Calibrating Carbon Dioxide Sensors

Carbon Dioxide (CO2) is an important compound to measure because the concentration helps tell us about the occupancy and ventilation rate in a space. However to accurately determine these metrics, we need the sensor to be accurate.

We conduct calibration efforts in two different types of environments to assess the CO2 sensors onboard the BEVO Beacon:

1. controlled, environmental laboratory chamber
2. mock home environment

For both these environments, the process is the same and shown below:

Environmental Chamber

The environmental chamber doesn't represent a real-world environment, but helps us calibrate these devices with greater ease and confidence.

Each of the four experiments (3 for model generating, 1 for testing) were conducted in the following manner:

1. Start data collection on the Beacons and reference monitor
2. Run for 30 minutes with no source
3. Researcher enters the space (source) at minute 30 and works in a seated position, respiring normally
4. Researcher exits the chamber, closing the door behind them, at minute 60.
5. Run for 30 minutes with no source
6. Door to the chamber is opened slightly at minute 90 to increase the penetration of lab space air
7. 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. 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 CO2 calibration efforts in the environmental chamber.

Mock Home Environment

Since we have the equipment and UTest House, we perform the calibration for CO2 again but now in an environment that the devices are likely to be deployed to. Beacons are setup in the open kitchen area in close proximity to the reference monitor.

Each of the four experiments (3 for model generating, 1 for testing) were conducted in the following manner:

1. Start data collection on the Beacons and reference monitor
2. Run for 15 minutes with only one researcher as the source
3. Inject CO2 from a gas cylinder in an area directly above the Beacons and reference monitor until concentrations on the reference monitor indicate approximately 2000 ppm.
4. Allow the concentration to slowly decay for the remaining experiment time with the researcher still present, but not in close proximity.

During the experiments, four box fans spread around the combined kitchen, living, and dining area to help induce mixing. The ventilation system is turned off so "fresh" air is provided through infiltration only.

Univariate, linear least-squares regression models are created from data gathered during each of the first three experiments and the two model parameters -- intercept and slope -- are averaged across these three experiments. 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 CO2 calibration efforts in the UTest House.