How long should a mercury vapour black light be allowed to cool before attempting to restart after being cut off?

The correct answer is that a mercury vapor black light should be allowed to cool for approximately 5 minutes before attempting to restart after being turned off. This cooling period is important because mercury vapor lights operate at high temperatures, and immediate restart may not only affect the longevity of the bulb but can also create safety hazards.

When the light is turned off, the internal components, including the mercury and electrodes, need time to cool down. This cooling phase helps prevent heat-related damage and ensures consistent performance when the light is turned back on. Allowing sufficient cooling time reduces the risk of thermal stress that can result in premature failure, flickering, or erratic performance of the light.

In comparison, shorter cooling periods might not provide adequate time for the light to return to a safe operating temperature, which is critical for both safety and functionality. Thus, adhering to the 5-minute recommendation is essential for optimal equipment performance in liquid penetrant testing applications.