Omega CDE-440 manual Conductivity Cells, Electrical, Mechanical, Chemical

3.Conductivity Cells

Cell is composed basically by two boards or two concentric cylinders(electrodes), with well defined geometry. The space between the boards or cylindres creates a solution collumn, being so the measurement is independent from the total solution volume.

The boards or cylindres geomety used will be related to the solutions concentration for a better reading of different samples we vary the distance (and;or) the board section.

The relation “distance” (d) over the “section” (s) determines the Cell Constante (K) . ds = K .

In order to obtain good results it is necessary to choose the specific cell for the desired application. Three factors are extremelly important in order to choose the right cell: the Electrical, the Mechanical and the chemical.

!Electrical

When reading an electrolyte concentrationof high resistance, caused by low quantity of ions, the greater sensibility will be obtained with bigger electrodes and with a small distance between them. Consequently, we shall diminish the capacitive reatance of the cell-solution system, increasing the polarization tension frequency supplied by the equipment.

We will need the inverse when the electrolite is very concentrate: small electrodes with long distances in order to avoid saturation and the polarization effect at the electrodes.

The Cell will present low "K" for low conductivity electrolites and high "K" for those of high conductivity, considering that the equipment must have an automatic exchange of the adequate frequency.

!Mechanical

Some significant factors are: the pressure, temperature, flow speed and the solid suspension presence. only when all these factors are considered that will be achieved a satisfactory performance with a minimum maintenance. When the systems temperature and pressure are higher than the specified values, it must be installed a cooling system with a restraining valve before the cell contact.

In places where the flow speed is very low, it must be provided with a flow increase for a solution exchange. solid presence will influence because of obstruction that they can cause at the electrodes or even a super conduction (in metal cases).

!Chemical

It is necessary to consider the chemical aggressiveness and clogging caused at the electrodes, this can modify the geometry, altering the constant value.

In these cases, the electrodes must be chemically aggressive and must be cleaned periodically.

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