Thermal, Flow Measuring Device with Diagnostic Function

Abstract

Thermal flow measuring device (1), especially for determining and/or monitoring the mass flow (Φ.sub.M) and/or the flow velocity (v.sub.F) of a flowable medium (3) through a pipeline (2), comprising at least three sensor elements (4a,4b,4c) and an electronics unit (9), wherein each of the at least three sensor elements (4a,4b,4c) is at least partially and/or at times in thermal contact with the medium (3), and includes a heatable temperature sensor (5a,5b,5c), and wherein the electronics unit (9) is embodied to heat each of the three sensor elements (4a,4b,4c) with a heating power (P1,P2,P3), to register their temperatures (T1,T2,T3), to heat at least two of the at least three sensor elements (4a,4b,4c) simultaneously, to ascertain the mass flow (Φ.sub.M) and/or the flow velocity (v.sub.F) of the medium (3), from a pairwise comparison of the temperatures (T1,T2,T3) and/or heating powers (P1,P2,P3) of the at least three sensor elements (4a,4b,4c) and/or at least one variable derived from at least one of the temperatures (T1,T2,T3) and/or heating powers (P1,P2,P3), to provide information concerning a change of the thermal resistance of at least one of the at least three sensor elements (4a,4b,4c), from a response to an abrupt change ΔP of the heating power supplied to at least one of the at least three sensor elements, to provide information concerning a change of the inner thermal resistance of the at least one sensor element, and in the case that a change of the inner and/or outer thermal resistance occurs in the case of at least one of the at least three sensor elements (4a,4b,4c), to perform a correction of the measured value for the mass flow (Φ.sub.M) and/or the flow velocity (v.sub.F) and/or to generate and to output a report concerning the state of the at least one sensor element mass flow (Φ.sub.M) and/or the flow velocity (v.sub.F).

Claims

1. Thermal, flow measuring device (1), especially for determining and/or monitoring mass flow (ΦM) and/or flow velocity (vF) of a flowable medium (3) through a pipeline (2), comprising at least three sensor elements (4a,4b,4c) and an electronics unit (9), wherein each of the at least three sensor elements (4a,4b,4c) is at least partially and/or at times in thermal contact with the medium (3), and includes a heatable temperature sensor (5a,5b,5c), and wherein the electronics unit (9) is embodied to heat each of the three sensor elements (4a,4b,4c) with a heating power (P1,P2,P3), to register their temperatures (T1,T2,T3), to heat at least two of the at least three sensor elements (4a,4b,4c) simultaneously, to ascertain the mass flow (ΦM) and/or the flow velocity (v.sub.F) of the medium (3), from a pairwise comparison of the temperatures (T1,T2,T3) and/or heating powers (P1,P2,P3) of the at least three sensor elements (4a,4b,4c) and/or at least one variable derived from at least one of the temperatures (T1,T2,T3) and/or heating powers (P1,P2,P3), to provide information concerning a change of the thermal resistance of at least one of the at least three sensor elements (4a,4b,4c), from a response to an abrupt change ΔP of the heating power supplied to at least one of the at least three sensor elements, to provide information concerning a change of the inner thermal resistance of the at least one sensor element, and in the case that a change of the inner and/or outer thermal resistance occurs in the case of at least one of the at least three sensor elements (4a,4b,4c), to perform a correction of the measured value for the mass flow (ΦM) and/or the flow velocity (vF) and/or to generate and to output a report concerning the state of the at least one sensor element (4a,4b,4c).

2. Thermal, flow measuring device as claimed in claim 1, wherein the electronics unit (9) is embodied to determine the flow direction (3a) of the flowable medium (3).

3. Thermal, flow measuring device as claimed in claim 1, wherein the electronics unit (9) is embodied, without interruption, to ascertain the mass flow (ΦM) and/or the flow velocity (vF), to determine the flow direction (3a) of the medium (3) and/or to provide information concerning the outer and/or inner thermal resistance of at least one of the at least three sensor elements (4a,4b,4c).

4. Thermal, flow measuring device as claimed in claim 1, wherein at least one of the at least three sensor elements (4a,4b,4c) has a first embodiment (4a,4b) with reference to geometry, construction and material, and at least a second of the at least three sensor elements (4a,4b,4c) has a second embodiment (4c) different from the first.

5. Thermal, flow measuring device as claimed in claim 1, wherein at least two (4a,4b) of the at least three sensor elements (4a,4b,4c) are arranged at a first position within the pipeline (2) equivalent with reference to locally surrounding flow (3a) of the medium (3), and wherein at least one (4c) of the at least three sensor elements (4a,4b,4c) is arranged at a second position within the pipeline (2) different from the first position with reference to locally surrounding flow (3b) of the medium (3).

6. Thermal, flow measuring device as claimed in claim 1, wherein at least one (4c) of the at least three sensor elements (4a,4b,4c) is arranged with reference to the longitudinal axis of the pipeline in direct vicinity before or behind a bluff body (16), or other flow influencing module.

7. Thermal, flow measuring device as claimed in claim 6, wherein the bluff body (16) has a cross sectional area in the form of a triangle, a rectangle, a parallelogram, a trapezoid, a circle or an ellipse.

8. Thermal flow measuring device as claimed in claim 1, wherein at least one of the at least three sensor elements (4a,4b,4c) includes a housing (6), especially a housing of metal, especially stainless steel or Hastelloy, wherein in the interior of the housing (6) at least the temperature sensor (5, 7), especially an RTD resistance element (14), is arranged in such a manner that the housing (6) and the temperature sensor (5,7) are in thermal contact.

9. Thermal, flow measuring device as claimed in claim 1, including exactly three sensor elements (4a,4b,4c), wherein at least one of the three sensor elements (4c) is arranged in direct vicinity of a bluff body (16), or other flow influencing module.

10. Thermal, flow measuring device as claimed in claim 9, wherein the first (4a, S1) and the second (4b, S2) sensor elements are arranged symmetrically on oppositely lying sides of an imaginary axis parallel to the pipeline (2), wherein the third (4c, S3) sensor element is arranged on the imaginary axis, and wherein between the imaginary connecting line through the first (4a,S1) and the second sensor element (4b,S2) and the third sensor element (4c, S3) a bluff body (16) is arranged, whose separation from the third sensor element (4c,S3) is less than that from the imaginary connecting line.

11. Thermal flow measuring device as claimed in claim 1, wherein the electronics unit (9) includes a memory unit (9a), in which at least one reference for a response of the sensor element (4,7) to an abrupt change (ΔP) of the supplied power in the functional state is stored.

12. Thermal flow measuring device as claimed in claim 1, wherein the electronics unit (9) is so embodied that it can register at least 100 measured values in a time interval of typically less than 100 ms.

13. Method for operating, in a normal operating mode (22) and in a diagnostic mode (24), a thermal, flow measuring device (1) for determining and/or monitoring mass flow (ΦM) and/or flow velocity (vF) of a flowable medium (3) through a pipeline (2) and having at least three sensor elements (S1,S2,S3) and an electronics unit (9), especially as claimed in at least one of the preceding claims, wherein in the normal operating mode (22) at least one (S1) of the at least three sensor elements (S1,S2,S3) is heated with a tunable (P1) heating power, and its temperature (T1) registered, and the mass flow (ΦM) and/or the flow velocity (vF) of the medium (3) is determined, and wherein in the diagnostic mode (24) at least steps are performed as follows: a first sensor element (S1) is heated with a first heating power (P11) and its temperature (T11) registered, a second sensor element (S2) is heated with a second heating power (P12) and its temperature (T12) registered, the temperature (TM) of the medium is registered by means of a non-heated, third sensor element (S3), from the heating power (P11, P12) and/or temperature (T11,T12) of the first (S1) or second (S2) sensor element and/or at least one variable derived from at least one of these variables, mass flow (ΦM) and/or flow velocity (vF) of the medium (3) are/is continuously determined, from a pairwise comparison of the temperatures (T11,T12, . . . ) and/or heating powers (P11, P12, . . . ) of the first (S1) and/or second (S2) sensor element and the temperature (TM) of the third sensor element (S3) and/or from at least one variable derived from the temperatures (T11,T12, . . . ) and/or heating powers (P11, P12, . . . ), information is derived concerning a change of the thermal resistance of at least one of the at least three sensor elements (S1,S2,S3), from a response to an abrupt change ΔP of the heating power supplied to at least one of the at least three sensor elements, information is derived concerning a change of the inner thermal resistance of the at least one sensor element, and correction of the measured value for the mass flow (ΦM) and/or the flow velocity (vF) is performed and/or a report concerning the state (D1,D2,D3) of at least one of the at least three sensor elements (S1,S2,S3) generated and output.

14. Method as claimed in claim 13, wherein the flow direction (3a) of the medium (3) is ascertained from a pairwise comparison of the temperatures (T11,T12, . . . ) and/or heating powers (P11, P12, . . . ) and/or from at least one variable derived from the temperatures (T11,T12, . . . ) and/or heating powers (P11, P12, . . . ).

15. Method as claimed in claim 13, wherein the mass flow (ΦM) and/or the flow velocity (vF), the flow direction (3a) of the medium (3) and/or information concerning the state (D1,D2,D3) of at least one of the at least three sensor elements (S1,S2,S3) are ascertained without interruption and at the same time.

16. Method as claimed in claim 13, wherein, before the simultaneous heating of at least two of the at least three sensor elements (S1,S2,S3) with a tunable heating power (P1, P2,P3), a reconciliation (12) of the measured media temperatures (T1,T2,T3) is performed, and, in given cases, a temperature correction term (ΔTkor,1,2, ΔTkor,1,3, ΔTkor,2,3) calculated and applied to all following measurements.

17. Method as claimed in claim 13 wherein a comparison of the power coefficients PC of the at least three sensor elements (S1,S2,S3) is performed.

18. Method as claimed in claim 13, wherein the response of a characteristic measured variable of the sensor element (4,7) dependent on the heating power (P), especially the temperature (T) or the electrical resistance (R), is evaluated.

19. Method as claimed in claim 18, wherein the response of the temperature (T(t)) and/or of 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 response of the temperature (T(t)) and/or of the resistance (R(t)) of the at least one sensor element (4,7) with at least one reference response of the temperature (T(t)) and/or of the resistance (R(t)), a change of the thermal resistance of the at least one sensor element (4,7) is deduced, and wherein upon the exceeding of a predeterminable limit value for the change of the thermal resistance a report concerning a malfunction of at least one sensor element (4,7) is generated and output.

20. Method as claimed in claim 18, wherein the gradient of the temperature (T) and/or of the resistance (R) is ascertained, and wherein by means of a comparison of the gradient of the response of the temperature (T) and/or of the resistance (R) and/or a variable of the at least one sensor element (4,7) derived therefrom with the gradient of at least one reference response of the temperature (T) and/or of the resistance (R), a change of the thermal resistance of the at least one sensor element (4,7) is deduced, and wherein upon the exceeding of a predeterminable limit value for the change of the thermal resistance a report concerning a malfunction of at least one sensor element (4,7) is generated and output.

21. Method as claimed in claim 13, wherein a time interval (17) for recording the response of the temperature (T) and/or of the resistance (R) is selected in such a manner that it is less than the time, which the heat supplied by means of the abrupt change of the heating power (ΔP) requires, in order to travel from the interior of the sensor element (4,7) to its surface.

22. Method as claimed in claim 13, wherein the flow measuring device (1) has exactly three sensor elements (S1,S2,S3), comprising method steps as follows: in the normal operating mode (22), the first sensor element (Si) is heated with a first heating power (P11), its temperature (T11) registered, and the mass flow (ΦM) and/or the flow velocity (vF) determined, in a first time interval (24a) of the diagnostic mode (24) the first (S1) and the second sensor elements (S2) are heated, the mass flow (ΦM) and/or the flow velocity (vF) are/is determined based on the first sensor element (S1), and a comparison of the power coefficients PC(S1,S3) and PC(S2,S3) performed, in a second time interval (24b) of the diagnostic mode (24) the first (S1) and the third (S3) sensor elements are heated, the mass flow (ΦM) and/or the flow velocity (vF) are/is determined based on the first (S1) sensor element, and a comparison of the power coefficients PC(S1,S2) and PC(S3,S2) performed, and the direction (3a, R2) of the flowing medium (3) ascertained therefrom, and in a third time interval (24c) of the diagnostic mode (24) the second (S2) and the third (S3) sensor elements are heated, the mass flow (ΦM) and/or the flow velocity (vF) are/is determined based on the second (S2) sensor element, and a comparison of the power coefficients PC(S2,S1) and PC(S3,S1) performed, and the direction (3a,R3) of the flowing medium (3) ascertained therefrom, wherein based on the comparisons of the power coefficients (PC(S1,S2), PC(S1,S3), PC(S2,S1), PC(S2,S3),PC(S3,S1), PC(S3,S2)) in the three time intervals (24a,24b,24c), information is derived concerning the state (D1,D2,D3) of at least one of the at least three sensor elements (S1,S2,S3) and a correction of the measured value for the mass flow (ΦM) and/or the flow velocity (vF) performed and/or a report concerning the state (D1,D2,D3) of the at least one sensor element (S1,S2,S3) generated and output.

23. Method as claimed in claim 22, wherein at least one change of the heating power supplied to at least one of the at least three sensor elements (S1,S2,S3), especially for the case, in which an earlier not heated sensor element (S1,S2,S3) is heated, or for the case, in which an earlier heated sensor element (S1,S2,S3) is no longer heated, is in the form of an abrupt change (ΔP) of the supplied heating power supplied to the respective sensor element (S1,S2,S3), and wherein, from the response of the particular sensor element (S1,S2,S3) to this abrupt change (ΔP) of the heating power, information is derived concerning the inner thermal resistance of the respective sensor element (S1,S2,S3).

Description

[0093] The invention as well as its advantages will now be explained in greater detail based on the appended drawing, the figures of which show as follows:

[0094] p FIG. 1 a schematic view of a thermal, flow measuring device according to the state of the art,

[0095] p FIG. 2 schematic drawings of two typical sensor elements,

[0096] p FIG. 3 a schematic drawing of a thermal flow measuring device with three sensor elements according to claim 4 and/or claim 5,

[0097] p FIG. 4 a schematic drawing of a thermal flow measuring device with three sensor elements, of which one is arranged behind a bluff body,

[0098] p FIG. 5 a graph of the power coefficient characteristic curves as a function of the Reynolds number in the case of different flow directions,

[0099] p FIG. 6 (a) temperature change as a function of time in response to an abruptly changed heating power (b) an electrical equivalent circuit diagram of a sensor element such as shown in FIG. 2,

[0100] p FIG. 7 a graph showing how different changes of the inner thermal resistance lead to different temperature gradients in response to an abruptly changed heating power to a sensor element.

[0101] p FIG. 8 a block diagram of a possible evaluation method.

[0102] p In the figures, equal features are provided with equal reference characters. The apparatus of the invention bears the reference character 1 in its totality. Primes on reference characters indicate different examples of embodiments.

[0103] p FIG. 1 shows a thermal, flow measuring device 1 according to the state of the art. Sealedly integrated in a pipeline 2 flowed through by a medium 3 are two sensor elements 4,7, in such a manner that they are at least partially and at least at times in thermal contact with the medium 3. Each of the two sensor elements 4,7 includes a housing 6,6a, which, in this case, is embodied cylindrically, and in which are arranged respective temperature sensors 5,8. Especially, the two temperature sensors 5,8, of each of the two sensor elements 4,7 should be in thermal contact with the medium 3.

[0104] p In this example, the first sensor element 4 is embodied as active sensor element such that it has a heatable temperature sensor 5. Of course, a sensor element 4 with external heating element, such as mentioned above, likewise falls within the scope of the present invention. In operation, it can correspondingly be heated to a temperature T1 by delivery of a heating power P1. The temperature sensor 8 of the second sensor element 7 is, in contrast, not heatable and serves for registering the temperature T.sub.M of the medium.

[0105] p Finally, the thermal, flow measuring device 1 includes also an electronics unit 9, which serves for signal registration, —evaluation and —feeding, or power supply. Known are thermal, flow measuring devices 1 with more than two sensor elements 4,7, as well as also the most varied of geometric embodiments and arrangements of the respective sensor elements 4,7.

[0106] p Shown in FIG. 2 are schematic, perspective views of two sensor elements, such as can be used, for example, for the flow measuring device shown in FIG. 1. Both are basically embodied as active sensor elements 4 and can, when required, be heated. The two housings 6, 6a have, in each case, the shape of a cylindrical, pin-shaped jacket. The end faces 10, 10′ protrude during operation at least partially and/or at times into thermal contact with the medium 3. Used for the construction of the sensor elements 4, 4′ materials are usually materials distinguished by a high thermal conductivity.

[0107] p For simplification in the case of both sensor elements 4,4′, the second ends lying opposite the end faces 10,10′ and, for example, secured in a housing of the electronics unit or to a sensor holder are not shown. The same holds for the illustration of FIG. 1.

[0108] p The sensor element 4 in FIG. 2a is closed on its end 10 with a plug 11, which is usually joined with the housing 6 or by welding. This plug as well as a spacer 12 following thereon form in this example a one-piece, monolithic component, which is in mechanical and thermal contact with the inside 13 of the pin-shaped housing 6. There are, however, also two-part embodiments known. Soldered on the spacer 12 is, furthermore, a resistance element 14, in such a manner that a good thermal contact and correspondingly a good heat conduction are assured. The second surface 14a of the resistance element 14 lying opposite the soldered connection is freely exposed in this example.

[0109] p A second embodiment of a typical sensor element is shown in FIG. 2b. The spacer 12′ forms a press fit with the housing 6′ in the form a pin-shaped jacket. Usually, it is pushed, starting from the end 10′, into the housing 6′ by means of the plug 11′ during the manufacturing. Plug 11′ is then bonded to the housing 6′ by welding, for example, using a laser welding method. Spacer 12′ has the shape of a cylinder with a groove 15′ extending along its longitudinal axis. A resistance element 14′ is soldered in the groove 15′. The second surface 14a′ of the resistance element 14′ lying opposite the soldered connection is likewise freely exposed in this example.

[0110] p Often, in a later manufacturing step, hollow spaces are filled with a suitable fill material (not shown) of a lesser thermal conductivity, in such a manner that, among other things, also the surfaces 14a, 14a′ of the respective resistance element 14,14′ lying opposite the respective soldered connections are covered by the respectively used fill material. Not shown, furthermore, are any required connecting cables.

[0111] p Often, the resistance element 14,14′ is a platinum element, for example, a PT10, PT100, or PT1000 element, which is arranged on a ceramic support. Frequently used for the spacer 12,12′ is copper, while the housing 6, 6′ is composed of stainless steel. Optionally, the housing can, moreover, be provided with a coating on the outer surface.

[0112] p Shown in FIGS. 3 and 4 are two possible arrangements, or embodiments, of a thermal, flow measuring device 1 in a two-dimensional, sectional illustration through the pipeline. The macroscopic flow direction 3a of the medium 3 is shown by an arrow. The thermal, flow measuring device 1′ of FIG. 2 includes three active sensor elements 4a,4b,4c containing, in each case, a heatable temperature sensor (not shown). The first sensor element 4a and the second sensor element 4b have an equal geometric embodiment with a circularly shaped cross sectional area, and are arranged at two positions 3b within the pipeline 2 equivalent with reference to the local flow surrounding them. The third sensor element 4c has a second geometric embodiment different from the first with a square-shaped cross sectional area. Moreover, the third sensor element 4c is arranged at a second position 3b′ within the pipeline 2 with locally surrounding flow different from the first position 3b. The local flow profiles are indicated by arrows.

[0113] p FIG. 4 shows a further, thermal, flow measuring device 1″ in a two-dimensional, sectional illustration. The medium 3 flows in the same direction 3a as in the example of FIG. 2. Also, this thermal, flow measuring device 1″ includes three active sensor elements 4a′, 41b′ and 4c′. Similarly as in FIG. 2, the first 4a′ and the second 4b′ sensor elements are arranged symmetrically on oppositely lying sides of an imaginary axis parallel to the pipeline, while the third sensor element 4c′ is arranged on the imaginary axis, wherein between the imaginary connecting line through the first 4a′ and second 4b′ sensor element and the third sensor element 4c′ a bluff body is arranged, whose separation from the third sensor element 4c′ is less than that from the imaginary connecting line. The first sensor element 4a′ and the second sensor element 4b′ are, furthermore, equivalently embodied. The bluff body 16 has a triangular, cross sectional area. It is understood, however, that other geometric embodiments are possible for the bluff body 16. Bluff body 16 influences the flow profile 3a, so that a local flow 3b″ results for the third sensor element 4c′, which is changed compared with the local flows surrounding the sensor elements 4a′ and 4b′.

[0114] p Different local flows 3b,3b′,3b″ surrounding the various sensor elements 4a,4b,4c,4a′,41b′,4c′ result in different cooling rates for them. The characteristic curves or functional, determinative equations referenced for determining the mass flow and/or the flow direction differ correspondingly. Moreover, due to different arrangements within the pipeline 2 or due to different geometrical embodiment, these characteristics curves, or functional relationships likewise differ for a forwards-, or backwards, directed flow 3a. These differences enable, for example, a reliable direction detection and correspondingly a more exact ascertaining of the mass flow and/or the flow velocity. By way of example, FIG. 5 shows a characteristic curve for the power coefficient of a calibrated heated sensor element S with reference to a passive, thus non-heated, sensor element SM as a function of the Reynolds number Re for a forwards directed and for a backwards directed flow. At the point of reversal of the flow direction (Re=0), there is an abrupt change of the power coefficient. Moreover, the power coefficient for a forwards directed flow lies in the range of 20-30%, while the power coefficient for a backwards directed flow amounts to 50-60%. Correspondingly, based on this characteristic curve, the flow direction can be exactly determined, even when a sensor element exhibits only a small drift.

[0115] p FIG. 6a shows, by way of example, temperature as a function of time in response to an abrupt change of the supplied heating power for a sensor element such as shown in FIG. 2. The following description concerns, without limitation of the generality, exclusively the evaluation of temperature as the characteristic measured variable. The respective assumptions and results can, however, be transferred, in simple manner, also to other characteristic measured variables, such as, for example, the electrical resistance.

[0116] p For the point in time t.sub.start=0, the power supplied to the at least one sensor element is abruptly changed from a first value P.sub.1 to a second value P.sub.2. Typically, the power jump amounts to about ΔP=50=500 mW. Preferably, while performing the power jump, and the power loss on the sensor element is kept constant. Alternatively, however, also a constant electrical current- or voltage signal can be used. The temperature response caused by the power jump is then measured in suitable time intervals. The sampling rate amounts typically to ≦1 ms, in order that a sufficient number of measured values is assured for the small time interval, within which the response occurs.

[0117] p The time interval 17 of interest for analyzing the response is indicated in FIG. 6a by an encircling line. A typical time interval is, for instance, 100 ms. In this time span, the temperature change as response to the power jump is determined only by the geometric construction of the sensor element 4 as well as the heat propagation resulting within the sensor element, thus by the thermal resistances and heat capacities of the respectively used materials. The dependence of the heat transport on the individual components and material transitions can be shown, for example, by an equivalent circuit diagram, such as shown e.g. in FIG. 6b. To the left and above is a sketch of a sensor element 14,14′ with integrated resistance element in the form of a platinum thin-film element 19 arranged on a ceramic support 18. Sensor element 14,14′ is represented in the equivalent circuit diagram as a heat source. Each component of the sensor element is represented by an electrical resistor 20a-f and a capacitor 21a-f connected in parallel. The influence of the flowing fluid is likewise taken into consideration in the form of an electrical resistance R.sub.fluid 20g. For a sensor element, such as one shown in FIG. 2, there results, then, a resistor and a capacitor for the platinum element (R.sub.platinum, C.sub.platinum) 20a, 21a, for the ceramic support (R.sub.ceramic, C.sub.ceramic) 20b, 21b, for the soldered connection (R.sub.solder, C.sub.solder) 20c, 21c between the resistance element 14,14′ and the spacer 12,12′, for the spacer 12,12′ (R.sub.copper, C.sub.copper) 20d, 21d, for the housing 6,6′ (R.sub.steel, C.sub.steel) 20e, 21e and, in given cases, for a coating of the housing 6,6′(R.sub.coating, C.sub.coating) 20f, 21f. Further noted in the equivalent circuit diagram are the temperatures on the respective components, namely the temperature of the sensor element T.sub.sensor, the temperature of the environment T.sub.ambient and the temperature on the surface of the sensor element T.sub.surface.

[0118] p By choosing a measurement duration, which is less than the time required for the heat transport from the heating unit to the surface of the sensor element, it can be assured that the respectively recorded measured values, for example, for the temperature, are independent of external influences, especially independent of changes of the mass flow or the flow velocity. This enables advantageously that the diagnostic function can be performed in the ongoing operation of the flow measuring device. Ideally, the diagnostic function can even be performed in parallel with determining the mass flow and/or the flow velocity.

[0119] p For diagnosing the functional ability of the at least one sensor element, ideally, the first derivative, or the gradient, of the temperature is considered. In the present example, thus, the rate of increase of the temperature is analyzed. This changes with sensor drift. If the sensor drift is brought about only by a change of the inner thermal resistance, then the rate of increase of the temperature changes with changes of the inner thermal resistances and/or capacitances according to the equivalent circuit diagram of FIG. 6b. In the case, in which, for example, the resistance element 14,14′ of the at least one sensor element 4,4′ loses its bonding, the thermal resistance R.sub.solder between the spacer 12,12′ and the resistance element 14,14′ increases due to the formation of a thin air layer. Since air is a good electrical insulator with a small thermal thermal conductivity, the forming of the air layer makes the rate of increase of the temperature greater. The reason for this is that the heat outgoing from the resistance element 14,14′ can no longer be transferred as rapidly to the spacer 12,12′. Correspondingly, the rate of increase of the temperature measured on the sensor element 4,4′ rises. Similar considerations can be performed for each of the resistances 20a-g as well as capacitors 21a-f shown in the equivalent circuit diagram. Besides the temperature, suited as measured variable is, moreover, especially the temperature gradient normalized to the supplied heating power.

[0120] p Furnished advantageously in a memory unit 9a integrated within the electronics unit 9 are then reference curves, or reference values, for characteristic, predeterminable, discrete points in time, so that the respective measured values can be compared. If a predeterminable deviation is detected between a reference and a measurement, a report and/or warning is generated and output for the customer. In such case, the allowable deviations can, in each case, be matched specifically to an application or to the particular requirements of the flow measuring device. In this way, the customer can, depending on the accuracy requirements predetermined by it, choose between different limit values for the maximum allowable deviation between a measured value and the associated reference value.

[0121] p FIG. 7 shows, by way of example, different curves for temperature gradient as a function of time in a time interval of 100 ms after a power jump. The individual curves correspond to different, equally-constructed sensor elements, in the case of which the quality of the soldered connections between the spacer 12,12′ and the resistance element 14,14′ varies.

[0122] p In addition to the temperature gradients, for example, the time constant τ as well as the end value t.sub.end of the response of the temperature as response to a power jump can be ascertained. By means of these additional variables, additional diagnoses can be made in combination with the mass flow and/or the flow velocity measurements ascertained at the same time or in the case of known external process conditions, such as, for example, during a so-called zero point measurement, by means of plausibility check relative to a desired-and actual value of the time constant τ or of the temperature rise ΔT=t.sub.end−t.sub.start. For example, information concerning fouling, accretion-formation and/or material removal on the at least one sensor element can be derived, as based on a change of the outer thermal resistance. For this, however, likewise sensor-specific characteristic values of the time constant τ or of the temperature rise ΔT=t.sub.end−t.sub.start of the response as a function of the mass flow, the flow velocity or a variable mathematically related to the mass flow and/or the flow velocity must be furnished in the electronics unit.

[0123] p FIG. 8 shows, finally, a block diagram of an option for a method for operating a flow measuring device 1. The shown steps are for the example of a flow measuring device 1 with three sensor elements 4a,4b and 4c, especially for a flow measuring device 1″ such as shown in FIG. 4. Advantageously, the mass flow, or the flow velocity, can be determined continuously and with high accuracy of measurement. Additionally, the flow direction of the medium within the pipeline can be determined and information concerning the state of at least one of the at least three sensor elements provided. Ideally, it can even be ascertained, which of the three sensor elements 4a,4b,4c exhibits a change of thermal resistance.

[0124] p In the following for purposes of simplicity, the first sensor element, e.g. the sensor element referred to in FIG. 4 with 4a′, is referred to with S1, the second sensor element, e.g. 4b′, is referred to with S2 and the third, e.g. 4c′, with S3.

[0125] p In the normal operating mode 22, at least S1 is heated to a first temperature T11 by delivery of the heating power P11. S2 and S3, in contrast, remain unheated and serve for registering the temperature T.sub.M of the medium. Of course, in principle, each of the three sensor elements S1, S2, S3 can be heated, or can remain unheated in the normal operating mode 22. From the heating power P11, the temperature T11 of the heated sensor element S1 as well as the temperature T.sub.M of the medium, then the mass flow Φ.sub.M, or the flow velocity v.sub.F, can be determined.

[0126] p Before the so-called diagnostic mode 24 is activated, optionally a temperature reconciliation 23 can be performed. In such case, the temperatures of the two unheated sensor elements S2 and S3 are compared. In the case, in which a deviation ΔT.sub.2,3 of the measured values for the temperature T.sub.M of the medium obtained by means of the two sensor elements is detected, a so called temperature correction term can be ascertained and applied in all following measurements, for example, in such a manner that T(S3)+ΔT.sub.kor,2,3=T(S2).

[0127] p In the diagnostic mode 24, different options are available. The basic idea is to supply, in different time intervals, two of the three sensor elements with equal or different heating powers and to leave one of the sensor elements unheated. From a pairwise comparison of the temperatures and/or heating powers of the two heated sensor elements and the temperature of the unheated sensor element and/or from at least one variable derived from the temperatures and/or heating powers, then information concerning the state of at least one of the at least three sensor elements can be provided and/or a correction of the measured value for the mass flow and/or the flow velocity performed and/or a report concerning the state of at least one of the at least three sensor elements generated and output. Moreover, in the case of each change of the supplied heating power to be performed for each of the three sensor elements, an analysis 25 of the response can be performed. Alternatively, the analysis 25 of the response can also be executed only when required, wherein otherwise the diagnostic mode 24 is completed without a response analysis. By performing a response analysis 25, information can be derived concerning whether the inner or the outer thermal resistance of at least one of the sensor elements S1, S2, S3 has changed. It can thus not only be determined, which of the three sensor elements S1, S2, S3 shows a sensor drift, but, instead, also, the cause of the sensor drift. If no change of the inner thermal resistance is ascertained by means of the response analysis 25, then, for example, a change of the outer thermal resistance can be deduced.

[0128] p In the block diagram shown in FIG. 8, in a first time interval 24a, S1 and S2 are heated, while S3 remains unheated. In a second (third) time interval 24b (24c), then S1 and S3 (S2 and S3) are heated, while, in turn, S2 (S1) remains unheated. Before each changing of the heated sensor elements, optionally anew a temperature reconciliation 23 can be performed, and, in given cases, a further temperature correction term ΔT.sub.kor,1,2 or ΔT.sub.kor,1,3 ascertained. These options are indicated by the arrows connecting the different intervals 24a-c and the section for the temperature reconciliation 23. Then, for example, before the first time interval 24a, a response analysis 25, such explained as based on FIGS. 5 and 6, can be performed with reference to the second sensor element S2, and, indeed, upon switching the heating power P12 on. Upon start of the second time interval 24b, in turn, a response analysis can be performed for the sensor element S2 upon turning the heating power P12 off, or for the third sensor element upon turning the heating power P32 on. Before start of the third time interval 24c, then either a response analysis can be performed with reference to the first sensor element S1 (turnoff procedure) or with reference to the second sensor element S2 (turnon procedure). These are indicated by the respective arrows between the individual time intervals 24a-c and the response analysis 25. A proviso for this procedure is that the different heating powers are, in each case, abruptly turned on or turned off, as the case may be.

[0129] p An opportunity for winning information concerning the state of at least one of the at least three sensor elements based on a pairwise comparison of the temperatures and/or heating powers results from calculating the respective power coefficient PC(S1,S2), PC(S1,S3), PC(S2,S3), PC(S3,S2), PC(S3,S1) and/or PC(S2,S1) and the respective decision coefficients in each of the time intervals 13a,13b,13c. From a comparison of the different decision coefficients, in turn, it can be ascertained, for which of the three sensor elements S1, S2, S3 the thermal resistance has changed. Sometimes this may not work. Depending on size of the change of the thermal resistance of a given sensor element, either, in case the change is only small, a correction of the ascertained measured value for the mass flow Om and/or the flow velocity vF can be performed. If, however, the change is greater than a predeterminable limit value, then a report concerning the state of the respective sensor element S1,S2,S3 or that the thermal resistance of at least one of the at least three sensor elements S1, S2, S3 has changed, is generated and output. In the case in which the response analysis 25 was likewise performed, it is possible, furthermore, to distinguish between a change of the inner thermal resistance and the outer thermal resistance. In the case, in which it is known, for which of the at least three sensor elements S1,S2,S3 a change of the thermal resistance has taken place, measurement operation corresponding to the normal operating mode 22 can be performed with the remaining two functional sensor elements, until the drifted sensor element is serviced.

[0130] p Depending on configuration, all three time intervals 24a-c do not need to be performed, because, for example, either none of the sensor elements S1,S2,S3 show a change of thermal resistance, or already in the first or second time interval, it is clear, for which of the sensor elements S1, S2, S3 a change of the thermal resistance has occurred.

[0131] p To the extent that all three time intervals 24a,24b and 24c are passed through, then three different statements of diagnostic information D1, D2 and D3 are obtained, which result from a comparison of the power coefficients ascertainable in each of the time intervals 24a-c, for example, based on the respective decision coefficients. In given cases, the diagnostic information D1, D2 and D3 moreover contains the results of the various response analyses 25. By comparing the diagnostic information D1, D2 and D3, it can, finally, in given cases, be ascertained, for which of the three sensor elements S1, S2 or S3 the inner or outer thermal resistance has changed. In the case of a change of the outer thermal resistance, when thus, for example, a fouling or accretion has occurred, the customer can simply, and on-site, perform a cleaning procedure, without having to replace the affected sensor element.

[0132] p In the example shown here, a direction detection 3a is, furthermore, performed in the second 24b and third 24c time intervals of the diagnostic mode 24. Since the flow diagram shown here is tailored for a sensor of FIG. 4, the locally surrounding flow 3b″ of S3 is different from the locally surrounding flows 3a of S1 and S2, so that a direction detection 3a can be completed most effectively, when one of the two heated sensor elements is S1 or S2 and the second heated sensor element is S3.

[0133] p In the third time interval 24c, S2 and S3 are heated. Correspondingly, for a continuous determining of the mass flow Φ.sub.M and/or the flow velocity v.sub.F, at least for this time interval, and alternation from S1 to S2 should occur. Before the alternation, there is, consequently, an opportunity especially for a temperature reconciliation 23. Somewhat the same holds in the case, in which a change of the thermal resistance of S3 is detected. The customer should, however, in given cases, be informed by means of a report that maintenance of the thermal, flow measuring device 1 is necessary.

[0134] p If there results from the diagnostic mode 24 that the (inner or outer) thermal resistance of S1 has changed, then one can switch for the normal mode 22 from S1 to S2, so that a continuing correct and exact determining of the mass flow Φ.sub.M and/or the flow velocity v.sub.F is assured.

[0135] p Depending on need of the customer, the diagnostic mode 24 and/or the direction detection 3a are/is activated. It is, however, likewise possible to perform the diagnostic mode 24 and/or a direction detection 3a continuously and in parallel with determining the mass flow Φ.sub.M and/or the flow velocity v.sub.F. Also, the customer can choose, whether the response analysis 25 should be executed within the diagnostic mode 24 continuously or only when required. The direction detection 3a is e.g. in FIG. 5 repeatedly performed in the time intervals 13b und 13c and the information won concerning the flow direction R.sub.2 and R.sub.3 can be compared, for example, with one another for checking the measurement results. In these time intervals, the third sensor element S3 arranged behind the bluff body 16 is heated, this being a feature, which is especially advantageous for the accuracy of the direction detection 3a. A comparison of two sensor elements arranged at equivalent positions and equally embodied does not, normally, lead to an exact direction detection.

[0136] p For evaluating the diagnostic information D1, D2 and D3 won in the different time intervals 24a-c of the diagnostic mode 24, it can be assumed with reference to a change of the outer thermal resistance that a fouling and/or accretion formation on at least one of the three sensor elements results in a negative shifting of the respective power coefficients compared with the normal state, while the occurrence of an abrasion leads to a positive shifting.

[0137] p If an arrangement and/or embodiment of the thermal, flow measuring device other than that utilized for the diagram of FIG. 8 is selected, the individual steps must, in given cases, be slightly modified. Independently of the number of sensor elements as well as their arrangement and/or embodiment, the basic procedure of changing between a normal mode 22 and a diagnostic mode 24 remains. Likewise there remains the opportunity, optionally, to perform a temperature reconciliation 23 and/or a direction detection 3a. Moreover, each method of the invention utilizes a pairwise comparison of the temperatures and/or heating powers of two heated sensor elements and the temperature of a third non-heated sensor element and/or a variable derived from at least one of the temperatures and/or heating powers. In such case, different sensor elements can be heated in different time intervals. Each change of the heating power supplied to one of the sensor elements can accompanied by a response analysis 25.

[0138] p In its totality, a flow measuring device of the invention and/or the application of a method of the invention thus offers advantages as follows: [0139] 1) A sensor drift brought about by a change of the inner or outer, thermal resistance can be detected independently of external influences, such as, for example, a non-constant flow of the medium as a function of time [0140] 2) It can be ascertained which sensor element has a sensor drift. [0141] 3) The cause of sensor drift is detectable, i.e. it can be detected whether a change of the inner or outer thermal resistance has occurred. [0142] 4) The diagnostic function can be performed in ongoing operation, thus under process conditions. [0143] 5) No additional installations are necessary. [0144] 6) The interruption of measurement operation for performing a response analysis amounts to maximum ≈1s, in case the performing of such analysis is not, in any event, a component of the diagnostic mode; a pairwise comparison of heating powers and/or temperatures can even occur in parallel with measurement operation. [0145] 7) Evaluation of a plurality of characteristic variables associated with the response helps, in given cases, with detection of a change of the outer thermal resistance. [0146] 8) Measured value evaluation during performance of the diagnostic function is simple to implement.

LIST OF REFERENCE CHARACTERS

[0147] 1 thermal flow measuring device [0148] 2 pipeline, respectively measuring tube [0149] 3 medium [0150] 3a macroscopic flow direction [0151] 3b local flow surrounding a sensor element [0152] 4 active sensor element [0153] 4a,4b,4c different arrangements/embodiments of an active sensor element [0154] 5 heatable temperature sensor [0155] 5a,5b,5c heatable temperature sensors of the sensor elements 4a,4b,4c [0156] 6,6a housing [0157] 7 passive sensor element [0158] 8 temperature sensor [0159] 9 electronics unit [0160] 9a memory unit of the electronics unit [0161] 10 end of a sensor element [0162] 11 plug [0163] 12 spacer [0164] 13 inner side of the pin-shaped housing [0165] 14 resistance element [0166] 15 groove of the spacer [0167] 16 bluff body [0168] 17 time interval of interest for the response [0169] 18 ceramic support of a resistance element [0170] 19 platinum element of a resistance element [0171] 20,20a-g electrical resistances of an equivalent circuit diagram of the heat transport through a sensor element [0172] 21,21a-f capacitances of an equivalent circuit diagram of the heat transport through a sensor element [0173] 22 normal operating mode [0174] 23 temperature reconciliation [0175] 24 diagnostic mode [0176] 24a,24b,24c first, second, third time interval of the diagnostic mode [0177] 25 response analysis [0178] S1 first sensor element, e.g. 4a [0179] S2 second sensor element, e.g. 4b [0180] S3 third sensor element, e.g. 4c [0181] Pxy heating power supplied to sensor element Sx in the time interval y [0182] Txy temperature of the sensor element Sx in the time interval y [0183] PC power coefficient [0184] DC decision coefficient [0185] D1,D2,D3 diagnostic information [0186] R.sub.2, R.sub.3 flow direction of the medium# [0187] Φ.sub.M mass flow [0188] v.sub.F flow velocity [0189] T.sub.M temperature of the medium [0190] ΔT.sub.kor,x,y temperature correction term for reconciliation between sensor elements x and y [0191] ΔT temperature rise in reference to the heating power supplied one of the sensor elements