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Q: Conductivity Info
A: Conductivity Info

Basic Theory
Conductivity is the ability of a solution to conduct electric current. The principle by which instruments measures conductivity is simple - two plates (cells) are placed in
the sample, a potential is applied across the plates and the current is measured.
Generally, the potential is in the form of a sine wave. Conductivity (C) is determined from the voltage and current values according to Ohm's Law:

C (siemens) =1/R=I (amps)/E (volts)

Since the charge on the ions in solution facilitates the conductance of electrical current, the conductivity of a solution is proportional to its ion concentration.
Note: Some solutions may not show a direct correlation to concentration; ionic interactions can alter the linear relationship between conductivity and concentration, especially in some highly concentrated solutions like sulfuric acid.

The basic unit of measurement for conductivity is Siemens (S). Since cell geometry affects conductivity values, standardized measurements are expressed in specific conductivity units (S/cm) to compensate for variations in electrode dimensions. For most solutions this measurement unit is much too large and either μS/cm or mS/cm are used instead.
The corresponding terms for specific resistivity (R = 1/C) are ohm-cm, Kohm-cm and Mohm-cm. Generally users of ultra pure water prefer to use resistivity units of Mohmcm or Kohm-cm, because measurement in this unit tends to spread the scale out into the range of interest. In these applications, the use of conductivity has the advantage of an almost direct relationship with impurities, especially at low concentrations. Hence, a rising conductivity reading shows increasing impurities in the given solution. The draw back to conductivity is that it is a non-specific easurement; it cannot distinguish between various types of ions and the reading is proportional to the combined effect of all ions present most conductivity electrodes only have two plates, usually made of platinum or carbon (graphite). The four plate or 'Bull's Eye' design results in higher accuracy for measuring pure water.

The following shows optimum conductivity ranges for cells of three different constants:

Applications
Since conductivity is proportional to the concentration of ions in solution (in most cases), conductivity measurement is a convenient way to determine the concentration of total dissolved solids (TDS) and the salinity of solutions. Common areas that conductivity sensors are used are:
Boiler blowdown
Desalination
Reverse osmosis
Salinity testing
Water / Wastewater treatment

Conductivity Cells
ASI offers conductivity sensors with a two electrode cell configuration (see figure 1) using platinum, titanium, or graphite. Our four-electrode cell or "Bulls Eye" (see figure 2) design uses a reference voltage to compensate for any polarization or fouling of the electrode plates. The reference voltage ensures that measurements indicate actual conductivity independent of electrode condition, resulting in higher accuracy for measuring pure water.
The conductivity cells for each electrode have specific cell constants (K) that are used in the determination of conductivity (C).


C = Cell Conductance x Cell Constant (K)
The cell constant (K) is the electrode separation distance (d) divided by the electrode area (a), so for a 1 cm cube of
liquid: K = d/a = 1 cm-1
Temperature Compensation all conductivity measurements are temperature dependent.
The degree to which temperature affects conductivity varies from solution to solution and can be compensated for using a temperature compensation meter with a thermistor in the conductivity electrode.

Conductivity Meter Calibration And Cell Maintenance 
Conductivity meters and cells should be calibrated to a standard conductivity solution. Selecting standards is very important, you should always choose one that has the approximate conductivity of the solution to be measured. In order to verify proper operation of your electrode, you should select another standard either above or below your first standard.
The conductivity of some common solutions is shown in the table below:


A polarized or fouled electrode must be cleaned to renew the active surface of the cell. In most situations, hot water with a mild liquid detergent is an effective cleanser.
Acetone easily cleans most organic matter, and hypochlorous solutions will remove algae, bacteria, or molds. To prevent cell damage, abrasives or sharp objects should not be used to clean an electrode. A cotton swab also works well for cleaning.


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