Classification description of thermal mass flowmeter

Thermal mass flowmeters can be divided into: constant temperature difference method flowmeter and constant power method flowmeter. The constant power method (temperature measurement method) is to provide heat to the platinum thermal resistance with constant power to heat it to a temperature higher than that of the gas, and the fluid flow will take away part of the heat on the surface of the platinum thermal resistance. The greater the flow rate, the greater the temperature drop. The temperature that changes with the fluid flow, can reflect the gas flow. There are the following two implementation methods: (1) Only one platinum resistance is heated, and the temperature difference is measured by the principle of thermal diffusion. Principle: Similar to the structure of the constant temperature differential flowmeter, two metal platinum resistors are also added to the measuring pipeline, one is a temperature measuring resistor used to measure the temperature of the measured fluid, and the other is used to measure the velocity of the measured fluid. Speed ​​measuring resistor. Add a constant power to the heater to heat the speed-measuring platinum resistance. When the fluid is at rest, the temperature difference between the speed-measuring platinum resistance and the temperature-measuring platinum resistance is ΔT21=TS2-TS1**. With the flow of the medium, the surface of the two platinum resistances The temperature difference decreases. The greater the flow rate of the fluid, the smaller the temperature difference between the two platinum resistors. The platinum resistance is connected in the Wheatstone bridge, and the temperature of the platinum resistance is different, so that the resistance of the platinum resistance presents different resistance values, so that the bridge is unbalanced, and the fluid flow is reflected by detecting the voltage difference of the bridge. The problem with this constant power mass flowmeter: if the density of the fluid isρ, the flow rate isμ, the heat taken away by the fluid for heating the platinum resistance is Q, and the temperature difference between the temperature measuring platinum resistance and the speed measuring platinum resistance is △T21, then there is a relationship: Q/ΔT21=k1+k2•(ρ•μ) k3 In the formula, for a fluid with a certain composition, k1, k2, k3 are constants. In a pipeline crossing S, the mass flow rate qm=ρ•μ•S. During the measurement process, the speed-measuring platinum resistance is heated by the current I, and in the thermal equilibrium state, the heating power of the current and the heat taken away by the speed-measuring platinum resistance are in a state of balance, that is, Q=I2•RS2. Therefore, the mass flow qm has a one-to-one relationship with Q/ΔT21, which can be expressed as: qm=f〔I2•RS2/ΔT21] When the heating current I remains unchanged, when calculating the mass flow rate of the fluid by measuring the temperature difference ΔT21 of the fluid, the change of the speed measuring platinum resistance RS2 with temperature is ignored, which will cause errors. (2) Heat two symmetrical platinum resistors, and calculate the temperature difference based on the principle of heat balance. The structure of the sensor is to symmetrically fix two *identical platinum resistors on both sides of the heat source and place them in the fluid. A constant current source (constant voltage source) is used to heat the heat source, and the fluid flow makes the temperature of the two platinum resistors different. The platinum resistance is connected in the Wheatstone bridge, and the temperature of the platinum resistance is different, so that the resistance of the platinum resistance presents different resistance values, so that the bridge is unbalanced, and the fluid flow is reflected by detecting the voltage of the bridge. The principle of the sensor is further analyzed from the perspective of heat transfer. Assuming that the fluid is a uniformly distributed Newtonian fluid, take one-dimensional measurement as an example: as shown in Figure 1, the heat source R is placed in the center of the sensor substrate, and two identical temperature detection chips (thin-film type) are symmetrically placed on both sides of it. Platinum resistance) The heat exchange between the S1 and S2 sensors and the fluid is mainly carried out by convection, and the heat exchange between the heat source and the temperature detection chip can be carried out by conduction and convection. When the fluid velocity is zero, that is, when the fluid is at rest, the streamline field near the surface and the resulting temperature field are symmetrically distributed with respect to the heat source. Due to the symmetry in the structure, the heat exchange by conduction through 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, the temperature difference: ΔT21=TS2-TS1=0. When the fluid flows, convective heat transfer is mainly between the fluid and the platinum resistor. Due to the difference in local convective heat transfer coefficients, the distribution of the streamline field near the substrate surface and the corresponding temperature field relative to the central heat source change, resulting in a tendency Asymmetric distribution of sex. According to the thermal boundary layer theory, it can be seen that the cooling rate of the upstream temperature detection chip surface is higher than that of the downstream chip surface at this time, that is, the heat transfer coefficient of the platinum resistance S1 is greater than that of S2, so TS2>TS1, the temperature difference temperature difference: ΔT21=TS2 - TS1>0. And the value of ΔT21 increases with the increase of the fluid flow rate. If the fluid flow direction is changed, the sign of ΔT21 will change accordingly. The temperature redistribution on the surface of the chip caused by convection can be calculated by using the heat balance equation, and the relationship between the temperature difference and the flow rate can be obtained.
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