What is conductivity?
- Conductivity is the ability of a material to transfer an electric charge from one point to another.
- Resistance is the inverse of conductance.
- The units used from this property are Mhos OR more commonly used Siemens.
- Mhos and Siemens can be used interchangeably.
- Materials that are in the liquid phase and conduct electrical current are call electrolytes.
How Conductivity is Measured?
There are two ways conductivity in liquids or slurries is measured :
- Contacting (or Electrode) Measurement.
- Toroidal (Inductive or Electrodeless) Measurement.
Contacting OR Electrode Type Measurement (Fig. 2):
- The two-electrode methodology uses two opposing electrodes.
- The anode is supplied with a known current, which is picked up by the cathode when placed in an electrolyte.
- The amount of current picked up by the cathode is dependent upon the conductance of the electrolyte.
The cell constant is the distance between the electrodes divided by the Area of the electrodes.
K (cm-1) = Distance between electrodes (cm) / Area of electrodes (cm)
The smaller the cell constant the higher the signal that will be returned to the meter.
Low conductivity solutions use a small cell constant and high conductivity solutions will use a larger cell constant sensor.
Two Electrode Methodology:
Its main drawback are:-
- The sensor is susceptible to coating and corrosion, which drastically lowers the reading.
- In strongly conductive solutions there can also be polarization effects, which results in non-linearity of the measurement.
The Four Electrode Methodology:
- It utilizes the same two electrode measuring scheme, but it also includes additional two electrode system to act as a reference point for the measuring circuit, for use in applications were light coats from the process can occur.
- The minimum range for this type of electrode is approximately 5000 micro S/cm.
Toroidal Type Measurement (Fig. 2):
- A toroidal conductivity measurement is made by passing an AC current through a Toroidal drive coil, which induces a current in the electrolyte solution.
- This induced solution current, in turn, induces a current in a second toroidal coil, called the pick-up toroid.
- The amount of current induced in the pick-up toroid is proportional to the solution conductivity.
Advantages of Toroidal Type Conductivity:
- The toroidal coils are not in contact with the solution.
- The insertion-style Toroidal sensor can be completely coated by a solid OR oily contaminant in the process, with essentially NO lowering of the reading until the coating displaces a significant volume of the surrounding liquid.
- The polymeric material housing the Toroids can be chosen to be compatible with corrosive solutions,
Drawbacks of Toroidal Type Conductivity Measurement :
- It lacks the sensitivity of contacting measurement.
- Toroidal sensors are also typically larger than contacting sensors, and the solution current induced by the Toroid occupies a volume around the sensor. Hence, Toroidal sensors need to be mounted in a larger pipe.
Temperature Effects on Conductivity Measurement (Fig. 3):
- Temperature does have a significant effect on the conductivity of solutions.
- The same solution concentration at different temperature has a drastically different conductivity.
- In order to measure conductivity temperature affect must be compensated.
For Temperature compensation follow the steps shown in Fig. 3
The temperature coefficients of the following electrolytes generally fall in the ranges shown below:
- Acids 0 – 1.6%/°C
- Bases 8 – 2.2%/°C
- Salts 2 – 3.0%/°C
- Fresh water 0%/°C
Calibration of Conductivity:
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.
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, reference instrument in a closed flow loop.
Applications of Conductivity Measurements (Fig. 4):
Advantages of Conductivity Measurement:
- Conductivity offers Fast, Reliable, Nondestructive, Inexpensive &
- Durable means of measuring the ionic content of a sample.
- Reliability and repeatability are excellent.
Disadvantages of Conductivity Measurement:
- The principle drawback of conductivity is that it is a nonspecific
- Measurement – it cannot distinguish between different types of ions,
- giving instead a reading proportional to the combined effect of all
- ions present. Therefore it must be applied with some pre-knowledge
- of the solution composition OR used in relatively pure (single solute) solutions to be successful.