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How should pressure and flow sensors be selected?

Both pressure sensors and flow sensors can be used to measure the flow rate of air.

In many applications, both types of sensors are usually used in combination with flow-limiting devices to generate a pressure difference. Some "air flow" sensors are called "differential pressure" sensors because of their calibration methods rather than based on their internal technologies. The following explanations are intended to clarify the differences between these two types of sensors, explain their distinctions, and indicate which type is more suitable for specific applications.

What is an air flow sensor?

In the simplest terms, an air flow sensor, more accurately known as an air mass flow sensor, is a device with two pressure ports, from which gas flows to the other port (see Figure 1). Inside the sensor, there is an induction element with a heated surface. When the gas flows through the sensing element, heat is transferred from the upstream to the downstream. This generates a thermal imbalance proportional to the mass of the flowing material, which can be measured by electronic circuits.

It is important to remember that the sensor measures the mass flow rate under standard conditions, not the actual volume of gas passing through. Although most sensors compensate for the influence of temperature, changes in atmospheric pressure can affect the density of gases, thereby influencing the output results. In addition, mass flow sensors must be calibrated for specific gas mixtures because different gases have different thermal properties.

Calibrate the mass flow sensor so that its output is proportional to the pressure drop between the two ports, because it is precisely this pressure drop that drives the flow through the sensor. This might cause some confusion because these sensors are usually sold as differential pressure sensors, while their internal technology is actually measuring flow.

What is a differential pressure sensor?

Traditional differential pressure sensors also have two pressure ports; However, there is no gas flow between these two ports. On the contrary, there is a MEMS diaphragm between the two ports for measuring the pressure difference. The deflection of the diaphragm is measured by the piezoresistive device implanted in the silicon wafer, and the electronic circuit converts this into an output signal.

The main differences between pressure sensors and air quality flow sensors

Flow path

The most obvious difference between pressure flow sensors and mass flow sensors lies in the presence or absence of gas flow paths. In order for the mass flow sensor to work properly, there must be gas passing through it. Any restrictions in the flow channel, such as dirt or liquid, will change the aerodynamic resistance, thereby affecting the output. In contrast, the pressure sensor is a "dead end". The only gas flow in its pipeline system is a small amount of gas caused by the compression or expansion of gas under high pressure. The dirt or liquid in the pipeline system will only cause output differences when the pipeline is almost completely blocked. The contamination in the flow channel eventually adheres to the inner surface of the mass flow sensor and may also affect the heat transfer to the sensing element, thereby influencing the output.

An air flow sensor should only be used when the gas passing through it contains no pollutants.

Qualitative and resolution

Because the mass flow sensor is a thermosensitive device, it is more stable than the stress-based pressure sensor at zero flow (or zero pressure difference). However, the above-mentioned failure mode will affect the slope of the sensor output. All failure modes of the pressure sensor tend to affect the zero pressure offset of the equipment. The slope of the pressure sensor seldom changes. In addition, the output of the sensing element of the mass flow sensor at low flow rates is higher than that at high flow rates. This means that even if the output has been corrected to a linear signal, the resolution of the mass flow sensor at extremely low flow rates will still be better than that at high flow rates. The output of the pressure sensor is naturally close to linear within its working range, so the resolution will not change.

Compared with equivalent pressure sensors, mass flow sensors have better resolution and stability at very low flow rates.

Anti-pollution property

Contamination in the flow channel can affect the output of the mass flow sensor in various ways. Even if a very thin layer of liquid or dirt forms on the surface of the sensing element, it will interfere with heat transfer and cause slope errors. In addition, if the sensor is used in a bypass configuration, as mentioned earlier, any factor that increases the flow resistance in the pipeline will affect the measurement results. When the pipeline is clogged, additional pressure is required to allow the same flow rate to pass through, which will change the relationship between flow rate and pressure. In contrast, there is almost no air flow in the pipeline of the differential pressure sensor. The only movement is a small amount of air intake and exhaust to generate pressure changes. Severely clogged pipelines may cause frequency response problems in high-frequency applications; However, the output of the sensor will be correct. By simultaneously using pressure sensors and mass air flow sensors for the same measurement, an almost foolproof system can be created. Since most failure modes in pressure sensors will affect the offset, while most modes in flow sensors will affect the slope, it is unlikely that these two devices will fail simultaneously in the same way.

The slope of the pressure sensor will be more stable than that of the mass air flow sensor and is less likely to be affected by contamination.

Zero-point automatic calibration technology

Automatic zeroing is a pressure sensor calibration technology based on sampling output under known reference conditions, which allows for additional correction of external output errors, including offset errors, offsets caused by thermal effects (offset changes), and offset drift. If this technology can be implemented in applications, it will be a simple method to gain the advantages of pressure sensors while avoiding the problems of mass flow sensors.

Power consumption

The heater in the mass flow sensor requires electricity to function properly and needs a short period of time to preheat and stabilize. In contrast, the simple resistance Wheatstone bridge in most pressure sensors consumes much less current and can stabilize rapidly. A typical flow sensor may require a current of 10 mA to 15 mA, while a pressure sensor of the same performance only needs 2 mA. The output of a pressure sensor usually remains stable within a range of 2 ms or less, while a flow sensor may require 35 ms. This greatly reduces the effectiveness of the power supply cycling strategy adopted for energy conservation.

Pressure sensors are usually preferred in low-power applications.

Frequency response

The sensing element of the pressure sensor is a mechanical diaphragm. It usually has a frequency higher than 10 kHz. In practical applications, the sensor response is usually limited to approximately 1 kHz provided by electronic devices. In contrast, airflow sensors respond more slowly to rapidly changing airflows and tend to average out the rapid changes - recall the difference in preheating times. It is slightly more difficult to accurately quantify the frequency response of the mass flow sensor. However, in most cases, it may be lower than 100 Hertz. This difference may affect the performance in the application.