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MFC PRINCIPLES
Mass Flow Controllers (MFCs) are ud wherever accurate measurement and control of a mass flow of gas is required independent of flow pressure change and temperature change in a given range. Mass Flow Meters (MFMs) are ud wherever accurate measurement of gas is required without control of the flow, which is done by another device. To help understand how an MFC works, it can be parated into 4 main components: a bypass, a nsor, an electronics board and a regulating valve:
The bypass, the nsor, and one part of the electronics board are the measurement side of the mass-flow controller and make a Mass Flow Meter. The regulating valve and the other part of the electronics board are the controlling side of the mass-flow controller and exist only on a Mass-Flow Controller. So every Mass-Flow Controller includes a Mass-Flow Meter.
MEASUREMENT PRINCIPLES
The flow is divided between a heated nsing tube (the nsor), where the mass flow is actually measured, and a flow restrictor or bypass, where the majority of flow pass. The bypass is designed in a way that flow thru the nsor and the bypass is always proportional to the flow range for which the mass-flow controller is built. The nsor is designed to deliver an output voltage almost proportional to the gas flow circulating thru it, which is due to the bypass design being proportional to the total flow circulating thru the mass-flow meter or controller. The electronics board amplifies and linearizes the nsor signal so the output of the+ electronics board, named ?readout?, gives a signal pr
oportional to the total flow circulating thru the mass-flow meter or controller. Most of the time this signal is a 0-5 V signal; 0 volts indicating ?no flow?, 5 volts indicating full scale flow of the device. Full scale is the maximum flow for which the mass-flow controller is designed and calibrated to work with at high accuracy. This information is always written on stickers, which are on the top of the cover and the side of the mass-flow stainless steel ba. Also written on the sticker is the gas for which the mass-flow controller is calibrated.
Why u a bypass? Becau the nsor element can only measure small flow (typically 5 sccm), the bypass/restrictor allows the controller to control and measure greater amounts of flow. On a 5 sccm full scale mass-flow controller, there is no bypass, all of the gas would flow thru the nsor. On a 100 sccm full scale mass-flow controller, the bypass is adjusted so that when 100 sccm flows thru the controller, 5 sccm will flow thru the nsor tube and 95 sccm will flow thru the bypass.
SENSORS PRINCIPLES
Basically, the nsor us the thermal properties of a gas to directly measure the mass flow rate. The nsor us the basic principle that each gas molecule has a specific ability to pick up heat. This property, called the "specific heat" (Cp), directly relates to the mass and physical structure of the molecule and can be determined experimentally. The specific heat is well known for many gas and is generally innsitive to changes in temperature or pressure.
By adding heat to a gas and monitoring the change in temperature, the mass flow rate can be determined. To illustrate this concept, take the ca of cool gas flowing through a heated tube. Mathematically, the heat loss can be described by the First Law of Thermodynamics,
It is important to realize that both the specific heat and the flow rate determine the amplitude of the heat flux. As the mass and physical structure of molecules vary widely from gas to gas, so does the specific heat Cp. For the same molar flow rate, the heat flux can differ significantly for different gas. If this heat flux is monitored, the amplitude can be converted into an electrical signal. Given that the specific heat is known for the gas, the mass flow rate can then be determined directly from the electrical signal.
The MFC nsor includes a capillary tube wound with two heated resistance thermometers, measuring the change in temperature distribution created by the gas flowing inside this tube:
Sensor schematic
For zero flow, the upstream and downstream temperature will be equal. The windings are heated electrically to 80°C above ambient temperature. When the gas is flowing, the upstream region cools down whereas the downstream region heats up causing a temperature gradient along the length of the tube (e the nsor temperature profile figure).
Sensor temperature profile
The coils of the heating resistances are made with a thermal nsitive wire so that the temperature differences due to the flow are directly converted into resistance changes. The resistance changes are converted to a voltage by a simple Wheatstone bridge.
