How to improve the accuracy of thermal gas mass flow meter?
How to improve the accuracy of thermal gas mass flow meter?
1. Selection and optimization of tube type
The tube type is the key factor that affects the sensitivity of the sensor under ideal conditions. The correct selection of vibration tube type has an important influence on the sensitivity and zero drift of thermal gas mass flowmeter. The tube type of the gas mass flowmeter needs to be high sensitivity and small zero drift, but in fact the two are contradictory. The sensitivity is high, and the torsional rigidity of the vibration tube is required to be small, and the torsional rigidity is small, that is, the force that makes the tube return to the equilibrium position is small, and the zero point is easy to drift. Therefore, we should choose the appropriate tube type in order to obtain better results.
2. Influence and inhibition of weak factors
a. Stress
The mass flowmeter used in the air filler measures the pressure of the gas at no less than 20mpa, and the pressure is much greater than atmospheric pressure. High pressure will change the internal stress of the vibrating tube, and then improve the equivalent stiffness of the elastic tube, which will increase the vibration frequency of the vibrating tube and reduce the phase difference between the two tubes. When the gas flows through the vibration tube, there is a pressure drop, so that the force of the vibration tube is no longer symmetrical, which will also affect the phase difference. The test results show that the pressure will cause the flow reading to be low (negative error). The effect of pressure can be corrected by calibration, using software.
b. Temperature
The mass flowmeter used in the air filler has a working temperature far below room temperature, and the effect of temperature must be considered; The change of medium temperature or ambient temperature will change the Young’s modulus of the measuring vibration tube and affect the structure of the zero drift. The temperature coefficient of Young’s modulus can be compensated by electronic circuit and digital technology to reduce its influence. Because the zero drift effect is formed by the geometry of the vibration tube and the asymmetry of the structural parts, it can not be reproduced, so it is difficult to reduce and eliminate.
3. Generation mechanism and inhibition of zero drift
The key to zero drift comes from the sensor part of the flow meter, and the main reasons are:
a. Asymmetry and attenuation of mechanical vibration, including: the correlation of fixed stress at the end of the tube, the correlation change of the stiffness of the vibrating tube, the inconsistency of the resonance frequency of the double tube and the internal attenuation of the tube wall material;
b. The density viscosity of the fluid is related to the change, mainly affecting the zero position. The main reason is that the vibration tube structure is unbalanced, so even if the resonant frequency of the two tubes is adjusted uniformly when the empty tube is filled with liquid, it may also produce zero drift due to fluid density changes, and the vibration attenuation caused by viscosity is also related to frequency, and it may produce zero drift when flowing.
If you want to reduce the zero drift, it is necessary to require the manufacturer to design a reasonable tube structure, and the use of appropriate materials, improve its manufacturing process, and require installation, as far as possible to reduce the internal stress, reduce the noise of the environment, to have a strong and stable support. The final zero must be performed at the installation site. When the installation changes or the temperature changes significantly, we need to readjust.
