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Mass Flow Controllers Theory guide

Design and operating principles
The basic design of a mass flow controller includes a flow rate sensor, bypass, valve and electrical circuits. The gas, which eneter from the inlet, first splits to flow past the sensor or through the bypass. At the sensor, the mass flow rate is detected as a proportional change in temperature and converted by the bridge circuits to an electrical signal. This signal passes through the amplification and correction circuits, and is output to the outer portion of the unit as a linear voltage between 0 and 5V. At the same time, it also send to the comparison control circuit. The comparison control circuit compares the flow rate setting signal from the outer portion of the unit(0 to 5V) and the actual flow rate signal from the sensor and sends a difference signal to the valve driving circuit. The flow rate control valve moves as appropriate to make the difference signal approach zero. In other words, the unit controls the flow so that it is always at the set flow rate.

Flow rate sensor
The mass flow rate sensor in a mass flow controller is called a thermal mass flow rate sensor. Horiba Stec's sensors consist of stainless steel capillary tubes wrapped in double exothermic resistance wire and a bridge circuit. Sending an electirc flow through the exothermic resistance wire makes it hotter. When fluid is passed through the heated capillary tubes, a temperature difference is created between the bottom and the top of the flow. This temperature difference is converted into an electrical signal by the bridge circuit, enabling measurement of the flow rate of the fluid.

Laminar flow element bypass
The sensor perceives the flow of gas as a change of temperature, and since capillary tubes are used, there is a limit to the flow rate that can be used. To increase the flow rate range, a bypass that has extremely similar differential pressure flow rate characteristics to the sensor (a laminar flow element bypass) is attached in parallel. This increase the flow rate change to the total combined flow rate of the sensor and the bypass. The linearity is superb, since the bypass has the same differential pressure flow rate characteristics as the sensor, which makes it the ideal element for this purpose.

Flow rate control valve
There are three types of valves that can be used in a mass flow controller: piezo actuator valves, thermal actuator valves and solenoid actuator valves. Horiba Stec, which pioneered the development of the piezo actuator, uses the distortion generated in piezo elements when voltage is applied to a pliezo stack to drive its piezo actuator valve. the use of metal diaphragm in the valve makes it possible to produce all metal, ultra clean mass flow controllers. A thermal valve is a valve whose aperture changes continuously in accordance with the amount of electric current passing through a heater wrapped around an expansion axle. Horiba Stec also manufactures solenoid valves that use a solenoid. Whatever the use, there is sure to be a valve in the Horiba Stec lineuo that is appropriate for it.

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