IC CONTROLS An Overview of Conductivity

What is Conductivity Electrolytic conductivity is a measure of the ability of a solution to carry a current. It is defined as the reciprocal or resistance, in ohms, of a 1cm3 of liquid at a specified temperature which is expressed as siemens/cm. As the range of the conductivity is very low it is frequently expressed as millionths of a siemens or microsiemens/cm (S/cm). Current in liquid is carried by ions. Any change in concentration of ions will change the conductivity of a liquid. Ions are formed in a liquid when a solid, such as a salt, is dissolved. NaCl (table salt) is dissolved to form Na+ and Cl- and will contribute to the overall conductivity of the solution.

Conductivity Terminology and Formulas Conductivity = Kcell 100-TC(25.0-TTC) R 100 Kcell = cell constant in cm-1 R = Measured resistance in ohms TC = temperature compensation factor as % change per °C TTC = measured temperature in °C

Conductivity Terminology and Formulas Kcell can range from 0.01 cm-1 to 50.0 cm-1 Conductance = 1 Resistance Conductivity = 1 = Conductance/cm Resistivity Cell Constant = Kcell = L A L = distance in cm between electrodes A = Area in cm2 of the electrodes

Conductivity Terminology and Formulas MEASUREMENT UNITS Resistance Ohm Conductance siemens Resistivity Ohmcm Conductivity siemens/cm

Conductivity for Commonly Used Chemicals Conductivity is an effective measurement of concentration of pure chemicals. As seen on the following graph, there are well know curves that can be applied to specific chemicals. However, these curves only work with pure chemicals. If other chemicals are added to the process they will influence the conductivity and the analyzer will not be able to track concentration accurately.

Conductance Data for Commonly Used Chemicals

Temperature Compensation Temperature compensation is critical to conductivity measurement. As you can see from the attached graph, temperature has a significant effect of about 2% per °C. Therefore, if a solution starts at a temperature of 25 °C and a conductivity of 1000 µS/cm and the temperature rises to 75 °C the conductivity will double to 2000 µS/cm. 75 °C - 25 °C = 50 °C change x 2% per °C = 100% change 1000 µS/cm + 100% change = 2000 µS/cm

Temperature Compensation

Temperature Compensation Temperature compensation is also specific to different types of chemicals. Even though 2% change per °C is a good average, the table on the next slide shows the variation between different chemicals. IC Controls analyzers are programmed to allow the user to change the % value in the configuration. This ensures temperature changes will accurately be reflected in the conductivity displayed and output from the transmitter or analyzer. For specific chemicals, like HCl and NaOH, IC Controls can supply these curves pre-programmed into the model 455 so concentration will be displayed very accurately.

Temperature Compensation SUBSTANCE % CHANGE PER °C Acids 1.0 to 1.6 Bases 1.8 to 2.2 Salts 2.2 to 3.0 Neutral Water 2.0

Temperature Compensation

Cell Constants To determine the amount of current that will flow through a known amount of liquid, the volume between the two electrodes must be exact and the current must be kept consistent and moderate. This is known as the cell constant. Any effective volume change alters the cell constant and current; too much volume will result in noise (low current) and too little volume results in electrolytic effects (high current). Cell constant recommendations will vary depending on the conductivity range of the solution. High conductivity requires a high cell constant and low conductivity requires a low cell constant. Choosing the right cell constant will ensure accurate conductivity. The table on the next slide shows the best cell constants for different application.

Cell Constants Cell Constant Design Range Lowest Range Highest Range 0.01/cm 10 µS/cm 0 µS/cm 100 µS/cm 0.02/cm 20 µS/cm 200 µS/cm 0.1/cm 1000 µS/cm 0.2/cm 2000 µS/cm 1.0/cm 10,000 µS/cm 2.0/cm 20,000 µS/cm 5.0/cm 5,000 µS/cm 500 µS/cm 50,000 µS/cm 20.0/cm 1,000,000 µS/cm

IC Controls offers a wide variety of sensor and analyzer styles to suit your conductivity needs. Go to the conductivity section of our website www.iccontrols.com to find Quality Water Solutions.