Specifications
Conductivity/
TDS/Salinity
About Conductivity/TDS/
Salinity Measurement
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).
The Eutech salinity pocket testers SaltTestr® series are specially
developed to provide direct readings in these applications.
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.
Units of Measurement
The basic unit of conductivity is the Siemens (S), formerly called the
mho. Since cell geometry affects conductivity values, standardized
measurements are expressed in specific conductivity units (S/cm) to
compensate for variations in electrode dimensions. Specific 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.
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About Conductivity, TDS &
Salinity Measurement
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.
Specific 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.
Eutech offers a wide range of conductivity meters for these
various applications. Models include the ECTestr series, COND 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 oers meters that allow the direct reading of TDS values.
These include the TDSTestr® series, 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.
Solution Conductivity
Absolute pure water 0.055 µS/cm
Power plant boiler water 1.0 µS/cm
Good city water 50 µS/cm
Ocean water 53 mS/cm
Distilled water 0.5 µS/cm
Deionised water 0.1 - 10 µS/cm
Demineralised water 0 - 80 µS/cm
Drinking water 0.5 - 1 mS/cm
Wastewater 0.9 - 9 mS/cm
Seawater 53 mS/cm
10 % NaOH 355 mS/cm
10 % H
2
SO
4
432 mS/cm
31 % HNO
3
865 mS/cm
Conductivity / TDS / Salinity