Reinventing Environmental Sensing with Air-Quality Sensors

Dramatic improvements in high-performance, cheap and easy-to-use sensors for light, pressure, sound, and temperature have led to significant advances in the monitoring field. These improvements are frequently due to the use of microelectromechanical systems (MEMS) technologies. For example, position and motion can now be measured by very small silicon-based accelerometers and gyroscopes.

However, vapor and gas measurements still require lots of development and are still far off from having the ability to sense things to the sensitivity of a human’s nose. Sensors for these compounds, in addition to volatile organic compounds and humidity, are very hard to improve and use. Furthermore, if numerous variables need to be monitored in one place then the issue can be aggravated even more.

Different analog interfaces are commonly used for sensors, including the three-wire voltage, four-wire voltage, and two-wire 4mA to 20mA current outputs. Integrating a set of different sensors can become a challenge for analog input/output (I/O). (Source: TE)

Figure 1: Different analog interfaces are commonly used for sensors, including the three-wire voltage, four-wire voltage, and two-wire 4mA to 20mA current outputs. Integrating a set of different sensors can become a challenge for analog input/output (I/O). (Source: TE)

In the last 10 years, progress has been made, as MEMs and semiconductors have been developed to allow for low-power and cost sensors for gases. Initially, however, these developments were made for application-specific impact and pressure sensors for vehicles airbags.

However, further research has opened up applications for these sensors for pressure, VOCs and CO2, with important uses in calibration, and many other functions.

Trend: Single-Point, Multi-Factor Sensing

These sensors have seen improvements in performance, accessibility, price, and charge, making them more useful for a variety of applications. They can now be used to measure multiple factors in a single location, including VOCs, CO2, humidity and temperature, all on a single-path sensor.

This is especially useful in lighting regulation, building automation, smart and connected houses, air quality measurements, security, motion and presence monitoring, and energy regulation.

Additionally, time to market is reduced, there are fewer costs and fewer interfaces, making them more convenient in commercial, industrial, institutional and residential applications.

This has led to the development of small boards and modules which allow the measurement of several factors in an integrated, calibrated, and easy capacity. For example:

TE Connectivity AmbiMate Sensor Module MS4 Series

TE Connectivity has developed a 16 mm x 30 mm unit within a PC board for fixed applications. It can measure various parameters through a passive infrared detector, such as light, humidity, and temperature. Furthermore, it enables microphone detection of noise and VOC sensing of quality and CO2 conditions.

The AmbiMate Sensor Module from TE Connectivity is a small PC board intended for fixed installations. (Source: Mouser)

Figure 2: The AmbiMate Sensor Module from TE Connectivity is a small PC board intended for fixed installations. (Source: Mouser)

The AmbiMate Sensor Module board is housed in a user-provided enclosure. Included on the board are sensors for (1) temperature; (2) relative humidity; (3) motion; (4) ambient light; (5) audio microphone (optional); (6) VOCs (optional); and (7) CO2 (optional). (Source: Mouser)

Figure 3: The AmbiMate Sensor Module board is housed in a user-provided enclosure. Included on the board are sensors for (1) temperature; (2) relative humidity; (3) motion; (4) ambient light; (5) audio microphone (optional); (6) VOCs (optional); and (7) CO2 (optional). (Source: Mouser)

A single, nominal 3.3 VDC supply is utilized, with only 10 m required to ensure minimal dissipation, high accuracy, and extended life from a battery. Accuracy for the temperature is ±0.3 °C in the 5-50 °C range, and 2% for humidity readings over 5-95% relative humidity (RH) range. The sensors for both processes have 1s update rates.

Gas sensing, however, can detect 0-1187 parts per billion, whereas CO2 readings can be obtained between 400-8192 parts per million, at a 60 second update rate. An interface allows queries and interruptions at any time, with an alarm output sensitivity of -25 to -19 dBV over a frequency range of 100 Hz-10 kHz.

Bosch Sensortec BME680 Integrated Environmental Sensor

VOCs in paints, lacquers, paint strippers, office equipment, glues, adhesives, and alcohol can be detected by the Bosch Sensortec BME680, which contains entrenched adjustment, and can monitor human breath for VOCs and other compounds.

BME680 can detect gas at a very high level of integration and complexity. Throughout its development process, power consumption was limited to make sure it would be both mobile and durable. It, therefore, functions at 1.71-3.6 V.

The Bosch Sensortec BME680 Integrated Environmental Sensor for mobile and wearable devices is a nearly invisible 3 mm × 3 mm metal-lid device, yet it includes an array of sensors, calibration, and I/O functions within the enclosure. (Source: Bosch)

Figure 4: The Bosch Sensortec BME680 Integrated Environmental Sensor for mobile and wearable devices is a nearly invisible 3 mm × 3 mm metal-lid device, yet it includes an array of sensors, calibration, and I/O functions within the enclosure. (Source: Bosch)

To facilitate connectivity between the BME680 and its host processor, the unit includes the three commonly used low-power options, which are the: (a) I2C interface; (b) four-wire SPI; and (c) three-wire SPI. (Source: Mouser)

Figure 5: To facilitate connectivity between the BME680 and its host processor, the unit includes the three commonly used low-power options, which are the: (a) I2C interface; (b) four-wire SPI; and (c) three-wire SPI. (Source: Mouser)

Firstly, an essential gas-sensor hot plate is warmed to a precise temperature, often 200-400 °C, and maintained for a period of time. Then, the resistance of the vapor-sensitive coating of the sensor is analyzed, which is then traced to an equivalent VOC concentration.

Table 1: The detailed data sheet for the BME680 includes a table of its tolerance and accuracy when sensing the most-common VOCs. (Source: Bosch)

Molar Fraction Compound Production Tolerance Certified Accuracy
5 ppm Ethane 20% 5%
10 ppm Isoprene (a.k.a., 2-Methyl-1,3 Butadiene) 20% 5%
10 ppm Ethanol 20% 5%
50 ppm Acetone 20% 5%
15 ppm Carbon Monoxide 10% 2%

 

Conclusion

With the development of integrated MEMs and semiconductor devices, original equipment manufacturers (OEMs) do not need to classify, purchase, integrate, and code numerous sensors from disparate origins. This has enabled the detection of previously difficult VOCs and CO2, with high performance and reliability.

This information has been sourced, reviewed and adapted from materials provided by Mouser Electronics.

For more information on this source, please visit Mouser Electronics.

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