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Measuring gas flow in applications demanding high precision and cost-efficiency is a tough task. Experience in the last few years has shown that microthermal flow sensors are better than other technologies. Industries with exacting demands, such as medical and automotive technology, have accepted that microthermal gas flow sensors provide their products pivotal advantages.
These manifest themselves in high long-term stability and precision even when flow rates are negligible, and the sensors’ suitability for reliable and cost-efficient mass production.
Where in the gas flow should a sensor be placed? And how should the flow guide be built to realize the ideal results while keeping the production processes simple? Experience with a number of Sensirion’s customers' applications and endurance tests for product certification provide a clear answer: In the majority of cases, a bypass configuration is preferable to positioning the sensor in the direct flow.
Methods of Gas Flow Measurement
There are many different techniques for measuring gas flow: float-type, differential pressure, ultrasonic, mechanical volumetric, coriolis, magnetic inductive and thermal flow metering, to name but a few. Metering methods without contact between gas and sensor require comparatively expensive technology and are therefore out of the equation for a number of applications. Whereas in the classic differential pressure technique, hysteresis effects and membrane fatigue can lead to drift issues and a lack of zero-point accuracy, because here the mechanical deflection of a sensor membrane over an orifice is used to measure the pressure drop.
Consequently, measuring methods based on thermal principles are extensive. In the simplest of these – the hot-wire anemometer – gas flow is established via the rate of cooling of an electrically heated wire with a temperature-dependent resistance. Advanced techniques use a heating element and as a minimum two temperature sensors, which measure the transport of heat through the gas. The term “microthermal flow sensors” is used when the sensor components are combined into a millimeter-scale silicon microchip.
Microthermal Flow Sensors
For many applications, microthermal sensors offer decisive benefits. The small size of the sensors and the use of standardized semiconductor manufacturing processes allow mass production with a reliably superior quality. Owing to the economies of scale, unit costs stay moderate.
The latest sensor elements deliver excellent precision compared to classic hot-wire anemometers, while a glass coating of the sensor element offers corrosion-resistance. All these benefits are extremely useful in many branches of industry. Over the last 10 years, leading industries have transitioned towards microthermal sensor technology for gas flow measurement, which is currently the predominant sensor type used for challenging medical, automotive and HVAC applications.
However, direct contact with the gas can result in challenges of its own. Flow speed measurement is only selective, which means that extrapolation to the total flow relies on the velocity distribution in the pipe and therefore inlet conditions become critical. A bend in the tube directly before the sensor, different types of surface structure within the tube, or corners and edges in the flow passage, will modify the measurement. Furthermore, heavily polluted gas flowing past the sensor element can result in soiling issues.
A good way to handle such challenges is to position the sensor chip in a bypass. An orifice, a Venturi, or another flow restrictor produces a pressure drop, which guides a small proportion of the gas via a bypass channel. While the microthermal flow sensor ensures reproducibility, high accuracy, and stability, mainly in the case of very low flow rates, a well-built pressure drop element guarantees that the resulting differential pressure is less vulnerable to variations in the inlet conditions. Furthermore, the arrangement of the bypass tapping ports is important. By using inertia effects and lessening the bypass flow, a smart bypass channel design will guarantee a clean gas flow to the sensor chip.
Using a bypass configuration helps to streamline the manufacturing process too. It permits the flow element to be molded and assembled independently of the sensor. Under the premise of tight production tolerances and by placing the pre-calibrated sensor at the end, it is often possible to skip the final calibration of the whole system.
Good Design for a Bypass Solution
How should a bypass configuration be engineered to achieve the desired results?
The flow restrictor’s task is to marginally increase the resistance in the gas flow and, as a result, produce a differential pressure. Physically speaking, this occurs in two ways.
First, friction between the gas and the flow restrictor’s surface areas (surfaces parallel to the flow) results in a pressure drop that grows linearly with the flow. Secondly, end faces and their edges produce turbulence and therefore a drop in pressure which increases quadratically with the flow. In practice, flow restrictors are always a blend of the two types and therefore their pressure/flow features are always a combination of quadratic and linear components.
Which of the two features prevails is established by the design of the flow restrictor. Usually, a linear characteristic is desirable because it boosts sensitivity at small flows, stabilizes the zero point, and lowers the pressure drop at high flow rates.
Thus, as a general rule of thumb, a pressure drop element should have as much surface exposure parallel to flow as possible and the smallest possible cross section area. Classic circular orifices are not well suited to the job; honeycomb structures are perfect, but expensive. An arrangement of blades, as shown in the figure, has proved to be a simple yet ideal design. It can be easily created using injection molding and its flow/differential pressure feature tends to be close to linear.
Thanks to inertia, most dust particles remain in the primary channel. This anticipated effect can even be considerably enhanced with a good bypass channel design. The tapping ports of the bypass should face backwards so that the gas needs to reverse to get to the sensor. It has also been observed that flow guides adjacent to the tapping ports keep the flow stable and laminar and thus decrease sensor signal noise. Eventually, the tapping ports should be small, preferably with a diameter of 0.6 mm.
Even though flow measurement using a bypass configuration is less sensitive to variations in the inlet conditions, it is still crucial to design the inlet path with care. Preferably, there should not be any sharp bends or edges in the pipe directly upstream of the flow element, and no abrupt variations in the pipe diameter.
Besides that, some form of resistance to the flow (such as a sieve), just upstream of the main flow restrictor and consistently distributed across the whole diameter of the pipe, can help to stabilize turbulence and other unnecessary influences.
Choice of Sensor
With the correct sensor, flow measurement in bypass is the most reliable and cost-effective technique. Microthermal differential pressure sensors, such as those made by sensor manufacturer Sensirion, are ideally matched to achieve the requirements for several reasons.
- The components are small, which helps to keep the size of the bypass small, thus decreasing the space required for flow measurement.
- Flow-based thermal differential pressure sensors have a high level of sensitivity and are highly stable at around zero. It is thus possible to accomplish a very wide measurement range (high dynamic range or high turn-down ratio).
- Regardless of the thermal flow measurement technique, the sensors are calibrated to measure the applied differential pressure. The sensors can thus be substituted and exchanged without recalibration.
- Sensirion provides a precise temperature compensation geared towards flow measurement in bypass.
The latter two points offer an additional advantage. In a number of cases, provided that the key channel has a good design and meets definite production tolerances, there is no need for final calibration of the whole system. The fact that sensors are supplied calibrated and temperature-compensated, and that tolerances in advanced injection molding are small, means that in numerous cases random testing of the pressure drop element will do.
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To measure gas flow with a high degree of repeatability and accuracy, while simultaneously keeping costs low, a bypass or differential pressure solution is usually most favorable. Compared with direct flow measuring methods, the effects of external conditions can be decreased, and the cleanliness of the gas around the sensor element can be considerably increased.
Furthermore, if a thermal differential pressure sensor offering high-level precision even when flow rates are minimal is selected, measurements around the zero point are very precise. In numerous cases, having the sensor calibrated for differential pressure integrated with appropriate temperature compensation will remove the need for extra calibration over the whole measurement range.
This information has been sourced, reviewed and adapted from materials provided by Sensirion Inc.
For more information on this source, please visit Sensirion Inc.