Note: Descriptions are shown in the official language in which they were submitted.
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A METER FOR MEASURING THE CONCENTRATION
OF WATER IN A WATER-INK MIXTURE
Ira ~. Goldberg
Kwang E. Chung
Thomas A. Fadner
BACKGROUND OF THE INVENTION
This invention relates to the field of meters and part-
icularly to the field of meters for measuring the concentration
of water in a water-ink mixture.
Lithographic printing presses use a mixture of ink and
aqueous dampening solution. The dampening solution is water
with small amounts of proprietary additives to enhance water
wetting of the printing plate. A continuous flow of ink and of
dampening solution are furnished to the press. These can be
supplied as a mixture of ink and dampening solutionj or as sep-
arate streams of ink and dampening solution depending upon press
configuration. In any case, the useful printing mixture on press
contains both ink and water. In keyless inking systems, for
economical~operation, the ink-water mixture should be~recirculated,
since only a fraction of the ink mixture is being consumed by the
printing~process at any given time.
In order to obtain the best printing results, the proper
amounts of water and of ink in the mixture must be maintained.
This can be done at start-up when a fresh water-ink mixture is
added by ...
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using the proper proportions for making the mixture. However,
over a period o~ time during printing the proportions of ink and
water can change in the recirculating mixture. At present, the
printer attending the press has no means of automatically and con-
tinuously monitoring the concentration of water in a recirculating
water-ink mixture. He must rely upon his experience and the print-
ing results to estimate the concentration of the water in the
mixture.
Meters are known which measure the dielectric constant of
a mixture to determine the moisture content of materials or the
density of a mixture. Some of these prior art meters use a
Wheatstone bridge-type circuit to measure dielectric properties
(e.g., U.S. Patent 3,696,299, dated October 3, 1972 to J. Pullman
and W. Weidlich). Others use two oscillators to obtain a frequency
differential which is related to the dielectric properties of the
material being tested (e.g., U.S. Patent 4,048,844, dated September
20, 1977 to A. Dunikowski et al). Still other meters use an
oscillator with peak detectors (e.g., U.S. Patent 4,399,404, dated
August 16, 1983 to R. Resh) or a logic circuit (e.g., U.S. Patent
4,130,796, dated December 19, 1978 to L. Shum). However, what
the printer needs is a simple meter which will automatically
provide a direct reading of the concentration of water in the ink.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a meter
for conveniently measuring the concentration of wat~r in a mixture
of water and ink.
According to the invention, the mixture to be measured forms
the dielectric between the plates of a capacitor in a special capac-
itor cell. This cell provides capacitance for an audio frequency
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oscillator. When the dielectric constant of the mixture changes
due to changes in the water content of the mixture, the output
frequency of the oscillator changes. The output frequency of
the oscillator is converted to a voltage which is proportional to
the output frequency of the oscillator. This voltage is then con-
verted to a second voltage which is proportional to the logarithm
of the first mentioned voltage. This second voltage is read out
in a display device. Because the concentration of water in water-
ink mixtures is proportional to the logarithm of the mixtures'
dielectric constant, the display device can be set to provide a
direct reading of the ratio of water in the water-ink mixture.
In a preferred embodiment, the special capacitor has an
inlet and an outlet and is coupled in series by appropriate tubing
to a recirculating water-ink mixture system at the press. ~he
meter can then provide a continuous reading of the water in the
mixture.
These and other objects and features of the invention will
be apparent from the following detailed description taken together
with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a graph showing the relationship between the
water concentration ton a linear scale) and the dielectric constant
(on a logarithmic scale) for water-ink mixtures of three di~ferent
types of inks;~
Figure 2 is a block diagram of the ink meter circuit
; according to the invention;
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Figure 3 is a schematic diagram of the ink meter circuit
according to the invention; and
Figure 4 is a preferred embodiment of a capacitor cell for
use in the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
A microwave cavity was used to study the dielectric pro-
perties and the microwave absorption of water and lithographic
ink mixtures. During these studies it was discovered that the
logarithm of the real component, ~', of the dielectric constant,
~= ~1 + iE", iS linear with the concentration of water in the
water-ink mixture. This relationship is defined by the following
equation:
log = log~O'+ KC, where ~1 is the real part of the com-
plex dielectric constant ~(shown above), Eol iS its value for ink
without added water, C is the concentration by weight of water (or
water with dampening solution), and k is a proportionality con~tant.
Additional testing showed that this relationship was re-
prcducible, and that it applied to many different types of lith-
ographic ink. Figure 1 shows this logarithmic relationship for
three different newspaper inks supplied by the Flint Ink Co.,
Detroit, MI, namely a blank ink, curve 2, a yellow ink, curve 4,
and a magenta ink, curve ~. These curves were obtained on mixtures
tested at 23 degrees C. Testing at temperatures ranging from 10
degrees C to 40 degrees C showed that the ink-water mixtures had
an anomalously small temperature dependence. These results were
surprising because water itself shows a large temperature depend-
ence. Apparently, this...
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anomalous behavior is related to the microstructure of the
water-ink emulsion. In any event, the small temperature de-
pendence and the reproducible phenomenological behavior (Fig-
ure 1) provides a theoretical basis for a meter to measure the
water concentration of water-ink mixtures as described below.
Figure 2 is a block diagram of a capacitance type meter
according to the invention for obtaining a linear readout of khe
concentration, C, of water (or dampening solution) in an ink-
water mixture. In this meter, the mixture acts as a dielectric
material for a capacitor 8. The capacitance of capacitor 8
depends upon the dielectric constant E ' of the mixture and (as
discussed above and as shown in Figure 1) this in turn depends
upon the concentration, C, of water in the mixture.
Capacitor cell 8 is a part of the circuit of an audio-
frequency oscillator 10. Oscillator 10 provides a square wave
frequency, f, which is inversely proportional to the dielectric
constant of the mixture in capacitor 8. This is expressed by the
equation~
f = 2RfC E ~ n[(l~ )] , where Rf is the feed-
back resistor defined by the type of ink being measured, ~ is the
value of the resistance of the positive feedback loop of the os-
cillator, and C is the capacitance of cell 8 in the absence of
the mixture.
Once the square wave signal, f, is generated, a frequency-
to-voltage converter 12 is used to convert the signal to A voltage,
E, whlch is proportional to -~-T- . As shown in Figure 1, the
o
logarlthm of the dielectric constant, is linear with the
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concentration of fountain solution, C. Therefore, El is con-
verted to base 10 logarithm by logarithmic amplifier 14. Be-
cause a limited logarithmic range is required, unit 14 can be
constructed from an inexpensive operational amPlifier and an
appropriate NPN transistor, or can be purchased as a modular
unit as shown later in the schematic diagram. In fact, at low
concentration of water (e.g., 0-20% water), the relationship
between water concentration and dielectric constant E ~ iS suff-
iciently linear to provide an accuracy of + 1% without a logar-
ithmic amplifier. Consequently, for meters used only for measur-
ing concentrations less than about 20% water, logarithmic ampli-
fier 14 can be eliminated. The output voltage, E2, is propor-
tional to the concentration of water, C, offset by some arbitrary
voltage as shown by the relationship: E2 ~ KC + log EoI . Log ~O'
is a constant for the particular type of ink being tested.
The final stage of the water-ink mixture meter is an
amplifier 16 for conditioning the signal from logarithmic ampli-
fier 14 and a display device 18 for reading out the signal.
Amplifier 16 has an adjustable offset voltage and an adjustable
gain from 0.95 to 20 in order to provide a voltage output of 0
volts when the concentration of water in the water-ink mixture
is 0 (C = 0) and 5 volts when its concentration is 50% (C - 50%).
A functional schematic diagram of the meter is shown in
Figure 3. The unit consists of an oscillator (multivibrator)
which uses amplifier Al shown at the upper left hand corner. A
three position switch is used to select the appropriate feedback
capacitors to set the frequency based on some reference point
(e.g., empty cell or known
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ink-water system). The output from the oscillator is amplified
(Amplifier A2) by an amount determined by the positive ~ain
setting, (50 ~ 2)/x where x is the resistance between the wiper
of the 50 k~ potentiometer and ground (in k~ ). This is a non-
critical setting and is needed only to ensure that there is a
proper signal amplitude to trigger the frequency-to-voltage con-
verter (Burr-Brown VF32) shown in the lower right corner of the
figure. This integrated circuit provides a voltage directly pro-
portional to the frequency. A reference voltage to input 1 pro-
vides a reference voltage for the logarithmic zero point.
This constant voltage is then fed into a logarithmic
amplifier (surr-Brown Log 100 module) shown at the bottom center of
Figure 3. This is connected such that a 3-decade span per 10 V
is maintained. A reference current to pin 14 is supplied by a
simple current regulated circuit shown below the log-amplifier.
The output of the unit is positive above a 1 V input (log 1 = 0),
and is inverted with the 0 to -1 gain amplifier A4. The gain
settings are switched with the appropriate feedback resistors for
the multivibrator circuit as shown by the dotted line. This amp-
lifier serves two functions. The first is to establish the pro-
portionality constant k using the appropriate gain settings as
described in the previous section, and the second is to establish
the zero point voltage to correct for log ~O'.
The output of A4 is fed into A3 which acts as a buffer for
the digital panel meter DM 3100L set for a 0 to 5 volt output.
The unit can be adjusted, for instance, to read 1 volt for 10%
water concentration. The four amplifiers (Al to A4) shown in
Figure 3 can
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be combined into a "~uad op-amp" integrated circuit such as
National Semiconductor LF 457.
With nominal modification the circuit can be adjusted to
operate at any frequency which responds linearly to the dielectric
constant of the ink. Frequencies which have been used for various
inks range from less than 4 kHz to over 300 kHz. However, care
should be taken in constructing the meter to minimize stray
capacitance and to use an amplifier with a sufficient gain-
bandwidth product and a negligible phase shift belo~ 3 MHz.
Figure 4 shows the design of a capacitor cell 8 for use in
the meter of the invention. This cell may be designed so that it
can be placed in series with, for instance, a 3/4 inch diameter
pipe 19 carrying the water-ink mixture of a large printing press.
The inner member 20 is supported within an outer member 22 and
electrically isolated from it by, for instance, nylon screws 24
and nylon spacers 26. Inner member 20 is a tube with closed ends,
and it serves as one of the plates of capacitor 8. Oscillator
10 is connected to inner member 20 by metal screw 28 and connector
30. It is insulated from outer member 22 by spacer 26 and
insulating bushing 32.
Outer member 22 serves as the other plate of capacitor
cell 8. It is electrically connected to oscillator 10 through
connector 34. In this design a space of about 0.2 inch i9 pro-
vided for the flowing water ink mixture which, as such, becomes
the dielectric of the capacitor cell.
The facing surfaces of members 22 and 20 are coated with
a thin (0.005 - 0.010 inch) non-conducting polymeric layer
(e.g., teflon* . . . . . . . . . . . . . . . . . . . . .
* trade mark
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cr poly~inyl chloride) to eliminate electrolYtic conductance and to
prever,t corrosion.
Numerous variation5 and modification5 can ~e made without
departing from the invention. For example, the capacitor cell 8 can
be designed as a pair of flat parallel plates rather than the
concentric tube desi~n illustrated in Figure 4. The mixture being
measured can include ir,~s other than lithographic in~ if the relation
between the dielectric constant and the water concentration of the
mixture is c-imilar to that for mixtures containing lithographic in~c.
The measurements c~n be made of a flowing stream or simplY of a batch
of mixture. ~ccordingly~ it should ~e undenstood that the specific
form of the invention described above is illustrative only and is not
intended to limit the ~.cope of the invention.
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