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Conductivity Meter
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- 2. Contents • Introduction • Principleof Conductivity Meter • Conductivity Meter Types • Contacting-type Conductivity Meter • Inductive Conductivity Meter • Benefits of Inductive Conductivity Meter • Drawbacks of Inductive Conductivity Meter • Temperature Dependence • Calibration of Conductivity Meter • Conductivity Meter Applications
- 3. Introduction • A conductivitymeter measures the amount of electrical current or conductance in a solution. • Conductivity is useful in determining the overall health of a natural water body. • Conductivity meters are common in any water treatment or monitoring situation, as well as in environmental laboratories.
- 4. Introduction • A conductivitysystem measures conductance by means of electronics connected to a sensor immersed in a solution. • The analyzer circuitry impresses an alternating voltage on the sensor and measures the size of the resulting signal. • An integral temperature sensor incorporated into its circuitry adjusts the reading to a standard temperature • The units of conductivity are siemens per cm (S/cm).
- 5. Principle of ConductivityMeter • The common laboratory conductivity meters employ a potentiometric method and four electrodes. • Often the electrodes are cylindrical and arranged parallel. • The electrodes are usually made of platinum metal. • An alternating current is applied to the outer pair of the electrodes. • Conductivity could in principle be determined using the distance between the electrodes and their surface area. • Generally for accuracy a calibration is employed using electrolytes of well- known conductivity.
- 6. Conductivity Meter Types Twotypes of conductivity meters use in industry and Laboratory • Contacting-type Conductivity Meter • Inductive Conductivity Meter
- 7. Contacting-type Conductivity Meter •Most contacting conductivity sensors consist of two metal electrodes. • Usually stainless steel or titanium, in contact with the electrolyte solution. • The analyzer applies an alternating voltage to the electrodes. • The electric field causes the ions to move back and forth producing a current. • The charge carriers are ions, the current is called an ionic current.
- 9. Contacting-type Conductivity Meter •The ionic current depends on the total concentration of ions in solution and on the length and area of the solution. • Then multiplying the conductance by the cell constant corrects for the effect of sensor geometry on the measurement. • The result is the conductivity which depends only on the concentration of ions. • The cell constant is measured at the factory and the user enters the value in the analyzer when the sensor is first put in service.
- 10. Contacting-type Conductivity Meter •Some contacting sensors have four electrodes. • In the four-electrode measurement, the analyzer injects an alternating current through the outer electrodes. • Measures the voltage across the inner electrodes. • The analyzer calculates the conductance of the electrolyte solution from the current and voltage. • Two-electrode sensors are ideal for measuring high purity water in semi-conductor, steam electric power, and pharmaceutical plants.
- 11. Inductive Conductivity Meter •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.
- 12. Inductive Conductivity Meter •The voltage causes an ionic current to flow proportional to the conductance of the liquid. • The current in the receive coil depends on the number of windings in the . • 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.rive and receive coils.
- 14. Benefits of InductiveConductivity Meter • 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. • Inductive sensors tolerate high levels of fouling, they can be used in solutions containing high levels of suspended solids. • Inductive sensors are ideal for measuring solutions having high conductivity.
- 15. Benefits of InductiveConductivity Meter • High conductivity solutions produce a large, easily measured induced current in the receive coil. • They can be cleaned with soap or solvents and a brush. There are no electrodes so here is no possibility of their damage. • Inductive sensors usually have a fairly large hole which permits free flushing.
- 16. Drawbacks of InductiveConductivity Meter • They are restricted to samples having conductivity greater than about 15 μS/cm. They cannot be used for measuring low conductivity solutions. • Calibration is awkward because clearance in the calibration baths must be provided for the external field. Calibration is especially inconvenient if the sensor is already mounted. • Errors can come easily in the instrument.
- 17. Temperature Dependence • Theconductivity of a solution is highly temperature dependent therefore it is important to either use a temperature compensated instrument. • Calibrate the instrument at the same temperature as the solution being measured. • The conductivity of common electrolytes typically increases with increasing temperature. • Over a limited temperature range, the way temperature affects the conductivity of a solution.
- 18. Calibration of ConductivityMeter • Calibration leads to a more accurate reading. • To calibrate a meter, follow the instructions for that meter in general.These steps are easy and standardized. • The meter usually has a menu item that allows you to enter the calibration mode. • Change the setting on the side that you can adjust with a small screwdriver or tool. • Place the probe in a solution with a known conductivity value and temperature and set the meter to that conductivity.
- 19. Conductivity Meter Applications •The instrument is used in concentration Measurement. This is the simplest and one of the most widely used applications. • Conductivity meters are also used in Leakage detection. • Water used for cooling in heat exchangers and surface condensers. • Heat exchangers contains large amounts of dissolved ionic solids. • Leakage of the cooling water can result in potentially harmful contamination.
- 20. Conductivity Meter Applications •Used in Interface detection. If two liquids have appreciably different conductivity, a conductivity sensor can detect the interface between them. • In the pharmaceutical and food and beverage industries, piping and vessels are periodically cleaned and sanitized in a procedure called clean-in-place. • Conductivity is used to monitor both the concentration of the CIP solution, typically sodium hydroxide, and the completeness of the rinse. •
Editor's Notes
- #6 Potentiometry is one of the methods of electroanalytical chemistry. It is usually employed to find the concentration of a solute in solution. In potentiometric measurements, the potential between two electrodes is measured using a high impedance voltmeter.
- #8 Conductive ions, such as salts and metals, produce a path for current to flow. Therefore, high conductivity indicates high ionic concentration
- #10 The cell constant, K, is equal to the distance in cm between the probe's electrodes divided by the surface area of the electrodes in cm2. For solutions with low conductivities the electrodes can be placed closer together or made larger so that the cell constant is less than one. y using the appropriate probe, K=0.1 for low conductivity solutions, K=1 for normal solutions and K=10 for high conductivity solutions, accurate measurements across the full range of conductivity values can be made.