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Small gas mass flow: Measurement and control


iCenta Controls specialises in the supply, manufacturing and calibration of flow meters, level sensors, instrumentation and control systems


water including, hydrocarbons, slurries and other liquids. In this article we look at measurement


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instruments for relatively low gas flows, from a few cc/min to 500 or 7000 Ln/min; where the gas mass flow meter with the thermal bypass concept is the most common instrument. The two types are gas mass flow meters (MFM) and mass flow controllers (MFC). The latter is a flow meter equipped with a control valve and the user can provide a setpoint that determines a generated gas flow.


ur comprehensive portfolio from leading manufactuers covers sensors for a wide range of fluids other than


Typical applications for MFC's • PVD-and other processes in the semiconductor industry • Dosage of air, O2, N2 and CO2 in bioreactors •


process of beer •


suitable for another exotic, explosive, poisonous or inflammable gas. If this “K-factor” is applied, parameters such as viscosity, pressure and temperature will have an effect on the accuracy.


Air injection in ice-cream, for a creamy product and best taste •


Air and other gases in cookies and packaging applications • Laboratory applications, for instance


analytics and catalyst-research There’s a distinction between the MFC's based on a traditional capillary sensor and instruments with a MEMS (Micro-Electro- Mechanical System) sensor. Both sensor techniques have advantages and limitations.


Air/O2 dosage used in the fermentation Figure 4: Sensor of MEMS mass flow meter


The MEMS-based MFM/MFC has no LFE but a regular by-pass to insure a defined percentage of the total gas flows through the sensor. The bore of the sensor is much larger, so that for example the pressure drop is relatively low and the sensor less sensitive to pollution than a capillary. A MEMS sensor is more sensitive and due to the turndown of 1000:1 is obtainable (versus 50:1 for capillary sensors). This type of MFM/MFC must be calibrated with the gas as used in the application and will provide greater accuracy.


The biggest advantage of the MEMS sensor is, Figure 1: Various different MFM/MFC models


MEASUREMENT CONCEPT In the thermal bypass concept most of the measured gas flows through a bypass that generates a very small pressure drop. As a result, a small percentage of the gas runs through the sensor that runs parallel to the bypass. This measures the gas flow, which is representative for the total amount of gas.


however, that there is no (measurable) drift to affect accuracy. Capillary sensors exhibit effects of mechanical and thermal stress on the Platinum wires and aging of insulation materials. The MEMS sensors consist of two or three temperature sensors and a heater, vapor deposited as an extremely small molecular layer on a thin membrane. Other advantages are that the sensor is very


Figure 3 Generic overview capillary mass flow meter


Figure 2: Schematic representation of the measurement principle (source Vögtlin)


Mass flow meters (MFM) and mass flow controllers (MFC) are instruments that measure and regulate the gas mass flow in Kg/hr, it can also be expressed in units of normalized volume, such as ln/min (normal litre per minute) or SCFH (Standard Cubic Foot per Hour), where the reference conditions change for each unit.


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The capillary sensor consists of a thin steel tube (0.2 to 1 mm o.d.) around which 2 very thin platinum wires are wrapped. The platinum wires act as electrical resistance and are part of a Wheatstone bridge. The resistance value of the platinum wire depends on the temperature. A current pass through them heating them up. At zero flow conditions the resistance remains same, but when a flow passes through the sensor a difference in temperature will be detected by the electronics. An MFC with a capillary sensor is equipped with a Laminar Flow Element (LFE). This LFE creates laminar flow behavior, just like inside the sensor. This means that the relationship between the flow through the sensor versus the flow through the bypass is almost linear over the measured range. Given this linearity it’s possible to calibrate MFC's with a cheap simple gas like air and use a known factor to make the meter


fast: 50 msec response time. When you power the unit up is ready for use within one second, while capillary concepts take 15 minutes. The instrument does not need the zero to be adjusted on a regular basis. (for capillary MFC's it’s a standard routine). MEMS-based MFC's offer significantly better reliability of the measured and/or regulated flow.


Figure 5: Four Redy Mass flow controllers in a customer specific solution


iCenta Controls www.icenta.co.uk October 2018 Instrumentation Monthly


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