'of the resistor element 14' is also free in this example. Often, in a later manufacturing step, cavities are still filled with a suitable filling material of low thermal conductivity (not shown), such that inter alia the surfaces 14a opposite the respective solder joints, 14a 'of the respective resistive element 14,14' are covered by the filling material used in each case. Also not shown are any necessary connection cables.

Often the resistance element 14, 14 'is a platinum element, for example a PT10, PT100 or PT1000 element, which is arranged on a ceramic carrier. For the spacer 12, 12 'in turn copper is often used, while the housing 6, 6' made of stainless steel. Optionally, the housing may also be provided with a coating from the outer surface. In FIG. 3 a, the temperature as a function of time in response to a sudden change in the supplied heating power for a sensor element, as shown in FIG. 2, is shown by way of example. The following description refers without limitation to the generality exclusively to the evaluation of the characteristic measure of the temperature. However, the respective assumptions and results can be easily transferred to other characteristic variables, such as electrical resistance.

At time t _start = 0, the power supplied to the at least one sensor element is changed abruptly from a first value Pi to a second value P ₂ . Typically, the power jump is approximately Ap = 50-500mW. Preferably, during the

 Performing the power jump, the power loss at the sensor element kept constant. Alternatively, however, it is also possible to work with a constant current or voltage signal. The step response caused by the power jump

Temperature is then measured at appropriate intervals. The sampling rate is typically -Ξ1 ms in order to ensure a sufficient number of measured values for the small time interval over which the step response takes place.

The time interval 16 of interest for the analysis of the step response is marked in FIG. 3 by a circle. It is about a time interval of 100ms. In this period of time, the temperature change in response to the power jump is determined solely by the geometric structure of the sensor element 4 and the heat propagation occurring within the sensor element, ie by the thermal resistances and heat capacities of the materials used in each case. The dependence of the

Heat transport from the individual components and material transitions can for example by a, such as. B. shown in Fig. 3b, equivalent circuit diagram are shown. Top left is a sketch of a sensor element with integrated resistance element 14,14 'in the form of a arranged on a ceramic support 17 Platindünnschichtelements 18 shown. This is shown in the equivalent circuit shown as a heat source. Everyone Component of the sensor element is associated with an electrical resistor 19a-f and a capacitor 20a-f connected in parallel thereto. The influence of the flowing fluid is also in the form of an electrical resistance R _f | _Uid 19g considered. For a

Sensor element, as shown in Fig. 2, then arise in each case a resistor and a capacitor for the platinum element (R _p i _a tin. Cpiatin) 19 a, 20 a, for the ceramic carrier (Rceramic, C _cer amic) 19 b, 20 b , for the solder joint (R _SO ider, C _S0 | _the ) 19c, 20c between the resistance element 14,14 'and the spacer 12,12', for the spacer 12,12 'itself (Rco en C _cop per) 19d, 20d , for the housing 6.6 '(R _s teei _> C _ste ei) 19e, 20e and optionally for a coating of the housing 6, 6' (R _CO ating, C _CO atin _g ) 19f, 20f. Furthermore, in the equivalent circuit diagram, the temperatures surrounding the respective components are noted, namely the temperature of the sensor element T _sen sor, the temperature of the environment T _amb ient and the temperature at the surface of the sensor element T _surface .

By choosing the measurement duration, which is smaller than the time required for the heat transfer from the heating unit to the surface of the sensor element, it is possible to ensure that the respectively recorded measured values, for example for the temperature, are independent of external influences, in particular independent of changes in the mass flow or the flow rate. This advantageously allows the

Diagnostic function can be performed in the continuous operation of the flowmeter. Ideally, the diagnostic function can even run parallel to the determination of the

Mass flow and / or the flow rate can be performed.

To make a diagnosis about the functionality of the at least one

Sensor element is ideally considered the first derivative, or the gradient of the temperature. In the present example, therefore, the rate of rise of the temperature is analyzed. This changes with an occurring sensor drift. If the sensor drift is caused merely by a change in the internal thermal resistance, the rate of rise of the temperature changes with changes in the internal thermal resistances and / or capacitances according to the equivalent circuit diagram from FIG. 3b. In the event, for example, that the resistance element 14, 14 'of the at least one sensor element 4, 4' comes off, the thermal resistance Rsoider between the spacer 12, 12 'and the resistance element 14, 14' increases due to formation of a thin air layer. Since air is a good electrical insulator with a small thermal

Thermal conductivity is increased by the formation of the air layer the

Rate of increase of the temperature. The reason for this is that the from

Resistance element 14,14 'outgoing heat can not be passed as fast to the spacer 12,12'. Accordingly, the rate of increase of the temperature measured at the sensor element 4,4 'increases. Similar considerations can be made for each of in the equivalent circuit shown resistors 19a-g and for the capacitors 20a-f are performed.

 In addition to the temperature, the temperature gradient which is normalized to the supplied heating power is particularly suitable as the measured variable.

Reference curves or reference values at characteristic predeterminable discrete time points are then advantageously stored on a memory unit 9a integrated within the electronic unit 9, by means of which the respective measured values can be compared. If a predefinable deviation between the respective reference and a measurement is determined, a message and / or warning for the customer is generated and

output. In each case, the permissible deviations can be adapted specifically for an application or for the respective requirements of the flowmeter. This allows the customer depending on the one given by him

Select accuracy requirements between different limit values for the maximum permissible deviation between a measured value and the associated reference value.

In Fig. 4, finally, various curves for the temperature gradient as a function of time in a time interval of 100 ms after a power jump are finally shown by way of example. The individual curves correspond to different identical sensor elements, for which the quality of the solder joint between the spacer 12,12 'and the

Resistance element 14,14 'varies.

In addition to the temperature gradient, for example, the time constant τ and the end value t _{end of} the step response of the temperature can be determined in response to a power jump. By means of these additional variables can be in combination with the respective simultaneously determined mass flow and / or flow rate or known external process conditions, such as during a so-called zero point measurement plausibility checks with respect to a setpoint and actual value of the time constant τ or the temperature increase AT = t _end - t _start extra

Make diagnoses. For example, a statement about pollution,

Deposit formation and / or erosion are made on the at least one sensor element, which are based on a change in the external thermal resistance. However, sensor-specific characteristics of the time constant τ or of the temperature rise AT = t _end -t _{start of} the step response as a function of the mass flow, the flow velocity or one with the mass flow and / or the

Flow rate stored in a mathematically related quantities in the electronic unit. In its entirety, a flow meter according to the invention and / or the use of a method according to the invention offers the following advantages:

 1) A sensor drift caused by a change in the internal thermal resistance can be detected independently of external influences, such as, for example, a flow that is not constant over time, or deposits, dirt or erosion on the sensor element.

 2) The diagnostic function can be performed during operation, ie under process conditions.

 3) No additional installations are necessary.

4) The interruption of the measuring mode to carry out a diagnosis is a maximum of ^« 1 ms.

 5) By evaluating several associated with the step response

 Characteristics can be optionally next to a change in the internal thermal

 Resistance also close to a change in the external thermal resistance.

6) The measured value evaluation during the execution of the diagnostic function is easy to realize.

LIST OF REFERENCE NUMBERS

1 thermal flow meter

 2 pipe, or measuring tube

3 medium

 4 active sensor element

 5 heatable temperature sensor

6,6a case

 7 passive sensor element

8 temperature sensor

 9 electronics unit

 9a Memory unit of the electronic unit

 10 end face of a sensor element

 1 1 stopper

12 spacer

 13 inside of the pin-shaped housing

 14 resistance element

 15 groove of the spacer

 16 time interval interesting for the step response

17 ceramic carrier of a resistive element

 18 platinum element of a resistive element

 19,19a-g electrical resistors for an equivalent circuit of the heat transport through a sensor element

 20,20a-f capacitors for an equivalent circuit of heat transport through a

 sensor element

Claims

claims Thermal flow meter (1) for determining the mass flow rate and / or a flow rate of a fluid medium (3) in a pipeline (2), with at least  a sensor element (4,7) and an electronic unit (9) with a  Sample rate in the range of milliseconds or less  wherein the at least one sensor element (4,7)  at least partially and / or temporarily in thermal contact with the medium (3), and  a heatable temperature sensor (5),  the electronic unit (9) being designed to  to heat the at least one sensor element (4, 7) with a heating power (P),  to detect its temperature (T),  from the heat output (P) and / or temperature (T) and / or at least one derived from at least one of these variables size to determine the mass flow rate and / or the flow rate of the medium (3), and  From a step response of the at least one sensor element to a sudden change in the supplied heating power to generate a statement about the state of the at least one sensor element and / or output. Thermal flow meter according to claim 1, wherein the at least one sensor element (4,7) comprises a housing (6), in particular made of a metal, in particular stainless steel or Hastelloy, wherein inside the housing (6) at least the temperature sensor (5, 7), in particular an RTD resistance element (14), is arranged such that the housing (6) and the temperature sensor (5,7) are in thermal contact. Thermal flow meter according to claim 1 or 2, wherein the electronic unit (9) comprises a memory unit (9a) on which memory unit (9a) at least one reference for a reaction of the sensor element (4,7) to a sudden change of the supplied power (ΔΡ) in functional condition is stored. Thermal flow meter according to at least one of the preceding claims, wherein the electronic unit (9) is designed such that it can record at least 100 measured values in a time interval of typically less than 100 ms. Method for operating a thermal flowmeter (1) for determining the mass flow rate and / or a flow rate of a flowable medium (3) in a pipeline (2), with at least one sensor element (4,7) and an electronic unit (9) with a sampling rate in the range of milliseconds or less,  wherein the at least one sensor element (4,7) is heated with a heating power (P), and whose temperature (T) is detected,  wherein from the heating power (P) and / or temperature (T) and / or at least one of at least the heating power (P) and / or temperature (T) derived size of the Massed urchfluss and / or the flow rate of the medium (3) are determined .  wherein at a definable time (t) the heating power is changed abruptly (ΔΡ),  wherein a statement about the state of the at least one sensor element (4,7) is generated and / or output from the step response of the sensor element (4,7) to the abrupt change in the heating power (ΔΡ). Method according to claim 4, wherein the step response depends on the heating power (P) characteristic measured variable of the sensor element (4,7), in particular the temperature (T) or the electrical resistance (R), is evaluated. Method according to claim 5,  wherein the step response of the temperature (T (t)) and / or the resistance (R (t)) of the at least one sensor element (4, 7) is recorded as a function of time (t),  wherein by means of a comparison of the recorded step response the temperature (T (t)) and / or the resistance (R (t)) of the at least one Sensor element (4,7) with at least one reference jump response of the temperature (T (t)) and / or the resistor (R (t)) on a change in the thermal resistance of the at least one sensor element (4,7) is closed, and wherein in the case of exceeding a predetermined limit value for the change of the thermal resistance, a message about a malfunction of the at least one sensor element (4,7) is generated and output. 8. The method according to claim 5 or 6,  wherein the gradient of the temperature (T) and / or the resistance (R) is determined, and  wherein by means of a comparison of the gradient of the step response of the temperature (T) and / or the resistance (R) and / or derived therefrom size of the at least one sensor element (4,7) with the gradient of at least one reference jump response of the temperature (T) and / or the  Resistance (R) is closed to a change in the thermal resistance of the at least one sensor element (4,7), and  wherein in the case of exceeding a predetermined limit value for the change of the thermal resistance, a message about a malfunction of the at least one sensor element (4,7) is generated and output. 9. The method according to at least one of the preceding claims,  wherein the time interval (16) for recording the step response of the temperature (T) and / or the resistance (R) is selected to be smaller than the time required for the heat supplied by the abrupt change in the heating power (ΔΡ) to get from the inside of the sensor element (4,7) to its surface.