Thermal mass flow sensor for gases

Bypass principle

Working principle mass flow sensor (bypass)
Figure 1: Working principle mass flow sensor (bypass)

How do thermal mass flow sensors work?

Thermal mass flow instruments that use a bypass are what most people have in mind when they think of thermal mass flow instruments. The principle is shown in Figure 1.

In this configuration, the mass flow is measured in the bypass (or sensor channel) instead of the main flow channel itself. This sensor is operated in the laminar flow regime. 

The bypass is a capillary tube with thin metal walls, in two locations surrounded with metal wires that can heat the tube and measure (‘sense’) the temperature as well. The small diameter of the capillary tube allows for fast response times and low energy requirements, because only a small tube volume needs to be heated. However, a filter is required prior to the mass flow meter to prevent plugging of the capillary tube.

What is 'Laminar flow element'?

A laminar flow element – which is a flow obstruction in the main channel of the device – allows a proportional part of the fluid to flow through the bypass (or sensor channel), in parallel to the main flow channel.

The laminar flow element (or flow splitter) consists of a stack of stainless-steel discs with high-precision etched flow channels, having similar characteristics as the flow sensor. For higher flow rates, the discs are equipped with an additional section to filter out turbulence effects. Using this element ensures stable and reliable flow proportioning, even under varying process conditions.

Do you want to learn more laminar flow elements? Read our blog about this critical element for flow meters which use the bypass principle.

Temperature through mass flow sensor
Figure 2: Temperature through mass flow sensor

Mass flow sensor for gases; bypass principle

The gas flowing through the sensor is warmed by two heaters (RHT1 and RHT2 in Figure 1). The temperature of the tube is measured at the same two points, so these heaters are sensors as well. With no flow present, the temperature difference between the two points in the bypass will be zero. When the flow increases, the temperature at the first measuring point (RHT1) falls as fluid carries away the heat. At the same time, the temperature at the second measuring point (RHT2) becomes higher because the fluid carries heat to it. A higher flow will result in a larger temperature difference, which is directly proportional to the mass flow.

Bronkhorst devices with ‘bypass’ mass flow sensors for gases

A clean, dry gas application where higher accuracy as well as repeatability are important is a good application for a bypass device where a proportional part of the flow is passed through a flow restriction, such as in the EL-FLOW Select, EL-FLOW Prestige, IN-FLOW and EX-FLOW devices. EX-FLOW devices are suitable for gas flow applications in hazardous (ATEX) areas.

As a variant of the bypass principle described above, Bronkhorst designed a special configuration for low pressure-drop applications - the LOW-dP-FLOW series. The sensor only requires approx. 0.25 to 2.5 mbars. Furthermore, the larger flow channels minimise the risks of clogging and facilitate cleaning and purging. Therefore, this configuration is preferred for corrosive gas applications.


Where are these ‘bypass’ mass flow meters and controllers used?

Onsite calibration setup for flow meters

To calibrate mass flow meters onsite, Bronkhorst manufactured a mobile calibration setup with a set of calibrated EL-FLOW Select or EL-FLOW Prestige thermal mass flow meters used as a reference.

Hydrogen storage in metal hydride

To investigate which process conditions work best for the loading/unloading of hydrogen to metal hydrides for storage purposes, hydrogen flows and the process pressure must be measured and controlled accurately. For the introduction of hydrogen to the metal hydride, instruments are used from the IN-FLOW thermal mass flow meter series in combination with Vary-P valves.

Flow control and monitoring for diamond coating using CVD 

Mass flow controllers, like EL-FLOW Select, LOW-dP-FLOW or IN-FLOW series, play a key role in applications involving chemical vapour deposition (CVD) processes, including to produce diamond films using hydrogen and methane as precursors.

Are you looking for a thermal flow device? Let us know how we can help you. 

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