There are many different methods for measuring the gas flow. With some there is no actual contact between gas and sensor; these are, however, relatively expensive and thus out of the question for many applications. With differential pressure methods that gauge the pressure drop through mechanical deflection of the sensor diaphragm over an orifice, hysteresis effects and fatigue of the diaphragm lead to drift problems and a lack of zero-point accuracy.
Thermal measuring methods
Consequently, measuring techniques based on thermal principles are widespread. In the simplest of these, the hot-wire anemometer, the gas flow is determined via the rate of cooling of an electrically heated wire with a temperature dependent resistance. Advanced methods use a heating element and at least two temperature sensors which measure the transport of heat through the gas (see Fig. 1). These sensor elements determine the gas flow more precisely than traditional hot-wire anemometers, while a glass coating of the sensor element provides corrosion-resistance.
We refer to the term "microthermal flow sensors" when the sensor elements are integrated in a millimeter-scale microchip. Besides their small size, another decisive advantage is that they are produced using standardized manufacturing processes, thus enabling mass production with a consistently high quality and moderate unit costs. It is for these reasons that they have become the predominant sensor type used for demanding automotive, medical, and HVAC applications.
But direct contact with the gas can pose challenges of its own for thermal sensors: Seeing as the flow speed measurement is only selective, extrapolation of the flow speed is crucial. However, the velocity distribution in the pipe depends on the inlet conditions. A bend in the pipe immediately before the sensor, different types of surface structure inside the pipe, or corners and edges in the flow duct will alter the measurement result. Moreover, heavily polluted air can lead to soiling of the measuring cell.
Measuring in a round-about way: Bypass solution
A good way to tackle such challenges is to place the sensor chip in a bypass. In this case, an orifice, a Venturi nozzle or blades generate a differential pressure which guides a small proportion of the gas through a bypass duct (see Fig. 2). The microthermal flow sensor guarantees high accuracy, reproducibility, and stability, particularly in the case of very low flow rates. A well designed pressure drop element in the bypass ensures that the resulting differential pressure is less susceptible to changes caused by the inlet conditions. By using inertia effects and minimizing the bypass flow it is possible to ensure that only clean gas reaches the sensor chip.
Moreover, this type of bypass solution helps to simplify the manufacturing process. As the gas flow element can be manufactured independently of the sensor; with the sensor being installed at the end of the process. Under the premise of a well-conceived design and exact orifice tolerances, it is often possible to forgo final calibration of the entire system. Certain points have to be considered for this:
The orifice's job is to slightly increase the resistance in the gas flow and, as a result, generate a differential pressure. Physically speaking, this happens in two ways: Firstly, friction between the gas and the orifice's surface areas (surfaces parallel to the flow) leads to a drop in pressure that increases linearly with the flow. Secondly, end faces and their edges create turbulence and thus a drop in pressure which increases quadratically with the flow. In practice, orifices are always a mix of the two types and thus their pressure/flow characteristics are always a combination of linear and quadratic components (see Fig. 3).
Which of the two characteristics prevails is determined by the design of the orifice. Usually a linear characteristic is preferable because it increases sensitivity at small flows, stabilizes the zero point, and reduces the drop in pressure at high flow rates.
An orifice should therefore have as much surface exposure parallel to flow as possible and the smallest possible cross section area. Traditional circular orifices are not particularly well suited to the job; honeycomb structures are ideal, but expensive. An arrangement of blades as shown in Fig. 4 has proved to be a simple yet suitable design. It can be easily produced using injection molding.
Thanks to inertia, there are less dust particles in the bypass than in the main duct. The number of particles can be reduced even further when the tapping ports of the bypass face backwards so that the gas needs to rotate more than 90° to reach the sensor. Guide blades upstream of the tapping ports keep the flow stable and laminar and thus reduce sensor signal noise. And, finally, the tapping port should be kept small, ideally measuring 0.6 mm in diameter.
Even though flow measurement using a bypass method is less sensitive to changes in the inlet conditions, it is still important to design the inlet path with care. Ideally, there should be no sharp bends or edges in the pipe directly upstream of the measuring point, and no abrupt changes in the pipe diameter. Apart from that, some form of resistance to the flow, such as a sieve upstream of the sensor, distributed evenly across the entire diameter of the pipe, can help to stabilize turbulence and other undesired influences.
Choice of sensor
With the right sensor, flow measurement in the bypass is the most reliable and cost-effective method. Differential pressure sensors are ideally suited to meet the requirements: Their dimensions are small which helps to reduce the space required for flow measurement. The currently smallest differential pressure sensor on the market is the SDP3x from Sensirion. The sensor measures a mere 5 x 8 x 5 mm and can be installed in applications which previously lacked sufficient space for any sensory technology. It therefore opens up countless new integration and application possibilities, for example for inhalers. Despite its small size the SDP3x offers a high level of sensitivity together with no zero-point drift. It is thus possible to achieve a very wide measurement range. Its specific temperature compensation is geared towards the conditions of bypass flow measurement: It ensures the flows can be measured correctly over the entire temperature range. Consequently, no further temperature compensation is required for converting the differential pressure output signal to mass or volume flow. And the complicated process of characterizing the bypass system is not necessary. Final calibration of the entire system can often be dispensed with. The sensor can be reflow soldered, offers new functions such as multiple I2C addresses or interrupt functions, and has a very fast response time of 2 kHz with 16-bit resolution. All these factors make the SDP3x the perfect choice for high-volume but cost-sensitive applications that demand accurate gas flow measurement.