Conductivity Sensors

What Is Conductivity?

Conductivity is the measure of a solution's ability to pass or carry an electric current. The term conductivity is derived from Ohm's Law, E=I•R; where Voltage (E) is the product of Current (I) and Resistance (R); Resistance is determined by Voltage/Current. When a voltage is connected across a conductor, a current will flow, which is dependent on the resistance of the conductor.  Conductivity is simply defined as the reciprocal of the resistance of a solution between two electrodes.

How Do We Measure Conductivity?

There are two basic sensor styles used for measuring Conductivity: Contacting and Inductive (Toroidal, Electrodeless).

When Contacting Sensors are used, the conductivity is measured by applying an alternating electrical current to the sensor electrodes (that together make up the cell constant) immersed in a solution and measuring the resulting voltage. The solution acts as the electrical conductor between the sensor electrodes.

With Inductive (also called Toroidal or Electrodeless), the sensing elements (electrode coils) of an inductive sensor do not come in direct contact with the process. These two matched (identical coils) are encapsulated in PEEK (or Teflon) protecting them from the adverse effects of the process.   

What Makes a Solution Conductive? 

Ions present in the liquid (Na, Ca, Cl, H, OH) are responsible for carrying the electric current an electrical conductivity sensor detects.

Conductivity is only a quantitative measurement: it responds to all ionic content and cannot distinguish particular conductive materials in the presence of others. Only ionizable materials will contribute to conductivity; materials such as sugars or oils are not conductive.

Conductivity applications cover a wide range from pure water at less than 1x10^-7 S/cm to concentrated solutions with values greater than 1 S/cm. Such application examples are WIFI, demineralizer water, RO water, percent concentration, boiler blowdown, and TDS.

In general, the measurement of conductivity is a rapid and inexpensive way to determine the ionic strength of a solution. Water conductivity transmitters use conductivity to measure the purity of water or the concentration of ionized chemicals in water. It is a nonspecific technique, unable to distinguish between different types of ions, giving instead a reading that is proportional to the combined effect of all the ions present.

The accuracy of the measurement is strongly influenced by temperature variations, polarization effects at the surface of the contacting electrodes, cable capacitances, etc.

Yokogawa has designed a full range of precision water conductivity controllers, sensors, and instruments to cope with these measurements, even under extreme conditions.

Industrial Applications of Conductivity Sensors

• Conductivity Measurements for Boiler Blowdown
• Cooling Towers
• Edible Fat and Oil Processing
• Conductivity Measurements for Heat Exchangers
• Salt Dissolvers at Electrolysis Plants
• Conductivity Measurements for Clean-in-Place (CIP) System
• Conductivity Measurements for Pharmaceutical Water
• Brine Manufacturing
• Measuring Concentrations of Developing Solutions and Fluoric Acid
• Liquid Analyzer Measurements for Reverse Osmosis

How Do I Select The Right Sensor? 

When selecting an industrial conductivity sensor probe for an application, we need to consider the following:

• What is the measurement range? (This dictates which cell constant will be required).
• What is the process temperature? (We have standardized on Pt1000)
• What is the chemical makeup of the process? (This determines what material of construction we offer to ensure chemical compatibility).

What is a cell constant, and why do we need to be concerned about it?

The cell constant is mathematical value for a "multiplying factor" that is used to determine the measuring range of the sensor. This mathematical value is determined by the geometric design of the cell. It is calculated by dividing the distance (length) between the two measuring plates by the area of the plates (area of the plates is determined by the area of the outside-the area of the inside = area between the electrodes).

The raw conductivity value is then multiplied by the cell constant which is why we see the unit µS (micro siemens)/cm.

Yokogawa offers four cell constants: 0.01, 0.1, 1.0, and 10.0, which provide accurate of the entire Measurement range of 0-2,000,000 µS. These values are known as the nominal cell constant, whereas the printed cell constant on the sensor can vary slightly (you will see 0.0198 instead of 0.02) is the specific cell constant for that sensor. 

One of the problems that occur when an incorrect cell constant is used is Polarization. 

The first example shows a solution with the correct cell constant where the ions are free to travel from one plate to the other.

The second example shows the same cell constant being used in a highly conductive solution.  When the voltage alternates (switches polarity), the ions cannot freely move to the other plate because the ion density is too high.  This results in fewer ions contacting the correct plate which will result in a false low reading.

However, for the ISC40 Inductive Sensor, there is only one cell factor (constant). It covers the entire conductivity measurement range 0-2,000 S/cm. But on only the low end (below 50µS) does the accuracy of the sensor suffer.