Conductivity/TDS About Conductivity/TDS Measurement
Conductivity, TDS and Salinity - 23595 About Conductivity Measurement
The Eutech salinity pocket testers SaltTestr™ series are specially developed to provide direct readings in these applications.
Introduction to Conductivity, TDS and Salinity
Electrical Conductivity (EC) meters measure the capacity of ions in an aqueous solution to carry electrical current. As the ranges in aqueous solutions are usually small, the basic units of measurements are milliSiemens/cm (mS/cm) and microSiemens/cm (µS/cm). Conductivity is used widely to determine the level of impurities in water supplies for domestic consumption as well as industrial use. Industries that employ this method include the chemical, semi-conductor, power generation, hospitals, textile, iron and steel, food and beverage, mining, electroplating, pulp and paper, petroleum and marine industries.Specifi c applications include chemical streams, demineraliser output, reverse osmosis, stream boilers, condensate return, waste streams, boiler blowdown, cooling towers, desalinisation, laboratory analysis, fruit peeling and salinity level detection in oceanography.
The Principle of Conductivity Measurement
The principle by which instruments measure conductivity is simple – two plates are placed in the sample, a potential is applied across the plates (normally a sine wave voltage), and the current is measured. Conductivity (G), the inverse of Resistivity (R) is determined from the voltage and current values according to Ohm’s law. G = I/R = I (amps) / E (volts) Since the charge on ions in solution facilitates the conductance of electrical current, the conductivity of a solution is proportional to its ion concentration. In some situations, however, conductivity may not correlate directly to concentration. The graphs below illustrate the relationship between conductivity and ion concentration for two common solutions. Notice that the graph is linear for sodium chloride solution, but not for highly concentrated sulfuric acid. Ionic interactions can alter the linear relationship between conductivity and concentration in some highly concentrated solutions. SolutionConductivity Absolute pure water0.055 µS/cmPower plant boiler water1.0 µS/cmGood city water50 µS/cm
Ocean water53 mS/cm
Distilled water0.5 µS/cm
Deionised water0.1 - 10 µS/cm
Demineralised water0 - 80 µS/cm
Drinking water0.5 - 1 mS/cm
Wastewater0.9 - 9 mS/cm
Seawater53 mS/cm
10 % NaOH355 mS/cm
10 % H
2 SO
4 432 mS/cm31 % HNO
3 865 mS/cm Eutech o ers a wide range of
conductivity meters for these various applications. Models include the ECTestr™ series, EcoScan CON 6, CyberScan handheld CON 11 and CON 110, CyberScan waterproof CON 400 and CON 410 and CyberScan bench meters CON 510, CON 1500, CON 6000 as well as the handheld multi-parameters PC 10, PC 300, PC 510 and colour touchscreen research-grade bench series PC 6000, PC 6500 and PCD 6500. The total TDS is a mass estimate and is dependent on the mix of chemical species as well as the concentration while conductivity is only dependent on the concentration of chemical species. Some applications require the measurement of Total Dissolved Solids (TDS) in mg/L, parts per million (ppm) or parts per thousand (ppt). The TDS concentration can be obtained by multiplying the conductivity value with a factor which is empirically determined. Eutech o ers meters that allow the direct reading of TDS values. These include the TDSTestr™ series, EcoScan TDS 6, CyberScan standard handheld CON 11, CON 110 and waterproof handheld CON 410. Bench meters for advanced level laboratory research are the CyberScan CON 510, CON 1500 and colour touchscreen research-grade CON 6000.
Units of Measurement
The basic unit of conductivity is the Siemens (S), formerly called the mho. Since cell geometry aff ects conductivity values, standardized measurements are expressed in specifi c conductivity units (S/cm) to compensate for variations in
electrode dimensions. Specifi c conductivity (C) is simply the product of measured conductivity (G) and the
electrode cell constant (L/A), where L is the length of the column of liquid between the
electrode and A is the area of the
electrodes. Salinity measurements are common in industries like agriculture, aquaculture, hydroponics, food, pools and spas where it is necessary to monitor the salt level constantly. The values are usually read in parts per thousand (ppt) or % (1 ppt = 1 gram per litre). page | 36
