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Mass Flow Meter Usage Considerations - Focus on Microfluidic Control Technology

With the development of industrial production, the requirements for the accuracy and range of flow measurement are getting higher and higher, and the flow measurement technology is changing with each passing day. In order to adapt to various uses, various types of flow meters have come out one after another. There are more than 100 types of flow meters that have been put into use today, and thermal mass flow meters are among the more widely used ones.

Thermal mass flow meters can be divided into:
Constant temperature difference method (power consumption measurement method) Flowmeter and constant power method (temperature measurement method) flowmeter. From the development history of thermal gas mass flowmeters, thermostatic differential flowmeters were first applied to the measurement of media in the actual industry. This is because the thermostatic type is easier to realize than the constant power type, so the mass flow rate of gas is now produced. The manufacturer of the meter has more manufacturers using the principle of constant temperature difference, including Kurx, Sirren, and Brooks. However, with the continuous improvement of production requirements, the thermostatic differential flowmeter has been difficult to meet the needs of some special production, which makes the constant power flowmeter become a key research direction. 1 principle of constant temperature difference: constant temperature difference method (power consumption measurement method) is the temperature of the heating element is higher than the temperature of the gas, when the gas flows through a part of the heat, keep the temperature difference between the heating element and the measured gas constant at a certain temperature difference, control And measuring the power provided by the heat source, the power consumption increases as the flow rate increases, and the power consumption reflects the gas flow rate. Two metal platinum resistors are added in the measuring pipeline. A platinum resistor adds a small current (the current is below 4mA, which will not cause the resistance heating). It is used to measure the temperature of the measured fluid and is called temperature resistance. Another platinum resistance leads to larger currents (currents generally above 50mA) and is used to measure the velocity of the fluid under test, known as the speed-measuring resistance. According to the principle of thermal diffusion, the amount of heat that the heated object is carried away by the fluid is related to the temperature difference between the heated object and the fluid, the flow velocity of the fluid, and the nature of the fluid. During operation, the temperature resistance continuously detects the temperature of the medium. The resistance of the speed measurement self-heats to a constant temperature higher than that of the fluid. When the fluid flows, the resistance of the speed-measuring resistor changes due to the decrease of the surface temperature of the heat-dissipating speed-measuring resistor and the Wheatstone bridge is unbalanced. A feedback circuit composed of a Wheatstone bridge feeds a temperature difference back to the processor to increase the heater current (which may also be a voltage) to keep the temperature difference constant. The flow rate of the fluid is proportional to the added current (voltage): the greater the flow rate of the fluid, the greater the current (voltage) added to maintain a constant temperature difference. Therefore, the relationship between the heating current (voltage) and the mass flow can be calibrated through experiments, and the fluid mass flow can be calculated through the current (voltage). There are problems with the constant-temperature differential gas mass flowmeter: As the flow of the medium increases, the heat of the speedometer platinum resistance is rapidly taken away. The constant-temperature differential flowmeter requires a rapid increase in the power of the platinum resistance heat resistance to ensure a constant temperature difference. . However, because the increase of energy is influenced by the power of the circuit itself and the large allowable current of the speed measurement platinum resistance zui, the large measurable flow rate is limited.

2 constant power method constant power method (temperature measurement method) is to provide constant heat to the platinum thermal resistance to provide heat, so that it is heated to a temperature higher than the gas temperature, fluid flow away a part of the platinum thermal resistance surface heat, the greater the flow, the temperature drop The greater the measurement of the temperature as a function of fluid flow, may reflect the gas flow. There are two implementations: (1) Only one platinum resistor is heated, and the temperature difference is measured by the principle of thermal diffusion. Principle: Similar to the structure of the thermostatic differential flowmeter, two metal platinum resistances are also added to the measuring pipeline. One is a temperature measuring resistor for measuring the temperature of the fluid to be measured, and the other is used for measuring the velocity of the fluid to be measured. Speed resistance. A constant power is applied to the heater for speed-measuring platinum resistance heating. When the fluid is at rest, the speed platinum resistance and temperature measurement platinum resistance surface temperature difference ΔT21=TS2-TS1zui is large. With the flow of the medium, two platinum resistor surface temperatures The difference decreases. The greater the fluid flow, the lower the temperature difference between the two platinum resistors. The platinum resistance is connected in the Wheatstone bridge. The temperature of the platinum resistance makes the resistance of the platinum resistance show different resistances, which makes the bridge unbalanced and reacts the fluid flow by detecting the voltage difference of the bridge. The constant power mass flowmeter has problems: if the density of the fluid is ρ, the flow rate is μ, the heat of the heated platinum resistance is taken away by the fluid is Q, and the temperature difference between the temperature measurement platinum resistance and the speed measurement platinum resistance is ΔT21. There is a relationship:

Q/ΔT21=k1+k2•(ρ•μ)k3

In the formula, k1, k2, and k3 are constants for a given fluid.

In the pipeline that crosses S, the mass flow qm = ρ • μ • S. During the measurement process, the speed-measuring platinum resistance is heated by the current I. In the thermal equilibrium state, the heating power of the current is in equilibrium with the heat taken by the speed platinum resistance, that is, Q=I2•RS2. Therefore, the relationship between mass flow qm and Q/ΔT21 has a one-to-one correspondence and can be expressed as:

Qm=f[I2•RS2/ΔT21]

When the heating current I is constant and the mass flow rate of the fluid is calculated by measuring the temperature difference ΔT21 of the fluid, the variation of the speedometer platinum resistance RS2 with the temperature is ignored and an error is caused.

(2) Heat the two symmetrical platinum resistors and calculate the temperature difference from the principle of heat balance.

The structure of the sensor is that two identical platinum resistors are symmetrically fixed on both sides of the heat source and placed in the fluid. A constant current source (constant voltage source) is used to heat the heat source. The fluid flow makes the temperature of the two platinum resistors different. The platinum resistance is connected in the Wheatstone bridge. The temperature of the platinum resistance makes the resistance of the platinum resistance show different resistances, which makes the bridge unbalanced and reacts the fluid flow by detecting the voltage of the bridge. The principle of the sensor is further analyzed from the viewpoint of heat transfer. Assuming that the fluid is a uniformly distributed Newtonian fluid, take a one-dimensional measurement as an example: As shown in Figure 1, the heat source R is placed at the center of the sensor substrate, and two identical temperature detection chips are symmetrically placed on both sides (film type The heat exchange between the S1 and S2 sensors and the fluid is carried out mainly by convection, and the heat exchange between the heat source and the temperature detection chip can be conducted by conduction and convection.

When the fluid velocity is zero, ie when the fluid is at rest, the streamlined field near the surface and the resulting temperature field are symmetrically distributed with respect to the heat source. Due to the structural symmetry, the heat exchange through the heat conduction of the substrate is always symmetrical with respect to the heat source. At this time, the platinum resistance temperature of the temperature sensing chip satisfies TS1=TS2, that is, temperature difference: ΔT21=TS2-TS1=0.

When the fluid flows, the convection heat transfer is mainly between the fluid and the platinum resistance. Due to the difference in the local convection heat transfer coefficient, the flow field and the corresponding temperature field near the substrate surface change with respect to the distribution of the central heat source, resulting in a tendency Asymmetrical distribution of sex. According to the theory of the thermal boundary layer, it can be seen that the surface temperature of the upstream temperature detection chip is higher than the surface temperature of the downstream chip, that is, the heat transfer coefficient of the platinum resistance S1 is greater than the heat transfer coefficient of the S2, so the TS2>TS1, temperature difference in temperature difference: ΔT21=TS2 -TS1>0.

And the value of ΔT21 increases as the fluid flow rate increases. If you change the direction of fluid flow, ΔT21 changes the sign accordingly.

Using the heat balance equation, the temperature redistribution on the surface of the chip due to convection can be calculated, and the relationship between the temperature difference and the flow rate can be obtained.

Comparison of Thermal Gas Mass Flow Meters

3 Comparison and Selection of Two Options

The great advantage of the gas mass flowmeter with constant temperature difference principle is that it responds faster than the constant power type gas mass flowmeter. Because the constant power type measurement value is obtained from the actual temperature change, the thermal inertia of the measurement tube quality and the detection element quality will reduce the response speed; the temperature distribution of the constant temperature difference type does not change, and is not affected by the thermal inertia of the detection element and the like.

For the constant power gas mass flowmeter, it uses a constant power to heat the platinum resistance, which has the following advantages over the constant-temperature differential gas mass flowmeter:

(1) The zui large flow rate of the constant power type flowmeter is large. With the increase of the medium flow rate, the heat of the platinum thermal resistance being heated is rapidly taken away. For a thermostatically-differential flowmeter, it requires a rapid increase in the energy of the heated resistor to ensure a constant temperature differential. However, since the increase in energy is influenced by the power of the circuit itself and the influence of the current to be allowed by the heated platinum resistor zui, its zui value is limited. The constant power type of flow measurement is easy to achieve, usually constant power principle of the flow meter on the air zui large measurable speed of 488m / s, while the constant temperature difference principle of the flow meter zui large measurable speed can only reach 38m / s.

(2) The constant power flow meter is not easily affected by dirty and wet media. In order to make it respond to the temperature quickly and maintain a constant temperature difference, the platinum resistors are generally thinner, while the constant power meters can be made coarser (the same size is not the same for each production plant). In this way, when measuring for dirty and wet media, the dirty material may have a short-term adhesion to the platinum resistance. (Any manufacturer has polished the platinum resistor, and the long-term attachment is not easy to produce.) For finer platinum resistors, their attachments will have a greater impact on the heat dissipation of the heated platinum resistors, and when they are severe, their measurement accuracy will be greatly reduced. Constant power measurements on dirty and wet media will be much better.

(3) The thermostatic differential flowmeter does not compensate for temperature changes, but the constant power flowmeter automatically compensates for the full range of temperature changes. As we all know, the heat flow rate and the equilibrium constant are changed by the change of temperature, generally only in the range of 30°C. When the measured gas temperature range exceeds this value, the heat flow coefficient and the equilibrium constant of the gas are both It will change. Affected by the circuit design, all the constant-temperature differential flowmeters on the market currently do not compensate for the temperature, and FCI's constant-power flowmeters can automatically compensate for the heat flow coefficient and the balance constant within the full-temperature range. . The temperature range of the medium is difficult to maintain constant due to climate or other reasons.

(4) There is a significant difference between constant power and constant temperature difference in high temperature resistance. At present, the constant power zui can achieve a high temperature resistance of 454 °C, while the constant temperature difference of the flow meter is generally within 260 °C, which for the measurement of superheated steam, its adaptability is very different.

4 Conclusion

From the comparison of the above aspects, it can be seen that the constant power thermal gas mass meter has a significant advantage over the constant temperature differential heat mass flow meter. In the actual production, the constant temperature differential response speed is fast and easy to implement, so most of the current products use this principle. However, compared with the constant power mass flowmeter, it has a very obvious advantage, and it has become the direction of further research and development.

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