Chris King
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In many research and production processes the important variable is mass and not volume. Measurements of volumetric flow are not as reliable as mass flow measurement due to the effects that changes in temperature or pressure have on the density of a fixed volume of gas.

Unlike volumetric flow measurement devices such as purge meters (variable area meters) or turbine meters, thermal mass flow meters (MFM) and mass flow controllers (MFC) are relatively immune to fluctuations in temperature and pressure of the incoming flow. The MFM/MFC is capable of providing direct measurement of mass flow, as opposed to most other method that measure volumetric flow and require separate measurements for temperature and pressure in order to calculate density and, ultimately, the mass flow.
The MFM/MFC actually measures and controls the flow on a molecular level and so is able to provide an extremely accurate, repeatable, and reliable delivery of gas into the process.

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Originally developed for the semiconductor industry, MFMs/MFCs are now widely used for applications in research laboratories, pilot plants, and continuous processes. The thermal mass flow meter and mass flow controller advantages of low flow accuracy and repeatability, relative immunity to fluctuations of inlet flow temperature and pressure, and a complete PID control loop in a compact package have helped to improve productivity and reduce costs in a variety of analytical, industrial process, and OEM applications.

Principle of Thermal mass flow measurement

EL-FLOW Prestige mass flow meter/controller product tutorial

James Walton
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The selection of a mass flow meter/controller depends fully on the application and customer requirements. There is isn’t a good or bad decision as long as you are aware of the specific characteristics of the different options for your application. As a vendor of a broad portfolio of sensor technologies, we compare the pros and cons of each technology per customer case. Occasionally we discuss with our customers how we can define the main differences between a Thermal By-Pass and a Differential Pressure Flow device. We sometimes come across these differential pressure devices in the field across a small set of applications due to their inherent limitations, however, when we do get asked the question it is always good to have a good understanding of the technology in question.

Range and options: Thermal By-Pass flow devices cover a much wider range of application conditions, for example we can cover from Vacuum to 700 bar when a differential pressure device typically operates between atmospheric and 10 bar.

Physical Behaviour: Where most end users are interested is usability. This is because it dictates how the instrument will directly affect their process to hopefully achieve the desired goal, either to increase or decrease something. Talking about usability is not always in terms of functionality, options, on-board screens or other extras, it is how the instrument is physically designed to handle the behaviour of gas flow as it passes through the instrument t to derive a useful reading. The internal structure of a thermal by-pass flowmeter is based around creating a predictable and repeatable split in the flow between the laminar flow element (LFE) and the by-pass sensor. The better the LFE works, the more predictable the flow of gas is and the more accurate the split of the flow and therefore the performance of the flowmeter. With the split of a thermal by-pass instrument being based on mechanical dimensions the absolute temperature and pressure virtually do not influence the split. With pressure based instruments the viscosity in the LFE is directly influencing the reading, viscosity strongly depends on temperature and pressure, and this may lead to the instrument being susceptible to subtle variations in the flow.

Accuracy: Overall accuracy in a thermal by-pass instrument is dependent on just one sensor (measuring direct thermal mass flow), on the other end of the scale pressure based instruments need to calculate mass flow from the measured volume flow, temperature and pressure. This could mean using up to 4 sensors. When measuring with this many sensors the individual errors will add up. Pressure based instruments have to measure temperature at two positions; one in the pressure sensor to compensate for temperature errors in the pressure sensor and one in the LFE to correct the pressure drop at actual gas temperature.

Conclusions: Thermal by-pass instruments cover a much larger range of flows and pressures; they are also less complex requiring fewer sensors to generate the same data. It is also easier to define the working parameters of the thermal by-pass instruments, fewer sensors means fewer combined limitations and inaccuracies. The internal design and basic principle of the differential pressure devices appears to require a more complex system of measuring to get to the same end point. As always we are happy to discuss and talk about the differences between all of the flow meters available, in the coming weeks we will be looking closely at other flow meter technology including; Coriolis, CTA and MEMS. If you have any requests for a topic of discussion then please get in touch and let me know.

James

How an MFC works
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