Page 363 - Sensors and Control Systems in Manufacturing

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flow vertically up through the flow cell. The light from a fixed source
in the spectrophotometer is collimated by the coated lens, passes
through the acidic solution, and then is focused by the second lens.
The amount of light absorbed by the indicator depends on the
acidity of the solution. The absorption spectrum of the indicator
reflects the concentration of hydrogen ions in the solution, which is
how acidity is measured.
The LANL sensor’s sensitivity range—acid concentrations of 4 to
12 molar—is unmatched by any other acidity measurement technique
in current manufacturing. Present techniques are unsuitable for mea-
suring solutions with a negative pH or for measuring chemical pro-
cesses. With the LANL sensor, high acid concentrations can be mea-
sured in 3 min or less, which is 50 times faster than measuring with
manual sampling and titration. The sensing material can function in
highly acidic solutions for four to six months with calibration less
than once a week, and needs to be replaced only once or twice a year.
A sensor using similar principles has recently been developed from
measuring lower acidity concentrations.
The sensor is selective in its measurements because the small
pores of the polymer allow only hydrogen ions to pass through the
indicator. Metal ions do not form a complex with the indicator under
acidic solutions. The sensor is reusable—that is, chemically revers-
ible. In short, no comparable product for measuring high acidity is
reversible or as sensitive, fast, accurate, and selective as the LANL
optical high-acidity sensor.
The prime use for the sensor is monitoring highly acidic chemical
processes and waste solutions. High-acidity processes are common in
preparing metals from ore-mining operations, treating fuel elements
from nuclear power plants, manufacturing bulk acid, and metal fin-
ishing including passivation and electroplating.
For highly acidic processor applications, the LANL sensor will
save companies thousands of dollars by improving efficiency and
decreasing the time devoted to acid measurements. The sensor can be
used on manufacturing lines, allowing control and waste manage-
ment adjustment to be made before acidity fluctuations become a
problem. The sensor will improve efficiency at least 25 percent by
eliminating the need to reprocess material processed incorrectly on
the first try because its true acidity was not known and controlled.
Higher efficiency will mean lower cost and minimal waste products
generated; the sensor itself generates no caustic waste. Finally, the
sensor’s waste monitoring capabilities will help ensure that any dis-
charged waste is environmentally benign.
For the 500 acidity measurements done at LANL in 1991, the sen-
sor saved $99,500, a 99.5 percent savings in labor costs in the first
year. And, because the sensor generates no waste, 20 L a year of caus-
tic waste was avoided, a 100 percent reduction.