Glass Level Gauges
GLASS LEVEL GAUGE
pressure: 21,5 MPa (3000 psi) - temperature: 370 °C (700 °F)
drawing shows a typical Bicolor Glass Level Gauge, used for measurement
of water level in high pressure boiler steam drums. This instrument
consists of a stainless steel body having trapezoid cross section
and machined to have a longitudinal chamber and front holes uniformly
distributed on the two not parallel faces.
On each hole a special high temperature resistant aluminium silicate
transparent cylindrical glass is tightly pressed between a metallic
cover and the body by means of bolts and nuts. A mica shield protect
the glass from the corrosive attack of the boiler basic water.
Belleville washers fitted between cover plates and glasses provide
compensation of their different thermal expansion, giving guarantee
of tightness at any operating condition.
The chamber is connected to vessel with cross fittings and flanged,
threaded or welded ends.
For severe conditions (over 8MPa - 1116 psi water/steam pressure)
water column and expansion joint are recommended.
Each Bicolor glass level gauge shall be fitted with top, bottom
and drain shut-off valve. Automatic shut-off ball check can be supplied.
A special back illuminator (suitable also explosion proof) gives
a sharp level reading by means of the contrast between bright color:
green in lower part (liquid) and red in upper part (steam) of level
Bicolor level gauges can be equipped with long glasses instead of
cylindrical glasses (as in Transparent
Nevertheless we do not recommend bicolor long glass level gauges
for steam drums over 10 MPa (1450 psi).
BONETTI Bicolor Glass Level Gauges are designed and manufactured
in accordance with ASME BOILER AND PRESSURE VESSEL CODE - SEC. I.
in operation, bicolor level gauge shows to the observer port holes
lighted green in the water zone and port holes lighted red in the steam
zone. In case of long glass gauge, the window appears part green and part
This result is obtained exploiting the following optical principle:
different colors (in this case: red and green) have a different
index of refraction when passing obliquely through different media
(in this case: glass, water and steam).
For the purpose, the gauge body has a trapezoid section with glasses
placed on the non parallel faces.
An illuminator with special red and a green filters is fitted on
the gauge at the opposite side with respect to the observer. This
special illuminator conveys light through the filters obliquely
to the back glasses of the level gauge.
Said filters allow crossing only to red and green rays.
Such colored rays reach, through the back glass, the media inside
When the gauge contains steam, green rays are considerably deviated
and prevented from emergingby the observer side; then only red light,
whose rays are smoothly deviated by steam, passes through the whole
internal hole, reaching the observer. Conversely when rays find water,
red rays are considerably deviated and lost inside the internal
part of level gauge, green rays can reach the front glass and seen
by the observer.
Level Gauges can be outfitted with Remote Transmission of Signal
by means of: Fiber Optics System "Red Cherry"®,
(image on the left) or Closed circuit
TV, or through the
use of special "periscope" mirrors