Page 99 - Instrumentation Reference Book 3E

84 Measurement of level and volume

Counter flange Flange module container’s dimensions after it is built. This can
be a time-consuming task. The values can be used
.-*- direct volumetric calibration of the container by
4
Flange joint to calculate the volume corresponding to differ-
ent levels. From this is compiled a conversion
chart for manual use or a “look-up table” for
computer use.
By far the most accurate method, however. is a
Wet side
which a suitable fluid (usually water) is pumped
in and out, to provide two readings, the tank
passing it through an accurate flow-metering sta-
tion. Whilst this is in process level data are
recorded, enabling the conversion factors to be
provided for each level measurement value. These
results will often require correction for tempera-
ture as has been already discussed. Another volu-
(a) metric calibration method is to pump the liquid
into a tanker vessel, usually a truck, which can
then itself be weighed and the tare of the empty
vessel subtracted, to produce an accurate measure
of volume.
Highly accurate level measurement requires
continuous monitoring of the various error
sources described earlier so that ongoing correc-
tions can be made. A continuous maintenance
program is needed to clean floats and electrodes
and to remove unwanted sediment.
In many instances the use of hand dipping is
seen as the ongoing calibration check of the level
measurement level. For this, rods or tapes are
used to observe the point where the contents wet
the surface along the mechanical member.
Obviously this cannot be used for dry substances;
for those the rod or tape is lowered until the end
TF rests on the surface.
1 In each case it is essential to initially establish
a permanent measurement datum, either as the
bottom of the tank where the rod strikes or as
a fiducial mark at the top. This mark needs to be
Figure 5.5 Repulsion of like magnetic poles providing related to the transducer system’s readout and to
non-immersed sensing and toggle action to a float sensor the original calibration.
One commercial form Courtesy, BESTA Ltd

The use of the indirect process of determining 5.4 Methods providing
volumetric or mass contents, based upon a level full-range level measurement
measurement, means that a conversion coeffi-
cient, or chart of coefficients, must be prepared Methods used to measure or control level in a
so that the level measurements can be converted container can be divided into those that measure
into the required measurement form. a continuous range of level and those that mea-
Calibration tables for a large fabricated tank sure a small change or point level. These full
are most easily prepared using the original range or continuous level methods have found
engineering construction drawings. This, how- wide acceptance.
ever, is not an accurate or reliable method. This
is especially true because of the difficulty in 5.4.1 Sight gauges
obtaining accurate “as-built’’ details on many
vessels other than custom-built process reactor A simple externally mounted sight-glass can be
vessels. used for reading the level of contents within a
A more accurate, and traceable, method closed container, such as a steam boiler. This
(known as “strapping”) is to actually survey the generally consists of a tube of toughened (usually