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Figure A1
A very simple means by which liquid level is
measured in a vessel is by the gauge glass method (Figure
A1). In the gauge glass method, a transparent tube is
attached to the bottom and top (top connection not needed in
a tank open to atmosphere) of the tank that is monitored. The
height of the liquid in the tube will be equal to the height
of water in the tank.
Figure A1 (a) shows a gauge glass which is
used for vessels where the liquid is at ambient temperature
and pressure conditions. Figure A1 (b) shows a gauge glass
which is used for vessels where the liquid is at an elevated
pressure or a partial vacuum. Notice that the gauge glasses
in Figure A1 effectively form a "U" tube manometer
where the liquid seeks its own level due to the pressure of
the liquid in the vessel.
Gauge glasses made from tubular glass or
plastic are used for service up to 450 psig and 400°F. If it
is desired to measure the level of a vessel at higher
temperatures and pressures, a different type of gauge glass
is used. The type of gauge glass utilized in this instance
has a body made of metal with a heavy glass or quartz section
for visual observation of the liquid level. The glass section
is usually flat to provide strength and safety. Figure A2
illustrates a typical transparent gauge glass.
Figure A2
Another type of gauge glass is the reflex
gauge glass (Figure A3). In this type, one side of the glass
section is prism-shaped. The glass is molded such that one
side has 90-degree angles which run lengthwise. Light rays
strike the outer surface of the glass at a 90-degree angle.
The light rays travel through the glass striking the inner
side of the glass at a 45-degree angle. The presence or
absence of liquid in the chamber determines if the light rays
are refracted into the chamber or reflected back to the outer
surface of the glass.
Figure A3
When the liquid is at an intermediate level
in the gauge glass, the light rays encounter an air-glass
interface in one portion of the chamber and a water-glass
interface in the other portion of the chamber. Where an
air-glass interface exists, the light rays are reflected back
to the outer surface of the glass since the critical angle
for light to pass from air to glass is 42 degrees. This
causes the gauge glass to appear silvery-white. In the
portion of the chamber with the water-glass interface, the
light is refracted into the chamber by the prisms. Reflection
of the light back to the outer surface of the gauge glass
does not occur because the critical angle for light to pass
from glass to water is 62-degrees. This results in the glass
appearing black, since it is possible 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 A4). This type is especially useful in areas of
reduced lighting; lights are usually attached to the gauge
glass. Operation is based on the principle that the bending
of light, or refraction, will be different as light passes
through various media. Light is bent, or refracted, to a
greater extent in water than in steam. For the portion of the
chamber that contains steam, the light rays travel relatively
straight, and the red lens is illuminated. For the portion of
the chamber that contains water, the light rays are bent,
causing the green lens to be illuminated. The portion of the
gauge containing water appears green; the portion of the
gauge from that level upward appears red.
Figure A4
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