DE102023126508A1 - Thermal flow meter - Google Patents

Abstract

The invention comprises a thermal flow meter (1) for determining and/or monitoring a measured variable, in particular a mass flow, of a medium flowing in a pipe (2), which thermal flow meter (1) comprises: two thermal sensor elements (S1, S2), two balancing resistance elements (A1, A2) and an electronic measuring/operating circuit (3) are interconnected to form a bridge circuit in such a way that - in a heating branch (HZ), the first balancing resistance element (A1) and the first thermal sensor element (S1) are connected in series, - in a measuring branch (MZ), the second balancing resistance element (A2) and the second thermal sensor element (S2) are connected in series, and - the heating branch (HZ) and the measuring branch (MZ) are electrically connected in parallel, and wherein the measuring/operating circuit (3) comprises a current control unit (4), which current control unit (4) is arranged to regulate a ratio formed from a heating current intensity (Ihtr) of an electrical heating current flowing in the heating branch (HZ) during a measuring operation divided by a measuring current intensity (Imeas) of a measuring current (Imeas) flowing in the measuring branch (MZ) during the measuring operation to a current control constant (K).

Description

The invention relates to a thermal flow meter for determining and/or monitoring a measured variable, in particular a mass flow, of a medium flowing in a pipe.

Thermal flow meters are used in process automation to determine and/or monitor the mass flow of a medium flowing in a pipe, e.g., a measuring tube, especially a fluid, e.g., a gas or a liquid or a mixture thereof. Thermal flow meters are known from the state of the art and are used, for example, in DE 10 2013 105 992 A1 , the DE 10 2016 121 110 A1 , or the DE 10 2019 110 876 A1 described. A thermal flowmeter comprises at least two thermal sensor elements, including at least one heating sensor element that is (actively) heated, and at least one temperature-detecting sensor element that is designed to detect the temperature of the medium. The mass flow rate can be determined from a heating power required to maintain a temperature difference between the heated heating sensor element and the temperature-detecting sensor element. If necessary, both thermal sensor elements are designed for both heating and temperature detection and can serve as the heating sensor element and/or the temperature-detecting sensor element, depending on the control by a measuring/operating circuit of the thermal flowmeter. In particular, the two thermal sensor elements can be constructed essentially identically.

Resistance elements (also known as RTD elements, or resistance temperature detectors), are often used as such thermal sensor elements, particularly so-called PTC thermistors (PTC for positive temperature coefficient). The latter are characterized by the fact that their electrical resistance increases with increasing temperature, especially in the first order linearly. Platinum is widely used among PTC thermistors because platinum's resistance overall has a maximum quadratic dependence on temperature. The resistance elements are generally designed to have a certain nominal resistance at a reference temperature, e.g., a reference temperature of 0°C. They are available under the designations Pt10 (10 ohms), Pt100 (100 ohms), and Pt1000 (1 kOhm). The resistance element is designed, for example, as a so-called wound resistance element or as a resistive layer applied, especially structured, to a substrate, e.g., using thin- or thick-film technology. In a (thin-)film sensor, for example, a resistive structure with connecting wires is applied to a carrier substrate and insulated by a cover layer.

The two thermal sensor elements are interconnected by an electronic measuring/operating circuit, often forming a bridge circuit, such that a heating branch with the heating sensor element is electrically connected in parallel with a measuring branch with the temperature detection sensor element. A larger current should flow in the heating branch than in the measuring branch.

The current ratio is set, for example, via a ratio of the two electrical resistance values of the two thermal sensor elements, i.e., via a high-resistance measuring branch and a low-resistance heating branch. The ratio is often in the range of approximately 1:10; i.e., a heated Pt10 and an unheated Pt100, or a heated Pt100 and an unheated Pt1000, etc. As a result, the control, with a common bridge voltage, results in an electrical current in the heating branch with the heating thermal sensor element that is approximately 10 times higher than the electrical current in the measuring branch.

• - Es müssen unterschiedliche Sensortypen bewirtschaftet werden;
• - Ein Wechsel zwischen beheiztem und unbeheiztem thermischen Sensorelement durch eine entsprechende Ansteuerung der Mess-/Betriebsschaltung ist nicht möglich;
• - Die Mess-/Betriebsschaltung muss exakt auf das Verhältnis der Widerstandswerte beiden thermischen Sensorelemente abgestimmt werden. Dies schränkt die Flexibilität und den Anwendungsbereich stark ein.
• - Die Wahl eines beheizten thermischen Sensorelements mit vergleichsweise kleinem elektrischen Widerstandswert (z.B. Pt10) führt dazu, dass sich die Sensitivität (gemessen in Widerstandsänderung pro Temperaturänderung) entsprechend verringert, bspw. um den Faktor 10 gegenüber einem Pt100. Dies erschwert eine genaue messtechnische Bestimmung der Sensortemperatur bzw. der Differenztemperatur der beiden thermischen Sensorelemente. Entsprechend ist die Sensitivität des thermischen Durchflussmessgeräts verringert.

However, this combination of a high- and low-resistance thermal RTD sensor element in the bridge circuit has a number of disadvantages:

• - Different sensor types must be managed;
• - It is not possible to switch between heated and unheated thermal sensor element by controlling the measuring/operating circuit accordingly;
• - The measuring/operating circuit must be precisely matched to the resistance ratio of the two thermal sensor elements. This severely limits flexibility and the range of applications.
• - The choice of a heated thermal sensor element with a comparatively small electrical resistance value (e.g. Pt10) leads to a reduction in the sensitivity (measured in resistance change per temperature change) is reduced accordingly, for example, by a factor of 10 compared to a Pt100. This makes it difficult to accurately determine the sensor temperature or the temperature difference between the two thermal sensor elements. The sensitivity of the thermal flow meter is correspondingly reduced.

The invention is based on the object of providing a thermal flow meter which does not have the disadvantages mentioned above.

• - zwei als Widerstandselemente ausgebildete thermische Sensorelemente, die zum beheizt werden und/oder zum Erfassen der Temperatur des Mediums eingerichtet sind,
• - ein erstes Abgleichs-Widerstandselement und ein zweites Abgleichs-Widerstandselement;
• - eine elektronische Mess-/Betriebsschaltung, die dazu ausgebildet ist
  • -- ein erstes thermisches Sensorelements zu beheizen,
  • -- mittels eines unbeheizten zweiten thermischen Sensorelements die Temperatur des Mediums zu erfassen,

wobei die beiden thermischen Sensorelemente, die zwei Abgleichs-Widerstandselemente und die elektronische Mess-/Betriebsschaltung unter Bildung einer Brückenschaltung derart miteinander verschaltet sind, dass

• - in einem Heizzweig das erste Abgleichs-Widerstandselement und das erste thermische Sensorelement in Reihe geschaltet sind,
• - in einem Messzweig das zweite Abgleichs-Widerstandselement und das zweite thermische Sensorelement in Reihe geschaltet sind, und
• - der Heizzweig und der Messzweig elektrisch parallel geschaltet sind,

und wobei die Mess-/Betriebsschaltung eine Stromregelungseinheit umfasst,
welche Stromregelungseinheit dazu eingerichtet ist, ein Verhältnis gebildet aus einer Heiz-Stromstärke eines in einem Messbetrieb in dem Heizzweig fließenden elektrischen Heizstroms geteilt durch eine Mess-Stromstärke eines in dem Messbetrieb in dem Messzweig fließenden Messstroms auf eine Stromregel-Konstante zu regeln.The task is solved by a thermal flow meter for determining and/or monitoring a measured variable, in particular a mass flow, of a medium flowing in a pipe, which thermal flow meter comprises:

• - two thermal sensor elements designed as resistance elements, which are designed to be heated and/or to detect the temperature of the medium,
• - a first trimming resistance element and a second trimming resistance element;
• - an electronic measuring/operating circuit designed to
  • -- to heat a first thermal sensor element,
  • -- to measure the temperature of the medium by means of an unheated second thermal sensor element,

wherein the two thermal sensor elements, the two balancing resistance elements and the electronic measuring/operating circuit are interconnected to form a bridge circuit in such a way that

• - in a heating branch, the first balancing resistance element and the first thermal sensor element are connected in series,
• - in one measuring branch, the second balancing resistance element and the second thermal sensor element are connected in series, and
• - the heating branch and the measuring branch are electrically connected in parallel,

and wherein the measuring/operating circuit comprises a current control unit,
which current control unit is configured to control a ratio formed from a heating current intensity of an electrical heating current flowing in the heating branch during a measuring operation divided by a measuring current intensity of a measuring current flowing in the measuring branch during the measuring operation to a current control constant.

Within the scope of the invention, the current control unit enables the two thermal sensor elements to now also have similar electrical resistance values. This is because the ratio of heating current to measuring current is not (exclusively) adjusted via the ratio of the electrical resistance values, but can be adjusted using the current control unit. Therefore, even essentially identical thermal sensor elements can be used in the bridge circuit. The selection of two thermal sensor elements with similar electrical resistance values minimizes unwanted asymmetries, which, for the reasons stated above, has a positive effect on the design of the measuring/operating circuit as well as the measuring accuracy of the thermal flowmeter. The current control unit regulates the electrical current in such a way that a constant ratio - namely the current control constant - is established between the heating current and the measuring current during measuring operation when determining and/or monitoring the measured variable.

In one embodiment of the invention, the bridge circuit has a first conductor node, which first conductor node is arranged in the heating branch between the first trimming resistance element and the first thermal sensor element, wherein the bridge circuit has a second conductor node, which second conductor node is arranged in the measuring branch between the second trimming resistance element and the second thermal sensor element, wherein the bridge circuit has a third conductor node, which third conductor node is arranged between the two trimming resistance elements, and wherein the bridge circuit has a fourth conductor node, which fourth conductor node is arranged between the two thermal sensor elements.

In one embodiment of the invention, the measuring/operating circuit comprises a voltage regulation unit, wherein the voltage regulation unit is designed to supply the bridge circuit with a bridge voltage, which bridge voltage depends on a voltage difference between a first voltage and a second voltage, and wherein the voltage regulation unit in particular has a differential amplifier and the first voltage is supplied to a non-inverting first input of the differential amplifier and the second voltage is supplied to an inverting second input of the differential amplifier or vice versa.

In the context of this application, "voltage difference between a first voltage and a second voltage" means, for example, that the second voltage is subtracted from the first voltage when forming the voltage difference. For example, the applied bridge voltage is proportional to the voltage difference. Thus, the first voltage is applied to the non-inverting input and the second voltage to the inverting input, or, conversely, the second voltage is applied to the non-inverting input and the first voltage to the inverting input.

• - einem Verhältnis gebildet aus der ersten Spannung geteilt durch eine an dem zweiten Leiterknoten anliegenden Spannung geteilt durch
• - ein Verhältnis gebildet aus der zweiten Spannung geteilt durch eine an dem ersten Leiterknoten anliegenden Spannung auf einen Skalierungsfaktor zu skalieren.

In one embodiment of the invention, the measuring/operating circuit comprises a voltage scaling unit which is designed to calculate a ratio formed from

• - a ratio formed from the first voltage divided by a voltage applied to the second conductor node divided by
• - to scale a ratio formed from the second voltage divided by a voltage applied to the first conductor node to a scaling factor.

The (dimensionless) scaling factor Gain is therefore a quotient of two quotients as follows: $$Gain=\left(\frac{U_{+}}{U_{LK2}}\right)/\left(\frac{U_{-}}{U_{LK1}}\right)$$

eingestellt, und damit letztendlich ein Skalierungsfaktor Gain > 1.The scaling factor is set, for example, by means of a voltage amplifier and/or a voltage attenuator of the voltage scaling unit. For example, a voltage amplifier is arranged between the second conductor node and the non-inverting input of the differential amplifier. Alternatively or additionally, a voltage attenuator is arranged between the first conductor node and the inverting input of the differential amplifier. The voltage amplifier can be used to increase the ratio of the first voltage U ₊ divided by the voltage U _LK2 applied to the second conductor node, and the attenuator can be used to decrease the ratio of the second voltage U to the voltage U _LK1 applied to the first conductor node. The voltage amplifier or attenuator can be used to adjust the ratio $\frac{U_{+}}{{\mathrm{<figure-callout\; id="2"\; label="U\; L\; K"\; filenames="DE102023126508A1\_0011.png"\; state="\{\{state\}\}">U</figure-callout>}}_{LK}}\text{ bzw}.\frac{U_{-}}{{\mathrm{<figure-callout\; id="1"\; label="U\; L\; K"\; filenames="DE102023126508A1\_0011.png"\; state="\{\{state\}\}">U</figure-callout>}}_{LK}}$

set, and thus ultimately a scaling factor Gain > 1.

For a scaling factor Gain < 1, a voltage attenuator is arranged between the second conductor node and the non-inverting input of the differential amplifier and/or a voltage amplifier is arranged between the first conductor node and the inverting input of the differential amplifier.

In one embodiment of the invention, the scaling factor deviates from the current control constant by less than 10%, in particular less than 5%, preferably less than 2%, wherein in particular the scaling factor is greater than 1 if and only if the current control constant is greater than one and the scaling factor is less than 1 if and only if the current control constant is less than one.

In the context of this application, the current control constant is used as a reference value when specifying the percentage deviation of the (dimensionless) scaling factor from the (dimensionless) current control constant.

Particularly preferably, the scaling factor deviates from the current control constant by less than 1%, i.e. it essentially corresponds to the current control constant.

The above-mentioned setting of scaling factor and current control constant to a “sufficiently equal” value (i.e. with a sufficiently small deviation) in the above-mentioned Within the scope of the invention, the limits are preferably implemented by means of the physical design of the measuring/operating circuit, in particular by means of the circuit elements used therein (e.g., the aforementioned voltage amplifier and/or attenuator, as well as the current source and ammeter mentioned below), and thus in hardware terms. If necessary, a remaining deviation between the current control constant and the scaling factor is additionally reduced, in particular at least halved, using a calculation method, e.g., implemented in software, especially in a microcontroller.

By adapting the scaling factor to the current control constant of the aforementioned embodiment, the voltage control unit responsible for readjusting the bridge voltage can operate flawlessly with the differential amplifier to fulfill the balancing condition of the bridge circuit. The current control constant is preferably >1. In the simplest case, for example, the voltage across the unheated thermal sensor element is amplified by essentially the same factor using a voltage amplifier (which, as mentioned above, is arranged between the second conductor node and the non-inverting input of the differential amplifier of the voltage control unit). This allows the balancing conditions for the bridge circuit to be met, so that no current flows from the heating branch into the measuring branch and vice versa.

In one embodiment of the invention, the current control constant is greater than 1 and less than 200, in particular greater than 5 and less than 100, and preferably greater than 10. Preferably, the current control constant is less than 60.

In one embodiment of the invention, the current control unit comprises a, in particular controllable, (direct) current source and an ammeter.

In one embodiment of the invention, the current control constant and the scaling factor are greater than 1, wherein the current source is arranged in the measuring branch between the second balancing resistance element and the second conductor node and in particular the ammeter is designed to measure the heating current intensity and in particular the ammeter is arranged in the heating branch between the third conductor node and the first conductor node.

Of course, the invention also includes variants with a current control constant and a scaling factor gain of less than 1. For a current control constant of < 1, for example, a current source is arranged in the heating branch between the first balancing resistance element and the first conductor node.

In one embodiment of the invention, either, in particular preferably, the fourth conductor node is at ground potential and the bridge voltage is supplied to the third conductor node or the third conductor node is at ground potential and the bridge voltage is supplied to the fourth conductor node.

In one embodiment of the invention, at a first reference temperature, in particular at a first reference temperature between 18 and 25 degrees Celsius, the first thermal sensor element has a first electrical resistance temperature coefficient and the second thermal sensor element has a second electrical resistance temperature coefficient,
wherein the first electrical resistance temperature coefficient and the second electrical resistance temperature coefficient in linear order are at least 1 × 10 ^-3 /Kelvin, in particular at least 3 × 10 ^-3 /Kelvin,
and wherein in particular the first thermal sensor element and the second thermal sensor element comprise a cold-conducting material, preferably platinum and/or nickel.

The thermal sensor elements are therefore PTC thermistors and are comparatively highly temperature-dependent. For example, Pt/Ni have an electrical resistance-temperature coefficient that, in linear order, is approximately 3.85 × 10 ^-3 /Kelvin. The two thermal sensor elements are preferably designed as Pt100, meaning they have an electrical resistance of 100 ohms at a reference temperature of 0°C.

In one embodiment of the invention, at a second reference temperature, in particular a second reference temperature between -10 and + 15 degrees Celsius,
the first thermal sensor element has a first electrical resistance value and the second thermal sensor element has a second electrical resistance value,
wherein the first electrical resistance value deviates from the second electrical resistance value by less than 10 ohms, in particular less than 5 ohms, preferably less than 2 ohms.

The two thermal sensor elements therefore have almost identical electrical resistance values. In particular, the first thermal sensor element and the second thermal sensor element are essentially identical in design.

In one embodiment of the invention, at the first reference temperature, the first trimming resistance element has a third resistance temperature coefficient and the second trimming resistance element has a fourth electrical resistance temperature coefficient,
wherein the third electrical resistance temperature coefficient and the fourth electrical resistance temperature coefficient in linear order are at most 0.2 × 10 ^-3 /Kelvin, in particular at most 0.1 × 10 ^-3 /Kelvin, preferably at most 0.05 × 10 ^-3 /Kelvin.

The two trimming resistor elements therefore have a sufficiently temperature-independent electrical resistance value.

All electrical resistance values and/or resistance value temperature coefficients at the respective first/second reference temperature specified in this application always refer to temperature coefficients/electrical resistance values that are present outside of a measuring operation, i.e. when the measuring/operating circuit is switched off and the first thermal sensor element is not (actively) heated.

• - die beiden thermischen Sensorelemente in ein Lumen des Rohres hineinragen, und insb. mediumsberührend sind.

In one embodiment of the invention, the thermal flow meter is designed such that when determining and/or monitoring the measured variable

• - the two thermal sensor elements protrude into a lumen of the pipe and are in particular in contact with the medium.

When determining and/or monitoring the measured variable of the medium using thermal sensor elements in contact with the medium, there is direct thermal contact between the two thermal sensor elements and the medium. This improves the thermal coupling of the thermal sensor elements to the medium and increases the measurement accuracy and/or response time of the thermal flowmeter.

In one embodiment of the invention, the thermal flow meter is configured such that the two balancing resistance elements are arranged outside the lumen of the tube when determining and/or monitoring the measured variable.

In contrast to the two thermal sensor elements, the influence of the temperature of the medium on the two - preferably essentially temperature-independent - balancing resistance elements is preferably minimized.

In one embodiment of the invention, the two thermal sensor elements are each a respective resistance structure applied to a respective electrically insulating substrate, in particular applied using thin-film technology, with a respective electrically insulating cover, in particular a cover layer.

In this embodiment, the thermal flowmeter is preferably free of probes, inside which the thermal sensor elements would otherwise be located and which would otherwise be immersed in the medium during the determination and/or monitoring of the measured variable. Instead, the thermal sensor elements extend directly into the lumen of the measuring tube. Thus, during the determination and/or monitoring of the measured variable, the substrate (or a layer opposite the cover, especially the back layer applied to the substrate) and the cover come into direct thermal contact with the medium.

In one embodiment of the invention, a displacement resistance element is arranged in the measuring branch between the second conductor node and the second thermal sensor element.

In particular, the displacement resistance element has a fifth electrical resistance temperature coefficient at the first reference temperature, wherein the fifth electrical resistance temperature coefficient in linear order is at most 0.2 × 10 ^-3 /Kelvin, in particular at most 0.1 × 10 ^-3 /Kelvin, preferably at most 0.05 × 10 ^-3 /Kelvin.

In particular, the thermal flow meter is also designed such that the displacement resistance element is arranged outside the lumen of the pipe when determining and/or monitoring the measured variable.

Preferably, the influence of temperature is also minimized for the displacement resistance element.

In one embodiment of the invention, at the second reference temperature, the first trimming resistance element has a third electrical resistance value and the second trimming resistance element has a fourth electrical resistance value, wherein the fourth electrical resistance value is at least 5 times, in particular at least 10 times, preferably at least 30 times, the third electrical resistance value.

The invention is explained in more detail with reference to the following figures, which are not to scale. Like reference numerals denote like features. Where necessary for clarity or otherwise deemed appropriate, reference numerals have been omitted from the figures.

• 1: Eine (schematische) perspektivische Ansicht auf Messrohr mit einem thermischen Durchflussmessgeräts mit einer Brückenschaltung nach dem Stand der Technik;
• 2: Eine Schnittansicht auf ein Messrohr eines thermischen Durchflussmessgeräts 1 in einer Ausgestaltung der Erfindung; und
• 3: Details der Mess-/Betriebsschaltung 3 eines erfindungsgemäßen thermischen Durchflussmessgerät 1 nach einer Ausgestaltung der Erfindung.

They show:

• 1 : A (schematic) perspective view of a measuring tube with a thermal flow meter with a bridge circuit according to the prior art;
• 2 : A sectional view of a measuring tube of a thermal flow meter 1 in an embodiment of the invention; and
• 3 : Details of the measuring/operating circuit 3 of a thermal flow measuring device 1 according to an embodiment of the invention.

In 1 is a simplified schematic diagram of a bridge circuit as used in a thermal flowmeter according to the state of the art, with a perspective view of a measuring tube 2. In a lumen 7 (see 2 ) of the measuring tube 2, the two thermal sensor elements S1, S2 are arranged such that they lie in a plane perpendicular to the flow direction qm. A normal vector of the plane thus coincides with the flow direction qm.

The bridge circuit further comprises a (differential) amplifier and two balancing resistor elements, which are arranged outside the measuring tube. A first balancing resistor element A1 is connected in series with the first thermal sensor element S1 in a heating branch HZ, and a second balancing resistor element A2 is connected in series with the second thermal sensor element S2 in a measuring branch MZ. According to the 1 In the bridge circuit shown, thermal sensor elements S1, S2 with different electrical resistance values R _S1 , R _S2 are used to achieve different electrical currents in the measuring branch MZ and the heating branch HZ. To avoid this, an improved measuring/operating circuit 3 is presented within the scope of the invention, which in one embodiment is shown in the following 3 is shown in more detail.

2 further shows a top view of a cross section of the measuring tube 2 and a thermal flow meter 1 arranged thereon in one embodiment of the invention. The measuring/operating circuit 3 is arranged, for example, in a housing 8 for accommodating the measuring/operating circuit 3. The two thermal sensor elements S1, S2 are preferably designed as thin-film sensor elements and protrude into a lumen 7 of the measuring tube 2. A spatial distance between the two thermal sensor elements S1, S2 within the aforementioned plane, i.e. perpendicular to the flow direction qm, is between 0.5 mm and 10 mm, so that the two thermal sensor elements S1, S2 also fit into a sufficiently small measuring tube 2, for example a DN 8 measuring tube 2. The exact spacing and arrangement of the thin-film sensor elements depends on the design of the thermal flowmeter 1, the diameter of the measuring tube 2, the temperature homogeneity of the flowing medium across the cross-section of the measuring tube, and the required measurement accuracy. For measuring tubes 2 with larger nominal diameters, a correspondingly larger spacing between the thermal sensor elements S1, S2, e.g., up to 30 mm, is common.

It is about 2 The embodiment shown is a probe-free variant of a thermal flow meter 1, in which the thermal sensors designed as thin-film sensor elements Sensor elements S1, S2 come into direct contact with the flowing medium. This increases heat transfer between the medium and the thermal sensor elements S1, S2, reduces the response time of the thermal flow meter 1, and increases its accuracy.

The electronic measuring/operating circuit 3 is configured to operate the thermal sensor elements S1, S2 and to provide flow measurement values. In order to measure the mass flow of a flowable medium through the measuring tube 2, for example, the first thermal sensor element S1 in the medium flowing through the measuring tube 2 is heated in such a way that a temperature difference compared to the medium temperature remains constant. The second thermal sensor element S2 serves to measure the temperature of the medium. Assuming constant media properties (e.g. density or composition), the mass flow of the medium can be determined from the heating current required to maintain the temperature difference and/or the heating power Q required to maintain the temperature difference. For example, a power coefficient PC is considered, which is proportional to the amount of heat Q introduced via the heating power divided by a temperature difference between the first thermal sensor element S1 and the second thermal sensor element S2: PC ∼ Q/ |T _S1 - T _S2 |.

The thermal flowmeter 1 outlined here is exemplary; the skilled person will assemble a number of thermal sensor elements S1, S2 according to their requirements and arrange them in the measuring tube 2 in a desired manner. Methods for operating such thermal flowmeters 1 with thermal sensor elements S1, S2 are state of the art.

Of course, the invention is not limited to the 2 However, the combination of the probe-free solution shown in 3 measuring/operating circuit 3 explained below with the 2 The probe-free solution shown provides a synergistic effect, since the aforementioned high accuracy can be achieved even with small nominal diameters of the measuring tube 2. The solution according to the invention specifically allows the use of thermal sensor elements S1, S2 with similar electrical resistance values R _S1 , R _S2 , whereby, as mentioned above, the accuracy of the thermal flowmeter 1 is correspondingly high.

3 finally shows a schematic circuit diagram of the measuring/operating circuit 3, in an embodiment of the thermal flow meter 1 according to the invention.

As in 1 As indicated, the two thermal sensor elements S1, S2 are arranged inside the measuring tube 2, i.e. a lumen 7 of the measuring tube 2, wherein the two balancing resistance elements A1, A2 and the displacement resistance element Soffset are preferably arranged outside the measuring tube 2. The two balancing resistance elements A1, A2 and the displacement resistance element Soffset should react as little as possible to temperature changes, e.g. of the medium, and therefore, among other things, be as temperature-independent as possible. This is achieved, for example, by a corresponding selection of a third or fourth electrical resistance temperature coefficient TC _A1 , TC _A2 of the first or second balancing resistance element A1 or A2, respectively, as well as a selection of a fifth electrical resistance temperature coefficient TC _offset of the displacement resistance element Soffset. The resistance temperature coefficients are given at a first reference temperature T1, e.g. room temperature, e.g. a room temperature

• - der erste Leiterknoten LK1 in dem Heizzweig HZ zwischen dem ersten Abgleichs-Widerstandselements A1 und dem ersten thermischen Sensorelement S1,
• - der zweite Leiterknoten LK2 in dem Messzweig MZ zwischen dem zweiten Abgleichs-Widerstandselement A2 und dem zweiten thermischen Sensorelement S2,
• - der dritte Leiterknoten LK3 zwischen den beiden Abgleichs-Widerstandselementen A1, A2, und
• - der vierte Leiterknoten LK4 zwischen den beiden thermischen Sensorelement S1, S2 angeordnet ist. Bevorzugt liegt der vierte Leiterknoten LK4 auf Massepotential.

The first thermal sensor element S1 and the first balancing resistor element A1 are connected in series in a heating branch HZ, and the second thermal sensor element S2 and the second balancing resistor element A2 are connected in series in a measuring branch MZ, with the heating branch HZ and the measuring branch MZ being connected in parallel. The bridge circuit therefore has first to fourth conductor nodes LK1, LK2, LK3, LK4, of which

• - the first conductor node LK1 in the heating branch HZ between the first balancing resistance element A1 and the first thermal sensor element S1,
• - the second conductor node LK2 in the measuring branch MZ between the second balancing resistance element A2 and the second thermal sensor element S2,
• - the third conductor node LK3 between the two balancing resistance elements A1, A2, and
• - the fourth conductor node LK4 is arranged between the two thermal sensor elements S1, S2. Preferably, the fourth conductor node LK4 is at ground potential.

A bridge voltage Vbridge is supplied to the bridge circuit via a bridge, which depends on a voltage difference between a first voltage U ₊ and a second voltage U _- , for example Vbridge ∝ |U ₊ - U _- |

The thermal sensor elements S1, S2 and the adjustment resistance elements A1, A2 have the following electrical resistance values at a second reference temperature T2: S1 First electrical resistance value R _S1 e.g. Pt100 S2 Second electrical resistance value R _S2 ≈ R _S1 e.g. Pt100 A1 Third electrical resistance value R _A1 A2 Fourth electrical resistance value R _A2

The specified electrical resistance values R _S1 ,R _S2 , R _A1 R _A2 at the second reference temperature T2 always refer to electrical resistance values R _S1 ,R _S2 , R _A1 , R _A2 that are present outside of a measuring operation.

The fourth electrical resistance value R _A2 is at least 5 times, in particular at least 10 times, preferably at least 30 times, the third electrical resistance value R _A1 . This ratio (at least a factor of 5) leads, due to Ohm's law, directly to a measuring current Imeas in the second trim resistance element A2 in the measuring branch MZ that is lower by the corresponding factor compared to a heating current Ihtr in the heating branch HZ.

In particular, the first electrical resistance value R _S1 , the second electrical resistance value R _S2 , the third electrical resistance value R _A1 and the fourth electrical resistance value R _A2 are preferably matched to one another in such a way that $$R_{A1}\ll {\mathrm{<figure-callout\; id="1"\; label="R\; S"\; filenames="DE102023126508A1\_0011.png"\; state="\{\{state\}\}">R</figure-callout>}}_S\approx {\mathrm{<figure-callout\; id="2"\; label="R\; S"\; filenames="DE102023126508A1\_0011.png"\; state="\{\{state\}\}">R</figure-callout>}}_S$$

For example, at the second reference temperature T2, the first electrical resistance value R _S1 is preferably at least 4 times the third electrical resistance value R _A1 . Depending on the ratio of R _A2 to R _A1 , the following may also apply: $${\mathrm{<figure-callout\; id="2"\; label="R\; S"\; filenames="DE102023126508A1\_0011.png"\; state="\{\{state\}\}">R</figure-callout>}}_S<{\mathrm{<figure-callout\; id="2"\; label="R\; A"\; filenames="DE102023126508A1\_0011.png"\; state="\{\{state\}\}">R</figure-callout>}}_A$$

For example, at the second reference temperature T2, the fourth electrical resistance value R _A2 is preferably at least 1.5 times the second electrical resistance value R _S2 .

The invention comprises a current control unit 4, here for the sake of clarity with a current source 41 in the measuring branch MZ and an ammeter 42 in the heating branch HZ. This current control unit 4 regulates the electrical currents in such a way that a constant ratio is always established between a heating current intensity Ihtr of an electrical heating current flowing in the heating branch HZ during a measuring operation and a measuring current intensity Imeas of a measuring current flowing in the measuring branch MZ during the measuring operation, namely the current control constant K=Ihtr/Imeas.

To regulate the bridge voltage, the voltage regulation unit 5 has a differential amplifier 51 with a non-inverting first input, to which, for example, the first voltage U ₊ is applied, and an inverting second input, to which, for example, the second voltage U _- is applied. If, for example, the voltage difference between the first voltage U ₊ and the second voltage U _- is positive, the bridge voltage Vbridge is increased, for example by a voltage regulation unit 5 in 3 shown npn transistor section becomes more conductive. However, if the voltage difference becomes negative, the voltage regulation unit 5 causes the npn transistor to block more in order to achieve the equilibrium of the bridge circuit or to establish the balancing condition. The differential amplifier 51 regulates the bridge voltage via the npn transistor of the voltage regulation unit 5 in 3 so always in such a way that the voltage difference between the first voltage U ₊ and the second voltage U _- is regulated to a voltage value of 0V.

For this purpose, the scaling factor Gain or the voltage U _LK1 at the first conductor node LK1 and the voltage U _Lk2 at the second conductor node LK1 are used: $$Gain=\left(\frac{U_{+}}{U_{LK2}}\right)/\left(\frac{U_{-}}{U_{LK1}}\right)=\frac{U_{+}\ast U_{LK1}}{{\mathrm{<figure-callout\; id="2"\; label="U\; L\; K"\; filenames="DE102023126508A1\_0011.png"\; state="\{\{state\}\}">U</figure-callout>}}_{LK}\ast U_{-}}$$

The matching condition can therefore be reformulated as: $$\begin{array}{l}0=U_{+}-U_{-}=U_{-}\left(\frac{U_{+}}{U_{-}}-1\right)\\ 1=\frac{U_{LK2}}{U_{LK1}}Gain\end{array}$$

Alternatively to the 3 In addition to the NPN transistor shown, a PNP transistor can also be used to achieve the balancing condition for the bridge circuit; in this case, for example, the inverting and non-inverting inputs of the differential amplifier 51 would be interchanged. The latter is achieved by applying the first voltage U ₊ to the inverting input and the second voltage to the non-inverting input, so that the differential amplifier 51 forms a difference between the second voltage _U- and the first voltage U ₊ .

To ensure that the balancing condition for the bridge circuit is met and that the differential amplifier 51 of the voltage control unit 5, which is responsible for readjusting the bridge voltage Vbridge, can function properly, the voltage across the second (here unheated) thermal sensor element S2 is amplified by a scaling factor Gain using a voltage scaling unit 6. The scaling factor Gain preferably essentially corresponds to the current control constant K.

For example, the voltage scaling unit 6 comprises a voltage amplifier which, as shown in 3 shown is arranged between the second conductor node LK2 and the non-inverting input of the differential amplifier 51. Thus, a voltage of the second conductor node LK2 amplified by the same factor Gain ≈ K is supplied to the non-inverting input of the differential amplifier 51 as the first voltage U ₊ . The second voltage U _- at the inverting input corresponds to the voltage at the first conductor node LK1, so that $\frac{U_{-}}{{\mathrm{<figure-callout\; id="1"\; label="U\; L\; K"\; filenames="DE102023126508A1\_0011.png"\; state="\{\{state\}\}">U</figure-callout>}}_{LK}}=1$

Selbstverständlich umfasst die Erfindung mutatis mutandis auch die vorstehend genannten Varianten mit einem zusätzlichen oder alternativen Spannungs-Abschwächer (elektronischer Spannungsdämpfer). Mittels eines Spannungs-Verstärkers bzw. - Abschwächers wird jeweils das Verhältnis $\frac{U_{+}}{U_{LK2}}$

vergrößert bzw. $\frac{U_{-}}{U_{LK1}}$

verkleinert. Somit wird insg. ein Skalierungsfaktor Gain > 1 erreicht.The total scaling factor gain of the 3 The embodiment shown with only one voltage amplifier of the voltage scaling unit 6 thus results in $$Gain=\left(\frac{U_{+}}{U_{LK2}}\right)/\left(\frac{U_{-}}{U_{LK1}}\right)=\frac{U_{+}}{{\mathrm{<figure-callout\; id="2"\; label="U\; L\; K"\; filenames="DE102023126508A1\_0011.png"\; state="\{\{state\}\}">U</figure-callout>}}_{LK}}$$

Of course, the invention also encompasses, mutatis mutandis, the above-mentioned variants with an additional or alternative voltage attenuator (electronic voltage attenuator). By means of a voltage amplifier or attenuator, the ratio $\frac{U_{+}}{{\mathrm{<figure-callout\; id="2"\; label="U\; L\; K"\; filenames="DE102023126508A1\_0011.png"\; state="\{\{state\}\}">U</figure-callout>}}_{LK}}$

enlarged or $\frac{U_{-}}{{\mathrm{<figure-callout\; id="1"\; label="U\; L\; K"\; filenames="DE102023126508A1\_0011.png"\; state="\{\{state\}\}">U</figure-callout>}}_{LK}}$

reduced. This results in a scaling factor Gain > 1.

The current control constant is preferably greater than 1. The current control constant K is typically selected such that the heating current range required for the application in question can be covered and, at the same time, a measuring current range results that is small enough so that no undesirable self-heating occurs on the unheated thermal sensor element S2. This measuring current range is typically in a range < 1 mA, for example with a Pt100 as the second thermal sensor element S2. At the same time, however, the measuring current should not be selected too small, since an excessively large current control constant K also places increased demands on the noise behavior and other parameters, such as an offset of the voltage amplifier of the voltage scaling unit 6, in order to ensure stable and precise control by means of the voltage control unit 5 and its differential amplifier 51.

The current control constant K is, for example, between 1 < K < 200, in particular between 5 < K < 100, preferably between 10 < K < 60.

The invention also encompasses variants in which the current control constant K < 1, for example, by interchanging the position of the current source 41 and the ammeter 42 in the current control unit 4. In this case, the scaling factor Gain is also correspondingly < 1. This is achieved, for example, by means of a voltage attenuator (electronic voltage attenuator) of the voltage scaling unit 6, which voltage attenuator is arranged between the second conductor node LK2 and the non-inverting input of the differential amplifier 51, or by means of a voltage amplifier which is arranged between the first conductor node LK1 and the inverting input of the differential amplifier 51.

In the measuring branch MZ, the displacement resistance element Soffset is connected in series with the second thermal sensor element S2, for example, by being arranged between the second conductor node LK2 and the second thermal sensor element S2. The task of the displacement resistance element Soffsett is to specify a constant temperature offset for temperature control of the thermal flowmeter 1. This temperature control is carried out by means of the differential amplifier 51 of the voltage control unit 5 and the associated control of the bridge voltage V _bridge . The displacement resistance element Soffset has a fifth electrical resistance value R _offset at the second reference temperature T2. This fifth electrical resistance value R _offset divided by the first resistance value temperature coefficient TC _S1 results in a temperature difference ΔT (i.e. temperature of the heated sensor element S1 minus temperature of the non-heated second thermal sensor element S2), which is regulated between the two thermal sensor elements S1, S2, i.e. ΔT = R _offset /TC _S1 .

The fifth electrical resistance value R _offset here is, for example, 10 ohms to achieve a temperature difference ΔT of approximately 30 Kelvin. Generally, the fifth electrical resistance value R _offset and the first resistance temperature coefficient TC _S1 are matched to one another such that the temperature difference ΔT is at least greater than 3 Kelvin, preferably at least greater than 10 Kelvin.

The 3 The schematic circuit diagram shown shows the circuit components of the measuring/operating circuit 3 that are essential for the invention. The measuring/operating circuit naturally also includes, for example, for providing flow measurement values, further circuits as described in 3 Components not shown here, including multi-wire circuits, e.g. four-wire circuits, e.g. for detecting a voltage drop across the first trim resistance element A1, the first thermal sensor element S1, the second thermal sensor element S2 and/or the displacement resistance element Soffset.

• - Die Regelung auf die konstante Temperaturdifferenz ΔT zwischen den beiden thermischen Sensorelementen S1, S2 reagiert schnell.
• - Sie ist mittels der Brückenschaltung über die Mess-/Betriebsschaltung 3 implementiert, und somit quasi (hauptsächlich) als Hardware. Eine zusätzliche Regelung, bspw. mittels eines Microcontrollers und entsprechendem Regelalgorithmus mittels einer in dem Microcontroller hinterlegten Software, ist nicht mehr vonnöten, bzw. höchstens kaum. Letzteres für den Fall, dass eine ausschließlich hardware-technisch implementierte Anpassung zwischen Stromregel-Konstante K und Skalierungsfaktor Gain, bei der eine kleine Abweichung in den vorstehend genannten Grenzen verbleibt, noch zusätzlich rechnerisch verkleinert wird.
• - Mittels der Wahl von Soffset ist die geregelte Temperarturdifferenz zudem variabel einstellbar. Denkbar ist hier auch, ein einstellbares Verschiebungs-Widerstandselement Soffset, sprich mit einem einstellbaren elektrischen Widerstandswert zu verwenden.
• - Die Wahl der zwei Sensorelemente S1, S2 mit ähnlich grossem elektrischen Widerstandswerten R_S1, R_S2 minimiert dabei ungewünschte Asymmetrien, was sich positiv auf die Auslegung der Mess-/Betriebsschaltung 3 sowie die Messgenauigkeit des thermischen Durchflussmessgeräts 1 auswirkt.
• - Die variable Wahl der Einstellung des Stromverhältnisses zwischen beheiztem und unbeheiztem Sensorelement (Heiz-Stromstärke Ihtr zu Mess-Stromstärke Imeas, über die Stromregelungseinheit bzw. die damit assoziierte Stromregel-Konstante K) kann auf die Sensorcharakteristik und die jeweilige Anwendung hin optimiert werden.
• - Die Mess-/Betriebsschaltung 3 ist durch eine entsprechende Beschaltung flexibel an die Anforderungen der jeweiligen bestimmungsgemäße Anwendung anpassbar.

The advantages of the invention are summarized below:

• - The control to the constant temperature difference ΔT between the two thermal sensor elements S1, S2 reacts quickly.
• - It is implemented via the bridge circuit across the measuring/operating circuit 3, and thus (primarily) in hardware. Additional control, e.g., via a microcontroller and a corresponding control algorithm using software stored in the microcontroller, is no longer necessary, or at most, barely necessary. The latter applies in the case where an exclusively hardware-implemented adjustment between the current control constant K and the scaling factor Gain, in which a small deviation remains within the aforementioned limits, is further reduced computationally.
• - By selecting Soffset, the controlled temperature difference can also be variably adjusted. It is also conceivable to use an adjustable displacement resistance element Soffset, i.e., one with an adjustable electrical resistance value.
• - The selection of the two sensor elements S1, S2 with similar electrical resistance values R _S1 , R _S2 minimizes unwanted asymmetries, which has a positive effect on the design of the measuring/operating circuit 3 and the measuring accuracy of the thermal flow meter 1.
• - The variable selection of the setting of the current ratio between heated and unheated sensor element (heating current Ihtr to measuring current Imeas, via the current control unit or the associated current control constant K) can be optimized to the sensor characteristics and the respective application.
• - The measuring/operating circuit 3 can be flexibly adapted to the requirements of the respective intended application by means of appropriate wiring.

For the sake of simplicity, the invention has always been explained using the same thermal sensor element S1 for heating and the same thermal sensor element S2 for measuring the temperature. As mentioned at the beginning, the invention also encompasses, mutatis mutandis, thermal flow measuring devices 1 in which the role of the two thermal sensor elements S1, S2 is interchangeable. For this purpose, the circuit topology of the measuring/operating circuit 3 is kept adaptable, for example by means of switching elements not shown, so that, for example, the high-ohmic second balancing resistance element A2 can be connected to the measuring branch MZ (with, for example, the current source 41 of the current control unit 4) and the low-ohmic first balancing resistance element A1 can be connected to the heating branch HZ. This solution is particularly suitable for responding to changes in the flow direction qm of the medium.

Reference signs and symbols

1

flow meter

2

Pipe

3

Measuring/operating circuit

4

Current control unit

5

Voltage control unit

51

differential amplifier

6

Voltage scaling unit

7

Lumen

8

Housing

S1, S2

thermal sensor elements

A1, A2

Adjustment resistor elements

Fabric

Displacement resistance element

HZ

Heating branch

MZ

measuring branch

Your

Heating current

Imeas

Measuring current

K

Current control constant

Vbridge

Bridge voltage

Gain

Scaling factor

U+, U-

first, second voltage

sqm

Flow direction

LK1, LK2, LK3, LK4

first to fourth ladder nodes

ULK1, ULK2

Voltage at the first/second conductor node

TCS1, TCS2, TCA1, TCA2

first-fourth electrical resistance value-temperature coefficient

RS1, RS2, RA1, RA2

first-fourth electrical resistance value

Roffset

fifth electrical resistance value

TCoffset

fifth electrical resistance temperature coefficient

ΔT

regulated temperature difference

T1, T2

first, second reference temperature

QUOTES CONTAINED IN THE DESCRIPTION

This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This 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 10 2013 105 992 A1 [0002]
• DE 10 2016 121 110 A1 [0002]
• DE 10 2019 110 876 A1 [0002]

Claims (17)

A thermal flowmeter (1) for determining and/or monitoring a measured variable, in particular a mass flow, of a medium flowing in a pipe (2), comprising: - two thermal sensor elements (S1, S2) designed as resistance elements, which are designed to be heated and/or to detect the temperature of the medium, - a first balancing resistance element (A1) and a second balancing resistance element (A2); - an electronic measuring/operating circuit (3) designed to: -- heat a first thermal sensor element (S1), -- detect the temperature of the medium by means of an unheated second thermal sensor element (S2), wherein the two thermal sensor elements (S1, S2), the two balancing resistor elements (A1, A2), and the electronic measuring/operating circuit (3) are interconnected to form a bridge circuit such that: - in a heating branch (HZ), the first balancing resistor element (A1) and the first thermal sensor element (S1) are connected in series, - in a measuring branch (MZ), the second balancing resistor element (A2) and the second thermal sensor element (S2) are connected in series, and - the heating branch (HZ) and the measuring branch (MZ) are electrically connected in parallel, and wherein the measuring/operating circuit (3) Current control unit (4) comprises, which current control unit (4) is configured to regulate a ratio formed from a heating current intensity (Ihtr) of an electrical heating current flowing in the heating branch (HZ) during a measuring operation divided by a measuring current intensity (Imeas) of a measuring current (Imeas) flowing in the measuring branch (MZ) during the measuring operation to a current control constant (K). Thermal flow meter (1) according to Claim 1 , wherein the bridge circuit has a first conductor node (LK1), which first conductor node (LK1) is arranged in the heating branch (HZ) between the first balancing resistance element (A1) and the first thermal sensor element (S1), wherein the bridge circuit has a second conductor node (LK2), which second conductor node (LK2) is arranged in the measuring branch (MZ) between the second balancing resistance element (A2) and the second thermal sensor element (S2), wherein the bridge circuit has a third conductor node (LK3), which third conductor node (LK3) is arranged between the two balancing resistance elements (A1, A2), and wherein the bridge circuit has a fourth conductor node (LK4), which fourth conductor node (LK4) is arranged between the two thermal sensor elements (S1, S2). Thermal flow meter (1) according to Claim 1 or 2 , wherein the measuring/operating circuit (3) comprises a voltage regulation unit (5), wherein the voltage regulation unit (5) is designed to supply the bridge circuit with a bridge voltage (Vbridge), which bridge voltage (Vbridge) depends on a voltage difference between a first voltage (U ₊ ) and a second voltage (U _- ), and wherein the voltage regulation unit (5) in particular has a differential amplifier (51) and the first voltage (U ₊ ) is supplied to a non-inverting first input of the differential amplifier and the second voltage (U-) is supplied to an inverting second input of the differential amplifier, or vice versa. Thermal flow meter (1) according to Claim 3 , wherein the measuring/operating circuit (3) has a voltage scaling unit (6), which voltage scaling unit (6) is designed to scale a ratio formed from - a ratio formed from the first voltage (U ₊ ) divided by a voltage (U _LK2 ) applied to the second conductor node (LK2), divided by - a ratio formed from the second voltage (U _- ) divided by a voltage (U _LK1 ) applied to the first conductor node (LK1) to a scaling factor (gain). Thermal flow meter (1) according to Claim 4 , wherein the scaling factor (gain) deviates from the current control constant (K) by less than 10%, in particular less than 5%, preferably less than 2%, and wherein in particular the scaling factor (gain) is greater than 1 if and only if the current control constant (K) is greater than one and the scaling factor (Gain) is less than 1 if and only if the current control constant (K) is less than 1. Thermal flow meter (1) according to at least one of the preceding claims, wherein the current control constant (K) is greater than 1 and less than 200, in particular greater than 5 and less than 100, and preferably greater than 10. Thermal flow meter (1) according to at least one of the preceding claims, wherein the current control unit (4) comprises a, in particular controllable, (direct) current source (41) and an ammeter (42). Thermal flow meter (1) according to at least one of the preceding claims, wherein the current control constant (K) and the scaling factor (Gain) are greater than 1, and wherein the current source (41) is arranged in the measuring branch (MZ) between the second balancing resistance element (A2) and the second conductor node (LK2) and in particular the ammeter (42) is designed to measure the heating current intensity (Ihtr) and in particular the ammeter (43) is arranged in the heating branch (HZ) between the third conductor node (LK3) and the first conductor node (LK1). Thermal flow meter (1) according to at least one of the preceding claims, wherein either, in particular preferably, the fourth conductor node (LK4) is at ground potential (GND) and the bridge voltage (V _bridge ) is supplied to the third conductor node (LK4) or the third conductor node (LK3) is at ground potential (GND) and the bridge voltage (V _bridge ) is supplied to the fourth conductor node (LK4). Thermal flow meter (1) according to at least one of the preceding claims, wherein at a first reference temperature (T1), in particular at a first (T1) reference temperature between 18 and 25 degrees Celsius, the first thermal sensor element (S1) has a first electrical resistance temperature coefficient (TC _S1 ), and the second thermal sensor element (S2) has a second electrical resistance temperature coefficient (TC _S2 ), wherein the first electrical resistance temperature coefficient (TC _S1 ) and the second electrical resistance temperature coefficient (TC _S2 ) are in linear order at least 1 × 10 ^-3 /Kelvin, in particular at least 3 × 10 ^-3 /Kelvin, and wherein in particular the first thermal sensor element (S1) and the second thermal sensor element (S2) have a cold-conducting material, preferably platinum and/or nickel. Thermal flow meter (1) according to at least one of the preceding claims, wherein at a second reference temperature (T2), in particular a second reference temperature between -10 and + 15 degrees Celsius, the first thermal sensor element (S1) has a first electrical resistance value (R _S1 ) and the second thermal sensor element (S1) has a second electrical resistance value (R _S2 ), and wherein the first electrical resistance value (R _S1 ) deviates from the second electrical resistance value (R _S2 ) by less than 10 ohms, in particular less than 5 ohms, preferably less than 1 ohm. Thermal flow meter (1) according to at least one of the preceding claims, wherein the first reference temperature (T1) the first adjustment resistance element (A1) has a third resistance temperature coefficient (TC _A1 ) and the second adjustment resistance element (A2) has a fourth electrical resistance temperature coefficient (TC _A2 ), wherein the third electrical resistance temperature coefficient (TC _A1 ) and the fourth electrical resistance temperature coefficient (TC _A2 ) in linear order are at most 0.2 × 10 ^-3 /Kelvin, in particular at most 0.1 × 10 ^-3 /Kelvin, preferably at most 0.05 × 10 ^-3 /Kelvin. Thermal flow meter (1) according to at least one of the preceding claims, wherein the thermal flow meter (1) is designed such that, during the determination and/or monitoring of the measured variable, - the two thermal sensor elements (S1, S2) protrude into a lumen (7) of the tube (2) and, in particular, are in contact with the medium. Thermal flow measuring device (1) according to at least one of the preceding claims, wherein the thermal flow measuring device (1) is set up in such a way that in the determination and/or monitoring To measure the measured variable, the two balancing resistance elements (A1, A2) are arranged outside the lumen (7) of the tube (2). Thermal flow meter (1) according to at least one of the preceding claims, wherein the two thermal sensor elements (S1, S2) are each a respective resistance structure applied to a respective electrically insulating substrate, in particular applied using thin-film technology, with a respective electrically insulating cover, in particular a cover layer. Thermal flow meter (1) according to at least one of the preceding claims, wherein in the measuring branch (MZ) between the second conductor node (LK2) and the first fourth conductor node (LK4) a displacement resistance element (S_offsett) is connected in series with the second thermal sensor element (S2), which displacement resistance element (S_offsett) is arranged in particular between the second conductor node (LK2) and the second thermal sensor element (S2). Thermal flow meter (1) according to at least one of the preceding claims, wherein at the second reference temperature (T2) the first adjustment resistance element (A1) has a third electrical resistance value (R _A1 ) and the second adjustment resistance element (A2) has a fourth electrical resistance value (R _A2 ), wherein the fourth electrical resistance value is at least 5 times, in particular at least 10 times, preferably at least 30 times, the third electrical resistance value.

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