Level DetectorsLEVEL DETECTORSAnother type of gauge glass is the reflex gauge glass (Figure 3). In this type, one side of theglass section is prism-shaped. The glass is molded such that one side has 90-degree angles whichrun lengthwise. Light rays strike the outer surface of the glass at a 90-degree angle. The lightrays travel through the glass striking the inner side of the glass at a 45-degree angle. Thepresence or absence of liquid in the chamber determines if the light rays are refracted into thechamber or reflected back to the outer surface of the glass.Figure 3 Reflex Gauge GlassWhen the liquid is at an intermediate level in the gauge glass, the light rays encounter anair-glass interface in one portion of the chamber and a water-glass interface in the other portionof the chamber. Where an air-glass interface exists, the light rays are reflected back to the outersurface of the glass since the critical angle for light to pass from air to glass is 42 degrees. Thiscauses the gauge glass to appear silvery-white. In the portion of the chamber with thewater-glass interface, the light is refracted into the chamber by the prisms. Reflection of the lightback to the outer surface of the gauge glass does not occur because the critical angle for lightto pass from glass to water is 62-degrees. This results in the glass appearing black, since it ispossible to see through the water to the walls of the chamber which are painted black.A third type of gauge glass is the refraction type (Figure 4). This type is especially useful inareas of reduced lighting; lights are usually attached to the gauge glass. Operation is based onthe principle that the bending of light, or refraction, will be different as light passes throughRev. 0 Page 3 IC-03
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