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What is a mass flow meter? What is a mass flow controller?
Mass flow meter (MFM) is an instrument that accurately measures gas flow rate, and its measurement value is not inaccurate due to temperature or pressure fluctuations, without the need for temperature and pressure compensation.
The Mass Flow Controller (MFC) not only has the function of a mass flow meter, but more importantly, it can automatically control the gas flow rate. Users can set the flow rate according to their needs. MFC automatically maintains the flow rate at the set value, even if there are fluctuations in system pressure or changes in ambient temperature, it will not deviate from the set value. Simply put, a mass flow controller is a gas flow stabilizing device that can be manually set or automatically controlled by connecting with a computer.
What are the main advantages of MFM / MFC?
(1)The measurement and control of flow should not be inaccurate due to temperature or pressure fluctuations.
For most flow measurement and control systems, it is difficult to avoid pressure fluctuations and temperature changes in the environment and medium. For ordinary flow meters, fluctuations in pressure and temperature will lead to significant errors; For MFM / MFC, they can generally be ignored.
(2)Automation of measurement control
MFM / MFC can output standard electrical signals based on flow measurement values. This makes it easy to achieve digital display of traffic, automatic measurement of cumulative traffic, automatic data recording, computer management, and so on. For MFC, automatic control of flow can also be achieved. Usually, analog MFC/MFM input and output signals are 0~+5V or 4~20mA. Digital MFC/MFM is also equipped with RS232 or RS485 digital serial communication ports, which can be easily connected to computers for automatic control.
(3)Accurately and quantitatively control flow
MFC can accurately control the amount of gas given, which is particularly useful for flow control in many process flows and proportional control of different gases.
(4)Wide application range
Our product has a wide working pressure range, from vacuum to 10MPa; Can be applied to various gas media (including some corrosive gases, such as HCL); Our product has a wide flow range, with a minimum flow range of 0-5 sccm and a maximum flow range of 0-200 slm. The resolution of flow display can reach 0.1% of full scale, and the flow control range is 2-100% of full scale (range ratio -50:1), so it is widely used in many fields.
What is the concept of the working pressure difference range of a MFC?
MFC is equipped with a gas flow control valve, which can adjust the flow rate through the controller from zero to the full range of measurement. During operation, a pressure drop, also known as pressure difference, is generated between the inlet and outlet of the controller. The working pressure difference range of MFC is usually 0.1~0.3MPa. If the pressure difference is lower than the minimum value (0.1 MPa), it is possible that the control cannot reach the full range value; If it is higher than the maximum value (0.3MPa), it is possible that the flow rate cannot be less than 2% F.S when closing. When users use MFC, regardless of whether the reaction chamber they are working in is vacuum or high-pressure, it should be ensured that the pressure difference between the inlet and outlet of the MFC is maintained within the required range, and the air pressure should be relatively stable.
How to customize a low-pressure drop product for a MFC using volatile liquid evaporation vapor with low air pressure?
Some users use volatile liquid evaporation vapor as a gas source (such as acetone), which has a very low pressure and cannot function properly with ordinary MFC. At this point, the user needs to make a special declaration to the supplier to customize a low pressure drop MFC.
Usually, MFC with a working pressure difference below 0.01MPa can be customized, and the working pressure difference range of MFC will also decrease. When using volatile liquid evaporation vapor, do not allow the vaporized substances to condense in the MFC, as condensation can affect the normal operation of the MFC. To prevent condensation, the pipeline and MFC can be heated appropriately, but the working environment temperature of MFC usually should not exceed 50 °C. If the user wants the working environment temperature of MFC to exceed 65 °C, special high-temperature MFC products need to be purchased.
Can the MFC be used to control corrosive gases and special gases? How to choose sealing materials?
MFC can be used to control various gases, including corrosive gases and special gases, but it should be noted that for the use of corrosive gases and special gases, appropriate selection of models and sealing materials is required. When placing an order, users need to make a special declaration to the supplier regarding the use of gas to avoid serious losses caused by incorrect selection or sealing materials< br>
Usually, the wet materials used for MFC to contact the working gas are 316L stainless steel, polytetrafluoroethylene, and sealing materials, which are further divided into two categories: metal seals and rubber seals. The materials for metal sealing are generally corrosion-resistant materials such as stainless steel, gold, or nickel. MFC with metal sealing can generally be used for any gas, including various corrosive gases and special gases. For highly corrosive gases such as boron tribromide, boron trichloride, hydrogen fluoride, and metal oxide gases used in MOCVD, it is recommended to use metal sealed MFC.
Due to the high price of metal sealed MFC, rubber sealed MFC is usually used in situations with low requirements. Unless otherwise stated, MFCs are sealed with fluororubber and can be used for most acidic and alkaline corrosive gases, such as halogenated dry gases such as hydrogen chloride. Other special corrosive gases require the use of special rubber. Ammonia gas - requires the use of ammonia resistant rubber, such as ethylene propylene rubber, chloroprene rubber, or nitrile rubber; Organic solvent vapor (acetone, toluene, etc.) - requires the use of silicone rubber; Boron tribromide, boron trichloride, hydrogen fluoride, etc. - perfluorinated rubber is required.
How to convert the mass flow rate of different gases?
The product is usually calibrated for nitrogen and the flow specification is determined based on the nitrogen flow rate when leaving the factory. When using the same specification of MFM/MFC to measure different gases, the actual flow value may be different when the flow detection value is the same. We provide the mass flow conversion coefficients of different gases relative to the calibration gas (nitrogen) in the instruction manual. If you are using a standard factory product (displayed according to nitrogen calibration) and need to know the actual mass flow rate of the gas used, first find the conversion coefficient of the actual gas used in the product manual. During the measurement process, multiplying this coefficient by the displayed flow rate value is the actual mass flow rate of the gas used; On the contrary, when determining the range of the purchased product, dividing the maximum expected flow rate of the actual gas used by the conversion coefficient is the corresponding flow rate value of the nitrogen calibration product. Air, O2, H2, CO, HCL, HF and other gases have a conversion coefficient of about 1.0, and can generally be directly displayed as products calibrated with nitrogen without the need for conversion.
How does an analog MFC connect to a computer?
Computers can be directly connected to MFC, or they can be connected to MFC through a flow meter. For analog MFC, it is generally necessary to install A/D and D/A conversion boards on the computer. Analog/digital conversion is used to receive flow measurement values, and digital/analog conversion is used to provide 0-5VDC flow setting signals. This type of conversion board is easily available in the industrial control product market; If the MFC is directly connected to the computer, the user needs to provide a ± 15V power supply (the power supply must have strong anti-interference ability); Connect the "settings" line to the computer's D/A output; Connect the "flow detection" cable to the A/D input terminal of the computer; Connect the MFC signal ground to the computer signal line. If MFC is connected to a computer through a flow meter, connect the "peripheral" line of the flow meter to the computer's D/A output; Connect the "flow detection" cable to the A/D input terminal of the computer; Connect the signal ground of the display device to the computer signal line. Of course, corresponding software is also required. |
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