Dyne's Lab Manager, Grant Lobdell's article When is Loading Detrimental to Sprinkler Performance? was published in the Sept/Oct issue of Sprinkler Age Magazine. 

Excerpt from the article:

The 2017 edition of the National Fire Protection Association (NFPA 25) “Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems,” Section 5.2.1.1.1 states that any sprinkler that shows signs of “loading detrimental to sprinkler performance” during the annual floor level inspection shall be replaced. Figure 1 shows an example of a previously installed sprinkler with loading.  Is that loading detrimental to the sprinkler’s performance?  The concern with loading is that it could insulate the thermal element, reducing the heat transferring to it and thus affecting release time.  Using a plunge oven test method similar to those described in Underwriters Laboratories Standard 199 - “Automatic Sprinklers for Fire-Protection Service” and Factory Mutual 2000 Approval “Standard for Automatic Control Mode Sprinklers for Fire Protection,” Dyne Technologies, LLC measured the response times of various sprinklers with different amounts of loading to help give an idea of how sprinkler thermal sensitivity is affected and when performance is no longer acceptable.