Thermal flowmeter has high measurement accuracy, wide application range, low installation requirements, reliable instrument operation and low maintenance rate. It has been widely used in petroleum, chemical, metallurgy, heat, power, food and other fields.
Overview
Thermal gas mass flow meter is a meter that uses the principle of heat conduction to measure flow. The instrument uses the constant temperature difference method to accurately measure the gas mass flow. It has the advantages of small size, high digitization, convenient installation and accurate measurement. The sensor part consists of two reference-grade platinum resistance temperature sensors. Using a bridge loop, one sensor measures the flow temperature and the other maintains a constant temperature difference above the temperature of the fluid, allowing flow measurement under high temperature and high pressure conditions. Thermal gas mass flow meters have the following technical advantages:
Real mass flow meter, no temperature and pressure compensation is needed for gas flow measurement, measurement is convenient and accurate. The gas mass flow or standard volume flow can be obtained.
Wide range ratio, can measure gas with flow rate up to 100Nm / s bottom to 0.5Nm / s, can be used for gas leak detection.
Good seismic performance and long service life. The sensor has no moving parts and pressure sensing parts, and is not affected by vibration to the measurement accuracy.
Easy installation and maintenance. Where site conditions permit, non-stop installation and maintenance can be achieved. (See safety precautions)
Digital design. Integrated digital circuit measurement, accurate measurement and convenient maintenance.
Using RS-485 communication, or HART communication, can realize factory automation and integration.
Working principle:
The sensor part of the thermal gas mass flowmeter consists of two precision platinum thermistors (see Figure 1). The resistor is enclosed in a stainless steel tube and does not directly contact the fluid medium, but has good heat transfer performance. One of them is a medium temperature sensing element; the other is a heating element. When the gas flows through the heating element. Remove heat. The amount of heat removed is directly proportional to the flow velocity of the fluid and the density of the fluid. The flow meter consists of two temperature sensors (see Figure 2). A certain amount of heating power P is applied to one of the sensors. Increase its temperature to the measured value T2. The other sensor measures the gas temperature T1. The mass flow of the gas can be determined based on the temperature difference between the heated sensor and the gas (ΔT2T1) and the heating power P.
This is called King's Law. K1 and K2 depend on the sensor's geometry and gas characteristics, such as thermal conductivity, viscosity, and specific heat capacity. K3 is related to the Reynolds number.
The values of these coefficients are unique to the flow meter and the gas, so the ITMF flow meter must be calibrated according to the gas to be measured. In practical applications, there are two methods for measuring the mass flow of gas: constant power method or constant temperature difference method.
In order to maintain a fixed temperature difference between Ts and Ta, the circuit must provide more power to the heating element. The output power in the circuit is directly related to the mass flow of the fluid, and the corresponding mass flow can be obtained by measuring the output power of the circuit. Because the heat on the heating element is mainly taken away by the flow of gas molecules, not the effect of heat conduction. Therefore, this flow meter is almost independent of the composition of the fluid within a certain range of operating conditions.