A sight glass is a device which allows people to see the level of fluids or gases in a system, or to look inside a closed system to see what is happening inside. Glass panels which allow for internal views are sometimes referred to as viewing glasses. Sight glasses are used on many different types of equipment and they add to the safety and ease of operation. Like other equipment parts, a sight glass is replaceable if it becomes damaged or compromised in some way. A very simple sight glass can consist of a glass tube which is linked with the contents of something such as a tank. The level of fluid in the tube indicates the level of fluid in the tank. The back of the sight glass may be patterned or striped to make it easier to see the fluid level. Larger tubes may also offer people an opportunity to identify contaminants in the fluid which might interfere with the function of the device.