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BME680: VOC Measurement for Air Quality Testing

Learn how the BME680 sensor measures volatile organic compounds (VOCs) for air quality testing. Explore VOC detection methods, sensor calibration, limits, and more.

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BME680: VOC Measurement for Air Quality Testing

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  1. BME680measuring atmospheric volatile organic compound (VOC) concentration Group 7 Natalia M., Matt Y., David K., Zac L.

  2. What is a VOC? Examples: Carbon Dioxide • Organic compounds are molecular compound containing Carbon • VOCs are organic chemical compounds which can become gaseous/vapor under normal indoor atmospheric conditions of temperature and pressure • VOCs is characterized to have an initial boiling point less than or equal to 250° C measured at a standard atmospheric pressure of 101.3 kPa • Since the BME680 can measure VOC concentrations, people can use this sensor to test air quality. This may be useful when doing housework because the BME680 can detect VOCs from paints (ex. formaldehyde), cleaning supplies, glues, and many more everyday items. Formaldehyde

  3. General VOC Measurement • Current VOC sensors are made up of semiconducting metal oxides, conductive polymers, and carbon black-polymer composites • Single VOC measurement: • One of the most accurate ways to measure a specific VOC is to apply a known correction factor in order to to calculate the relative value for the sensor calibration. • To avoid interference, no other VOCs should be present in the studied environment. Although only having one VOC is ideal, this is very hard to maintain for many indoor and outdoor conditions outside of a laboratory. • For greater accuracy the sensor can be calibrated to the target VOC using a calibration gas of the target compound. • Mixed VOC measurement • Used to detect VOCs in uncontrolled environments to determine air quality. • The measurement is used as a relative indicator of the presence and level of VOCs in the environment. • Some VOCs will influence the reading more than others. This is known as cross-interference. Because of this, sensor measurements are only a reference indication to the air quality.

  4. BME680 VOC Measurement • The BME680 measures VOCs via a resistance sensor. “The sensor can have 10 programmable gas sensor heater set-points. A set-point consists of a target heater resistance, heater-on time and optionally an initial heater current” • The gas sensor in the BME680 has two steps: 1) Heating the gas sensor hot plate to a target temperature. This temperature is usually between 200 °C and 400 °C. It must be maintained for a certain amount of time 2) Measuring the resistance of the gas sensitive layer • The gas resistance ADC (Analog to Digital Conversion) value is then readout. The calculation of gas resistance consists utilizes three steps: 1) Reading the gas ADC value and gas ADC range (gas_r&gas_range_r below) 2) Reading the range switching error from register address 0x04 <7:4> (signed 4 bit) 3) Converting the ADC value into gas resistance in units of ohm • The conversion is done as follows: var1 = (1340.0 + 5.0 * range_switching_error) * const_array1[gas_range]; gas_res= var1 * const_array2[gas_range] / (gas_r - 512.0 + var1);

  5. DETECTION LIMITS & Errors • STRANGE DETECTION LIMIT: Although CO2 is a VOC, the BME680 sensor does not detect it (???) • The response time is about < 1 s for new sensors. Various power modes have different response times. Ultra-low power mode can have a 92 s response time. • The sensor-to-sensor deviation: +/- 15% +/- 15 • The typical resolution of gas sensor resistance measurement is approximately 0.08%, with a range of 0.05-0.11% • The RMS noise is about 1.5% • The general operating ranges for the BME680 are as follows: • -40‒+85 °C, 0‒100% r.H., 300‒1100 hPa

  6. Measuring Indoor Air quality • The sensor has a special feature where it can output a calibrated measure called Indoor Air Quality (IAQ) • How it works: “the measure is calibrated according to the general conditions of where the sensor has been running. The sensor will set an IAQ value of 25 for “good” air quality for the area and 250 for a typical polluted air event, typically calibrating these over the last ~4 day sensing period.” • Calibrating against scientific units is very challenging. It requires the use of specialist equipment (costly)

  7. Sources • https://www.remote-research.org/hardware/bme680/ • https://www.bosch-sensortec.com/bst/products/all_products/bme680 • https://ae-bst.resource.bosch.com/media/_tech/media/datasheets/BST-BME680-DS001.pdf • https://www.epa.gov/indoor-air-quality-iaq/technical-overview-volatile-organic-compounds • https://pdfs.semanticscholar.org/7172/f91ae757113bb88bcee992ec1917edce5cac.pdf • Google Image Searches for Pictures

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