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Thermal Flow Measurement: High-Flow Sensor for Laboratory Analytics and Industry

Thermal Flow Measurement: High-flow Sensor For Laboratory Analytics And Industry
Sensirion extends the possibilities of thermal flow measurement in laboratory analytics and industry. With the help of a new design, engineers have succeeded in developing a high-flow sensor with significantly higher liquid flow rates of up to 1 liter per minute. The Swiss manufacturer is thus advancing into a new dimension of accurate flow measurement and expanding their portfolio.

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

Although measuring higher flow rates with Coriolis mass flow meters or ultrasonic flow meters is accurate, it’s not an option for many applications: the technologies are costly and take up space. Low-cost paddlewheel sensors, on the other hand, are too inaccurate for many requirements and don’t provide a calibrated output signal. Users also complain about a lack of process reliability. Sensirion now closes the gap with a cost-effective liquid flow sensor for laboratory analysis and industry that accurately measures higher flow rates in the usual Sensirion quality, just 5 cm long and 7 g in weight.

Thermal limitation overcome by steel membrane

The company relies on its proven operation mode: A microscopic heating element adds a tiny “heat cloud” to the liquid, which is deformed by the flow. Two highly sensitive microsensors measure the temperature before and after the heating element and record the deformation of the heat cloud, which manifests itself in a temperature difference. The microchip converts the value to a fully calibrated and linearized flow rate and provides it as an output signal. To not disturb the flow, and to protect the microsensor chip from influences caused by the liquid, Sensirion places it outside the flow channel. The flow rate is measured “through the channel wall,” so the heat from the heater, and the signals from both temperature sensors pass through the membrane. Thus, the thermal properties of the membrane significantly influence the sensor performance.

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

In the LD20-2600B liquid flow sensor (for flow rates up to 1000 ml/h), Sensirion uses a polymer membrane whose thermal conductivity, heat capacity, and heat transfer coefficient are low. However, if higher flow rates are to be measured, heat must be introduced more effectively into the flow channel. For this reason, Sensirion developers have equipped the SLF3x family with a steel membrane. In the SLF3S-1300F, this ensures a larger measuring range – up to 65 ml/min – and a wider flow rate range (graphic), since the characteristic curve of the steel membrane is flatter than that of the polymer membrane. Now, the newSLF3S-4000B opens up completely new dimensions thanks to additionally optimized hydrodynamic properties: this high-flow sensor is even capable of measuring flow rates of up to 1000 ml/min.

The characteristic curve of the steel membrane is flatter than that of the polymer membrane. Thus, sensors with steel membrane cover a larger flow rate range.

Hydrodynamic limitation overcome by W-shaped channel cross-section

The prerequisite for the thermal measuring principle is a laminar liquid flow profile. This ensures that individual fluid layers don’t mix with each other, because swirling streamlines disturb the temperature profile and falsify the sensor signal. An important indicator is the Reynolds number (Re), which describes the flow pattern by means of four variables: channel diameter, flow velocity, density, and the fluid’s dynamic viscosity. A lower Reynolds number (up to 2300) leads to laminar flow, a higher one (above 3000) to turbulent flow. In principle, low liquid flow velocities favor laminar flow profiles.

To accurately measure high flow rates, either higher flow velocities or larger channel cross-sections are required. But both factors increase the Reynolds number. To overcome this hydrodynamic limitation, Sensirion’s engineers employed a design trick: they laid out the new channel profile in a W-shape. This creates the possibility of positioning the MEMS chip at the narrower side stream (with laminar flow), where it can demonstrate its performance.

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

Physical modeling further curbs influences

While designing the new high-flow sensor, Sensirion’s developers discovered that several disturbing influences have a greater impact on the measurement accuracy of W-profile sensors than that of conventional round profile sensors. For example, fluid temperature has a greater impact on the measured value because it changes not only the thermal but also the hydrodynamic properties – because sometimes the fluid’s thermal conductivity depends on the temperature. For example, increased fluid temperature reduces viscosity, which has a greater influence on the measured value in the W-profile (via the Reynolds number).

Consequently, creating an ideal measurement environment remains challenging even after overcoming thermal and hydrodynamic limitations. To include further influences in the calculation of flow rates, Sensirion also uses physical models that are incorporated in the in-house calibration and ensures that sensors function reliably and accurately under all conditions.

Complete portfolio: single source supply

With the new SLF3S-4000B high-flow sensor, Sensirion is moving into a new measurement dimension and now covers an extended measurement range from nanoliters to liters per minute. With the same look and feel as the existing SLF3x flow sensor family, the SLF3S-4000B offers several advantages. Users can continue to use existing cables or software for readout without customization, thus eliminating reprogramming processes. The complete liquid flow sensor portfolio benefits users who want to source their sensor technology from one specialist in automation solutions and fluid systems. But we are only at the beginning of the journey: Sensirion wants to raise the bar higher, and has its sights set on flow rates of 20 liters per minute. Initial field studies are planned.

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Headquartered in Staefa, Switzerland, Sensirion AG is a leading manufacturer of digital microsensors and systems. Its product range includes gas and liquid flow sensors, differential pressure sensors and environmental sensors for the measurement of humidity...

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