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A Comprehensive Look at Conductivity Measurement

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A Comprehensive Look at Conductivity Measurement
Presented at the 67th Annual International Water Conference, Engineers Society of Western Pennsylviania, Pittsburgh, Oct 2006
A Comprehensive Look at Conductivity Measurement
in Steam and Power Generation Waters
DAVID M. GRAY – Mettler-Toledo Thornton, Inc., Bedford, MA
IWC-06-29
KEYWORDS: conductivity, cation conductivity, degassed cation conductivity, cycle chemistry, TOC,
total organic carbon
ABSTRACT: Conductivity has been the simplest, oldest, most common, most reliable and lowest cost
measurement used for determining water purity. Nevertheless, this lowly measurement has provided the
means for protecting and assuring efficient operation of billions of dollars worth of power and steam
generation equipment throughout the world for over three-quarters of a century. Recent innovations in
measurement accuracy, temperature compensation, and sample conditioning have enabled this basic
detection method to provide more information and to expand its use even further. From the simplest TDS
meter to the most sophisticated ion chromatograph, conductivity measurement is the core technology.
The widespread use of this measurement often outpaces the understanding of it. Provided here
are the fundamentals and application of conductivity measurement as used in power and steam
generation, including ionic conductance, cell constant, measuring techniques and temperature effects. In
addition, various means of sample conditioning are discussed, covering the measurements of cation
conductivity, degassed cation conductivity, pH and CO2 calculations and even TOC (total organic carbon)
measurements. It is hoped this discussion will assist those applying conductivity measurement in any of
its incarnations to have a greater understanding and to obtain better results.
Although conductivity is non-specific,
responding to all ions, some ions are more
conductive than others. Figure 1 illustrates the
relative conductances of commonly encountered
cations and anions. The much higher
conductivity of hydrogen and hydroxide ions
makes conductivity much more sensitive to the
acids and bases in cation and specific
conductivity samples as will be discussed later.
It is also the reason that conductivity correlates
closely with pH in some samples.
One practice for determining total
dissolved solids in low pressure industrial boilers
takes into consideration the high conductance of
the hydroxide ion in high pH phosphate and/or
caustic treatment. The practice takes a grab
sample and adds a weak acid such as gallic
acid. The acid neutralizes the excess hydroxide
conductance without adding much conductance
of its own and allows the other ionic content of
the sample to be measured without the
dominating influence of hydroxide ions.
CONDUCTIVITY THEORY
Conductivity is an intrinsic property of a
solution—the ion-facilitated electron flow through
it. Electrons will not flow through water by
themselves but their charge can be carried by
ions. Conductivity is nearly proportional to ionic
concentration and is therefore a good indication
of water purity. The more ions, the more
conductive is the solution. Conductivity will not
detect non-ionic materials such as oils or many
other organics.
When a voltage is applied to two
electrodes immersed in a solution, ions between
them will be attracted by their opposite charge
and will move toward them to produce a current.
To prevent ion migration and electrochemical
reactions in the sample, AC voltage is always
used for conductivity measurements. The
polarity changes frequently enough that ions
don’t move or react significantly.
-1-
Equivalent Ionic Conductances
350
300
2
S cm /equivalent
250
200
150
100
50
0
H+
K+
NH4+
1/2Ca++
Na+
OH-
1/2SO4--
Cl-
HCO3-
Acetate-
Figure 1 - Conductance of Common Ions at 25 °C
CONDUCTIVITY SENSORS
siemens/cm
(S/cm).
However,
some
organizations and countries now adhere to strict
SI units of siemens/m (S/m) which are 100 times
smaller. Further confusion can arise when using
micro and milli prefixes when computer font
changes or incorrect abbreviation may covert “µ”
into “m”. Suffice it to say that considerable
vigilance is required in communicating
conductivity units accurately.
The
accuracy
of
conductivity
measurements is limited by the uncertainty of
the cell constant. Most conductivity sensors
used parallel plates as in Figure 2a more than
40 years ago. Balsbaugh and Thornton
pioneered the now common coaxial conductivity
sensor design shown in Figure 2b in the 1960s.
It provides the equivalent fixed length/area ratio
in a much more robust configuration. Quality
equipment suppliers calibrate and certify the
precise cell constant of each individual sensor to
allow entry of that value into the associated
analyzer to normalize the measurement.
The size of the electrodes immersed
and the spacing between them directly affect the
conductance measured. The cell constant is
based on the geometry of the two electrodes
used to measure the electrical conductance of
the fluid. It is the length between electrodes
divided by the cross-sectional area of fluid
between them, as illustrated in Figure 2a for a 1
-1
cm constant sensor. Lower cell constants
(closer spacing, larger cross-section) are
needed to provide good signals to the
measuring instrument for low conductivity
samples. Higher cell constants (wider spacing,
small cross-section) are needed to measure
high conductivity samples. The measuring
instrument must “know” what cell constant is
connected
and
normalize
the
readout
accordingly.
-1
The cell constant has units of cm and
results in conductivity having units of
1 cm
1 cm
1 cm
Figure 2a - Cell Constant Derivation
Figure 2b - Coaxial Conductivity Sensor
-2-
Presented at the 67th Annual International Water Conference, Engineers Society of Western Pennsylviania, Pittsburgh, Oct 2006
Ultimate traceability is to ASTM
potassium chloride solutions or to NIST
Standard Reference Materials.[1,2] Good cases
have also been made for using pure water as a
standard.[3,4] Precise certification of cell
constants involves meticulous work in a
standards laboratory and rigorous ISO9001
documented manufacturing procedures with
suitable apparatus.
A particularly thorough system for
calibrating and certifying cell constants has been
developed, with traceability as illustrated in
Figure 3. Laboratory standard sensors are
repeatedly calibrated in ASTM solutions and
then confirmed in a recirculating ultrapure water
ASTM D1125
Primary Standard
and Pure Water
at 15, 25, 40 °C
Standards Lab
Cell Constants
loop in the laboratory at three controlled
temperatures: 15, 25 and 40°C. Because of the
high conductivity temperature coefficient, the
three temperatures assure performance at three
significantly different conductivity levels in the
pure water range: 0.0314, 0.0550 and 0.1130
µS/cm respectively. These standard sensors are
then installed in a temperature controlled
recirculating ultrapure water loop in series with
production sensors as shown in Figure 4. Both
the cell constant and the RTD (resistance
temperature detector) temperature compensator
are calibrated precisely, in an installation very
similar to that of final use in a sample panel.
Production Cell
Constants
Figure 3 - Cell Constant Traceability
Mettler-Toledo Thornton, Inc.
Figure 4 - Cell Constant and Temperature
Compensator Calibration & Certification Facility
measuring circuit to reduce this effect to
maintain specifications.
In addition, the modest resistance and
temperature coefficient of RTDs makes them
vulnerable to leadwire resistance errors. Some
systems require temperature calibration after
installation. This requires the use of a certified
thermometer and a uniform temperature bath,
plus the time to reach complete temperature
equilibrium for every conductivity sensor—an
ordeal at best. Worse yet, this calibration may
be ignored, causing conductivity errors as high
as 5% of reading per degree C error in the
temperature
measurement.
Well-designed
systems handle this through the use of the
three-wire RTD measuring technique that
compensates for leadwire resistance and
eliminates the need for on-site calibration.
Cell constants are quite stable over time
in pure waters. However, if a sensor is
measuring samples carrying a significant
amount of corrosion products, it may be
necessary to establish a regular cleaning and
The calibration data is provided either
on the cell label, for manual entry into the
measuring instrument, or, with a Smart Sensor
system, the data is stored in the sensor'
s
memory for automatic reading by the measuring
instrument as soon as it is connected. Both
methods are provided with certificates of
calibration traceable to ASTM.
INSTALLED ACCURACY
Installed accuracy of conductivity
equipment requires first that the cell constant
and temperature compensator accuracies be
precisely certified at the manufacturer'
s site. But
high accuracy must be preserved in an
installation even if it involves hundreds of feet
separation between sensors and instruments.
The AC conductivity signal is vulnerable to
leadwire capacitance which increases with cable
length. A good conductivity system provides
specially shielded cable that works with the
-3-
exchangers. Conductivity also responds to the
treatment chemicals intentionally added.
verification procedure. Small volume flow
chambers should be used to produce enough
flow velocity that particles are carried through
the sensor and discharged. If there is some
accumulation, sensors should be cleaned with
dilute acid on a schedule determined by
experience. Sample line filters are not
recommended since the filtered particles would
accumulate and act as an ion sponge, greatly
slowing response downstream.
Cell constant re-verification is also
necessary. A convenient on-site procedure is
desirable. This can be done using a "plant
standard" sensor that is kept clean and used
only for such verifications. A comparison can be
made using a portable system with the plant
standard sensor in a temporary flow chamber
piped in series with the sensor being verified.
However, the standard sensor itself must be
periodically calibrated. It can be returned to the
manufacturer for re-certification or verified inhouse. Instruments that provide wide range of
measurement are most convenient for this.
Some
instrumentation
has
been
validated to measure accurately from pure water
to well over 147 µS/cm, the lowest of the ASTM
standard solutions, with a single cell constant.[5]
With such a system, direct calibration is greatly
simplified. Despite the gap between the
calibration point and the range of measurement,
this provides the best accuracy. Standards lower
than about 100 µS/cm have too high an
uncertainty (as a percentage of their value) due
to variable contamination from carbon dioxide in
air as well as the container. This uncertainty
would generate much larger errors than those
caused by the difference between calibration
point and measurement range. Conductivity
standard solutions are available from many
sources. Care should be taken in selecting and
using standards to obtain reliable results.
Specific (direct) conductivity measurement in
high pressure boiler cycles is typically
dominated by treatment chemicals such as
ammonia, phosphates, amines, etc. at ppm
concentrations which should be well above the
ppb concentrations of any contaminants.
Therefore, specific conductivity, often with
corroboration by pH, is generally used to monitor
and control treatment chemical concentrations.
Cation (acid) conductivity is the technique of
continuously conditioning the sample by passing
it through a hydrogen form cation exchange
cartridge. It enhances the sensitivity to
contaminants by two means:
• Ammonia or amines that are intentionally
present at ppm levels are removed. This
eliminates their high conductivity. The lower
ppb concentrations and conductivity of
contaminants can then be “seen” by the
measurement.
• Contaminant corrosive salts are converted
to their respective acids which are typically
three times as conductive as the original salt
because of the highly conductive hydrogen
ion.
This simplicity and detection sensitivity
have made cation conductivity the most widely
used measure of contamination in the cycle. The
upper left of Figure 5 illustrates the specific
conductivity sample containing the ammonia and
traces of sodium chloride and bicarbonate as
typical contaminants. It has a relatively high
conductivity due to the amount of ammonia
present, represented by the left bar of Figure 6.
The conductivity due to sodium chloride (or
other contaminants) is almost insignificant. The
sample passes through the cation exchanger
and the resin “R” retains the ammonia, sodium
and any other cations, but always with an
excess of resin in the hydrogen form near the
outlet. The exiting sample consists of very dilute
hydrochloric, carbonic (and other) acids and
yields the cation conductivity value represented
by the middle bar in Figure 6. A comparison of
the two bars shows the enhanced sensitivity for
contaminant detection noted above.
Although cation conductivity is widely
applied, there has been little standardization of
the cation exchange cartridge and this can add
many variables to the measurement.
In
particular, the sample flow velocity through the
cartridge must be high enough to promote
CONDUCTIVITY APPLICATIONS
Conductivity measurement responds to
all ionic contaminants in a sample, whether they
are minerals or dissolved gases such as carbon
dioxide (in the form of carbonic acid, bicarbonate
or carbonate). Contaminants come from many
sources. They can be deionizer regenerant
chemicals or other materials leaching from
inadequately rinsed or fouled resins, air
(including CO2) not completely removed by
deaerators, carry-over from drum boilers or
leaks from condensers or other heat
-4-
turbulent flow and good exchange. If the
exchange is not complete, the measurement can
be ambiguous. Reasonable guidelines for the
technique have been defined in an ASTM
standard.[6]
Specific
NH4+, OH-,
Na+, H+, Cl-,
HCO3-
Degassed Cation
H+, Cl-, OH-
CO2
R–NH4+
Degas Unit
R–Na+
Cation Exchanger
+
R–H
Cation
H , Cl , CO2, HCO3-, OH+
-
Figure 5 - Specific, Cation and Degassed Cation Conductivity Measurements
Conductivity (µS/cm)
4
3
NH3
2
1
0
HCl
CO2
NaCl
CO2
Specific
Cation
HCl
Degassed Cation
Figure 6 - Typical Specific, Cation and Degassed Cation Conductivity Response
Degassed cation conductivity is useful in
plants where carbon dioxide is present in the
sample and cation conductivity limits are
especially
low.
A
degas
unit
allows
measurement of cation conductivity with the
carbon dioxide removed as shown on the right
side of Figure 5. Carbon dioxide is
acknowledged to be far less corrosive than other
contaminants. Degassing can be done with a
“reboiler” which heats the sample to boiling to
drive off the carbon dioxide, with a sparging
scheme which flows pure nitrogen gas across a
thin film of the sample or with a membrane
contactor that pulls the carbon dioxide through a
membrane. Recent studies have been made on
the performance of “reboilers” that showed good
results.
[7]
One
evaluation
compared
performance of reboiler, nitrogen sparger, and
membrane systems for carbon dioxide removal
and they all performed reasonably well.[8]
-5-
effects can produce temperature coefficients as
high as 5 to 7% of conductivity value per °C.
Because of this, guidelines and specifications
reference conductivity values to 25 °C which
dictates sample temperature control and/or
compensation. The relationship is shown in
Figure 7, for neutral salt contaminants
exemplified by sodium chloride. Conductivity
increases with increasing amounts of sodium
chloride. However, the temperature coefficient
(slope) of the curves increases with decreasing
sodium chloride concentration. Also, it can be
seen that at conductivity values above 5 µS/cm,
the ionization of the water is insignificant and all
curves above that are parallel at about 2% per
°C. Below that, the non-linearity with both
concentration
and
temperature
requires
specialized temperature compensation. It should
be noted that very few laboratory instruments
have more than rudimentary 2% per °C
temperature compensation that is unsuitable for
low conductivity samples.
Where a reboiler is used and the sample is
measured near 100 °C, extremely accurate
temperature compensation is required. Some
systems re-cool the sample to near 25 °C for
this
reason.
Although
not
a
precise
measurement, degassed cation conductivity
does serve a very useful purpose.
TEMPERATURE COMPENSATION
Temperature Compensation is required
because conductivity is greatly influenced by
temperature. All dissolved ions become more
mobile and conductive at higher temperatures
largely because water becomes less viscous.
The increase is at a rate of approximately 2% of
conductivity value per °C for most ions. In
addition, in pure waters the increasing ionization
of the water itself with temperature is
significant—there are more hydrogen and
hydroxide
ions
to
conduct
at
higher
temperatures. The combination of these two
10.00
0.1
4605 ppb
2289 ppb
1.00
1
437 ppb
2 06 ppb
90 ppb
0.10
10
21 ppb
Resistivity (Mohm-cm)
Conductivity (µS/cm)
1131 ppb
0 p pb
0.01
NaCl Concentrations
0
10
20
100
30
40
50
60
Temperature (°C)
70
80
90
100
Figure 7 – Figure 7 - Temperature Effects on Pure Water with Neutral Salt Contamination
High purity water compensation has
traditionally been recognized as consisting of
two separate components: the properties of the
pure water and the properties of the impurities in
the water. This is appropriate for neutral salt
impurities since the ionization of water changes
only with temperature, not with dilute salt
concentration. Similarly, the mobility of salt ions
varies with temperature, independent of the
ionization of water. A quick visualization of these
effects is given by equations 1 and 2 which take
place independently of each other in very dilute
solutions. The combination of these independent
effects was quantified decades ago by General
Electric for use with nuclear boiling water reactor
(BWR) measurements where no treatment
chemicals were added. It must be noted that this
compensation is appropriate only for BWR
samples and for polished makeup water where
low levels of neutral salts are the anticipated
contaminants.
Conventional pure water sample
H2O
↔
H+ + OH-
-6-
(1)
NaCl
→
Na+ +
Cl-
conductivity samples because the hydrogen ion
has the dominant concentration and the
dominant ionic conductance. The anion is of
secondary importance. Thus sulfuric, formic,
acetic, carbonic and other acids would have
essentially the same conductivity temperature
characteristics.
The first cation conductivity temperature
compensation was developed using the best
curve matching that could be developed at that
time.[9] In the intervening years, a number of
other attempts have been made at providing this
compensation but most have failed to achieve
reasonable accuracy. A very close curve match
to hydrochloric acid in pure water across the full
ranges of temperature and concentration was
finally developed at Thornton. A comparison of
the compensation of several instrument
algorithms, with hydrochloric acid data as the
reference, is given in Figure 8. A vast
improvement was achieved, indicated by the
results with Instrument E.[10] It is apparent that
temperature compensation is by far the greatest
source of error in cation conductivity
measurement with most instruments installed in
U.S. power plants today.
Temperature compensation algorithms
have also been developed for specific
conductivity samples dominated by ammonia or
amines, and various instruments were
evaluated.
Similar
wide
variations
in
performance were again observed.[11]
While temperature compensation with
accurate algorithms can eliminate substantial
errors in the measurement, it should be stressed
that sample temperature is still important. Since
the sample first passes through a cation
exchange cartridge, the consistent performance
of that exchange is critical. The flow rate and
temperature ranges of the sample and the
dimensions of the cartridge, as well as the type
of resin can all influence cation conductivity
results.
Note that the cation conductivity
temperature compensation algorithm is also
appropriate for degassed cation conductivity
since carbonic is just one more of the acids that
may be present and all have similar temperature
effects.
(2)
CATION CONDUCTIVITY TEMPERATURE
COMPENSATION
Of
greatest
importance
is
the
temperature compensation accuracy of cation
conductivity which has become more critical with
the wide acceptance of equilibrium phosphate
and oxygenated treatment chemistries. The
benefits of both of these treatments depend
heavily on maintaining very low cation
conductivity levels.
The
challenge
of
pure
water
temperature compensation is to provide
conductivity readout referenced to 25 °C
regardless of the actual sample temperature.
This can eliminate the cost of expensive and
occasionally unreliable secondary sample
cooling equipment and can preserve the
measurement if a chiller fails.
Cation conductivity sample
H2O
↔
H+ + OHHCl
→
H+ + Cl-
(3)
(4)
A cation conductivity sample behaves
differently from makeup water. It has had
ammonia and/or amines removed and any salts
converted to acids as noted previously. The
ionization of acids and water cannot be neatly
separated for compensation since they have the
hydrogen ion in common. See equations 3 and
4. The additional hydrogen ions from the acid
suppress the hydroxide ion of water. This
interaction between water and acid ionization
and its variation with temperature require an
entirely different approach to temperature
compensation.
The
relationship
for
compensation is much more complex and has
pushed the limits of mathematical modeling for a
number of years. The standard is to be able to
compensate trace hydrochloric acid in pure
water across the ranges of concentration and
temperature to obtain the value at 25 °C.
Hydrochloric acid is representative of cation
-7-
% Error
25
20
15
10
5
0
-5
Instrument A
Instrument B
Instrument C
Instrument D
Instrument E
-10
-15
-20
Compensated Conductivity Error (%)
-25
0
10
20
30
40
50
60
Temperature (°C)
70
80
90
100
Figure 8 - Cation Conductivity Temperature Compensation Performance at 0.1 µS/cm
conditions, or upsets, their effects must be
accommodated.
Refinement of the conductivity and pH
correlation has been accomplished in various
ways by several organizations and was
discussed previously.[12] Specific conductivity is
the primary influence while cation conductivity is
used to trim for the presence of small amounts
of mineral and/or carbon dioxide contamination.
These algorithms still assume that the primary
specific conductivity (and pH) influence is
ammonia or amines and that the contaminants
have lower concentrations. Generally, the pH
must be within 7.5-10.5 and specific conductivity
must be greater than cation conductivity,
especially at low conductivity levels.
Specific conductivity is dominated by
hydroxide and ammonium (or amine cation)
which are at the highest concentrations.
Hydroxide ion conductivity is 3 times that of
other ions (except hydrogen, which is
suppressed at the high sample pH). Cation
conductivity is dominated by the hydrogen ion
which is about 7 times as conductive as other
ions (except hydroxide, which has a suppressed
concentration at the low pH in the sample at this
point) so it makes little difference just what the
mix of anions is among chlorides, bicarbonates,
sulfates, or others.
pH calculation algorithms provide very
accurate determinations when the sample
composition complies with the conditions above.
This excellent performance is acknowledged by
its use in plants around the world, especially in
Europe. However, it is also important to be
aware of the errors that can be produced when
PH CALCULATION FROM CONDUCTIVITY
The correlation of pH and conductivity of
ammonia has been used for decades to
compare cycle chemistry measurements. For a
given ammonia concentration in water there is a
definite pH and conductivity value which can be
calculated from dissociation and conductance
data. Because conductivity measurement is
typically more reliable than high purity pH
measurement, specific conductivity is often used
as the primary variable to control ammonia feed
although pH is also measured. There are two
reasons for conductivity’s higher reliability and
accuracy:
1. Conductivity is linear with concentration
whereas pH is logarithmic. pH therefore has
less resolution. For example, a change of
only 0.3 pH represents a two-fold (100%)
change
in
both
concentration
and
conductivity in cycle chemistry ranges.
2. pH reference electrode junction potential is
notoriously less stable in low conductivity
samples and that instability is frequently
greater than ± 0.1 pH, depending on the
electrode system used.
Inherent in the simple correlation above
is the assumption that there is nothing else
present but ammonia and water. Any traces of
carbon dioxide and/or mineral contaminants
must be negligible. Under many operating
conditions, this is a reasonable assumption.
However, as these trace contaminants grow in
concentration during plant startup, unusual
-8-
conditions outside the range for accurate pH
calculation as well as conditions exceeding cycle
chemistry guidelines.
The availability of both calculated and
electrode pH measurements allows more
accurate
calibration
of
the
electrode
measurement when operation is within the
conditions for accurate computation of pH. The
electrode system should previously have been
calibrated at two points using buffer solutions to
set up the span response. The final one-point
trim calibration eliminates variations in electrode
diaphragm/junction potential in buffer solutions
and can easily be done more frequently. It
greatly enhances the accuracy, reliability and
diagnostic
capability
of
the
electrode
measurement, as well as saving considerable
time.
operation goes well outside normal operating
conditions.
To eliminate this risk, multi-channel,
multi-parameter instrumentation can measure
specific and cation conductivities, compute pH
from them, and simultaneously measure from a
pH electrode as shown in Figure 9. This kind of
instrumentation covers both situations: it
provides highly accurate calculated pH
measurement under normal conditions and can
give a warning based on a pH electrode
measurement
when
conditions
become
abnormal.
The multi-parameter instrument can also
display and alarm on the difference between the
calculated and the measured pH values. This
kind of diagnostic can identify the need for pH
electrode maintenance or calibration, or warn of
Figure 9 - Two pages of the display of a multi-parameter instrument configured with sensors for Specific,
Cation and Degassed Cation Conductivity plus a direct pH electrode
CALCULATED CARBON DIOXIDE & ANIONS
FROM CONDUCTIVITY
tables in memory easily makes the conversion to
display, alarm and output ppb concentrations of
chlorides or sulfates.
As noted previously, plants may have
carbon dioxide present in steam and condensate
samples and it is worthwhile to distinguish it
from more corrosive contributors to the cation
conductivity. Degassed cation conductivity
provides this. Taken a step further, carbon
dioxide concentration can be inferred from the
difference between cation conductivity and
degassed
cation
conductivity.[13]
Again,
mutiparameter instrumentation can combine the
measurements by interpolating ASTM standard
tables in its memory to provide display and
output signals for carbon dioxide as shown in
the second screen of Figure 9.
Yet another parameter that can be
obtained is the anion concentration in the
degassed cation conductivity sample. If the
conductivity at that point is assumed to be all
due to chlorides or sulfates, a conversion to
concentration can be done. The same
instrumentation including ASTM conversion
TOTAL ORGANIC CARBON FROM
CONDUCTIVITY
Most organics are nonconductive and
cannot be sensed by conductivity alone.
Nevertheless, organics frequently foul resins
and can be corrosive and need therefore to be
detected and removed. The simplest method for
TOC (total organic carbon) detection is based on
high intensity ultraviolet oxidation of a
continuously flowing sample. Organics are
oxidized to carbon dioxide. Conductivity
measurements are made both upstream and
downstream of the UV lamp. The first
measurement accounts for any conductive
contaminants already in the sample while the
second measurement detects the increased
conductivity due to added carbon dioxide. The
conductivity measurements and their difference
-9-
than the TOC concentration since the carbon
dioxide oxidation product would otherwise be
neutralized by alkaline materials and would not
produce the expected increase in conductivity.
For these reasons this technology has been
recommended primarily for the clean end of
makeup water treatment systems.[14] However,
more recent evaluations on ammonia-treated
cycle chemistry samples measuring after a
cation exchange cartridge show promising
results with a direct conductivity TOC sensor.
Clean cation resin allows organics to pass
through and lowers the pH and conductivity to a
range where this TOC measurement technique
excels. This type of TOC sensor is also available
for use with the multiparameter instrumentation
previously
noted.
are compared using a correlation curve to
produce a consistent TOC measurement.
Figure 10 shows the flow path of this
kind of TOC sensor. Oxidation takes place in a
quartz coil surrounding the high intensity UV
lamp. It can be seen that the sample flows
continuously and there are no solenoid valves,
moving parts, membranes or reagents.
Response time is particularly fast, taking less
than 60 seconds for the sample to pass from the
inlet to the final conductivity sensor. The simple
design reduces maintenance to simple
replacement of the UV lamp on an annual basis.
This technology does have application
limitations. It is used only on pure water samples
with conductivity 2 µS/cm and TOC of 0.05 1000 ppb. Also, alkalinity should be much less
Figure 10 – Flow path of a direct conductivity TOC sensor
CONCLUSION
as significant to plant equipment reliability and
life as the detection and correction of upset
conditions.
Accurate
application-specific
conductivity temperature compensation has
greatly aided these improvements. A number of
sample conditioning techniques plus multiparameter instrumentation allow conductivity
measurements to obtain even more specific
information about contaminants.
In the past, conductivity measurements
have been viewed as only trend indicators to
detect major changes in operating conditions.
Today, there is more understanding of long term
corrosion mechanisms. Observing the absolute
conductivity limits of the applicable EPRI or
ASME guidelines and turbine manufacturer
specifications over long-term operation can be
REFERENCES
1. Standard Test Methods for Electrical Conductivity and Resistivity of Water, D1125, American Society
for Testing and Materials, W. Conshohocken, PA, 1999.
2. Certificates of Analysis, Aqueous Electrolytic Conductance Standard Reference Materials 3190-3196,
3198-3199, National Institute of Standards and Technology, U. S. Department of Commerce,
Gaithersburg, MD.
3. K. Griffiths, D. Gray, A. Bevilacqua, and T. Light, "Ultrapure Water as a Fundamental Standard for the
Calibration of Conductivity/Resistivity Systems," Eskom Power Plant Chemistry Conference,
Johannesburg, South Africa, November, 1997.
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4. T. Light, S.Licht, A. Bevilacqua, K. Morash, “The Fundamental Conductivity and Resistivity of Water,”
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