Look closely at a sprinkler head and you'll see one of two thermal mechanisms: a colored glass bulb or a fusible link between two metal plates. This thermal element determines when the sprinkler activates — wrong temperature selection degrades fire performance and increases maintenance costs. NFPA 13's thermal classification system clarifies the selection.
Glass Bulb Color Codes
The liquid color inside the glass bulb tells you the sprinkler's operating temperature. NFPA 13's standard code:
| Color | Temperature (°C) | Temperature (°F) | Class |
|---|---|---|---|
| Orange | 57°C | 135°F | Ordinary |
| Red | 68°C | 155°F | Ordinary |
| Yellow | 79°C | 175°F | Intermediate |
| Green | 93°C | 200°F | Intermediate |
| Blue | 141°C | 286°F | High |
| Purple | 182°C | 360°F | Extra High |
| Black | 227-260°C | 441-500°F | Very Extra High |
This color system standardized in the 1970s and is identical worldwide. A firefighter seeing a blue glass bulb in a Turkish warehouse instantly knows it'll activate at 141°C.
Response Time Index (RTI)
How fast a sprinkler responds is a separate measure: Response Time Index (RTI). Unit: (m·s)^0.5. Lower RTI = faster response.
| Type | RTI (m·s)^0.5 | Typical Application |
|---|---|---|
| Quick Response (QR) | < 50 | Light Hazard: hotel, residential, office |
| Standard Response | 80-350 | Ordinary Hazard: manufacturing, retail |
| ESFR | < 50 (special design) | High-piled storage |
Quick response advantage: opens early and suppresses/controls the fire at its start. Disadvantage: slightly more sensitive to heat accumulation (summer air heating, etc.), so slightly higher false-activation risk. NFPA 13 recommends QR for Light Hazard but doesn't mandate it.
Temperature Selection Logic
A sprinkler's operating temperature should be at least 30°C (50°F) above the expected maximum ambient temperature at the ceiling. This reduces false discharge risk. Examples:
- Office (ceiling 27°C): 68°C red bulb (27 + 30 = 57 < 68 ✓)
- Attic (summer 48°C): 93°C green (48 + 30 = 78 < 93 ✓)
- Boiler room (steady 80°C): 141°C blue (80 + 30 = 110 < 141 ✓)
- Industrial oven vicinity (100°C): 182°C purple (100 + 30 = 130 < 182 ✓)
Temperature Selection in Turkey
Turkish attic temperatures can reach 55-60°C in summer. Putting standard 68°C bulbs in an attic increases summer accidental activation risk. In Turkey's southwestern cities (Antalya, Muğla, Mersin), attic sprinklers should use 79°C yellow or 93°C green for safety.
Fusible Link
An older sprinkler type but still current. Instead of a glass bulb, two metal pieces held by low-melting-point solder. As heat rises, the solder melts, the pieces separate, and the sprinkler opens. Advantage: more tolerant of mechanical shock. Disadvantage: can't hold high loads at low temperatures (weaker).
Sprinkler thermal data in SprinkCalc
Glass bulb temperature color codes, RTI classes, and sprinkler selection in SprinkCalc's info modules.
Learn MoreThree Common Mistakes
- Single temperature throughout the building: Using the same 68°C bulb in office + kitchen + boiler room. False activations happen in the kitchen.
- Low temperature in vacation properties: Coastal resort hotels need 79°C instead of 68°C.
- Confusing body color with bulb color: Sprinkler body color (chrome, brass, white) and glass bulb color are entirely separate.
Conclusion
Sprinkler thermal characteristics look simple, but they underlie 70% of false discharge events. Understanding the space, measuring maximum temperatures, and applying NFPA 13's ambient + 30°C rule is critical. If 4-5 different temperature classes are used in a building, the spare cabinet must cover this diversity.
Core reference: NFPA 13 - Installation of Sprinkler Systems (Chapter 6 Sprinklers). Original NFPA post: NFPA Today - Sprinkler Thermal Characteristics.