How should I use the device correctly?

When you first start testing, as the meter is just energized, the electrochemical sensor may have a transient reaction during warming up period, causing the value to rise within a few seconds. And it will the correct reading after the sensor is stable. The correct detecting method is to monitor for 5 minutes per test point and perform multiple points in one room, then average the number of points, which will be the average content in the air of the room.