Mass flow meter with thermal dispersion technology

Abstract

Mass flow meter for fluid media, the mass flow meter comprising: an unheated reference sensor with an electrical excitation input and an output, wherein the reference sensor measures the temperature of the fluid media, a heated active sensor with an electrical current input and an output, wherein the temperature difference ΔT between the output of the reference sensor and that of the active sensor is continuously determined, an electrical power source connected to the heater of the active sensor, a controller that controls the current applied to the heater of the active sensor to keep ΔT constant until the applied current reaches a predetermined value as the mass flow increases, and maintains this current value as the mass flow of the media continues to increase while ΔT decreases, the controller using a hardware-based closed-loop control to regulate the current applied to the heater of the active sensor, and digital circuits for determining the mass flow of the media when the electric current is changed relative to the mass flow changes, and for determining the mass flow of the media when ΔT is changed relative to the mass flow changes when the electric current applied to the active sensor is maintained at the predetermined value.

Description

Technical area

[0001] This device relates generally to mass flow meters using thermal dispersion technology, and more particularly to an adaptive system to allow the flowmeter to employ both constant current and constant temperature differential technologies as appropriate for any fluid flow rate. State of the art

[0002] Thermal flow meters are well-known for measuring fluid flow, and in particular for measuring the mass flow of gas. While any type of fluid flow can be measured using this technology, the following discussion will be simplified by focusing on the mass flow of gas. While other sensing elements may be applicable, the sensors will be treated as resistance temperature detectors (RTDs) for simplicity in the discussion.

[0003] Thermal flow meters measure the cooling effect of passing gas molecules. The sensor consists of a heated element, called the active element, and an unheated element, called the reference element. The temperature difference (ΔT) between the active element and the reference element is proportional to the mass flow of the gas.

[0004] It should be noted that fluids, in this case gases, can have a number of different properties. They can be dry or moist. The pressure under which the gas flows can vary greatly, and the gas can have different densities that are unrelated to pressure. Other variables may also exist. Liquids also have densities that can be greater or less than the density of water, so these properties must be taken into account when measuring flow.

[0005] DE 690 11 099 T2 discloses a mass flow meter for fluid media in which a control system switches between the relevant CTA and CCA operating modes in the known constant-power variant depending on an increase in mass flow. DE 33 09 404 A1 also describes a similar concept in principle.

[0006] There are two predominant measurement technologies that are generally used. With regard to the Fig. 2 and Fig. 3 are these: 1. Constant current, where the heated element receives a constant current and ΔT decreases as the mass flow increases. The constant current configuration allows measurements of all flow rates, including very high flow rates, can handle high liquid contents, and delivers a very robust measurement signal without ripple or other noise. A disadvantage of this type of application is that the response time is slower than desired in a small percentage of cases when flow rates change very rapidly. 2. Constant temperature difference (ΔT), where the heating element receives a variable current required to maintain a constant temperature difference ΔT between the active sensor and the reference temperature sensor. This variable current allows the sensors to respond quickly to changes in flow rate. A disadvantage of this type of application is that as flow rates increase, a maximum current delivered to the heated sensor can be generated, beyond which read signals may be inaccurate.

[0007] Relevant properties of a constant current sensor are: • it can tolerate high levels of moisture, • it is resistant to dirt, • it can measure very high currents, • it has an extended measuring range (maximum flow : minimum flow) of 1000 : 1, • it has better temperature compatibility, • It enables fail-safe operation as it can ensure an intact ΔT sensor signal, but • It has a relatively slow reaction time of 10 to 15 seconds.

[0008] Relevant properties of a constant ΔT sensor are: • It has a fast reaction time of about 1 second, but • it is less suitable for moist gas, • it is less suitable for high currents, and • it has a limited measuring range (e.g. 100 : 1).

[0009] There are several reasons why thermal dispersion technology is particularly suitable for measuring the mass flow of gases. These include: • it can measure the mass flow directly, • it has no moving parts, • it does not cause pressure drop, • it has a low flow sensitivity, and • it has a large measuring range. Description of the invention

[0010] The purpose of this concept is to create a thermal dispersion control technology, both in constant delta-T mode and constant current mode. As explained above, constant ΔT mode provides very fast response times, but has stability and accuracy issues in some applications, such as high flow rates. Constant current mode works in all applications but has a slower response time than constant ΔT mode.

[0011] The described measurement system uses adaptive measurement technology (AST) to combine the essential positive features of constant current and constant ΔT in a single instrument, resulting in such an instrument having the following properties: • it measures the mass flow directly, • it has no moving parts, • it does not cause pressure drop, • it is sensitive to low currents, • it has a wide measuring range, • it can accept high levels of moisture, • it is insensitive to dirt / dust / fine dust, • It can measure flow rates from 0.25 to 1000 standard feet per second (SFPS) (0.08 to 300 m / s), • it has a measuring range ratio of 1000 : 1, • It has a temperature operating range of -70°C to +450°C (-94°F to 850°F), • it has a response time of about one second, • it complies with ISO-14164 and many other international standards, and • It allows monitoring intact sensor signals that control a fail-safe mode. Short description of the drawing

[0012] The tasks, advantages and features of the described concept will become more understandable when reading the following description in conjunction with the attached drawing, in which: Fig. Figure 1 is a schematic representation of a known in situ thermal dispersion mass flow meter; Fig. Figure 2 is a graphical representation showing how the constant flow mass flowmeter operates; Fig. Figure 3 is a graph showing how a constant ΔT mass flow meter operates; Fig. 4 is a graphical representation showing how the instrument works according to the present concept, the Fig. 5A, Fig. 5B and Fig. 5C shows a combined schematic block diagram of the instrument according to the present concept; the Fig. 6A-6D show a combined circuit diagram of details of the system; Fig. 7A and Fig. 7B shows a combined circuit diagram of further details of the system; Fig. 8A and Fig. 8B shows a combined circuit diagram of further details of the system; Fig. 9A-9D show a combined circuit diagram of further details of the system; Fig. 10A-10C show a combined circuit diagram of further details of the system and Fig. Figure 11 is an exemplary curve showing a comparison between the present system and known competing systems. Best possible embodiment of the invention

[0013] The following definitions apply below: Constant Delta T (ΔT): This mode of operation uses a feedback loop of the current supplied to the heated sensor to maintain a constant temperature differential between the active meter and the reference RTD meter, regardless of changes in the surrounding fluid flow caused by different flow rates, temperatures, or pressure. Constant current: This operating mode keeps the current supplied to the heated sensor constant in all cases.

[0014] Fig. Figure 1 shows a conventional in-situ thermal dispersion mass flowmeter. The conduit wall is designated by reference numeral 11, and the fluid flow is represented by arrows 12. The flowmeter head 13 contains the usual electronics and provides power and operating signals required to operate the thermowells 14 and 15. Thermowell 14 contains the reference or unheated sensor, and thermowell 15 contains the active or heated sensor.

[0015] Fig. Figure 2 is self-explanatory and shows how a constant-flow flowmeter works. As the mass flow increases, ΔT decreases.

[0016] Similarly, Fig. Figure 3 graphically illustrates how a constant-ΔT flowmeter works. The current delivered to the active sensing heater increases as the flow rate increases. As shown here, the current reaches a maximum at around 400 SFPS (120 m / s), a loading level at which a flow rate reading becomes inaccurate. While this system can provide reasonably useful results up to 600 SFPS (180 m / s), accuracy can be compromised at these high mass flow rates. These flow rates are examples only, and for different installations, the current delivered to the heater to maintain constant-ΔT may be lower or higher than shown.

[0017] In Fig. Figure 4 shows that at lower flow rates, the instrument according to one embodiment of the invention maintains the voltage difference ΔT between two sensors constant, with one sensor heated by a separate heater and another unheated. This voltage difference is maintained even if the flow rate of the flow that has passed the instrument sensors has increased or decreased. The current supplied to the separate heater in the heated or active sensor is increased or decreased as required via a feedback loop to keep the voltage difference constant.

[0018] A limitation of the constant-ΔT approach is that there is a predetermined maximum heating power limit, and at higher flow rates above 200 SFPS (60 m / s), as shown in this example, when this maximum current limit is reached, the constant voltage difference is no longer maintained and actually decreases at higher flow rates. This would, of course, result in inaccurate mass flow rate readings in instruments that only employ a constant-ΔT operating function.

[0019] Unlike units that only use constant ΔT mode, the instrument described here will continue to operate when the maximum heater current is reached at higher flow rates. This instrument always measures both the heater current at the active sensor and the voltage difference across the two sensors. When the heater current at the active sensor reaches its maximum and the applied heater current remains constant, higher flow rates can still be accurately calculated by the internal software algorithm using the measured voltage difference across the sensors.

[0020] This aspect of the system enables a very fast response time when in constant voltage differential mode, yet allows continued operation at very high flow rates up to 1600 standard feet per second (SFPS) (480 m / s). Fig. Figure 4 shows, as an example, the mass flow at 1000 SFPS (300 m / s). Typical units that only use the constant ΔT mode are "maxed out" when they are no longer able to maintain the voltage difference between the sensors constant because the predetermined maximum current has been reached, but the mass flow rate continues to increase. Some instruments are limited to flow rates as low as 300 SFPS (90 m / s). The flow rate depends on the properties of the flowing medium as well as the instrument, so Fig. 4 shows 200 SFPS (60 m / s) for exemplary purposes.

[0021] The transition at which constant ΔT switches to constant flow (at 200 SFPS (60 m / s) in this particular example) depends on the available maximum flow, which is a predetermined value, and the heat transfer rate of the medium. A higher maximum flow shifts the transition point to the right or to higher SFPS, as shown in Fig. 4. A smaller maximum current shifts the transition point to the left.

[0022] Similarly, a medium with lower heat transfer will shift the transition to the right, while a medium with higher heat transfer will shift the transition to the left.

[0023] For example, natural gas transfers heat better than air, so the transition point will move to the left with natural gas as the medium compared to air. Hydrogen transfers heat even better, so if the medium is hydrogen, the transition point will move further to the left.

[0024] The present technology can also be configured to maintain a constant current supplied to the separate heater and allow the voltage difference across the sensors to decrease as flow rates increase. In some applications, particularly semi-wet and low-flow applications, this configuration provides better results. Operating the instrument in this manner can be used to optimize either response time or stability, depending on the application.

[0025] Another feature of this technology is that it can be reconfigured to the constant current configuration in the field without compromising the accuracy of the unit. This is useful because some customers do not know the specific requirements of their particular application until the unit is installed. Being able to change the instrument's configuration in the field without requiring recalibration at the factory is a major advantage for the end user.

[0026] The basic mathematical principles underlying the operation of mass flow rate technology are rather simple. For a given mass flow rate with a specific fluid composition and at constant temperature, there is a relationship between mass flow rate and heat transfer, characterized by the following simplified equation: Mass flow−K∗PdT where P is the heating current supplied to the active (heated) sensor; dT is the temperature difference between the heated sensor and the non-heated sensor, and K is a constant determined by calibration and is only valid for that particular mass flow rate, temperature, and specific fluid composition. For calibration purposes, the pressure in the line is subsumed into the mass flow rate factor. A functioning flowmeter requires a range of K values, which must be determined over the desired mass flow range in the desired fluid. The K values ​​must be curve-fitted using a linearization algorithm; examples of such algorithms are commercially available.

[0027] Unlike other flowmeters, this technology continuously measures both the heater current (P) and the temperature difference (ΔT) simultaneously and uses them in the linearization algorithm. This allows the flowmeter to be optimized for faster response time by varying the current to keep ΔT constant, or to optimize stability and accuracy by keeping the current constant and allowing ΔT to vary.

[0028] How Fig. 5B, the PID heater return control block 51 allows the heater current supplied to the active or heated sensor to be controlled to a constant ΔT value between the active sensor and the reference sensor (e.g. RTD) when operating at the lower end of the flow range, ie on the left side of Fig. 4. The heater current is set to a constant current when operating at the higher end of the flow range.

[0029] Although a separate heater for the active sensing element is a preferred embodiment, the system can operate by self-heating the active sensor by applying a higher current to the active RTD sensor.

[0030] Further with reference to Fig. 5, the system is clearly illustrated. The active sensor 15 and the reference sensor 14 are identified, and it is preferred that they are structurally identical. Thus, any two protective tubes 14, 15 ( Fig. 5A) can be incorporated into the present system. The heater can be a coil or any other form. For example, coils 14A and 15A denote the heater element in this figure. One of the coils is activated to make this sensor the active sensor, and the other sensor is the reference sensor.

[0031] As previously described, the system that is installed in the Fig. 5 to 10, continuously measures the value of the current supplied to the heater of the active sensor and the temperature difference ΔT between the active and the reference sensor.

[0032] Fig. Figure 4 is a graphical representation of the system's operating status from a very low SFPS value ≥ 0 (0 m / s) to a high SFPS value of 1000 (300 m / s) for the mass flow. At the lower end of the instrument range, shown as 0 to 200 SFPS (60 m / s), for exemplary purposes only, the system operates in constant ΔT mode. When the current value reaches the load value, shown at 400-600 SFPS (120-180 m / s) in the example from Fig. 3, the instrument switches to constant current mode operation.

[0033] As explained above, the typical constant-ΔT mode of operation provides a fast response time in the range of approximately one second. When an instrument operates in constant-current mode at low mass flow rates, the response time may be 10–15 seconds. However, this instrument provides a fast response time at low flow rates because it operates in constant-ΔT mode, and it also provides a relatively fast response time of ≤ 1 to 5 seconds at higher flow rates when operating in constant-current mode. This fast response time is due to the fact that the flowing medium carries heat away from the active sensor much more rapidly at high mass flow rates.

[0034] As explained above, there can be no sharp definition of where on the flow rate scale low flow rates change to high flow rates, as this depends on the factors that must be considered in generating the constant K during the calibration of an instrument in the factory. When calibrating an instrument, the manufacturer must consider the properties of the media expected to be encountered by the end user, with this information provided to the manufacturer.

[0035] Fig. Figure 5C is a continuation of the block diagram shown in the Fig. 5A and Fig. 5B. The output of the part of the system that is Fig. 5B leads to the FE digital board 53. The term “FE” indicates that signals from the flow element shown in Fig. 5A. ΔT and the current inputs, which are constantly monitored, are processed in board 53. The decision to switch to constant current mode when the mass flow rate approaches the load value or the transfer point, as shown in Fig. 4, or to switch to constant ΔT mode when the flow rate decreases, is made by control board 55. This functionality is controlled by the microprocessor in board 55. One purpose of board 53 is to convert analog signals from the sensors into digital signals used by control board 55.

[0036] As already explained, the end user can adjust the set point if the properties of the media change or if they vary. This is done using a computer / configurator 57, which is optionally plugged into a USB port in the board 55 by an operator. It should be noted that the boards 53 and 55 as well as the circuits of the Fig. 6-10 in a head of the type as in Fig. 1 as head 13. Alternatively, part or all of the circuitry may be located at a location remote from the instrument, which includes protective tubes 14 and 15 mounted through the wall of the conduit through which the media flows. The coupling or connections between the sensors and the circuitry included in the Fig. 6-10, as well as those shown in Fig. The blocks shown in Figure 5C may be wired connections or a wireless coupling.

[0037] The Fig. 6-10 show a circuit for operating the system according to Fig. 4 and Fig. 5 from an internal point of view. While the circuit is self-explanatory when viewed in conjunction with Fig. 5, details of the Fig. 6-10 are presented to increase understanding of this relatively complex circuit. The description may be more detailed than necessary, but the goal is to facilitate understanding without leaving any gaps.

[0038] With reference to Fig. 6A are T-type electromagnetic interference filters (L9-L13 and L15-L17 (ACH32C-104-T) used to filter electrical noise. U41 (TS5A23166) is a 2-channel single-pole, single-throw analog switch used to select or disable the ACT_SENSE and REF_SENSE signals, controlled by the CTRL_SW3_1 and CTRL_SW3_2 signals, respectively.

[0039] Fig. 6B makes Fig. 6A, where U20 (TS5A23159DGSR) is a 2-channel single-pole-double-throw analog switch used to connect point A or point C to ACT_EXC_OUT and point B or point D to REF_EXC_OUT, respectively controlled by CTRL_SW_2 and CTRL_SW_1 signals.

[0040] In relation to Fig. 6C and Fig. 10C, P3 and P1 are connectors that connect the FE Adaptive Sensing Technology (AST) analog board to the Fig. Connect to the FE digital board 53 shown in Figure 5C.

[0041] In Fig. 6D, U40 (TS5A3359DCUR) is a single-pole, triple-throw analog switch used to connect R70, R69, or R68 to point E, controlled by point G and point H. U43 (TS5A3166DBVR) is a single-pole, single-throw analog switch used to connect R67 to point E, controlled by a CTRL_ACT_ZERO signal. U39 (TS5A23166DCUR) is a dual single-pole-single-throw switch used to connect R66 (controlled by CTRL_REF_HI) or R65 (controlled by CTRL_REF_ZERO) to REF point F. U18 (LT1790BIS6-2.048#PBF) is a Low Dropout Voltage Reference chip that accepts +5V input and produces 2.048V output.

[0042] With reference to Fig. 7A, part AD5143BCPZ100-RL7, is a non-volatile digital potentiometer for adjusting the gains of the PID (proportional, integral, and derivative) circuits. R38, R30, and C22 form a low-pass filter circuit relative to SENSOR_RET, and R39, R42, and C21 form another low-pass filter for the REF signal from the reference RTD relative to A Ground. This filtered signal is designated REF_FILT and is connected to the +IN terminal of U6.

[0043] Similarly, with reference to REF, R40, R32, and C32 form a low-pass filter circuit, and with reference to A Ground, R40, R41, and C21 form another low-pass filter for the signal from the active RTD; this filtered signal is referred to as ACT_FILT and is connected to the -IN terminal of U6. U6 (AD8237ARMZ) is an instrumentation amplifier used to amplify the differential signal between the +IN terminal and the -IN terminal with a gain of 1 + R21 / R22. And U7A (OPA4313) is an operational amplifier used to compare the output of U6 and the CONST_DT_ADJUST (Delta Temperature Set Point) signal with a gain of -R3 / R14, and a low-pass filter formed by R3 and C16.

[0044] In Fig. In Figure 7B, R16, R4, C18, R17, U1B, and the upper DPA4313 operational amplifier (op-amp) form the proportional element (P) of the PID circuits. R36, R35 (100kΩ), R37, C1, U1C, and the middle DPA4313 operational amplifier (op-amp) form the integrating element (I) of the PID circuits. R34, C6, R24, R33, U1D, and the lower DPA4313 operational amplifier (op-amp) form the differentiating element (D) of the PID circuits. And R35(10k), R19, R20 and the right DPA4313 operational amplifier (op-amp) form a summing circuit that adds the P, I and D signals and generates the PID_OUT signal, which is used to set the heater current (see Fig. 5B).

[0045] Referring to Fig. 8A, U11B (DPA4313), R76, and Q1 form a heater current control circuit; when the differential voltage between pins 5 and 6 of U11B is positive, Q1 is turned on, allowing the heater current to flow from HEATER_LO to AH Ground if Q9 is also turned on. Similarly, U11A, R46, and Q9 form another heater current control circuit; when the differential voltage between pins 3 and 2 of U11A is positive, Q9 is turned on, allowing the heater current to flow from HEATER_LO to AH Ground if Q1 is also turned on. MAX_HTR_ADJUST is the direct current (DC) signal from an output of the digital-to-analog converter (DAC) (from the Fig. 5C), which is used to set the maximum heater current allowed in the system. If the voltage at point 5 is higher than either MAX_HTR_ADJUST or PID_OUT, or higher than both, the heater current is reduced until the differential temperature between the active RTD and the reference RTD equals the desired delta temperature (Delta-T) provided by the CONST_DT_ADJUST signal in Fig. 7A. R64 is used to detect the heater current. R1B, C26, R1A, C27, R47, and C8 form filter circuits for the signal detected at R64. R98, R97, and U11D form a heater open / off detection circuit. Q8 (FDC5612) is used to turn off the heater, controlled by the HEATER_SHTDWN signal.

[0046] In Fig. 8B, U5 (LMH6551MA) is a differential high-speed operational amplifier (Differential High Speed ​​Op Amp) that amplifies the sensed heater current, with the gain of 5, which is determined by R2C, R2D, R1C, R2A, R2B, R1D and R1A and R1B. Fig. 8A is set.

[0047] With reference to Fig. 9A, U3 (AD8237ARMZ) is an instrumentation amplifier used to amplify the voltage difference between the REF_FILT signal and the SENSRET_FILT signal with a gain of 1. U2A (DPA4313) is a buffer for the REF_FILT signal.

[0048] With reference to Fig. 9B, U42, U50, and related components in the upper left of the drawing form the pressure input analog-to-digital data acquisition circuit; U42 (ADS1112IDGST) is a 16-bit analog-to-digital converter (ADC), and U50 (OPA333AIDBVT) is a buffer for the pressure input. U53 (LTC2485IDD#PBF) is a 24-bit ADC used to convert the reference RTD's analog signal into ADC counts for further processing by the embedded software. U4 (LTC2485IDD#PBF) is a 24-bit ADC used to convert the heater current's analog signal into ADC counts for further processing by the embedded software. And U48 (TMP100MDBVREP) is a 16-bit ADC temperature sensor used to monitor the temperature around the FE-AST analog board.

[0049] In Fig. 9C, U2B (DPA4313) is a buffer for the active RTD.

[0050] With reference to Fig. 9D, U55 (LT1236AIS8-5#PBF) is a precision reference that accepts the +17V input voltage and regulates it to a stable +5V output. U54 (LTC2485IDD#PBF) is a 24-bit ADC used to convert the analog signal from the delta-R (the voltage difference between the active RTD and the reference RTD) into ADC counts for further processing by the embedded software.

[0051] In Fig. 10A, U56 (top), Q7, Q2, Q3, and their associated components form a voltage-to-current converter for excitation current for the active RTD. Likewise, U56 (bottom), Q5, Q6, Q4, and their associated components form a voltage-to-current converter for excitation current for the reference RTD.

[0052] With reference to Fig. 10B, U51 (ADR01BRZ) is a 10V voltage reference that takes the +17V voltage input and produces an output of 10V, which in turn feeds U56 (top) and U56 (bottom) into Fig. 10A. U49 (OP777ARMZ) is a buffer for the 10 Volt output and supplies higher currents to U56 (top) and U56 (bottom) in Fig. 10A.

[0053] The P1 block of Fig. 10C has already been described in the description of Fig. 6C discussed.

Claims

[1] Mass flow meter for fluid media, wherein the mass flow meter comprises: an unheated reference sensor with an electrical excitation input and an output, wherein the reference sensor measures the temperature of the fluid media, a heated active sensor with an electrical current input and an output, wherein the temperature difference ΔT between the output of the reference sensor and that of the active sensor is continuously determined, an electrical power source connected to the heater of the active sensor, a controller that controls the current applied to the heater of the active sensor to keep ΔT constant until the applied current reaches a predetermined value as the mass flow increases, and maintains this current value as the mass flow of the media continues to increase while ΔT decreases, the controller using a hardware-based closed-loop control to regulate the current applied to the heater of the active sensor, and digital circuits for determining the mass flow of the media when the electric current is changed relative to the mass flow changes, and for determining the mass flow of the media when ΔT is changed relative to the mass flow changes when the electric current applied to the active sensor is maintained at the predetermined value. [2] A method for measuring the mass flow of a fluid using a device comprising: an unheated reference sensor having an excitation input and an output, a heated active sensor having an electrical current input and an output, an electrical excitation source for the reference sensor, and an electrical current source for the heater of the active sensor, the method comprising: Maintaining the temperature difference (ΔT) between the reference sensor and the active sensor constant when the fluid mass flow increases up to a predetermined current value applied to the active sensor, Measuring the electric current value as it increases in relation to the increase in the mass flow of the media, Determining the mass flow relative to the current value applied to the heater of the heated sensor, Continuing to apply electrical current with the predetermined current value to the heater of the active sensor, Measuring the temperature difference ΔT as the mass flow continues to increase, with the current value applied to the heater of the active sensor remaining constant, and Determine the mass flow rate in relation to ΔT changes when the mass flow rate changes. [3] The method of claim 2, further comprising continuously measuring ΔT and the electrical current applied to the heater of the active sensor.

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