Indoor sprinkler systems are not just a critical firefighting device, they're also a valuable tool for screenwriters who can't think of any other way to resolve a scene. When in doubt, have the guy hold a lighter to the overhead sprinkler valve, then evacuate the building. In terms of plot contingency cliches, they're second only to air ducts.

But do sprinklers really work like that? Not really, as Trace Dominguez explains in today's DNews special.

Modern fire sprinklers typically use a heat-activated glass bead filled with liquid or a color-coded piece of metal. Inside the bead is a glycerin-based liquid which expands when heated to a specific temperature. The expanding liquid breaks the glass, triggering the release of water through the overhead pipes.

Most fire suppression systems of this type are strictly local. Triggering the sprinkler in an office or hotel room won't cause the entire system to go off. There are exceptions, though. So-called deluge systems will turn on all sprinklers at once, but they're typically reserved for places where fires could get serious fast, like power plants or airplane hangers.

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Industrial facilities like these typically have advanced systems anyway, and depending on the fire risk involved, might not use water at all. Some suppression systems use inert gas or foam designed for specific kinds of fires or sensitive areas.

When a water-based sprinkler is in place, it may be set up as a dry system in which nothing is in the pipes at all until a vacuum balance is upset. Dry systems are usually deployed in places where temperatures drop low enough to freeze water in the pipes. They're also used in museums, historical sites or anywhere else where a leak or accidental alarm could damage property.

You didn't ask but we're telling you anyway: The first proto-sprinkler systems, built for 19th century textile, simply used a network of perforated tubes suspended overhead. In the event of fire, water was manually poured into the pipes. Not that much different, really, from what we have now.

-- Glenn McDonald