An Mhz-19b CO2 sensor was mounted in the center of a high school classroom and 1 meter above the ground in accordance with ISO 1600-26. Readings from the sensor were logged to an SD card with an Arduino at a sample rate of about 1 read per 30 seconds. The readings were plotted for each day of measurement, and were collected for a week. Results showed CO2 concentrations to regularly exceed 1500ppm, and sometimes exceed 2000 ppm. Below is an example of the CO2 concentrations in the classroom over the course of a day.