New Possibilities for Thermal Flow Measurement

Sensirion is a Swiss manufacturer that offers sensor solutions to expand the possibilities of thermal flow measurement in both industry and the lab. The company has recently developed a high-flow sensor with significantly higher liquid flow rates of up to 1 liter per minute.

This allows Sensirion to progress into a new dimension of accurate flow measurement and grow its portfolio.

In Sensirion’s design, the microsensor chip is placed outside the flow channel.

In Sensirion’s design, the microsensor chip is placed outside the flow channel. Image Credit: Sensirion AG

Although measuring higher flow rates with Coriolis mass flow meters or ultrasonic flow meters is very accurate, it is not an option for many applications for two reasons: the technology is costly and takes up a lot of space.

The low-cost paddlewheel sensors, on the other hand, are too inaccurate for many requirements and do not provide a calibrated output signal. There is also a lack of process reliability because the measuring wheel can get jammed in the flow channel.

Sensirion closes this gap with an economical liquid flow sensor for both lab analysis and industry that provides accurate measurements of higher flow rates in the typical Sensirion quality, with the sensor being only 5 cm long and 7 g in weight.

Thermal Limitation Overcome by Steel Membrane

For this sensor, Sensirion relies on its proven mode of operation: A microscopic heating element enables the addition of a tiny “heat cloud” to the liquid, which becomes deformed by the flow.

The temperature is measured by two highly sensitive microsensors both before and after the heating element, and the deformation of the heat cloud is recorded, which manifests itself in a temperature difference between the two sensors. 

The value is converted by a microchip to a fully calibrated and linearized flow rate, and an output signal is provided. The microsensor chip is located outside the flow channel to avoid the flow being disturbed, as well as to protect the sensor chip from being affected by the liquid.

This means that the flow rate is measured “through the channel wall,” as the heat from the heater and the signals from both temperature sensors pass through the membrane. Consequently, the membrane’s thermal properties impact the sensor performance considerably.

The flow channel is largely made of polymer, and the membrane is made of stainless steel.

The flow channel is largely made of polymer, and the membrane is made of stainless steel. Image Credit: Sensirion AG

The LD20-2600B liquid flow sensor is designed for flow rates up to 1000 ml/hour. It utilizes a polymer membrane whose heat capacity, heat transfer coefficient, and thermal conductivity are low.

However, heat must be introduced more effectively into the flow channel to measure higher flow rates. To address this, developers at Sensirion have utilized a steel membrane in the SLF3x product family.

In the SLF3S-1300F, this provides a larger measuring range – up to 65 ml/minute – as well as a wider flow rate range (as shown in Figure 3) due to the characteristic curve of the steel membrane being flatter than that of the polymer membrane.

The SLF3S-4000B is the newest addition to the SLF3x family. It enables new dimensions due to further optimization of the hydrodynamic properties, resulting in the high-flow sensor being able to measure flow rates of up to 1000 ml/min.

The characteristic curve of the steel membrane is flatter than that of the polymer membrane

The characteristic curve of the steel membrane is flatter than that of the polymer membrane. Image Credit: Sensirion AG

Hydrodynamic Limitation Overcome by W-Shaped Channel Cross-section

A laminar liquid flow profile is required for the thermal measuring principle to guarantee that individual fluid layers do not mix in the flow. This is necessary because turbulence or swirling streamlines affect the temperature profile and alter the sensor signal.

The Reynolds number (Re) is a crucial indicator in this respect as it describes the flow pattern using four variables: channel diameter, density, flow velocity, and the dynamic viscosity of the fluid.

A lower Reynolds number (up to ca. 2300) leads to laminar flow, while a higher number (above ca. 3000) leads to turbulent flow. Theoretically, low liquid flow velocities favor laminar flow profiles, while instruments like liquid flow controllers or sensors mounted in the channel cause turbulence.

Either larger channel cross-sections or higher flow velocities are required for the accurate measurement of high flow rates. However, both factors increase the Reynolds number and, as a result, also increase the probability of turbulence.

To address this hydrodynamic limitation, the engineers at Sensirion laid out the channel profile in a W-shape when developing the new high-flow sensor. This means that the MEMS chip can be positioned at the narrower side stream (with laminar flow), allowing it to demonstrate its measurement performance.

Due to the W-shaped channel profile of the SLF3S-4000B, a pseudo-bypass with laminar flow is created away from the main flow.

Due to the W-shaped channel profile of the SLF3S-4000B, a pseudo-bypass with laminar flow is created away from the main flow. Image Credit: Sensirion AG

Physical Modeling Further Curbs Influence

During the development of the new high-flow sensor, Sensirion’s developers discovered that a number of disturbing influences have a greater impact on the measurement accuracy of sensors with a W-profile than that of conventional sensors with a round profile.

For instance, fluid temperature has a greater impact on the measured value because it alters the thermal and hydrodynamic properties, sometimes due to the thermal conductivity of the fluid being dependent on the temperature.

Increased fluid temperature leads to a decrease in viscosity, and this has a larger influence on the measured value in the W-profile (using the Reynolds number).

As a result, creating the perfect measurement environment is still a challenge despite the thermal and hydrodynamic limitations being addressed.

For other influences to be included in the calculation of flow rates, Sensirion employs physical models that are incorporated in the in-house calibration to guarantee that the sensor remains reliable and accurate under all conditions.

Complete Portfolio: Single Source Supply

The new SLF3S-4000B high-flow sensor has promoted Sensirion into a new measurement dimension, with a substantially extended measurement range from nanoliters per minute to liters per minute.

The SLF3S-4000B looks and feels the same as the existing three flow sensors of the SLF3x family, but it provides several advantages. Users can still use their existing cables or software for readout without any customization, meaning that no software reprogramming is required.

The complete liquid flow sensor portfolio enables users to source their sensor technology exclusively from one specialist in automation solutions and fluid systems.

Moving forward, the developers at Sensirion are aiming for flow rates of up to 20 liters per minute, with initial field studies already planned.


Produced from materials originally authored by Patrick Reith at Sensirion.

This information has been sourced, reviewed and adapted from materials provided by Sensirion AG.

For more information on this source, please visit Sensirion AG.


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