Thermal flow meter

Abstract

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

Description

Technical Field

[0001] The present invention relates to a heat flow meter for determining and / or monitoring the measurement variables of a medium flowing in a pipe, particularly the volumetric flow rate. Background Technology

[0002] Heat flow meters are used in process automation to determine and / or monitor the volumetric flow rate of a medium, particularly a fluid such as a gas or liquid or a mixture thereof, flowing in a pipe, such as a measuring tube. Heat flow meters are known from the prior art and are described, for example, in DE 10 2013 105 992 A1, DE 10 2016 121 110 A1, or DE 10 2019 110 876 A1. A heat flow meter includes at least two thermal sensor elements, including at least one heating sensor element that is (actively) heated, and at least one temperature sensing sensor element designed to detect the temperature of the medium. The volumetric flow rate can be derived from the heating power required to maintain the temperature difference between the heated sensor element and the temperature sensing sensor element. If necessary, both thermal sensor elements are designed for both heating and temperature sensing, and can be used—depending on the control of the heat flow meter's measurement / operation circuitry—as both a heating sensor element and / or a temperature sensing sensor element. In particular, the two thermal sensor elements can be substantially identical in design.

[0003] Resistive elements (also known as RTD elements, short for resistance temperature detectors) are typically used as such thermal sensor elements, particularly so-called PTC thermistors (PTC stands for positive temperature coefficient). The difference lies in the fact that their resistance increases linearly with increasing temperature, specifically with a first-order linear increase. Platinum is widely used in PTC thermistors because platinum generally exhibits a resistance that correlates with at most the square of temperature. Resistive elements are typically designed to have a specific nominal resistance at a reference temperature, such as 0°C. They are obtained by the names Pt10 (10 ohms), Pt100 (100 ohms), and Pt1000 (1 kiloohms). Resistive elements are designed, for example, as so-called wound resistive elements, or as resistive layers applied to a substrate, particularly a structured substrate, using thin-film or thick-film techniques. For example, in (thin) film sensors, a resistive structure equipped with connecting wires and applied to a carrier substrate is used and insulated by means of a capping layer.

[0004] Two thermal sensor elements are typically interconnected via electronic measurement / operation circuitry to form a bridge circuit, such that the heating branch with the heating sensor element is electrically connected in parallel with the measurement branch with the temperature sensing sensor element. In this case, a current larger than that in the measurement branch should flow in the heating branch.

[0005] The current ratio is set, for example, via the ratio of the two resistance values ​​of the two thermal sensor elements, i.e., via the high-resistance measuring branch and the low-resistance heating branch. This ratio is typically in the range of approximately 1:10; that is, for example, heated Pt10 and unheated Pt100, or heated Pt100 and unheated Pt1000, etc. Therefore, due to the adjustment, using the common bridge voltage, the current in the heating branch containing the heated thermal sensor element is approximately ten times larger than the current in the measuring branch.

[0006] However, this combination of high-resistivity thermal RTD sensor elements and low-resistivity thermal RTD sensor elements in the bridge circuit has several disadvantages: - Different sensor types need to be managed; It is not possible to switch between heated and unheated thermal sensor elements by properly controlling the measurement / operation circuitry; The measurement / operation circuit must be precisely matched to the ratio of the resistance values ​​of the two thermal sensor elements. This severely limits flexibility and the range of applications.

[0007] Choosing a heated thermal sensor element with a relatively low resistance value (e.g., Pt10) results in a corresponding decrease in sensitivity (which is measured as the change in resistance per temperature change), for example, a 10-fold decrease compared to Pt100. This makes it difficult to accurately determine the sensor temperature or temperature difference between two thermal sensor elements using measurement techniques. Consequently, the sensitivity of the heat flow meter is reduced. Summary of the Invention

[0008] The present invention achieves the objective of providing a heat flow meter that does not have the aforementioned drawbacks.

[0009] This objective is achieved by a heat flow meter for determining and / or monitoring the measurement variables of the medium flowing in a pipe, particularly the volumetric flow rate, the heat flow meter comprising: - Two thermal sensor elements, designed as resistive elements, configured to be heated and / or detect the temperature of the medium. - First balancing resistor element and second balancing resistor element; - Electronic measurement / operation circuit, which is designed as -- Heating the first thermal sensor element, -- The temperature of the medium is detected by means of an unheated second thermal sensor element. Two thermal sensor elements, two balancing resistor elements, and an electronic measurement / operation circuit are interconnected to form a bridge circuit, making... In the heating branch, the first balancing resistor element and the first thermal sensor element are connected in series. - In one measurement branch, the second balancing resistor element and the second thermal sensor element are connected in series, and - The heating branch and the measuring branch are connected in parallel. Furthermore, the measurement / operation circuit includes a current regulation unit. The current regulating unit is designed to adjust the ratio formed by dividing the heating current of the electric heating current flowing in the heating branch during the measurement operation by the measuring current of the measuring current flowing in the measuring branch during the measurement operation to the current regulating constant.

[0010] Within the scope of this invention, the current regulating unit allows two thermal sensor elements to have similar resistance values. This is because the ratio of heating current to measuring current is not (solely) set via the ratio of resistance values, but can be adjusted using the current regulating unit. Therefore, even substantially identical thermal sensor elements can be used in a bridge circuit. Selecting two thermal sensor elements with similar resistance values ​​minimizes unwanted asymmetry, which, for the aforementioned reasons, has a positive impact on the design of the measurement / operation circuit and the measurement accuracy of the heat flow meter. The current regulating unit adjusts the current such that during measurement operation, when the measured variable is determined and / or monitored, a constant ratio—i.e., the current regulating constant—is established between the heating current and the measuring current.

[0011] In one embodiment of the invention, the bridge circuit has a first conductor node arranged in the heating branch between the first balancing resistor element and the first thermal sensor element, a second conductor node arranged in the measurement branch between the second balancing resistor element and the second thermal sensor element, a third conductor node arranged between the two balancing resistor elements, and a fourth conductor node arranged between the two thermal sensor elements.

[0012] In one embodiment of the invention, the measurement / operation circuit includes a voltage regulation unit configured to supply a bridge voltage to the bridge circuit, the bridge voltage depending on the voltage difference between a first voltage and a second voltage, and wherein the voltage regulation unit specifically 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.

[0013] Within the scope of this application, "voltage difference between the first voltage and the second voltage" means, for example, subtracting the second voltage from the first voltage when calculating 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 is applied to the inverting input, or conversely, the second voltage is applied to the non-inverting input and the first voltage is applied to the inverting input.

[0014] In one embodiment of the invention, the measurement / operation circuit has a voltage scaling unit configured to scale a ratio formed by: - The ratio formed by dividing the first voltage by the voltage applied to the second conductor node, divided by - The ratio formed by dividing the second voltage by the voltage applied to the first conductor node.

[0015] Therefore, the (dimensionless) scaling factor G is calculated as the quotient of the two quotients as follows:

[0016] For example, the scaling factor is set using a voltage amplifier and / or voltage attenuator in the voltage scaling unit. For instance, the voltage amplifier is positioned between the second conductor node and the non-inverting input of the differential amplifier. Alternatively or additionally, the voltage attenuator is positioned between the first conductor node and the inverting input of the differential amplifier. The voltage amplifier is capable of amplifying the first voltage U. + Divide by the voltage U applied at the second conductor node LK2 The ratio, and the attenuator can reduce the second voltage U - With the voltage U applied at the first conductor node LK1 The ratio. Voltage amplifiers or attenuators are used to set the ratio. or Therefore, the scaling factor G is ultimately set to G>1.

[0017] For scaling factor G<1, the voltage attenuator is arranged between the second conductor node and the non-inverting input of the differential amplifier and / or the voltage amplifier is arranged between the first conductor node and the inverting input of the differential amplifier.

[0018] In one embodiment of the invention, the scaling factor deviates from the current regulation constant by less than 10%, particularly less than 5%, preferably less than 2%, wherein, in particular, the scaling factor is greater than 1 if and only if the current regulation constant is greater than 1, and the scaling factor is less than 1 if and only if the current regulation constant is less than 1.

[0019] Within the scope of this application, the current regulation constant is used as a reference value when the deviation of the (dimensionless) scaling factor relative to the (dimensionless) current regulation constant is specified as a percentage.

[0020] Preferably, the scaling factor deviates from the current regulation constant by less than 1%, that is, it is essentially consistent with the current regulation constant.

[0021] The aforementioned adjustment of the scaling factor and current regulation constant to "sufficiently similar" values ​​within the aforementioned limitations (i.e., with sufficiently small deviations) is preferably implemented within the scope of the invention by means of the physical design of the measurement / operation circuit, particularly by means of the circuit elements used therein (e.g., the aforementioned voltage amplifiers and / or attenuators, and the current sources and ammeters mentioned below), and thus in hardware. If necessary, any remaining deviation between the current regulation constant and the scaling factor is further reduced computationally using calculation methods, for example, in software, particularly in a microcontroller, especially by at least halving it.

[0022] By adjusting the scaling factor to the current regulation constant of the aforementioned embodiment, the voltage regulation unit responsible for adjusting the bridge voltage can operate faultlessly using a differential amplifier to satisfy the balance condition of the bridge circuit. A current regulation constant > 1 is preferred. In the simplest case, for example, the voltage across the unheated thermal sensor element device is amplified by substantially the same factor using a voltage amplifier (as previously described, located between the second conductor node and the non-inverting input of the differential amplifier of the voltage regulation unit). This ensures that the balance condition of the bridge circuit is satisfied, such that no current flows from the heating branch to the measuring branch, and vice versa.

[0023] In one embodiment of the invention, the current regulation constant is greater than 1 and less than 200, particularly greater than 5 and less than 100, and preferably greater than 10. Preferably, the current regulation constant is less than 60.

[0024] In one embodiment of the invention, the current regulating unit includes a (DC) current source, particularly an adjustable current source, and an ammeter.

[0025] In one embodiment of the invention, the current regulation constant and scaling factor are greater than 1, wherein the current source is arranged in the measuring branch between the second balancing resistor element and the second conductor node, and in particular, the ammeter is designed to measure the heating current, and in particular, the ammeter is arranged in the heating branch between the third conductor node and the first conductor node.

[0026] Naturally, the invention also includes variations having a current regulation constant and a scaling factor G less than 1. For a current regulation constant < 1, for example, the current source is arranged in the heating branch between the first balancing resistive element and the first conductor node.

[0027] In one embodiment of the invention, particularly 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.

[0028] In one embodiment of the invention, at a first reference temperature, particularly between 18 and 25 degrees Celsius, a first thermal sensor element has a first resistance-temperature coefficient, and a second thermal sensor element has a second resistance-temperature coefficient. Wherein, the first resistance value-temperature coefficient and the second resistance value-temperature coefficient are at least 1×10 in the linear order. -3 / Kelvin, specifically at least 3 × 10 -3 Kelvin, In particular, the first thermal sensor element and the second thermal sensor element comprise a cold-conducting material, preferably platinum and / or nickel.

[0029] Therefore, the thermal sensor element is a PTC thermistor and is relatively highly temperature-dependent. For example, Pt / Ni exhibits a linear order of approximately 3.85 × 10⁻⁶. -3 The resistance value in Kelvin is related to the temperature coefficient. The two thermal sensor elements are preferably designed as Pt100, meaning they have a resistance of 100 ohms at a reference temperature of 0°C.

[0030] In one embodiment of the invention, at a second reference temperature, particularly at a second reference temperature between -10 degrees Celsius and +15 degrees Celsius, The first thermal sensor element displays a first resistance value, and the second thermal sensor element displays a second resistance value. The deviation between the first resistance value and the second resistance value is less than 10 ohms, particularly less than 5 ohms, and preferably less than 2 ohms.

[0031] Therefore, the two thermal sensor elements have almost identical resistance values. In particular, the first and second thermal sensor elements are essentially identical in structure.

[0032] In one embodiment of the present invention, at a first reference temperature, the first balancing resistor element has a third resistance value-temperature coefficient, and the second balancing resistor element has a fourth resistance value-temperature coefficient. Among them, the third resistance-temperature coefficient and the fourth resistance-temperature coefficient are at most 0.2 × 10⁻⁶ in the linear order. -3 / Kelvin, specifically at most 0.1 × 10 -3 / Kelvin, preferably at most 0.05 × 10 -3 Kelvin.

[0033] Therefore, the two balancing resistors have sufficiently temperature-independent resistance values.

[0034] All resistance values ​​and / or resistance-temperature coefficients specified at the corresponding first / second reference temperatures within the scope of this application are always temperature coefficients / resistance values ​​that exist outside the measurement operation, i.e., when the measurement / operation circuit is turned off and the first thermal sensor element is not (actively) heated.

[0035] In one embodiment of the invention, the heat flow meter is designed such that during the determination and / or monitoring of the measured variable...

[0036] - Two thermal sensor elements protrude into the lumen of the tube and are in particular in contact with the medium.

[0037] When using thermal sensor elements that contact the medium to determine and / or monitor the measured variables of the medium, there is direct thermal contact between the two thermal sensor elements and the medium. This improves the thermal coupling between the thermal sensor elements and the medium, and increases the measurement accuracy and / or response time of the heat flow meter.

[0038] In one embodiment of the invention, the heat flow meter is arranged such that two balancing resistor elements are placed outside the lumen of the tube when determining and / or monitoring the measured variable.

[0039] Unlike two thermal sensor elements, the effect of the medium's temperature on two balancing resistor elements—which are preferably substantially temperature-independent—is preferably minimized.

[0040] In one embodiment of the invention, each of the two thermal sensor elements includes a corresponding resistive structure, which is applied to, and particularly to, a corresponding electrically insulating substrate using a corresponding electrically insulating covering, particularly a covering layer, using thin-film technology.

[0041] In this embodiment, the heat flow meter is particularly preferably probeless; the thermal sensor element would otherwise be arranged inside the probe, and the probe would otherwise be immersed in the medium when determining and / or monitoring the measurement variable. Instead, the thermal sensor element protrudes directly into the lumen of the measuring tube. Thus, when determining and / or monitoring the measurement variable, the substrate (or the layer applied to the substrate, particularly on the back side, opposite the cover) and the cover are in direct thermal contact with the medium.

[0042] In one embodiment of the invention, the displacement resistor element is arranged in the measurement branch between the second conductor node and the second thermal sensor element.

[0043] Specifically, the displacement resistor element exhibits a fifth resistance-temperature coefficient at the first reference temperature, where the fifth resistance-temperature coefficient is at most 0.2 × 10⁻⁶.-3 / Kelvin, specifically at most 0.1 × 10 -3 / Kelvin, preferably at most 0.05 × 10 -3 / Linear order of Kelvin.

[0044] Specifically, the heat flow meter is also configured such that when determining and / or monitoring the measured variable, the displacement resistance element is arranged outside the tube cavity.

[0045] Therefore, for displacement resistive elements, the effect of temperature is preferably minimized.

[0046] In one embodiment of the invention, at a second reference temperature, the first balancing resistor element has a third resistance value, and the second balancing resistor element has a fourth resistance value, wherein the fourth resistance value is at least 5 times, particularly at least 10 times, and preferably at least 30 times, the third resistance value. Attached Figure Description

[0047] The invention will be further explained with reference to the accompanying drawings, which are not drawn to scale, wherein the same reference numerals denote the same features. Reference numerals will not be repeated in the drawings for clear reasons, or if it seems reasonable for other reasons.

[0048] In the attached diagram: Figure 1 This is a schematic three-dimensional diagram of the measuring tube of a heat flow meter with a bridge circuit based on existing technology. Figure 2 : This is a cross-sectional view of the measuring tube of the heat flow meter 1 in an embodiment of the present invention; Figure 3 Details of the measurement / operation circuit 3 of the heat flow meter 1 according to an embodiment of the present invention are shown. Detailed Implementation

[0049] Figure 1 A simplified schematic representation of a bridge circuit, as used in a heat flow meter according to the prior art, is shown, with a perspective view of the measuring tube 2. In the lumen 7 of the measuring tube 2 (see...) Figure 2 In this configuration, two thermal sensor elements S1 and S2 are arranged such that they lie in a plane perpendicular to the flow direction qm. Therefore, the normal vector of the plane coincides with the flow direction qm.

[0050] Furthermore, the bridge circuit includes a (differential) amplifier and two balancing resistors arranged outside the measuring tube. The first balancing resistor A1 is connected in series with the first thermal sensor element S1 in the heating branch HZ, and the second balancing resistor A2 is connected in series with the second thermal sensor element S2 in the measuring branch MZ. According to... Figure 1The bridge circuit shown has different resistance values ​​R. S1 R S2 Thermal sensor elements S1 and S2 are used to achieve different currents in the measurement branch MZ and the heating branch HZ. To avoid this, an improved measurement / operation circuit 3 is presented within the scope of this invention, and is explained below. Figure 3 One embodiment is shown in more detail.

[0051] Figure 2 A top view of the cross-section of the measuring tube 2 and the heat flow meter 1 disposed thereon is further shown in an embodiment of the invention. The measuring / operating circuit 3 is arranged, for example, in a housing 8 for enclosing the measuring / operating circuit 3. Here, the two thermal sensor elements S1, S2 are preferably designed as thin-film sensor elements and protrude into the cavity 7 of the measuring tube 2. The spatial distance between the two thermal sensor elements S1, S2 in the aforementioned plane—i.e., perpendicular to the flow direction qm—is between 0.5 mm and 10 mm, such that the two thermal sensor elements S1, S2 are also fitted into a sufficiently small measuring tube 2—for example, a DN 8 measuring tube 2. The exact distance and arrangement of the thin-film sensor elements depend on the design of the heat flow meter 1, the diameter of the measuring tube 2, the temperature uniformity of the flow medium across the cross-section of the measuring tube, and the required measurement accuracy. For measuring tubes 2 with larger nominal diameters, for example, a correspondingly larger distance between the thermal sensor elements S1, S2—for example, up to 30 mm—is also common.

[0052] Figure 2 The embodiment shown is a probeless variant of the heat flow meter 1, wherein the thermal sensor elements S1 and S2, designed as thin-film sensor elements, are in direct contact with the flowing medium. This increases heat transfer between the medium and the thermal sensor elements S1 and S2, thereby reducing the response time of the heat flow meter 1 and / or increasing its accuracy.

[0053] Electronic measurement / operation circuit 3 is configured to operate thermal sensor elements S1 and S2 and provide flow measurement values. To measure the volumetric flow rate of the flowable medium through measuring tube 2, for example, the first thermal sensor element S1 flowing through the medium is heated such that the temperature difference relative to the medium's temperature remains constant. The second thermal sensor element S2 is used to measure the temperature of the medium. Assuming constant medium properties (e.g., density or composition), the volumetric flow rate of the medium can be determined based on the heating current required to maintain the temperature difference and / or the heating power Q required to maintain the temperature difference. For example, consider the power factor PC, which is proportional to the heat Q supplied via the heating power divided by the temperature difference between the first thermal sensor element S1 and the second thermal sensor element S2: .

[0054] The heat flow meter 1 outlined herein is exemplary; those skilled in the art will assemble multiple heat sensor elements S1, S2 as needed and arrange them in a desired manner in the measuring tube 2; methods for operating such a heat flow meter 1 with heat sensor elements S1, S2 are prior art.

[0055] Of course, this invention is not limited to Figure 2 The probe-free solution is shown below. However, the following... Figure 3 The measurement / operation circuit 3 explained in the text is related to... Figure 2 The combination of probe-free solutions shown results in a synergistic effect, as the aforementioned high accuracy can be achieved even with a small nominal diameter measuring tube 2. The solution according to the invention allows for precise use with similar resistance values ​​R. S1 R S2 The thermal sensor elements S1 and S2, as mentioned at the beginning, result in the corresponding high accuracy of the heat flow meter 1.

[0056] Figure 3 Finally, a schematic circuit diagram of the measurement / operation circuit 3 in an embodiment of the heat flow meter 1 according to the present invention is shown.

[0057] like Figure 1 As indicated, two thermal sensor elements S1 and S2 are also arranged inside the measuring tube 2, i.e., the cavity 7 of the measuring tube 2, wherein the two balancing resistor elements A1 and A2 and the displacement resistor element SO are preferably arranged outside the measuring tube 2. The two balancing resistor elements A1 and A2 and the displacement resistor element SO should respond as little as possible to, for example, temperature changes of the medium, and therefore should be as uncorrelated with temperature as possible. This is, for example, by correspondingly selecting the third resistance value-temperature coefficient TC of the first balancing resistor element A1 or the second balancing resistor element A2. A1 Or the fourth resistance value - temperature coefficient TC A2 And the fifth resistance value - temperature coefficient TC of the displacement resistor element SO. SO This is achieved by specifying the resistance value-temperature coefficient at a first reference temperature T1, such as room temperature.

[0058] The first thermal sensing element S1 and the first balancing resistor element A1 are connected in series in the heating branch HZ, and the second thermal sensing element S2 and the second balancing resistor element A2 are connected in series in the measuring branch MZ, wherein the heating branch HZ and the measuring branch MZ are connected in parallel. The bridge circuit therefore has first to fourth conductor nodes LK1, LK2, LK3, and LK4, wherein...

[0059] - The first conductor node LK1 is arranged in the heating branch HZ between the first balancing resistor element A1 and the first thermal sensor element S1. - The second conductor node LK2 is arranged in the measurement branch MZ between the second balancing resistor element A2 and the second thermal sensor element S2. - The third conductor node LK3 is arranged between the two balancing resistive elements A1 and A2, and - The fourth conductor node LK4 is arranged between the two thermal sensor elements S1 and S2. Preferably, the fourth conductor node LK4 is at ground potential.

[0060] The bridge voltage Vb is supplied via the bridge in the bridge circuit and is determined by the first voltage U. + Second voltage U - The voltage difference between them is determined, that is, for example, .

[0061] The thermal sensor elements S1 and S2 and the balancing resistor elements A1 and A2 exhibit the following resistance values ​​at the second reference temperature T2:

[0062] The specified resistance value at the second reference temperature T2 , , , It always refers to the resistance value that exists outside of the measurement operation. , , , .

[0063] Fourth resistance value It is the third resistance value At least 5 times, particularly at least 10 times, and preferably at least 30 times. According to Ohm's law, this ratio (at least a factor of 5) directly results in a measuring current Imeas in the second balancing resistor element A2 in the measuring branch MZ that is a factor lower than the heating current Ihtr in the heating branch HZ.

[0064] Specifically, the first resistance value R S1 The second resistance value R S2 The third resistance value R A1 and the fourth resistance value R A2 Preferably, matching is performed in the following manner:

[0065] For example, at the second reference temperature T2, the first resistance value R S1 Preferably, it is the third resistance value R A1 At least four times. Additionally, it depends on R. A2 With R A1 The following ratios also apply:

[0066] For example, at the second reference temperature T2, the fourth resistance value R A2 The resistance value is preferably the second resistance value R. S2 At least 1.5 times.

[0067] The present invention includes a current regulating unit 4. For clarity, a current source 41 in the measuring branch MZ and an ammeter 42 in the heating branch HZ are shown here. The current regulating unit 4 regulates the current so that a constant ratio is always maintained between the heating current Ihtr, which is the electric heating current flowing in the heating branch HZ during the measurement operation, and the measuring current Imeas, which is the measuring current flowing in the measuring branch MZ during the measurement operation, i.e., the current regulating constant K = Ihtr / Imeas.

[0068] To adjust the bridge voltage, the voltage regulation unit 5 has a differential amplifier 51, which has, for example, a first voltage U applied. + The non-inverting first input and, for example, the second voltage U applied - The inverting second input. For example, if the first voltage U + With the second voltage U - If the voltage difference between them is positive, then for example, by making Figure 3 The voltage regulation unit 5 shows an npn transistor section that becomes more conductive to increase the bridge voltage Vb. On the other hand, if the voltage difference becomes negative, the voltage regulation unit 5 causes the npn transistor to block more again in order to achieve bridge circuit balance or establish a balance condition. Differential amplifier 51 is connected via... Figure 3 The voltage regulation unit 5 uses an npn transistor to regulate the bridge voltage, always ensuring that the first voltage U... + Second voltage U - The voltage difference between them is adjusted to a voltage value of 0V.

[0069] For this purpose, a scaling factor G or voltage U is used at the first conductor node LK1. LK1 And a voltage U is applied at the second conductor node LK1. LK2 :

[0070] Therefore, the equilibrium condition can be reconstructed as follows:

[0071] As Figure 3 As shown in the diagram, instead of the npn transistor, the pnp transistor can also be used to achieve the balance condition of the bridge circuit; in this case, for example, the inverting and non-inverting inputs of the differential amplifier 51 will be interchanged. The latter will be achieved by switching the first voltage U... +A second voltage is applied to the inverting input and a second voltage is applied to the non-inverting input, such that the differential amplifier 51 thus has a second voltage U. - With the first voltage U + The difference between them.

[0072] To meet the balance conditions of the bridge circuit and ensure that the differential amplifier 51 of the voltage regulation unit 5, which is responsible for adjusting the bridge voltage Vb, operates correctly, the voltage amplification scaling factor G across the second—i.e., unheated—thermal sensor element S2 is applied using the voltage scaling unit 6. The scaling factor G preferably corresponds substantially to the current regulation constant K.

[0073] For example, such as Figure 3 As shown, the voltage scaling unit 6 has a voltage amplifier arranged between the second conductor node LK2 and the non-inverting input of the differential amplifier 51. This means that by the same factor The voltage at the amplified second conductor node LK2 is used as the first voltage U. + The voltage U at the inverting input of differential amplifier 51 is supplied. - The voltage corresponding to the first conductor node LK1 is such that .

[0074] Therefore, a single voltage amplifier with voltage scaling unit 6 Figure 3 The total scaling factor G of the embodiment shown is calculated as follows:

[0075] Naturally, the invention also includes the aforementioned variations with additional or alternative voltage attenuators (electronic voltage dampers). The magnitude of this ratio is amplified using a voltage amplifier. Or use an attenuator to reduce Therefore, the total scaling factor G > 1 is achieved.

[0076] The current regulation constant is preferably greater than 1. The current regulation constant K is typically chosen such that the required heating current range of this application can be covered, while simultaneously generating a measurement current range small enough that unwanted self-heating does not occur at the unheated thermal sensor element S2. This measurement current range, for example using Pt100 as the second thermal sensor element S2, is typically in the range of <1 mA. However, the measurement current should not be chosen too small, as an excessively large current regulation constant K also increases the need for noise behavior and other parameters—such as the offset of the voltage amplifier in the voltage scaling unit 6—in order to ensure stable and accurate regulation by means of the voltage regulation unit 5 and its differential amplifier 51.

[0077] The current regulation constant K is, for example, between 1 < K < 200, particularly between 5 < K < 100, and preferably between 10 < K < 60.

[0078] The invention also includes variations in which the current regulation constant K < 1, for example by swapping the positions of the current source 41 and the ammeter 42 in the current regulation unit 4. In this case, the scaling factor G is also < 1. This is achieved, for example, by means of a voltage attenuator (electronic voltage damper) in the voltage scaling unit 6, which 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 arranged between the first conductor node LK1 and the inverting input of the differential amplifier 51.

[0079] In the measuring branch MZ, the displacement resistor element SO 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 resistor element SO is to specify a constant temperature offset for temperature regulation of the heat flow meter 1. This temperature regulation is achieved by means of the differential amplifier 51 of the voltage regulation unit 5 and the associated adjustment of the bridge voltage Vb. The displacement resistor element SO exhibits a fifth resistance value R at the second reference temperature T2. SO The fifth resistance value R SO Divide by the first resistance value - temperature coefficient TC S1 Cause temperature difference (That is, the temperature of the heated sensor element S1 minus the temperature of the unheated second thermal sensor element S2), which is adjusted between the two thermal sensor elements S1 and S2, i.e. .

[0080] Here, the fifth resistance value R SO For example, 10 ohms, to achieve a temperature difference of approximately 30 Kelvin. Typically, the fifth resistance value R SO With the first resistance value - temperature coefficient TC S1 Matching, resulting in temperature difference It should be at least greater than 3 Kelvin, preferably at least greater than 10 Kelvin.

[0081] Figure 3 The schematic circuit diagram shown illustrates the key circuit components of the measurement / operation circuit 3 of the present invention. The measurement / operation circuit naturally includes, for example, components for providing flow rate measurements. Figure 3 Other components, which may not be shown, include multi-wire circuitry, such as four-wire circuitry, for example, to detect the voltage drop across the first balancing resistor element A1, the first thermal sensor element S1, the second thermal sensor element S2, and / or the displacement resistor element SO.

[0082] The advantages of this invention are summarized as follows: - Based on the constant temperature difference between two thermal sensor elements S1 and S2 Its regulation reacts quickly.

[0083] - It is implemented via a bridge circuit through the measurement / operation circuit 3, and is therefore practically (mainly) implemented in hardware. For example, the additional regulation system using a microcontroller and a corresponding regulation algorithm via software stored in the microcontroller is no longer necessary, or at most almost unnecessary. The latter applies if the adjustment between the current regulation constant K and the scaling factor G, implemented solely in hardware, leaves a small deviation within the aforementioned limitations, which is further reduced by calculation.

[0084] - By selecting SO, the regulated temperature difference can also be variably adjusted. It is also conceivable to use an adjustable displacement resistor element SO, i.e., one with an adjustable resistance value.

[0085] - Has a similarly large resistance value R S1 R S2 The selection of the two sensor elements S1 and S2 minimizes unwanted asymmetry, which has a positive impact on the design of the measurement / operation circuit 3 and the measurement accuracy of the heat flow meter 1.

[0086] - The variable selection of the current ratio between the heated and unheated sensor elements (heating current Ihtr relative to the measuring current Imeas, via the current conditioning unit or the associated current conditioning constant K) can be optimized for sensor characteristics and the corresponding application.

[0087] - The measurement / operation circuit 3 can be flexibly adapted to the needs of the corresponding intended application by means of an appropriate circuit system.

[0088] For simplicity, the present invention will always be explained using the same thermal sensor element S1 for heating and the same thermal sensor element S2 for measuring temperature. As mentioned at the beginning, the invention also includes a heat flow meter 1 with necessary modifications, wherein the functions of the two thermal sensor elements S1 and S2 are interchangeable. For this purpose, the circuit topology of the measurement / operation circuit 3 is kept adaptable, for example by means of a switching element not shown, such that, for example, the high-impedance second balancing resistor element A2 can be switched to the measurement branch MZ (e.g., using the current source 41 of the current regulating unit 4), and the low-impedance first adjusting resistor element A1 can be switched to the heating branch HZ. This solution is particularly suitable for responding to changes in the flow direction qm of the medium.

[0089] Appendix labels and symbols

Claims

1. A heat flow meter (1) for determining and / or monitoring a measured variable, particularly a volumetric flow rate, of a medium flowing in a pipe (2), the heat flow meter (1) comprising: - Two thermal sensor elements (S1, S2), which are designed as resistive elements and are configured to be heated and / or detect the temperature of the medium; - First balancing resistor element (A1) and second balancing resistor element (A2); - Electronic measurement / operation circuit (3), said electronic measurement / operation circuit (3) is designed to be -- Heating the first thermal sensor element (S1). -- The temperature of the medium is detected by means of an unheated second thermal sensor element (S2). The two thermal sensor elements (S1, S2), the two balancing resistor elements (A1, A2), and the electronic measurement / operation circuit (3) are interconnected to form a bridge circuit, such that: - In the heating branch (HZ), the first balancing resistor element (A1) and the first thermal sensor element (S1) are connected in series. - In the measurement 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 connected in parallel. Furthermore, the measurement / operation circuit (3) includes a current regulation unit (4). The current regulating unit (4) is configured to adjust the ratio formed by dividing the heating current (Ihtr) of the electric heating current flowing in the heating branch (HZ) during the measurement operation by the measuring current (Imeas) of the measuring current flowing in the measuring branch (MZ) during the measurement operation as the current regulating constant (K).

2. The heat flow meter (1) according to claim 1. in, The bridge circuit has a first conductor node (LK1), which is arranged in the heating branch (HZ) between the first balancing resistor element (A1) and the first thermal sensor element (S1). The bridge circuit has a second conductor node (LK2). The second conductor node (LK2) is positioned in the measurement branch (MZ) between the second balancing resistor element (A2) and the second thermal sensor element (S2). The bridge circuit has a third conductor node (LK3), which is arranged between the two balancing resistor elements (A1, A2). Furthermore, the bridge circuit has a fourth conductor node (LK4), which is arranged between the two thermal sensor elements (S1, S2).

3. The heat flow meter (1) according to claim 1 or 2. in, The measurement / operation circuit (3) includes a voltage regulation unit (5). wherein the voltage regulating unit (5) is configured to supply a bridge voltage (Vb) to the bridge circuit, the bridge voltage (Vb) being determined by a voltage difference between a first voltage (U + ) and a second voltage (U - ), And wherein the voltage regulating unit (5) in particular comprises a differential amplifier (51), and the first voltage (U + ) is supplied to a non-inverted first input of the differential amplifier and the second voltage (U - ) is supplied to an inverted second input of the differential amplifier, or vice versa.

4. The heat flow meter (1) according to claim 3. in, The measurement / operation circuit (3) has a voltage scaling unit (6) configured to scale to a scaling factor (Ga) by a ratio formed by: - By the first voltage (U) + Divide by the voltage (U) applied to the second conductor node (LK2). LK2 The ratio formed by dividing by - By the second voltage (U) - Divide by the voltage (U) applied to the first conductor node (LK1). LK1 The ratio formed.

5. The heat flow meter (1) according to claim 4. in, The scaling factor (Ga) deviates from the current regulation constant (K) by less than 10%, particularly less than 5%, and preferably less than 2%. Furthermore, in particular, the scaling factor (Ga) is greater than 1 if and only if the current adjustment constant (K) is greater than 1; and the scaling factor (Ga) is less than 1 if and only if the current adjustment constant (K) is less than 1.

6. The heat flow meter (1) according to at least one of the preceding claims, wherein, The current regulation constant (K) is greater than 1 and less than 200, particularly greater than 5 and less than 100, and preferably greater than 10.

7. The heat flow meter (1) according to at least one of the preceding claims, wherein, The current regulating unit (4) includes, in particular, an adjustable (DC) current source (41) and an ammeter (42).

8. The heat flow meter (1) according to at least one of the preceding claims, wherein, The current regulation constant (K) and the scaling factor (Ga) are both greater than 1. Furthermore, the current source (41) is arranged in the measuring branch (MZ) between the second balancing resistor element (A2) and the second conductor node (LK2), and in particular, the ammeter (42) is designed to measure the heating current (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).

9. The heat flow meter (1) according to at least one of the preceding claims. in, Particularly preferably, the fourth conductor node (LK4) is at ground potential (GND) and the bridge voltage (Vb) is supplied to the third conductor node (LK4), or The third conductor node (LK3) is at ground potential (GND) and the bridge voltage (Vb) is supplied to the fourth conductor node (LK4).

10. The heat flow meter (1) according to at least one of the preceding claims, wherein, At the first reference temperature (T1), especially at the first reference temperature (T1) between 18 degrees Celsius and 25 degrees Celsius, The first thermal sensor element (S1) has a first resistance-temperature coefficient (TC). S1 The second thermal sensor element (S2) has a second resistance-temperature coefficient (TC). S2 ), where the first resistance value - temperature coefficient (TC) S1 ) and the second resistance value-temperature coefficient (TC) S2 In linear order, it is at least 1×10 - ³ / Kelvin, especially at least 3 × 10⁻⁶ - ³ / Kelvin, In particular, the first thermal sensor element (S1) and the second thermal sensor element (S2) comprise a cold-conducting material, preferably platinum and / or nickel.

11. The heat flow meter (1) according to at least one of the preceding claims, wherein, At the second reference temperature (T2), particularly at the second reference temperature (T2) between -10 degrees Celsius and +15 degrees Celsius, the first thermal sensor element (S1) has a first resistance value (R). S1 ), and the second thermal sensor element (S1) has a second resistance value (R). S2 ), And among them, the first resistance value (R) S1 ) relative to the second resistance value (R) S2 The deviation is less than 10 ohms, particularly less than 5 ohms, and preferably less than 1 ohm.

12. The heat flow meter (1) according to at least one of the preceding claims, wherein, At the first reference temperature (T1), the first balancing resistor element (A1) has a third resistance-temperature coefficient (TC). A1 ), and the second balancing resistor element (A2) has a fourth resistance-temperature coefficient (TC). A2 ), Among them, the third resistance value-temperature coefficient (TC) A1 ) and the fourth resistance value-temperature coefficient (TC) A2 In linear order, it is at most 0.2 × 10⁻⁶. - ³ / Kelvin, specifically at most 0.1 × 10⁻⁶. - ³ / Kelvin, preferably at most 0.05 × 10⁻⁶. - ³ / Kelvin.

13. The heat flow meter (1) according to at least one of the preceding claims, wherein, The heat flow meter (1) is designed such that, when the measured variable is determined and / or monitored, - The two thermal sensor elements (S1, S2) protrude into the lumen (7) of the tube (2) and are in particular in contact with the medium.

14. The heat flow meter (1) according to at least one of the preceding claims, wherein, The heat flow meter (1) is configured such that, when the measured variable is determined and / or monitored, the two balancing resistance elements (A1, A2) are arranged outside the cavity (7) of the tube (2).

15. The heat flow meter (1) according to at least one of the preceding claims, wherein, The two thermal sensor elements (S1, S2) are each a corresponding resistive structure, which is applied to a corresponding electrically insulating substrate using a corresponding electrically insulating covering, particularly a covering layer, and especially using thin film technology.

16. The heat flow meter (1) according to at least one of the preceding claims, wherein, In the measurement branch (MZ), between the second conductor node (LK2) and the first fourth conductor node (LK4), a displacement resistor element (SO) is connected in series with the second thermal sensor element (S2), and the displacement resistor element (SO) is specifically arranged between the second conductor node (LK2) and the second thermal sensor element (S2).

17. The heat flow meter (1) according to at least one of the preceding claims, wherein, At the second reference temperature (T2), the first balancing resistor element (A1) has a third resistance value (R). A1 ), and the second balancing resistor element (A2) has a fourth resistance value (R). A2 ), The fourth resistance value is at least 5 times the third resistance value, particularly at least 10 times, and preferably at least 30 times.

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