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Air
supply
Pressure
regulator
Pnuematic
flow
controller
Rotameter
(flow
indicator)
one of these instruments. Second, these instruments often have some amount of
zero offset which cannot be fully calibrated out of the system. The zero offset is
caused by the residual backpressure in the pneumatic purge line when the
recirculation tank is empty. (The purge line backpressure cannot be taken into
account by the instrument manufacturer since it is custom fabricated for each
application.) Although this offset really does not hurt anything, it causes alot of
consternation because everyone would like the instrument to read zero atempty,
and the cause of the problem is not readily understood. Finally, the bubble tube
level sensor will not work on anon-vented recirculation tank.
Other Level Sensing Technologies
There are many other types of level sensing technologies available to the user;
however, each technology has some disadvantages which makes their use in CIP
applications generally impractical. Several typical types of instruments will
be reviewed.
Hydrostatic (diaphragm) level sensing technology has improved considerably
over the years; however, this type of instrumentation is still plagued with
excessive zero drift caused by the thermal shock that typical CIP systems are
exposed to. There have been some recent advances with ceramic elements which
aremuch more temperaturestable, but these materials aregenerally not recognized
as acceptable in pharmaceutical applications.
Capacitance probe–type sensors are not applicable in CIP systems since they
are sensitive to changes in dielectric constant of high purity water at varying
temperature causing unacceptable accuracy errors.
Ultrasonic level measurement technology will not provide accurate readings
in awash/recirculation tank. Any foaming of the CIP solution will cause substantial
instrument inaccuracy. This author has not tried applying this technology on a
purified water supply tank and will defer comment over the acceptability in
this application.
Typical non-contact radar systems are generally affected by any moisture or
condensation that forms or sprays on the instrument horn, causing substantial
accuracy problems. In addition, other factors such as low dielectric constant of the
P/I
Current to
pressure
transducer
Bubble tube level measurement system
FIGURE 4 Bubble tube level transmitter.
4-20 mA To
control system
Pressure
relief
valve
Fitting
on
tank
Andersen
Vessel