An Indigenous Device for Water Film Thickness Monitoring

TECHNOLOGY-24
An Indigenous Device for Water Film Thickness Monitoring
EXECUTIVE SUMMARY
In-house developed unique pulsating type position and conductance sensors were integrated for development of a device
for determination of water film thickness. The device was deployed for rapid measurement of variation in film thickness
across falling water layer sliding along a weir crest at different flow rates towards optimisation of conditions for avoidance
of flow separation. Measurements were needed in a water simulated facility to test the weir crest design of the thermal
baffle located inside the main reactor vessel of PFBR. Construction and operational details of the horizontally mountable
device for this application are described.
OUTLINE
Film thicknesses were determined by configuring a
laboratory made pulsating position sensor that enabled
measurement of linear displacement of a horizontally
mounted thin moving rod perpendicular to the vertically
falling water surface. A pair of tiny electrodes as
conductance sensor, mounted aboard the moving rod at
its tip, projecting in the direction of the rod, sensed airwater
interface in front as well as the water-solid surface
interface behind the film. The displacement during
movement of the rod from one interface to the other is the
measure of water film thickness. The device is shown
schematically in Figure 1.
The other end of the rod was fixed to a cylindrical ferrite
core which is partially placed within an inductor coil, and
the linear movement was effected manually by turning
slowly a teflon screw head, the non-threaded end of which
pushed the ferrite core and remained always in contact
with it by action of a spring. The inductor was part of an
oscillator circuit, the output of which was a train of pulses
with frequency at any instant was determined by the
position of the ferrite core and hence that of the tip of the
rod. Shift in pulse frequency was calibrated appropriately
against independently measured displacements prior to
deployment of the device.
The pair of closely placed electrodes for conductance
sensing at the tip of the bar was again a part of another
oscillator of different kind, the output pulse frequency of
which was governed by the conductance offered by the
medium that was in contact with the electrodes at a given
moment. Initially, during measurement, as the tip
approaches the water film from air space, the pulse
frequency is nearly zero. Frequency jumps as soon as the
tip touches the air-water interface. The design of the
sensor was such that, thereafter, it increased with
progressive movement towards a value determined by the
conductivity of water. The device had a 0.1 mm thick
stainless steel sheet fixed to the ends of its three legs,
which is pressed firmly against the solid surface, resulting
in the film flowing over the sheet in a small region.
Conductance, and hence the pulse frequency, increased
sharply when, on further advance of the tip, the electrodes
touched the stainless steel surface at the other end of the
film. With no further movement in the forward direction
being possible, the pulse frequencies in both conductance
and position channels attained upper limiting values.
Likewise, during retraction, the air-water interface was
trapped again by sudden fall of pulse frequency to near
zero in the conductance monitoring channel.
The device made use of laboratory made conductance and
position sensors. Oscillators were designed to provide
rectangular pulses of 5V amplitude. The pulses were fed
directly to a personal computer through its parallel port
and were recognised and counted simultaneously for a
given duration under software control for determination of
pulse frequencies. The pulse frequencies as a function of
time were displayed graphically in real time for both
conductance and position monitoring channels in two
separate windows. Response in both these channels for
three cycles of forward and reverse movements, as
acquired and viewed on-line, is shown in Figure 2.
Fig 1 : Schematic representation of the laboratory made water film
thickness measurement device
Fig. 2 : Pulse frequency data as functions of time from conductance
and position sensing channels in separate graphic windows,
as viewed in real time during measurement
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TECHNOLOGY-24
PERFORMANCE OF THE DEVICE
The device was designed for measurement of thickness of the film in the range 0 – 10 mm. Inherent standard deviation of
0.3 mm for was largely due to small ripples introduced by the hand-held dynamic device in addition to unavoidable factors
such as surface tension that manifests when the conductance sensing electrode tips touch or leave the air-water interface.
EXTRACTION OF WATER FILM THICKNESS DATA
The difference in measured pulse frequencies in position
monitoring channel, between the upper limiting value,
corresponding to the steel-water interface, and that at the
instant of sharp change in conductance as the rod enters or
leaves the air-water interface, provided data on displacement
from one interface to the other and, hence, the film thickness.
Thickness data in duplicate was obtained in one cycle of
forward and reverse movement. Three such cycles were
repeated in succession at a given location and flow rate. The
film thickness values are thus average of six data in each case.
The manner by which the change in frequency in the position
sensing channel corresponding to film thickness was derived is
further depicted in the Figure 3 by presenting the data from
both the channels together. Movement of the rod between two
interfaces across the film thickness is derived from the
frequency shift to displacement relationship established prior
to measurement.
The vertical lines correspond to onset or disappearance of
conductance signal. The points of intersections with the
position sensor output plot give the pulse frequencies
corresponding to the position of the tip at air-water interface,
with the mean value depicted by the lower horizontal line. The
frequency shift from the upper horizontal line, the water-solid
surface interface, provides data on displacement of the tip
between the interfaces and, thereby, the film thickness.
Fig. 3 : Time dependent pulse frequency data from position and
conductance monitoring channels during forward and
reverse movements of the rod in three cycles
BASIC APPROACH TO SENSING
Position sensing is realised through partial positioning of a ferrite rod within an inductor coil. The inductor is placed in the
timing circuit of a specially designed miniature R-C-L oscillator with fixed R and C. The digital pulse frequency at the
output changes as the inductance (L) offered by the coil varies sensitively with the movement of ferrite rod. Likewise, an
electrode pair is placed in the timing circuit of an appropriately designed R-C oscillator with fixed C to measure
conductance. The output digital pulse frequency changes as the conductance (1/R) varies. Similarity in approach to the
directly measurable primary signal and absence of any signal conditioning hardware led to development of the compact
device for water film thickness monitoring.
ACHIEVEMENT
The device has been used successfully for an important application as mentioned above. Designers of the weir crest could
experimentally determine the drag coefficient. The device design is of generic nature and can be adopted for thickness
monitoring of any conducting liquid film or solid layer, with much better precision for the latter. There is also a definite
scope for total automation, if needed.
PUBLICATIONS ARISING OUT OF THIS STUDY AND RELATED WORK
K. Veluswamy, G.R. Ravi Prasan, M.P. Pradeep, P. Sahoo, P. Selvaraj, P. Chellapandi and B. Saha,Design note Report No.
PFBR/31260/DN/1007/Rev.A ( 2004).
Further inquiries:
Shri B. Saha, Innovative Instrumentation Section
Electronics and Instrumentation Group, IGCAR, e-mail: saha@igcar.gov.in
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