Thermal gas mass flowmeters based on the bypass concept are widely used in small gas flows. This type of flow meter can have a control valve with a built-in control function. We then call this a thermal gas mass flow controller (MFC). In general terms, both flow meters with and without a control valve are called MFCs. This measuring technique is particularly suitable for low flow rates, from 0.5 SCCM (Standard Cubic Centimetre per Minute) to 1000 SLPM (Standard Litre per Minute). In principle, the technique can be used for any type of gas.

Typical applications of MFCs are found in:

The semiconductor industry

Highly accurate control of all types of gas under ultra clean conditions (UHP).

Biscuit machines

Lighten the dough.

Ice machines

To make the ice cream soft and creamy and for the taste.

Breweries

Air or oxygen injection into the wort
CO2 injection in filling processes

Analysis equipment

Sampling

Laboratoria

Bio-reactors
Catalysis research
Heart-lung machines
Blanketing applications
Consumption measurements of laboratory gases.

Bypass and sensor flow are directly proportional

A bypass thermal flowmeter is constructed from a "flowbody" in which a so-called laminar flow element (LFE) is placed. This is the by-pass. This LFE provides a laminar flow behaviour in the by-pass, and also a small pressure drop. Parallel to the LFE a channel with a sensor is placed. Because of the pressure drop over the LFE, a quantity of gas also flows through the sensor. The diameter of the sensor and the gas velocity are so low that the flow behaviour in the sensor is also laminar. This results in an almost constant ratio of the amount of gas flowing through the sensor and the bypass. Measuring the flow rate through the sensor is therefore representative of what flows through the entire instrument.

Measuring gas mass flow

The sensor in bypass flow meters consists of a thin tube (often called a capillary) with two temperature sensitive resistors wrapped around it. Both resistors give off heat to the tube, and thus also to the gas flowing through it. A temperature difference will occur between the two temperature sensitive resistors, proportional to the gas flow through the sensor. It is molecules that absorb the kinetic energy, so this concept measures the gas mass flow, which means that the exact amount of molecules that have passed through is known (apart from the inaccuracy of the measuring instrument). But there is no need to compensate for pressure and temperature. And if these are measured you can make statements about the gas composition.

The sensor is sensitive to contamination

Relative to the gas flow being measured, the flow through the sensor is very small. To give you an idea: for a 0-1 SLPM MFC, 0-10 SCCM will pass through the sensor, which is 1% of the total flow. For an instrument that can measure 0-1000 SLPM, also 0-10 SCCM will pass through the sensor, so only 0.001% of the total flow! Furthermore, the tube is very thin. A slight contamination of the sensor will immediately result in a huge measurement error, which is often difficult to solve because the whole instrument has to be cleaned. Therefore, for large flow rates, it is much wiser to use the immersion technique.

Schematic representation by-pass thermal flowmeter.

Illustration: schematic representation of a by-pass thermal flowmeter.

Photo: Thermal by-pass flowmeter based on MEMS technology.

The MEMS technology

A better, but also more expensive, alternative for the thin sensor tube is a MEMS sensor. This sensor has the heating resistor and the measuring resistor integrated on a chip. Therefore this type is completely insensitive to aging and does not need to be calibrated after use. Other advantages are:

even greater turndown
much less susceptible to contamination
extremely fast response

Unlike a capillary gauge, a MEMS-based instrument must be calibrated with the gas to be used. A positive consequence is the high accuracy.

The K-factor

The measurement concept is sensitive to the type of gas being measured. Unlike immersion thermal flow meters, MFCs allow for the use of K-factors. The K-factor is a multiplication factor for the use of other gases than the gas with which the flow meter is calibrated.

When the MFC is going to be used for SF6 for example, the instrument can be calibrated with air or nitrogen. This is of course much cheaper and more practical. It is also possible to calibrate an MFC for a mixture of gases in this way.

Summary

There are not many alternatives for measuring small gas flows. MFCs measure the gas mass flow directly, which is an advantage for accurate metering. MFCs are extremely suitable for building gas systems for small flows, with a significantly higher performance than for example VA meters. An important condition for the proper functioning of this type of flowmeter and flowcontroller is that the gas is clean and dry. In addition, such a system can be automated, also with Modbus or Profibus communication protocols.

By-pass thermal flowmeters FAQ

More information?

More information about our products or looking for a custom solution?
Please contact our Sales Engineers.

NL: +31 70 413 07 00
CN: +86-(0)10-60576210
USA: +(1) 973 383 0691
TW: +886-(0)3-5600560

Or use our contact form

Already know who you're dealing with? Contact your salesperson directly:

Look up salesperson

Any Questions?

More information about our products, services or looking for a custom solution?
Please contact our Sales Engineers.

We use cookies just to track visits to our website, we store no personal details. We also us commercial cookies, accept them for a better user experience. Please refer to our Privacy Policy.