Glass Bulbs contain a fluid that expands with rising temperatures. When the nominal temperature is reached, the internal pressure increase bursts the glass bulb into small particles, thus actuating the sprinkler. Glass bulbs can be adjusted for any actuating temperature up to 260°C / 500°F by calibrating their filling level. For the most common actuating temperatures a standardized colour code has been established. Norbulbs always actuate within ±3.5% of the nominal release temperature.