What the manufacturer of the glass bulb said is that the amount of air is the determining factor as to what temperature the bulb will activate. The larger the air bubble the higher the temperature needed for activation. I could not get from them the amount of air they use. What the manufacturer of the glass bulb was trying to say is that the bulb is designed the where the liquid in the bulb expands due to heating the expansion is absorbed by the compression of the air bubble. Once the air bubble is completely compressed all the force from the expansion is then transferred to the glass. If there is any damage to the glass, such as a scratch, then the glass will fail at a lesser pressure. Both TYCO and Viking said this is why the sprinkler head packaging changed in the mid 90’s. According to them they were having a large number of premature activations. They found the bulbs were being scratched during shipping, handling and cleaning after instillation. Now each head must be separated during shipping. Each head must have a plastic protective covering over the glass bulb until the time it is installed and placed in service. Also, that had information added to the code that the only accepted cleaning method was the used of compressed air or a vacuum. If the head is touched it must be replaced.

What the manufacturer of the glass bulb said is that the amount of air is the determining factor as to what temperature the bulb will activate. The larger the air bubble the higher the temperature needed for activation. I could not get from them the amount of air they use.

The study was conducted by Exponent and Viking Sprinkler Company. The information I have on their study is, Delayed Fracture of Glass Bulbs Used in Fire Sprinklers, Roland Huet, Scott Martorano, Nicoli Ames, Fire Technology, Volume 53, Pages 629-647, 2017. What was interesting from a general point of view is that they took the time to locate stress point on the bulb to where they believed the failure due to the expansion of the liquid would take place.

I agree with you about the first test of the hypothesis is to determine if a scratch or a cleaning compound with effect the operation of the glass bulb. Unfortunately, I have been unable to find any specific testing or study relating to this exact issue. I was hoping by posting on the form that’s some one would have some knowledge of a study.

Jim Mazerat
Forensic Investigations Group