GLASS LEVEL GAUGES
drawing shows a typical Reflex Glass Level Gauge.
This instrument consists of a metal body, machined to have an
internal chamber and one or more front windows (on one side only of
On each window a special high resistance plate reflex glass is
fitted with sealing joint and metal cover plates hold by bolts
The chamber is connected to vessel with cross fittings and flanged,
threaded or welded ends. Usually, between the instrument and its connecting
ends, valves are fitted to consent shut-off piping and to disassemble
the level gauge without to empty the vessel. Drain valves can
also be fitted to cross fittings device.
To avoid leakage in case of glass breakage, safety ball-check
device can be provided.
level gauges working principle is based on the light refraction
and reflection laws.
Reflex level gauges use glasses having the face fitted towards
the chamber shaped to have prismatic grooves with section angle of
90°. When in operation, the chamber is filled with liquid in the lower
zone and gases or vapors in the upper zone; the liquid level
is distinguished by different brightness of the glass in the
liquid and in the gas/vapor zone.The reflex level gauges do not need
a specific illumination: the day environmental light is enough. Only
during the night an artificial light must be provided.
The different brightness in the two zones is obtained as explained
This zone appears quite dark when the gauge is
in operation and lighted as above said.
Given the construction, most of the environmental light rays
incident on the external face of the glass are quite perpendicular
to said face and, therefore, not deviated by the glass. These
rays reach the glass/liquid interface with an inclination of
approx. 45°. The critical angle glass/liquid is always superior to
45°. Therefore the rays incident within the critical angle (practically
the totality) are refracted within the liquid and, since the
internal walls of the gauge chamber are not reflecting, the
rays cannot be seen from the outside. In fact the zone will appear
dark, nearly black, to the observer.
zone appears almost silver bright to the observer. As for the liquid
zone, the light rays reach the glass/gas-vapor interface with an
angle around 45°. Since this angle is greater than glass/gas-vapor critical
angle, the rays are not refracted , but totally reflected making
90° turn, thus reaching the nearest glass/gas-vapor interface again
with angle of 45°. For same reason they will be reflected
and turned by 90° towards the observer, to whom the zone will
appear silver bright.
Level Gauges can be used in most of the cases and offer great advantages
in terms of: low initial cost, low operating cost, easy level reading.
Reflex Level Gauges cannot be used in certain cases (where Transparent
Level Gauges must be used) as
the separation level between two liquids has to be read (interface),
besides the level indication, observation of the liquid color is
the process fluid is high pressure water/steam (since in this case
the glass must be protected from the chemical attack of the boiler
basic water by using mica shields),
the process fluid is such that can corrode the glass (e.g. high
temperature alkaline solutions or hydrofluoric acid) since mica
shields or Polytrifluorochloroethylene shields must be used
to protect the glass.
Bont's standard range of Reflex Level Gauges, starting from PN 16 up to PN 420/ ANSI 150 up to 2500 lb, and can be manufactured, depending on model & type, in different materials, as ASTM A105, ASTM A350 LF2, ASTM A182 F304 / F316, Monel, Hastelloy, Incoloy, Carpenter, Nickel, Titanium, Zirconium, Tantalum, Ebonite, PVC, Polypropropilene, PTFE etc.
Reflex Level Gauge Specifications