Slide 1
Conductivity
Conductivity measurement has widespread use in industrial
applications that involve the detection of contaminants in water
and concentration measurements.
Conductivity measures how well a solution conducts electricity.
The units of conductivity are Siemens/cm (S/cm)Conductivity
Conductivity measurements cover a wide range of solution
conductivity from pure water at less than 1x10-7 S/cm to values in
excess of 1 S/cm for concentrated solutions .
For convenience, conductivity is usually expressed in the units
of microSiemens/cm (S/cm, one millionth of a Siemen/cm) or
milliSiemens/cm (mS/cm, one thousandth of a Siemen/cm)
Conductive Solutions
Conductivity is typically measured in aqueous (water) solutions
of electrolytes. Electrolytes are substances that ionize separate
into charged particles called ions.The ions formed in solution are
responsible for carrying the electric current. Electrolytes include
acids, bases, and salts.
Conductivity is Non-specificA conductivity measurement responds
to any and all ions present in a solution. A solution cannot be
identified, or its concentration known, from conductivity alone .
In certain cases, the concentration of an electrolyte in solution
can be determined by conductivity if the composition of the
solution is known.
MEASUREMENT OF CONDUCTIVITY
CONTACTING CONDUCTIVITYTOROIDAL (INDUCTIVE) CONDUCTIVITY
CONTACTING CONDUCTIVITYContacting conductivity uses a sensor
with two metal or graphite electrodes in contact with the
electrolyte solution. An AC voltage is applied to the electrodes by
the conductivity analyzer, and the resulting AC current that flows
between the electrodes is used to determine the conductance.
Probe Constant:
The amount of current that flows between the electrodes depends
not only on the solution conductivity, but also on the length,
surface area, and geometry of the sensor electrodes. The probe
constant (also called "sensor constant" or "cell constant") is a
measure of the current response of a sensor to a conductive
solution, due to the sensors dimensions and geometry. Its units are
cm-1 (length divided by area), and the probe constant necessary for
a given conductivity range isbased on the particular conductivity
analyzer's measuring circuitry. Probe constants can vary from 0.01
cm-1 to 50 cm-1 and, in general, the higher the conductivity, the
large the probe constant necessary.Characteristics of Contacting
Conductivity
Contacting conductivity can measure down to pure water
conductivity. Its main drawback is that the sensor is susceptible
to coating and corrosion, which drastically lowers the reading. In
strongly conductive solutions.
Temperature Effects
The conductivity of a solution typically increases with
temperature. In moderately and highly conductive solutions, this
increase can be compensated for using a linear equation involving a
temperature coefficient (K), which is the percent increase in
conductivity per degree centigrade. The temperature coefficients of
the following electrolytes generally fall in the ranges shown
below:Acids 1.0 - 1.6%/CBases 1.8 - 2.2%/CSalts 2.2 - 3.0%/CFresh
water 2.0%/C
Temperature Compensation in High Purity Water
In solutions with a conductivity of 1 S/cm or less, the
conductivity increase with temperature is highly nonlinear.
Conductivity applications, at or below 1.0 S/cm, require high
purity temperature compensation to avoid large errors.
This occurs because the conductivity of water itself is a large
fraction of the overall conductivity.
Temperature compensation for these solutions must not only take
into account the increase in the conductivity of water, but also
the increase in conductivity of the solute (dissolved
electrolyte).
The increase in the conductivity due to the solute will also
depend upon what type of electrolyte is present, i.e., acid, base,
or salt.
CONDUCTIVITY CALIBRATION
Moderate to High Range Measurements
For conductivity measurements in excess of 100 S/cm, a
conductivity standard may be used to calibrate a conductivity loop.
The conductivity measurement may also be calibrated using grab
sample standardization.Care must be taken that the correct
temperature coefficient is being used in both the on-line
instrument and the referee instrument to avoid discrepancies based
on temperature compensation errors.
CONDUCTIVITY CALIBRATION
High Purity Water Measurements
Conductivity samples below 100 S/cm are highly susceptible to
contamination by trace contaminants in containers and by CO2 in
air.
As a result, calibration with a conventional standard is not
advisable.Many conductivity instruments designed for high purity
water measurements include a calibration routine for entering the
constant of the conductivity sensor.
The conductivity sensor used with this kind of instrument must
have its sensor constant accurately measured using a conductivity
standard in a higher range.
Once the sensor constant is entered into the instrument, the
conductivity loop is calibrated.
A second method is to calibrate the on-line instrument to a
suitably calibrated, referee instrument in a closed flow loop.
CONDUCTIVITY APPLICATIONS
Non-Specific ApplicationsNon-specific applications involve
simply measuring conductivity to detect the presence of
electrolytes.The majority of conductivity applications fall within
this category. They include monitoring and control of
demineralization, leak detection, and monitoring to a prescribed
conductivity specification. In most instances, there is a maximum
acceptable concentration of electrolyte, which is related to a
conductivity value, and that conductivity value is used as an alarm
point.
Concentration Measurements
Conductivity is non-specific, even though it can sometimes be
applied to concentration measurements if the composition of the
solution and its conductivity behavior is known.The first step is
to know the conductivity of the solution as a function of the
concentration of the specie of interest. This data can come from
published conductivity vs. concentration curves for electrolytes,
or from laboratory measurements. The conductivity of mixtures
usually requires a laboratory measurement, due to the scarcity of
published conductivity data on mixed electrolytes.Over large
concentration ranges, conductivity will increase with
concentration, but may then reach a maximum and then decrease with
increasing concentration.It is important to use conductivity data
over the temperature range of the process, because the shape of the
conductivity vs. concentration curve will change with temperature,
and a concentration measurement may be possible at one temperature
but not at another.
Some different cases of concentration measurements:
SUMMARY
Conductivity is the ability of a solution to conduct
electricity. Conductivity measurement can be applied to the full
range of water solutions, from high purity water to the most
conductive solutions known. Things to consider in applying
conductivity:1. Use contacting conductivity for low conductivity
applications in clean process streams.2. For accuracy in
applications approaching 1 S/cm, use contacting conductivity with
high purity water temperature compensation.3. Use toroidal
conductivity for dirty, corrosive, or high conductivity
applications.4. If a concentration measurement is required, the
full stream composition, as well as its conductivity behavior over
the desired concentration and temperature range must be known. If a
reliable estimate cannot be made from published data, data must be
gathered in the laboratory.
Inductive Conductivity is sometimes called toroidal or
electrodeless conductivity.
An inductive sensor consists of two wire-wound metal toroids
encased in a corrosion-resistant plastic body. One toroid is the
drive coil, the other is the receive coil.
The sensor is immersed in the conductive liquid. The analyzer
applies an alternating voltage to the drive coil, which induces a
voltage in the liquid surrounding the coil.
The voltage causes an ionic current to flow proportional to the
conductance of the liquid. The ionic current induces an electronic
current in the receive coil, which the analyzer measures.
The induced current is directly proportional to the conductance
of the solution.
Troidal CONDUCTIVITY
The current in the receive coil depends on the number of
windings in the drive and receive coils and the physical dimensions
of the sensor.
The number of windings and the dimensions of the sensor are
described by the cell constant.
As in the case of contacting sensors, the product of the cell
constant and conductance is the conductivity.
The walls of the tank or pipe in which the sensor is installed
also influence the cell constantthe so-called wall effect.
A metal (conducting) wall near the sensor increases the induced
current, leading to increased conductance and a corresponding
decrease in the cell constant.
A plastic or insulating wall has the opposite effect. Normally,
wall effects disappear when the distance between the sensor and
wall reaches roughly three-fourths of the diameter of the
sensor.
For accurate results, the user must calibrate the sensor in
place in the process piping
.
The inductive measurement has several benefitsFirst, the toroids
do not need to touch the sample. Thus, they can be encased in
plastic, allowing the sensor to be used in solutions that would
corrode metal electrode sensors.
Second, because inductive sensors tolerate high levels of
fouling, they can be used in solutions containing high levels of
suspended solids. As long as the fouling does not appreciably
change the area of the toroid opening, readings will be
accurate.
High conductivity solutions produce a large, easily measured
induced current in the receive coil. Inductive sensors do have
drawbacks. Chiefly, they are restricted to samples having
conductivity greater than about 15 S/cm.They cannot be used for
measuring low conductivity solutions.
Some features of CR200 THRONTON1. Measure four signals and
compute four measurements.2. Check setpoints against the
measurements.3. Control the relays.4. Update analog output
signals.5. Transmit measurement data over the communication port.6.
Display data (if not displaying menu).
Linear CompensationThe raw resistance measurement is compensated
by multiplication with a factor expressed as a % per C (deviation
from 25C). The range is 0 - 99%/C with a default value of 2%/C.
Standard CompensationThe standard compensation method includes
compensation for non-linear high purity effects as well as
conventional neutral salt impurities and conforms to ASTM standards
D1125 and D5391.
Temperature Compensation
Some types of temperature compensation:
SENSOR CALIBRATION:
Transmitter : 5081 RosemountSensor : 400-13
Calibration of 5081-C-HT1. With the sensor in a standard
solution of known conductivity value, allow the temperature of the
sensor to stabilize (10 min).2. To access the CALIbrAtE menu, press
the CAL button on the IRC.3. Press ENTER to access the CAL segment
with flashing prompt.4. Use the IRC editing keys to indicate the
conductivity values of the standard solution on the screen.5. Press
ENTER then EXIT to enter the standard solution value and return to
the main screen.
Sensor 0
From the main screen, press CAL, then press NEXT to enter the
SEnSOr 0 menu. Press ENTER to access the SEnSOr 0 sub-menu. With
the sensor attached and in air, press ENTER again to zero the
sensor. Press EXIT to return to the SEnSOr 0 sub-menu.
Temp Adj
1. Press NEXT and then ENTER to access the tEMP sub-menu with
flashing prompt. With the sensor in any solution of known
temperature, allow the temperature of the sensor to stabilize. Use
the editing keys of the IRC to change the displayed value as
needed.2. Press ENTER to standardize the temperature reading and
return to the tEMP AdJ screen.
Cell Constant
1. When the CALibrAtE sub-menu has been accessed, press NEXT
four (4) times and then ENTER to access the CELLCOnSt menu segment
with the flashing cell constant prompt.2. Using the arrow keys on
the IRC, enter your sensors cell constant as indicated on the
sensors tag or specificationsheet.3. Press ENTER to save the cell
constant into the transmitter memory and return to the CELL COnSt
sub-menu.
Temp Slope
1. Press NEXT to enter the tEMP SLOPE menu.The correct
temperature slope must be entered into the transmitter to ensure an
acceptable process variable measurement under fluctuating process
temperature conditions. Enter the slope in measured conductivity
units per degree temperature change using the IRCs arrow keys.
Press ENTER to enter the slope into memory; then press EXIT to
return to the main screen.2. If the temperature slope of the
process is not known but you wish to approximate it, refer to the
following guide and press ENTER to proceed on to tSLOPE sub-menu
with flashing prompt. Utilize the IRC editing keys to generate the
desired slope value. Press ENTER then EXIT to return to the main
screen.Acids: 1.0 to 1.6% per CBases: 1.8 to 2.2% per CSalts: 2.2
to 3.0% per CWater: 2.0% per C
Output calibration
Thank you for your attentionPresented by:Eng. Mohamed
SarhanWaiting Your feedback
