Page 101 - Instrumentation Reference Book 3E

86 Measurement of level and volume

Externa,
indicator column

Indicators turn
toward magnet
Float in srill tube
contalnr permanent
magnet at Itwid level

Figure 5.8 Schematic of magnetic level indicator
installation. Courtesy,Weka-Besta Ltd.

than simple glass sight gauges should be consid-
ered. Valves are usually incorporated to allow the
whole fitting to be removed without having to
depressurize the container. Figure 5.7(b) shows
a configuration incorporating these features.
A modern development of the above sight-
gauge concept is the magnetic level indicator
shown in Figure 5.8. As the float, containing a
bar magnet, follows the liquid surface in the still
tube the individual, magnetically actuated flaps
rotate to expose a differently colored surface that
is coated with luminous paint. The float is chosen
to suit the specific gravity of the fluid.
The magnetic action also operates individual
switches for control and alarm purposes. Discrimin-
ation is to around 5mm. Magnets must not be
operated beyond their Curie point, at which tem-
perature they lose their desired properties. As a
guide these systems can measure liquids under pres-
sures up to 300 bars at temperatures up to 400 "C.
In some circumstances it may be possible to
view the surface of the liquid from about but
not from the side. In this case a hook gauge,
Figure 5.9, can be used to enable the observer to
detect when the end of the dipstick rod just
breaks the surface.

5.4.2 Float-driven instruments
The magnetic indicator described above is one of
ib) a class of level indicators that use a float to follow
Figure 5.7 Sight-glasslevel indicator. (a) Basic the liquid surface. Where a float is used to drive a
schematic. (b) Sight-glass with automatic cut-off. Courtesy,
Hopkinsons Ltd. mechanical linkage that operates a remotely