Calibrating Total Volatile Organic Compound Sensors
The Total Volatile Organic Compound (TVOC) sensors required a more simplistic approach to calibrate because of their varied sensitivity to different compounds. Rather than calibrate these sensors by comparing to a reference or through the use of a gas standard, we instead calibrated the sensors to each other to ensure they were reporting similar values. We conduct calibration efforts in only the controlled, environmental laboratory chamber.
We use TVOC data gathered during calibration efforts for CO2 since the researcher we use as a source for CO2 also works as a source for VOCs. Most TVOC sensors are used in occupancy detection and therefore are sensitive to compounds that come from exhaled breath.
Each of the four experiments (3 for model generating, 1 for testing) were conducted in the following manner:
- Start data collection on the Beacons and reference monitor
- Run for 30 minutes with no source
- Researcher enters the space (source) at minute 30 and works in a seated position, respiring normally
- Researcher exits the chamber, closing the door behind them, at minute 60.
- Run for 30 minutes with no source
- Door to the chamber is opened slightly at minute 90 to increase the penetration of lab space air
- Experiment ended at minute 120
During the experiment, one rotating house fan is used to induce mixing and the ventilation system is not active -- the only ventilation provided to the space is by infiltration through small cracks between the double doors.
Univariate, linear least-squares regression models are created for each of the first three experiments and the two model parameters -- intercept and slope -- are averaged across these three experiments. We used the averaged output from all devices we tested as the reference concentration. The models are then applied to data from a fourth experiment to assess the model's performance. The image below represents an example outcome from the TVOC calibration efforts in the environmental chamber.
