When a glass bulb is tested in the Operating Temperature (Bath) Test, a partial fracture might occur. A partial fracture indicates that the glass bulb breaks and the liquid inside leaks out, while the bulb body remains in the sprinkler frame, shown in the figure below.

A glass bulb with partial fracture shall be indicated as operated, even if the operating time and temperature is not recorded.

According to UL standard, a partial fracture of a glass bulb occurring during the Operating Temperature (Bath) Test is a normal phenomenon. When a partial fracture of a glass bulb occurs, additional sprinkler samples with glass bulbs shall be tested by the Air Bath Test. In the Air Bath Test, no partial fracture shall occur, and each sample shall be examined for full operation.

The test procedure below is referred from UL 199 Standard:

UL 199 STANDARD FOR SAFETY: Automatic Sprinklers for Fire-Protection Service

32 Air Bath for Glass Bulb Sprinkler Test

32.1 When a partial fracture of a glass bulb occurs during the Operating Temperature (Bath) Test Section 31, sprinklers with a glass bulb heat responsive element shall fully operate when subjected to the air bath test described in 32.2.

32.2 Fifty sample sprinklers with a glass bulb heat responsive element shall be placed on their inlet in a programmable circulating air oven. The temperature in the oven shall be gradually increased to 20 ± 2°F (11 ± 1.1°C) below the marked temperature rating of the sprinklers. When this temperature is reached, the oven shall be maintained at a constant temperature for a period of 60 ± 5 minutes. The temperature shall then be increased at a constant rate of 1 ± 0.5°F (0.5 ± 0.3·C) per minute until the temperature in the oven is 25 percent higher than the marked temperature rating of the sprinklers or until all the sprinklers operate whichever occurs first. Each sample shall be examined for full operation.