Conductivity Sensor Proper Use
Electrolytic conductivity is defined as the ability of a liquid to conduct electrical current. In conductive solvents, dissolved ions are the principle conductors of electricity. By selecting the appropriate electrode, one can easily measure the electrical conductivity of liquids ranging from ultra-pure water to the most salty solutions.
How well a solution conducts electricity is dependent on the following parameters:
PASCO conductivity sensors determine the electrical conductivity of a solution by measuring the AC current flowing through a circuit when a 1 kHz AC voltage is applied to a 2-cell electrode submerged in the solution.
The accuracy of conductivity measurements depends on the following factors:
Polarization occurs because of migration of the ions in the solution to the surfaces of the electrodes. If a DC voltage is applied, then there will be a voltage drop across this region. The AC voltage and platinized electrodes are used to reduce the effects of solution polarization on the conductivity measurements. When a solution becomes completely polarized all current stops because the voltage outside of the boundary layer of oppositely charged ions falls to zero.
A 1 kHz AC voltage and platinization to minimize polarization effects. The electrodes are platinized, which is a process that greatly increases the surface area of the platinum electrodes, resulting in a considerably increase in the time for the surface to saturate with ions and decreasing the effect of ion shielding (polarization resistance) of the electrode.
The electrodes perform best when free of contaminants that would reduce the surface area of the electrodes or change the dielectric cell constant.
Contamination will make the probe more susceptible to polarization effects and change the conductivity of the solution being measured.
Conductivity has a substantial dependence on temperature. This dependence is usually expressed as percent/Celsius at 25 Celsius. Ultrapure water has the largest dependence on temperature, at 5.2%/oC. Ionic salts run about 2%/oC, with acids, alkalis, and concentrated salts solutions are around 1.5%/oC. Temperature variation causes frequent problems with conductivity measurements when the solution under testing has a rapid varying temperature. The change in conductivity is virtually instantaneous.
The length between the sensing elements, as well as the surface area of the metallic electrodes, determine a electrode cell constant, with units of length/area. The cell constant is a critical parameter affecting the conductance value produced by the cell and handled by the electronic circuitry. The cell constant is a geometric factor that converts between conductance and conductivity. The PASPORT Conductivity Sensor - PS-2116A assumes that a probe with a cell constant of 10 is connected to the probe. For solutions of low conductivity, one could use a 1.0 x probe or smaller to obtain a better signal to noise ratio, but one would have to divide the measured values by a factor of ten in software to obtain the correct values.
Below are the optimum conductivity range for cells with different cell constants:
Advantages and Disadvantages of Conductivity Measurement
In general, conductivity offers a fast, reliable, nondestructive, inexpensive and durable means of measuring the ionic content of a sample. Reliability and repeatability are usually excellent. Unlike measurement with ion-selective electrodes, such as pH sensors, the response of a conductivity sensor will not drift over time.
The principle drawback of conductivity measurements is that they are not ion-selective, giving a reading proportional to the combined effect of all of the dissolved ions. In order to determine the amount of total dissolve solids, one must have an idea of the ionic composition of the solution being measured.
Units of Conductivity
The units of measurement used to describe conductivity and resistivity are quite fundamental and are frequently misused. Once the units are known, various waters can be quantitatively described. The basic unit of resistance is the familiar ohm. Conductance is the reciprocal of resistance, and its basic unit is the Siemens [S], formerly called mho. In discussions of bulk material, it is convenient to talk of its specific conductance, or conductivity. This is the conductance as measured between the opposite faces of a one-centimeter cube of material. This measurement has units of Siemens/cm.
Note: ppm x 2 = conductivity
Select an appropriate solvent for the contaminants to which the electrode is exposed:
Clean cells by dipping or immersing the cell in the cleaning solution, agitating for two or three minutes, and rinsing first with tap water and then several times with distilled or deionized water.
Before measurement, immerse the probe in distilled water, gently tap out any trapped air bubbles, soak for at least an hour in distilled water and recalibrate.
For short-term storage, leave the cell immersed in deionized water. Any cell that has been stored dry should be soaked in distilled water for one hour before use to assure complete wetting of the electrodes.
If the black platinized coating appears to be wearing or flaking off the electrodes or if the cell constant has changed by 50%, the cell should be replaced or re-platinized.
Replatinization (Advanced Chemists Only)
If platinum black appears degraded, or if the values have changed by over 50% from nominal, replace the probe or replatinize according to the following procedure:
Creation Date: 12/7/2004
Last Modified: 07/3/2014