Device for Measuring the Electrical Conductivity of a Liquid Medium
This invention relates to a device for measuring the electrical conductivity of a liquid
medium, especially an aqueous medium according to the preamble of the main claim. These
conductivity measurement devices have been known for a long time from the prior art in different
circuit topologies and are used for the most varied applications. Here for example the industrial
process engineering is important in which the conductivity of the fluids is checked in order to
ascertain the concentration of dissolved salts in chromatography processes or the like. Typical
specific conductivities in die range between roughly 50 mS/cm and 200 mS/cm can be detected
here. In the field of control and monitoring of ultrafiltration processes conversely for example a
range between 5 jxS/an and 100 |iS/cm is especially relevant.
Another important application is the cleaning of process installations (for example the
monitoring of flushing and cleaning processes), these installations being typically flushed with
high-purity deionized water and the conductivity of this flushing water then being an indicator of
reaching a desired cleaning state. The conductivities to be detected here are much smaller, typically
less than 10 pmS/cm, certain requirements necessitating measurement errors or tolerances less than
0.5 jimS/cm. If for example, in this cleaning context, not only the flushing water, but also in
addition for example a cleaning solution are to be monitored (typically somewhat heated NaOH,
with conductivities of more than 100 mS/cm), in the conductivity measurement devices which are
suitable for this purpose there is for example a need for systems which encompass a wide range of
values of the conductivities of the relevant liquids to be detected.
The prior art fiirtherraore discloses different measurement principles for measuring die
conductivity in aqueous media. Thus for example to detect higher conductivity ranges inductive
measurement methods are conventional, in which there is no (galvanic) contact of measurement
electronics to the medium to be tested. But in particular in applications of the aforementioned type
in which for example high-purity water is to be monitored in the microsiemens range, contact
measurement methods are necessary, typically implemented by the feed and subsequent
measurement of a measurement current via electrodes into the medium.
Due to the low conductivity of the medium, even at high electrode voltages the effective
measurement currents are low, and moreover it can be assumed to be known that the current to be
fed is provided as an alternating signal in order to keep low a voltage drop on the electrodes on the
interface to the aqueous medium (relative to the actual voltage drop in the medium); electrolytic
degrading influences on the electrodes are also avoided by this alternating signal.
It is considered known from the prior art to use in tibie so-called 2-pole measurement the
same electrodes (as an electrode pair) both for the feed of the current and also for the measurement,
and for example at low conductivities and accordingly optimized electrode geometry (typically then
large-area electrodes with a small spacing are used) moderate injected voltages will yield easUy
measurable currents.
Then the specific conductivity (sigma) can be determined as:
sigma = G X k = k X FU,
the cell constant k m the dimension length'" (typically 1/cm) indicating the special conditions of the
respective measurement cell configuration, included electrode parameters and electrode geometries.
When the measurement range is expanded to higher conductivities, for example several 100
mS/cm, the voltage drop increases on the boundary layers of the electrodes, at die same time the
voltage drop decreases over the aqueous medium which is to be measured. To solve these problems
it can also be assumed to be known from the prior art to decouple the electrodes for a supplied
current from the measurement electrodes for measuring the resulting voltage drop in the medium, in
addition to increasing the measurement frequency (therefore for example a polarity reversal rate of
the injected alternating signal). This leads to tlie fundamentally known approach of the 4-pole
arrangement, by analogy to the generic 4-pole resistance measurement technology, a measurement
current as an excitation current being fed via a first electrode pair into the aqueous medium
(electrolyte) and then the measurement voltage (voltage drop) being detected via a second
measurement electrode pair which interacts simply via the liquid medium. Both electrode pairs are
thus diree-dimensionally spaced apart from one another and separated. Since on the measurement
and evaluation side there is typically a high internal resistance of the downstream detector
electrodes, small currents flow accordingly in the output-side measurement dicuit, with the
advantageous action that the voltage drops on the interfaces of the measurement electrode pak are
negligible. Since on the primary side (i.e. in the circuit which interconnects the injection electrodes)
a current which flows there can be measured and managed, the primary-side electrode voltage drop
. is not important.
While, as described above, the 4-pole technology is favorably suited for measliring the
electrical conductivity in aqueous media especially also for high conductivity values, it is
nevMtheless conventionally complex to implement this technology such that it aicompasses a large
conductivity range. In the practical implementation it has been found to be problematic that liie cell
constant (k) which is determined and calibrated for example generically using standardized
environments is dependent on the actual conductivity. In addition the interfaces of the electrodes.
which for example depending on the respective order of magnitude of the conductivity, cause
current density shifts on the electrodes and in the electrolyte, as well as alternating current effects,
play a part, such as for example parasitic capacitances (as loading of the secondary side voltage
electrodes) or crosstalk, therefore an unintentional alternating current coupling between the primary
side excitation circuit and secondary-side measurement circuit. Here for example in practice the
(often long) feed lines between the electrode and measuring amplifier significantly influence these
parasitic effects.
These problems are solved in the technologies which can be assumed firom the prior art by
evaluation-side measuring amplifiers which are specifically matched to different measurement and
value ranges of the conductivities to be detected and are suitably switched over (manually or
electronically); typical quantities to be influenced are respective firequencies, trigger curraits and
gains of a measuring amplifier. Integrated designs, for example microcontroller units, make
available comfortable infrastructures which make a multistage adaptability to various conductivity
ranges easily programmable.
One problem associated with this technological ^proach is in any case the necessary
occurrence of discontinuities or jumps at transition sites of these measurement ranges; if therefore
ihetc is an aqueous fluid to be measured in this transition range, potentially unusable measurement
results arise. Therefore the implementation of hysteresis during switchover is necessary, however
such (also can be favorably implemented with digital electronics) a hysteresis approadi entails the
problem that a value which lies in the hysteresis range is dependent on the direction firom which the
measured value changes so that a hysteresis interval or deviation means a potential measurement
error.
Just the aforementioned digital electronics in which for example digital processes
electronics also generates a (rectangular) primary-side trigger signal for injection into the aqueous
medium entails other problems of practical importance. Thus for example the harmonics of the base
frequency which ai-e associated with a rectangular pulse signal are especially gi'eatly influenced by
nonohmic interference, for example the aforementioned parasitic capacitances. This in turn can be
opposed only by a sinusoidal (at least low-harmonic) primary-side triggering; this distinctly
increases the hardware cost
Finally, the possible variation of a measurement frequency during switchover of the range
which can he assumed to be known (for example within the framework of an integrated-digital
design) entails the potential problem of ambiguity by parasitic capacitive effects: If for example at
very low conductivities a very high measurement frequency is used, these parasitic capacitive
influences can feign h i ^ conductivity, with the unwanted effect that the secondary-side measuring
amplifier does not recognize the appropriate measurement conditions and accordingly does not
undertake a pertinent measurement range selection or switchover.
The object of this invention is therefore to devise a generic device for measuring the
electrical conductivity of a liquid medium, especially an aqueous medium which is made and
intended for detection of electrical conductivity in a large detection range, especially in the region
between a few fiS/cm up to several 100 mS/cm, which is made structurally simple here, especially
on the measurement side and evaluation side avoids a plurality of discrete measurement and
evaluation ranges and/or discontinuous evaluation-side measurement signals, nevertheless offers a
measurement signal of high signal quality and measurement accuracy whidi corresponds to the
electrical conductivity.
The object is achieved by the device for measuring the electrical conductivity according to
the main claim; advantageous developments of the invention are described in the dependent claims.
In addition, protection Vk'ithin the framework of the invention is requested for a use of this device
for measuring the conductivity of water and an aqueous solution, and this device will cover an
uninterrupted conductivity range between 0.01 mS/cm and 500 mS/cm and beyond to more than 1
S/cm and will make it available as a hysteresis-free continuous measurement signal.
In a manner advantageous according to the invention, on the primary (injection) side an
arrangement is formed from constant voltage means, commutator means for the first (injection)
electrode arrangement and the ohmic resistance means as a series circuit in a (single) circuit brandi,
and the injection current can be tapped as a voltage drop over the resistance means (especially a
partial resistance of it). Thus on the primary side the DC voltage which has been produced by the
voltage source is applied via the commutator means as a modulator in clocked-changing polarity
with the clock frequency to the injection electrode pair (as the preferred implementation of the first
electrode arrangement). Depending on the conductivity of the aqueous medium (or the ohmic
resistance means which are made preferably as constant resistance) an (injection) current flows
which can be detected and evaluated as a voltage drop over the resistance means and is included in
the determination of the electrical conductivity. Equally on the evaluation side demodulation
occurs, in correct phase relation with the injection-side commutator means in the clock frequency
by the detection means a storage capacitor being charged whose capacitor voltage can be tapped as
the measurement signal, preferably quantified into a digital signal and likewise further processed
electronically for determining the electrical conductivity.
The configuration as claimed in the invention, especially the circuit branch which is made
on the injection side in a series connection advantageously enables the fact that over a wide range of
the electrical conductivity of the aqueous medium the excitation current dianges continuously
depending on the conductivity, without as claimed in the invention other active components in this
circuit branch being used (outside of the commutator means which are implemented in an otherwise
known manner for example by means of semiconductor switches or the like, furthermore preferably
without using an integrated circuit unit, or a programmable output of such a circuit unit).
Accordingly the invention enables a change of conductivity of for example over five powers of ten
to cause a change of the current flowing on the primary side in the circuit branch by roughly a factor
of 30 (1.5 powers of ten), continuously and without discontinuities. Thus the wide range of the
electrical conductivity which is to be measured in the aqueous medium, which range is intended as
claimed in the invention, can then be advantageously encompassed without the necessity of
choosing or switching over the range, and in addition advantageously only limited resolutions of
assigned analog-digital (A/D) converters being necessary, with which then in addition the hardware
cost can be reduced (and in addition the large series suitability of the invention is further increased
accordingly). On the described dimensioning example then for the described range or its detection
and evaluation by measurement engineering on the injection side and on the evaluation side one
typical 16 bit converter with the pertinent resolution would be sufBcient
On die injection side, for example this analog-digital converter unit would convert the
voltage signal which was tapped via the resistance means (or the partial resistance provided
according to the development) and which is proportional to the injection current in the circuit
branch and make it available for fiirther digital processing, likewise as on the evaluation side one
analog-digital converter unit for digitizing the capacitor voltage is assigned to the capacitance
means which interact with the second electrode arrangement (i.e. the measurement electrodes).
The invention is especially relevant and advantageous for practical implementation when
the advantages of simplification offered by the above described invention principle are
implemented using simple and economical electronic modules. In addition to the described,
potentially economical A/D converters which are available as mass-produced parts, for this purpose
especially the implementation of the evaluation means by means of suitable microcontrollers is
recommended, for example the injection and measurement signal which has been digitized
according to the development separately from such a controller unit likewise reducing the
(hardware) requirements for such an electronic component, such as for example the serial coupling
of the A/D converter units provided on the injection side and/or measurement side, which coupling
is provided according to the development; in this way advantageously the number of controller
terminals can be distinctly reduced, equally the primary side and secondary side can be interrogated
in a timely manner or synchronously and then further processed in terms of data.
In particular, the implementation of the commutator means which is advantageous
according to the development using electronic switching components, for example suitable (power)
transistors, leads to potentially disruptive higher frequency signal components in the primary side
circuit branch, optionally also on the measurement side. In addition a trigger signal for the
commutator means (in the clock frequency) influences this signal picture. Accordingly it is
provided aax)rding to the development that suitable filter means be provided in the primary-side
circuit branch and/or on the evaluation side/measurement side, for example in the form of a
capacitor in parallel to the measurement resistor. It is also advantageous and preferred according to
die development to use the (digital) filter means which are often integrated into the analog-digital
converter units to filter (matched for example to the clock frequency of the alternating signal) a
noise influence in this respect out of the measurement circuits (this applying equally for example to
the influences of a line frequency of an existing supply voltage or to other superimposed noise
alternating signals).
Another prefened embodiment of the invention calls for (noncapacitive) decoupling means,
especially ohraic resistances, to be provided between the electiodes of the second electrode
arrangement and the downstream measurement-side commutation. This development measure, in
contrast to traditional logic to connect the measurement-side voltage electrodes with low resistance
to the measurement capacitor and the downstream quantization or evaluation electronics, causes an
influence of the voltage electrodes (for example thickness and homogeneity of a passive layer) in
concert with charge effects and potential effects in commutator edge change to have much less an
effect on the secondary side measurement signal.
This measure would alternatively also be possible for example by an especially h i ^ -
resistance configuration of the commutator input; this design which is advantageous and simple in
terms of hardware according to the development using discrete semiconductor components would
require howevar additional cost here.
While within the scope of implementations as claimed in the invention it is advantageous
and useful on the secondary side (i.e. on the side of the second electrode arrangement and the
assigned detection means) to amplify and then suitably digitize the voltage signal arising via the
capacitor means with a constant gain (or to digitize it unaraplified), one version of the invention
which is advantageous according to the invention calls for the capacitor voltage to be amplified in
the form of amplifier units provided parallel to one another by different gains and accordingly to
offer the amplified capacitor voltage signal (simultaneously) at different levels; this high resistance
of operational amplifiers which are typically used for this purpose likewise little opposes this
parallel arrangement of two or more amplifier units, such as for example according to another
version, on the one hand the capacitor voltage signal is directly digitized and then made available
likewise continuously for further processing, as a capacitor voltage signal which has been amplified
by a predetermined gain before digitization in a separate, firmly assigned A/D converter.
In the further processing by the evaluation means then this version of a simultaneous
multiple measurement signal at different voltage (amplification) levels enables the maximum
possible flexibility in the evaluation and the further processing of the signal which has been
acquired: Thus for example at high capacitor voltage levels an unamplified signal can be digitized
and fiirther processed (for example an amplifier which potentially overdrives at such a high voltage
level remaining by controlled decrease in a high-resistance range which thus does not influence the
measurement voltage), while for example a low-level capacitor voltage which unamplified would
yield only a very noisy A/D conversion result can first be raised by the predetermined and known
gain. In one possible transition range in tum the signal of the two measurement branches (since
present continuously and in parallel) can be combined or computed with suitable weightings.
In tum the circumstance is advantageous and of practical importance that the A/D converter
units which are assigned on the primary side and secondary side, according to the development
separated fixtm a microcontroller control unit, acquire the same operating or reference voltage of a
supply voltage unit which is present for the entire device. This advantageously leads to the fact that
corresponding errors due to fluctuations or other effects of the reference voltage are equalized and
leave the measurement result unaffected. This also applies equally to the necessary quantization of a
10
temperature signal by means of an A/D converter which conditions the analog temperature signal of
a temperature sensor which is assigned to the aqueous medium according to another advantageous
development of the invention.
As a result, this invention surprisingly makes it possible easily, effectively and powerfiiUy
for a large continuous range of electrical conductivity of a liquid medium to be covered, typically
between 1 (.unS/cm and 1000 mS/cm without the necessity of complex and fault-susceptible
switching over of the range, without increased measurement engineering and evaluation
engineering cost on the secondary side, and witiiout the electrolytic and other effects associated
with the electrode pairs significantly influencing a measurement result Here the 4-pole technology
in which both the first and also the second electrode arrangement each have one electrode pair is
preferred, nevertheless constellations are conceivable in which more than two electrode pairs, on
the injection side and/or measurement side, are provided in the measurement cell and are wired in
the device as claimed in the invention.
According to the design, cable Influences or other capacitively active factors are
advantageously minor so that they have no adverse effects, in an essentially useful manner.
Likewise an electrical (total) power demand of the device is low, favorable especially in a power
supply which is isolated according to the development and which facilitates a low-capacitance
design. In isolation according to the developraMit, a coupling capacitance parallel to the isolation
barrier of less than 5 pF can be implemented; this in turn reduces the occurrence of noise potentials.
Other advantages, features and details of the invention will become apparent from the
following description of exemplary embodiments as using the drawings.
Figure 1 shows a schematic block diagram for Uliistration of the fundamental operating
11
structure of this invention;
Figure 2 show?s a representation analogous to Figure 1, but with additional circuit
components and modules which are advantageous for practical implementation, and
Figure 3 shows a current and voltage diagram, relative to the electrical conductivity, for
illustration of a conductivity dependency over a large detection range.
Figure 1 illustiates in tlie schematic the important functional components of the device for
measuring the electrical conductivity of an aqueous medium accordmg to a first embodiment of this
invention. Thus reference number iO describes a measurement cell which can be filled with the
liquid medium, and into which one electrode pair 12 projects as a first electrode arrangement and
another electrode pair 14 projects as a second electrode airangement, decoupled from one another,
in order to undertake injection in the manner of an otherwise known 4-pole arrangement via the
electrodes 12 and then to generate the measurement signal by means of the electrodes 14. For
purposes of more extensive disclosure, especially with respect to one possible practical
implementation on the exemplary embodiment, reference is made to the measurement cell which is
disclosed in DE 199 46 315 and which shows one possible example for implementation of the cell
10 which is shown in Figure 1.
On the injection side, i.e. as a circuit branch connected to the electrode pair 12 (and via the
DC potential coupled to the coupling capacitors 16 which decouple the electrodes), there is a
constant voltage source 18 which is connected via a first resistor 20 (as a working resistor) and a
second resistor 22 (measuring resistor) to the primary-side commutator means 24 which in turn,
triggered by a clock signal CLK, make an alternating signal for the electrode pair 12 with the clock
frequency CLK in the described manner from the constant voltage signal of the source 18. In eadi
12
*
commutator switching state there is an individual circuit branch on the injection side, the voltage
source 18 forming a series connection to the resistance means which have been formed from the
resistors 20,22 and to the electrode pair 12. A current I which flows in this circuit branch drops
over the resistor 22 and is tapped there as an injection-side, current-proportional injection signal
(U)I and supplied to an A/D converter unit 26.
On the measurement side is the potential of the electrode pair 14 (coupled or decoupled by
respective coupling capacitors 28 or coupling resistors 30 which are in series) over a secondary-side
commutator unit 32 via a measurement capacitor 34, the capacitor voltage U(U) being tapped as
the measurement signal and present at the A/D converter unit 26.
Since the secondary-side commutator unit 32, clocked with the clock frequency CLK and
operated synchronously to the primary-side unit 24, demodulates the signal which has been
modulated rectangularly for the injection in the illustrated manner on the output side there is a
capacitor DC voltage over the measurement c^acitor 34 as the measurement voltage U(U).
In the further processing as evaluation means both the current-proportional injection signal
U(I) and also the measurement signal U(U) are then present in quantized digital form which is to be
transmitted serially as AD (U(U)) and AD (U(I)) and are processed in an assigned microcontroller
unit 40. This microcontroller unit at the same time generates in an otherwise known manner the
alternating clock CLK for the primary-side and secondary-side commutators 24, 32 which, not
shown in the figures and in an otherwise known manner, are implemented by means of discrete
semiconductor switches (or as commutator transistors which have been integrated into a
semiconductor module, optionally with additional integrated means for compensation of the charge
injection and with delay elements for the clock signal in order to avoid transient short circuits when
13
the edge changes). From the ratio of the current injected on the primary side and vohage measured
on the secondary side (with consideration of the cell constant and in addition of necessary
calibration constants, including an offset for current, voltage and conductivity) the microcontroller
unit then either itself computes the value of the electrical conductivity or alternatively prepares the
individual quantized variables for external further processing via a suitably chosen protocols.
The schematic circuit diagram of Figure 2 which is made analogously to Figure 1 illustrates
additional components of the described exemplary embodiment which are useful in the practical
implementation, the same reference numbers describing identical functional components or
functionalities. This arrangement, with the exception of the measuring cell 10 and temperature
sensors whidi are assigned to it (symbolized with reference numbers 62,64), with correspondingly
short cable connecting lengths and small parasitic capacitances, is integrated into a body of cuboidal
shape 18 mm x 47 mm x 10 mm, flierefore implements an advantageously small compact size. The
block circuit diagram of Figure 2 shows details of the AID converter arrangement which is only
schematically suggested in Figure 1 as a plurality of converters; a first 16 bit converter unit 261
with an integrated digital filter unit (not shown iu detail) which is tuned to a line frequency of a
supply voltage unit 50 (more accurately: a DC-DC converter unit with electrical isolation; in one
practical implementation supplied by a higher-level feed unit with 5V DC voltage, then for example
an unregulated supply voltage of roughly lOV DC being present on the voltage source 18) receives
the injection current-proportional voltage signal U(I) which drops over the resistor 22 and converts
this 16 bit signal AD ((U)I) which has been quantified with a reference voltage REF of 2.7 V for the
microcontroller unit 40. The reference voltage REF is generated from a referaice voltage unit 52
which is in turn supplied from the unit 50 (like otherwise also the constant voltage unit 18).
14
Equally a second A/D converter unit 262 (which receives the same supply and reference
voltage REF as the unit 261) generates from the measurement voltage signal (U)U which is
dropping over the measurement capacitor 34 the 16 bit-quantified and serial measurement signal
AD (U(U)), in turned prepared on the microcontroller unit 40.
As a variation and an addition to tlie generic implementation of Figure 1, the exemplary
embodiment of Figure 2 additionally shows how the measurement voltage signal U(U) as a
capacitor voltage is present additionally at the input of a measurement amplifier unit 60, in the
exemplary embodiment a gain 21 is preset (relative to the unamplified capacitor voltage U(U). This
amplified signal Ua(U) is processed, analogously to the quantization of the unamplified
measurement signal and of the injection current signal, by a third A/D converter unit 263, in turn
resolution 16 bit and serial output as AD (Ua(U)) to the microcontroller unit 40, with the action that
according to Figure 2 the measurement voltage is present twice in parallel over the capacitance 34,
specifically on the one hand as an unamplified signal, on the other hand as raised in level by the
amplifier unit 60 for processing by the microcontroller unit 40.
In turn, in addition to the generic principle of Figure 1, Figure 2 shows still a fourth 16 bit
A/D converter 264 which converts a temperature signal which has been tapped from a temperature
sensor T into a corresponding serially digitized temperature signal AD (U(T))- This analog
temperature signal U(T) which solely for reasons of schematic simplification is shown as a
temperature-proportional voltage signal which drops over a temperature-variable resistor 62 relative
to a reference resistor 64, illustrates a temperature detection of the aqueous medium which is
contained in the measuring ceil 10, either the temperature sensor 62, suitably transferring heat,
being attached to the measuring cell or projecting into it for detection of a current fluid temperature.
15
The representation furthermore illustrates that the illustrated temperature measurement (as also the
conductivity measurement) is independent of a supply or reference voltage of the A/D converter
units 26i which is supplied in parallel: In the illustrated temperature measurement specifically the
digitized value depends also only on the ratio of the resistances 62,64, but not on the reference
voltage REF since tlie voltage divider 62/64 and the AD/converter 264 are between the same
voltage.
The microcontroller unit 40 then prepares the digitized signals of the converters 261 to 264
which have been obtained in this way for output via an interface module 80 for optical decoupling
of the digital signals (clock, digital in/out = DIO, in addition to the DC/DC converter unit 50 this
module 80 constituting a main source of parasitic coupling via the potential barriers, in total
roughly 5 pF), either the microcontroller unit 40 having undertaken signal processing, or however
only according to a desired transmission protocol, then the acquired signals being available for
further treatment by the unit 80.
Possible galvanic decoupling of the supply unit 50 from the primary-side and secondaryside
measurement circuits and from the pertinent A/D conversion is not shown in Figure 2.
In the specific implementation, structure of the measuring cell for example according to DE
199 46 315, cell constant 0.35/cm, the illustrated device would be at a clock frequency of
commutator changing of 10 kHz (this clock frequency in the range between roughly 1 and roughly
100 kHz has proven favorable on the one hand to be fast enough to reduce the disadvantageous
voltage drop on the electrodes, on the other hand of sufficiently low frequency for reducing
disadvantageous switching effects) is then combined with a typical repetition or interrogated rate of
the A/D conversion by the units 261 - 264 of 0.1 seconds (a typical conversion time of roughly 80
16
•
ms, after fliis time a respective measured value is available; with a clock rate of 30 kHz the digital
signal is clocked out from the converter via the serial interface), for a roughly synchronous
acquisition of the data. Since moreover, as explained as advantageous within the fi^mework of the
invention, all converters 261 - 264 are operated with the same supply and reference voltage, noise
influences do not affect the measurement result. A typical total power consumption of the circuit is
in the range of < 250 mW so that, as discussed, low coupling capacitances are sufficient for
galvanic decoupling,
A conductivity sigma of the fluid contained in the tank 10 would now be determined as
follows:
Sigma = ( i—i-ii • ) - siemai,
(1.1) AZ){i7{[/))-^Duo R^. *
,AD{Uiiy)-ADio 21 , .
Sigma = ( • ) - iiema.
(1.2) ADmU))-ADuo R,J *
and according to formula 1.1 and no amplification of the capacitor signal a large measurement
range can be covered, while, especially in the region of small conductivities, then as the amplified
signal (factor 21) according to formula 1.2, the data stream AD (Ua(U)) is available.
In the formulas ADio (as tlie offset for the current measurement), ADuo (as the offset for the
output-side voltage measurement), sigmao (as the offset for the conductivity, for example for taking
into account cable and installation effects) mean calibration constants wWch are set or selected
suitably and in the already described manner k means the cell constant and therefore takes into
account the geometry of the measuring cell. Since within the framework of the invention the cell
constant k is well defined via the measuring cell geometry, it can be reproducible defined via
17
suitable process control (as mechanical accuracy). To determine the constant ADuo and sigmao an
open state (i.e. air, therefore unfilled measuring cell) is induced, while for example the offset Dio
can be determined by a short circuiting of the electrodes. Accordingly the (traditionally
problematic) handling of liquid conductivity standaid media for calibration is eliminated. Since
moreover sigmao describes essentially an appaient conductivity by capacitances which are parasitic
in parallel to the electrodes 14 (see in tliis respect also the voltage decrease in the range between 0
)iS/cm and 20 jiS/cm in Figure 3), this conductivity in practical use is reduced to roughly sigmao =
2.5 |iS/cm.
A specific signal behavior of this circuit, especially a characteristic of the (injection-side)
current as well as, associated therewith, characteristics of the measurement-side measurement
voltages (amplified as Ua and unamplified as Un) illustrate the signal characteristics of Figure 3. It
is shown that over a very wide range of conductivities, firom roughly 2 {iS/cm to roughly 200
mS/cm, the injection-side current varies between roughly 0.01 and roughly 3.8 mA, while there are
voltages which are unamplified on the measurement side between roughly 10 raV and roughly 1700
mV; just the region of high conductivities makes the use of the amplified voltage signal beneficial
since here, at correspondingly low mjection-side current flows, an unamplified voltage
measurement signal no longer yields sufficient resolution. In fact, up to roughly 200 mS/cm the
resolution remains better than 0.6% of the measured value so that up to this value the amplified
branch can be fundamentally omitted.
In the practical implementation this behavior means that both in the left-side region of
Figure 3, at small specific conductivities, and also in the right-side region of large conductivities,
good resolutions can be achieved, only on the boundaries of the measurement range, i.e. at 2 |iS/cm
18
or 200 mS/cm (relative to the unamplified branch) the resolution reaching the described region of
0.6%, but in between for many measurement tasks it is already completely sufficient and thus
generally does not require the additional amplified branch which is shown in the exemplary
embodiment.
V-..,
19 I
s

Claims
1. A device for measuring the electrical conductivity of a liquid medium, especially an
aqueous medium witli
constant voltage means (18) which are connected to a first electrode aiTangement (12) and which
ai-e made and wired for injecting an alternating signal into the liquid medium,
detection means (34) which are connected to second electrode arrangement (14), especially an
electrode pair, which is separated from the first electrode arrangement and which is coupled via the
liquid medium especially in the manner of a four-pole configuration, and generate a measurement
signal which is influenced by the electrical conductivity with a clock frequency (CLK) of the
alternating signal, and
evaluation means (26,40) which from a current-proportional injection signal (U(I) of the current
injected by the first electrode anangement into the liquid raediuin and from the measurement signal
(U(Lr)) generate an output signal, especially a conductivity signal of the liquid medium and make it
available for further electronic processing,
characterized in that
the constant voltage means are made in an individual circuit branch wki^ has commutator means
(24) for the first electrode arrangement and ohmic resistance means (20,22) in a series connection,
the current-proportional injection signal (17(1) being tapped as a voltage drop over the resistance
means, especially over a partial resistor (22) of the resistance means.
2. The device as claimed in Claim 1, wherein a separate first analog-digital converter unit
(261) for generating the digital current-proportional injection signal (AD(U(I))) which is not
20
integrated into the evaluation means is assigned to the resistance means.
3. The device as claimed in Claim 1 or 2, wherein a separate second analog-digital converter
unit (262) for generating the digital measurement signal (AD(U(U)) which is not integrated into the
evaluation means is assigned to the capacitance means which interact with the second electrode
arrangement.
4. The device as claimed in Claim 2 or 3, wherein the first and/or the second analog-digital
converter unit generates a serial output signal for the evaluation means (40) and interacts with them
via a serial connecting line, especially via a serial connecting line which is assigned individually to
each of the first and second analog-digital converter units.
5. TTie device as claimed in one of Claims 2 to 4, wherein the first and/or the second analogdigital
converter unit has preferably integrated filter means which are tuned to the clock frequency
of the alternating signals and/or of a supply voltage signal.
6. The device as claimed in Claim 1,2,4, and 5, wherein a common voltage supply unit
(50) is assigned to the first and/or the second analog-digital converter unit and is used preferably in
addition as a supply voltage unit for the constant voltage means (18) and/or is integrated into them
and/or the first and/or the second analog-digital converter unit has a common reference voltage
source for converter quantization.
7. The device as claimed in one of Claims 1 to 6, wherein the constant voltage means (18)
have an analog control unit and/or are implemented without integrated digital circuit modules.
8. Hie device as claimed in one of Claims 1 to 7, characterized by temperature detection
means (62) which are made for interaction with the liquid medium and which interact with the
evaluation means via an assigned third analog-digital converter unit (264) such that the temperature
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of a medium influences the conductivity signal.
9. The device as claimed in one of Claims 1 to 8, wherein the individual circuit branch is
made such that the ohmic resistance means are set up to be invariable, especially constant with their
ohmic resistance value, free of hysteresis and not able to be discretely switched over or selected,
and/or there are not active and/or switching elements in the circuit branch which influence the
ohmic resistance value.
10. The device as claimed in one of Claims 1 to 9, wherein the detection means have
capacitor means (34) whose capacitor voltage determines the digital measurement signal which is
generated continuously and/or independently of the clock frequency of the alternating signal.
11. The device as claimed in one of Claims 1 to 10, wherein the measurement signal (U(U))
is present parallel and synchronously on a first and a second input of the detection means with
signal levels which have been amplified to different degrees from one another.
12. The device as claimed in one of Claims 1 to 11, wherein a repetition frequency and/or
cycle frequency of the generation of the digital current-proportional injection signal and/or of the
digital measurement signal is set up to the clock frequency of the alternating signal in a ratio of less
than 1:100, especially less than 1:500.
13. The device as claimed in one of Claims 1 to 12, characterized by decoupling means,
especially ohmic resistance means (30), between respective electrodes (14) of the second electrode
arrangement and commutator means (32) wdiich are connected downstream of them.
14. A use of the device as claimed in one of Claims 1 to 13 for conductivity measurement of a
liquid, especially of water, wherein the conductivity signal has an uninterrupted range of conductivity
between 0.001 raS/cm and 850 mS/cm, especially between 0.001 mS/cm and 1000 mS/cm.