How a Fire Sprinkler Works: The Glass Bulb
At the heart of all modern fire sprinklers is a heat-sensitive glass bulb. The bulb is filled with a glycerin based liquid dyed different colors depending on temperature and sealed leaving one small vapor bubble remaining. During a fire ceiling temperatures rise rapidly. The liquid absorbs the heat until it exceeds a specific temperature– most commonly 155° Fahrenheit – at which time the vapor shrinks and the liquid expands until the bulb breaks and a gush of flame-retarding water rushes past.
The time it takes for a bulb is dependent on temperature and bulb size. Bulbs are produced in two common sizes: 3 millimeter (mm) diameter or Quick Response and 5mm or Standard Response. A 3 mm bulb will break faster than a 5 mm bulb, though the time is measured in milliseconds and there is no discernible safety risk from using one versus the other.
Fire Sprinklers are commonly referred to by their bulb’s maximum temperature and are color coded.
  • Orange: 135°F (58°C)
  • Red: 155°F (68°C)
  • Yellow: 175°F (80°C)
  • Green: 200°F (94°C)
  • Blue: 286°F (141°C)
  • Purple: 360°F (182°C)
This image shows the various bulbs with easily marked temperature ratings below:
Fire Sprinkler Glass Bulbs

Determining the proper temperature requires consideration of ceiling temperatures, room size, and response time. But we’ll get into that in a future post.

fire sprinklers, temperature, glass bulb