On occasion, you may notice a 'step' appearing in your CO2 graph within the Tether portal.
This is the CO2 sensor recalibrating itself.
This will usually happen within the first two weeks of receiving a Tether device. Before a device is shipped to you it sits in a warehouse unpowered. This can cause the calibration of the CO2 sensor to drift. During the first two weeks of operation the CO2 sensor will constantly search for "fresh air" - which is around 400ppm. After two weeks the CO2 sensor will recalibrate itself and this can often result in a drastic sudden spike in a CO2 reading.
To force the Tether sensor to calibrate itself, take the sensor outside and press and hold the reset button for 5 seconds until a solid light appears. This will tell the sensor that the air is fresh/outside air (400ppm) and it will calibrate itself to that reading.
It is recommended to perform this action before you install a Tether sensor.
The readings taken before the first calibration will usually be high and not accurate. This is why it is recommended to reset the Tether device outside before installing.