Air mass flow meter (abbreviated as MFM) is used for precise measurement of gas mass flow.
A gas mass flow controller (abbreviated as MFC) is used for precise measurement and control of gas mass flow.
What is a mass flow meter?
Air mass flow meter, namely Mass Flow Meter (abbreviated as MFM). It is an instrument for accurately measuring gas flow. The measured value is not inaccurate due to temperature or pressure fluctuations. No temperature pressure compensation is required.
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What is a mass flow controller?
The mass flow controller (abbreviated as MFC) not only has the function of the mass flow meter. More importantly, it can automatically control the gas flow. That is, users can set the flow according to their needs. MFC automatically keeps the flow constant at the set value. Even if the system pressure fluctuates or the ambient temperature changes. It will not deviate from the set value.
Simply put, the mass flow controller is a steady flow device. It is a gas steady flow device that can be set manually or connected to a computer for automatic control.
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What does a mass air flow meter do?
Industrial Air mass flow meter is a meter used to measure the flow of industrial gas pipelines. The mass flow meter directly measures the mass flow of the medium passing through the flow meter. It can also measure the density of the medium and indirectly measure the temperature of the medium.
Since the transmitter is an intelligent instrument with a single-chip microcomputer as the core, more than a dozen parameters can be derived for users to use based on the three basic quantities mentioned above.
The mass flow meter has flexible configuration, powerful functions and high performance-price ratio. It is a new generation flow meter.
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What is the principle of a mass flow controller?
The mass flow controller is composed of a flow sensor, a diverter channel, a flow regulating valve and an amplifying controller.
The flow sensor uses the principle of capillary heat transfer thermometry to measure the mass flow of gas (without temperature and pressure compensation).
The flow signal measured by the sensor heating bridge is sent to the amplifier for amplification. The amplified flow detection voltage is compared with the set voltage, and then the difference signal is amplified to control the regulating valve. The closed loop controls the flow through the channel to make it equal to the set flow.
The flow divider determines the flow of the main channel.
A regulated power supply and a digital voltmeter are provided on the flow display instrument matched with the mass flow controller. Set potentiometer, peripherals, built-in conversion and three-position valve control switch, etc.
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How to understand the flow unit of a mass flow meter/mass flow controller?
Gas mass flow units are generally expressed in sccm (Standard Cubic Centimeter per Minute, standard milliliters per minute) and slm (Standard Liter per Minute, standard liters per minute).
This means that under different conditions of use, the indicated flow rate is the flow rate in the standard state.
The standard state requirements are: air pressure—101325Pa (760mm Hg); temperature—0℃ (273.15K). This is an important difference between this meter and other flow meters, and it is also the difference between sccm and slm from mL/min and L/min.
For most users, the expression method of volume flow is very habitual. Easy to use. But there are also users who need to know the mass of the medium flowing in a unit of time (such as g/min). This requirement is very easy to achieve. Because the gas density in the standard state is a constant. , Can be easily found, so it can be realized by simply doing a multiplication (multiplying the density by several slm). Therefore, the volume flow rate in the standard state is equivalent to the mass flow rate.
When to use a mass flow meter and when to use a mass flow controller?
Generally speaking, when only measuring or monitoring the flow, use a mass flow meter.
When you need to control the flow, use a mass flow controller.
For some measurement occasions, both can be used. For example, when measuring the flow capacity of capillary tubes, use a mass flow meter when measuring flow at a constant pressure.
When measuring pressure with constant flow, a mass flow controller is required.
Note that when the mass flow controller is in the cleaning state. It can also be used as a mass flow meter. It’s just that the pressure drop is larger than that of ordinary mass flow meters.
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What are the main advantages of mass flow meters/mass flow controllers?
(1) Flow measurement and control are not inaccurate due to temperature or pressure fluctuations.
For most flow measurement and control systems, it is difficult to avoid system pressure fluctuations and environmental and medium temperature changes. For ordinary flow meters, fluctuations in pressure and temperature will cause larger errors. For mass flow meters/mass flow controllers, it can generally be ignored.
(2) Automation of measurement control
The mass flow meter/mass flow controller can output the flow measurement value as a standard electrical signal output.
In this way, it is easy to realize digital display of flow, automatic measurement of accumulated flow, automatic data recording, computer management, etc.
For mass flow controllers, automatic flow control can also be realized.
Generally, the analog MFC/MFM input and output signals are 0~+5V or 4~20mA, and the digital MFC/MFM is also equipped with RS232 or RS485 digital serial communication ports. It can be easily connected to a computer for automatic control.
(3) Accurately control flow
The mass flow controller can accurately control the given amount of gas. This is particularly useful for flow control in many processes, and for proportional control of different gases.
(4) Wide scope of application
- With a wide working pressure range, our products can range from vacuum to 10MPa.
- Can be applied to a variety of gas media (including some corrosive gases, such as HCL).
- There is a wide flow range. The minimum flow range of our products can reach 0～5 sccm, and the maximum flow range can reach 0～200 slm.
- The resolution of the flow display can reach 0.1% of the full scale.
- The flow control range is 2~100% of full scale (range ratio is –50:1). Therefore, it is widely used in many fields.
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What are the disadvantages of mass flow meters? What are the fears of using mass flow controllers?
Relative to cheap rotameters, the main disadvantage of mass flow meters is that they are relatively expensive.
There are four fears when using a mass flow controller:
First fear of dust. If dust enters the flowmeter, it will block the sensor and the diverter, causing measurement errors. It is required to install a filter before the air inlet;
The second fear is that liquid and oil will enter the MFC. It will cause loss of control. If the outlet of the MFC is connected to a liquid tank, a check valve must be added to the outlet of the MFC to prevent the liquid from returning;
The third fear of using moist corrosive gas. Moist and corrosive gases will rust the stainless steel and cause damage to the MFC channel. Therefore, it is required to dry the gas and the MFC piping system before using the corrosive gas;
Fourth, fear not operating in accordance with safety rules when using highly corrosive, flammable, explosive, toxic, and other hazardous gases. It may cause product damage and other more serious losses. Correct use and safe operation are important to every user.
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Can mass flow controllers be used to control corrosive and special gases? How to choose sealing materials?
Mass flow controllers can be used to control various gases, including corrosive gases and special gases.
But it should be noted that for the use of corrosive gas and special gas, it is necessary to select the model and sealing material appropriately.
When ordering, the user needs to make a special statement to the supplier that the gas is used. In order to avoid serious losses due to wrong selection or selection of sealing materials.
Generally, the wet materials of the gas mass flow controller that contact the working gas are 316L stainless steel, polytetrafluoroethylene, and sealing materials. Sealing materials are divided into two categories: metal seals and rubber seals.
The material of the metal seal is generally corrosion-resistant materials such as stainless steel, gold or nickel.
In principle, MFC with metal seal can be used for any gas. Including various corrosive gases and special gases.
For strong 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 MFCs, rubber-sealed MFCs are usually used where the requirements are not high.
If there is no special statement, all MFCs are sealed with fluorine rubber. It can be used for most acidic and alkaline corrosive gases. For example, it can be used for halogenated dry gases such as hydrogen chloride.
Some other special corrosive gases require special rubber.
Ammonia-Ammonia resistant rubber, such as ethylene-propylene rubber, neoprene rubber or nitrile rubber is required. Organic solvent vapor (acetone, toluene, etc.)-silicone rubber is required. Boron tribromide, boron trichloride, hydrogen fluoride, etc.-need to use perfluoro rubber.
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Wu Peng, born in 1980, is a highly respected and accomplished male engineer with extensive experience in the field of automation. With over 20 years of industry experience, Wu has made significant contributions to both academia and engineering projects.
Throughout his career, Wu Peng has participated in numerous national and international engineering projects. Some of his most notable projects include the development of an intelligent control system for oil refineries, the design of a cutting-edge distributed control system for petrochemical plants, and the optimization of control algorithms for natural gas pipelines.