In a Lindberg fire detection system, what happens when the detector tube heats up?

• A. Gas is released and the pressure drops.
• B. Gas escapes reducing volume.
• C. The heating has no effect until a threshold.
• D. Gas is released from the inert gas, increasing pressure, which activates a pressure switch.

Answer: (D)

In this type of detector, the element is a sealed tube filled with inert gas at a small overpressure. When the tube is heated by heat from a fire, the gas expands and/or is released from its reservoir inside the detector, causing the pressure inside the tube to rise. That pressure increase acts on a diaphragm or piston that drives a pressure switch; once the gas pressure reaches the preset threshold, the switch closes and a fire warning signal is sent. So the key idea is that heating raises internal pressure by releasing gas, and this pressure rise activates the switch to trigger the alarm.