Overview
When using Awair devices, you may have noticed that your TVOC values change over time when compared to previous readings and other air quality monitors, even when placed in the same space. Let's explore why TVOC sensor measurements are not consistent and learn about the operating principles of the most widely used Metal Oxide Semiconductor (MOS) sensors.
What is TVOC?
First, let's briefly explain what TVOC means: TVOC stands for Total Volatile Organic Compounds, which refers to the total amount of various volatile organic compounds present in indoor air. TVOC includes hundreds of compounds such as formaldehyde, benzene, and toluene. These substances can be emitted from new furniture, paint, cleaning products, and even during cooking processes. Other compounds detected by our monitors are listed here.
How is TVOC Measured?
While various measurement methods exist, Awair devices use Metal Oxide Semiconductor (MOS) type sensors.
The measurement method typically compared with MOS type sensors is the PhotoIonization Detector (PID) type sensor.
MOS type sensors have lower absolute measurement accuracy compared to PID type sensors, but they offer superior durability with no need for separate maintenance, and are smaller in size, enabling miniaturization and very low cost. They even respond to various VOC gases, making them suitable for general home environments.
Basic Structure and Operating Mechanism of MOS Type Sensors
The core of MOS type sensors is a metal oxide layer (mainly tin oxide, tungsten oxide, zinc oxide, etc.), which acts as the sensing element.
1. Heating Stage: For the sensor to react to VOC gases, an internal heater heats the metal oxide layer to approximately 200-400°C (392-752°F). At this temperature, oxygen molecules are adsorbed onto the metal oxide surface, attracting electrons.
2. Clean Air Reaction: In clean air conditions, oxygen is adsorbed onto the sensor surface, increasing oxygen molecules and raising the sensor's electrical resistance.
3. Gas Reaction: When VOC gases are present, these gas molecules react with oxygen molecules on the sensor surface, reducing electrical resistance.
4. Signal Conversion: Resistance changes are converted into electrical signals and transmitted to the microprocessor.
5. Data Processing: The microprocessor interprets these signals using algorithms and converts them into TVOC concentrations in ppb (parts per billion) units.
Simply put, MOS sensors use the principle that "the more organic compounds in the air, the better electricity flows." They estimate TVOC concentrations by measuring changes in electrical resistance.
Main Reasons Why TVOC Values Differ
1. Limitations of Integrated Detection Method
The characteristic of MOS sensors being able to react to various gases, as mentioned in the measurement methods above, can become a disadvantage in situations requiring precise measurement.
MOS sensors cannot distinguish gases individually. Due to this characteristic, measurement values can be higher or lower than theoretical values depending on the types of gases present in the air.
For example, if formaldehyde: 100 ppb, ethanol: 250 ppb, and toluene: 150 ppb exist in the air, the TVOC value should be 100 + 250 + 150 = 500 ppb, but the actual sensor might measure 300 ppb.
This is because sensors react differently to each gas type, but since the sensor doesn't know what types of gases are present, it can only make measurements based on the changed electrical signals after reacting. This can cause significant differences in measurement values between different sensor manufacturers.
Awair devices are calibrated based on ethanol gas and calculates and outputs TVOC values using proprietary algorithms based on the ethanol gas values measured by the sensor.
2. Inter-sensor Variation
As mentioned earlier, Awair devices are calibrated based on ethanol gas. However, even within the same ethanol concentration, measurement errors occur between devices.
Based on ethanol gas 0.3 ppm = 300 ppb, a standard ±15% error occurs. And as concentrations increase, inter-device measurement errors can become larger.
This is the guaranteed error range for ethanol gas only, and larger errors can occur in real indoor environments where it exists as a mixture with other gases.
To reduce these basic inter-sensor errors, we provide additional sensitivity calibration through customer support.
3. Sensor Drift
Over time, the sensor's reference point changes, a phenomenon called 'drift.' A representative example is zero drift.
This is similar to how the zero point of an analog scale changes over time, requiring you to reset the zero point each time before weighing.
Awair devices have a built-in sensor calibration algorithm to resolve zero drift phenomena.
Sensor Calibration Algorithm
We want sensors to always react consistently over time. As explained earlier, in clean air conditions, resistance values remain high, but when VOC gases are detected, resistance values decrease.
However, during use, various factors cause the resistance value (baseline) in clean air conditions to rise. This means that if the value was 200 in clean air conditions before, it could become 300 in the same clean air conditions over time. These changes vary depending on the usage environment but can occur as quickly as one month later.
What would happen if the baseline changed but we maintained the previous baseline? It would inevitably be calculated as progressively lower values compared to before, and eventually, VOC gases could not be detected.
However, by automatically readjusting the baseline, there are no problems detecting VOC gases even as time passes.
However, through this process, values may differ between devices or gradually increase when used independently. This is because various real-world environmental factors have complex effects on the calibration process.
Conclusion
The inconsistency of TVOC values results from the complex interaction of natural changes in indoor environments and the technical limitations of MOS sensors. Once you understand how sensors operate, you can better understand the variability in their measurements.
While perfect measurement is difficult, understanding these limitations and interpreting them appropriately allows these sensors to still be useful tools for indoor air quality management.
The important thing is not to become too fixated on single measurement values, but to observe overall trends and patterns. It's better to understand TVOC sensors not as 'perfect measuring instruments' for our indoor environment, but as 'indicators that show trends.'