This helpful tool is installed on the outside liquid line (3/8" copper line) also called the high side of your a/c system. The glass window gives you an indication as to your a/c system performance related to your refrigerant. Under normal operating conditions the small glass window should appear clear or display a green dot. If bubbles form in the window it is indicative of low refrigerant or another potential system issue such as a leak. Along the perimeter of the glass window is a color chart which indicates moisture in the system. Under normal operating conditions it should display green in color. A yearly checkup of your a/c system by a licensed HVAC contractor is highly advised; the sight glass is only an indication of a potential problem; your system may still have other issues and cleaning requirements.