For cases where it is desired to measure the level of water in a boiler or steam generator application, a bi-color level gauge is often utilized. The device consists of a set of light emitters such as LEDs, and optical filters, one red and one green, that send light into the level gauge chamber through the water/steam towards the level gauge glass and the observer.
The optical principle at work is that different colors of light (which represent specific frequencies and wavelengths) have a different index of refraction when they pass through different media. When the level gauge chamber contains steam, the green rays of light deviate significantly and as a result, the observer sees red light on the level gauge glass. Similarly, when the level gauge contains water, the red light is deviated from reaching the observer while the green light passes through. Hence the level of water in the boiler can be established as a result of this color change observed from the variation in index of refraction. Green represents liquid water and red represents vapor or steam.
Bi-color level gauges may consist of a long glass or a series of portholes of exposed glass.
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