Conductivity measurement involves measuring how well a solution conducts electricity. There are two main types of conductivity sensors: 1. Contacting sensors which use electrodes in contact with the solution and can measure low conductivities. They are susceptible to fouling. 2. Inductive (toroidal) sensors which do not contact the solution and can be used in dirty applications. They require a minimum conductivity of 15 μS/cm. Proper calibration and temperature compensation are important for accuracy. Contacting sensors are calibrated using standard solutions while inductive sensors require in-situ calibration accounting for installation effects. Temperature compensation considers the nonlinear increase in water conductivity and solute type.

2. 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

3. 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 (μS/cm, one millionth of a Siemen/cm) or
milliSiemens/cm (mS/cm, one thousandth of a Siemen/cm)

4. 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.

5. Conductivity is Non-specific
A 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.

6. MEASUREMENT OF CONDUCTIVITY
•CONTACTING CONDUCTIVITY
•TOROIDAL (INDUCTIVE) CONDUCTIVITY

7. CONTACTING CONDUCTIVITY
Contacting 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.

8. 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 sensor’s
dimensions and geometry. Its units are cm-1 (length divided by area),
and the probe constant necessary for a given conductivity range is
based 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.

9. 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%/°C
Bases 1.8 - 2.2%/°C
Salts 2.2 - 3.0%/°C
Fresh water 2.0%/°C

11. Temperature Compensation in High Purity Water
In solutions with a conductivity of 1 μS/cm or less, the conductivity
increase with temperature is highly nonlinear.
Conductivity applications, at or below 1.0 μS/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.

12. CONDUCTIVITY CALIBRATION
Moderate to High Range Measurements
For conductivity measurements in excess of 100 μS/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.

13. CONDUCTIVITY CALIBRATION
High Purity Water Measurements
Conductivity samples below 100 μS/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.

14. CONDUCTIVITY APPLICATIONS
•Non-Specific Applications
Non-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.

15. 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.

16. Some different cases of concentration measurements:

17. 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 μS/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.

18. 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

19. 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.

20. The walls of the tank or pipe in which the sensor is installed also influence
the cell constant—the 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

21. .
The inductive measurement has several benefits
• First, 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
µS/cm.They cannot be used for measuring low conductivity solutions.

22. Some features of CR200 THRONTON
1. 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).

23. •Linear Compensation
The raw resistance measurement is compensated by multiplication
with a factor expressed as a “% per °C” (deviation from 25°C). The
range is 0 - 99%/°C with a default value of 2%/°C.
•Standard Compensation
The 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:

24. SENSOR CALIBRATION:

26. Transmitter : 5081 Rosemount
Sensor : 400-13

28. Calibration of 5081-C-HT
1. 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.

29. 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.

30. 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.

31. 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 sensor’s cell
constant as indicated on the sensor’s tag or specification
sheet.
3. Press ENTER to save the cell constant into the transmitter
memory and return to the CELL COnSt sub-menu.

32. 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 IRC’s 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 °C
Bases: 1.8 to 2.2% per °C
Salts: 2.2 to 3.0% per °C
Water: 2.0% per °C

33. Output calibration