DCU18 Contacting Conductivity

The electrical conductivity of the liquid to be measured should have a minimum conductivity of 5 µS cm (microsiemens/cm)

1ms/cm (millisiemen/cm) is equal to 1000µS/cm (microsiemens/cm)

There is no difference. Micromhos is more common in the U.S, while microsiemens is more commonly used in Europe.

Resistivity is simply the reciprocal of conductivity. Generally Resistivity measurements are used in low conductivity or pure water applications.

You will notice that whenever the conductivity of a solution is quoted it is usually referred to a temperature. This is because the conductivity of a solution varies not just with the amount of dissolved salts, but also with temperature. If the conductivity measurement is going to be used to determine for instance the concentration of sulphuric acid it is essential that the temperature effect is eliminated. This is achieved by measuring the solution temperature and compensating the conductivity measurement to the reading that would be given at a standard temperature. This is usually 25°C.

Different solutions will have different coefficients in the order of 1-3 %/°C. The temperature compensation will be quoted in %/°C and to a base temperature. When comparing different conductivity readings on different instruments it is essential that the temperature compensation is performed in the same way. A typical compensation rate of 2%/°C is generally used, but where more accuracy is needed a variable coefficient is essential.

It should also be noted that the conductivity of pure water below 0.1 uS/cm (10M°.cm) has a very large and non-linear temperature dependence. In order to obtain accurate readings in this range of measurement it is essential that the instrument has the correct temperature facilities.

The following maintenance information applies to the conventional electrode type of conductivity cell. The area of the cell that is sensitive to fouling are the electrode surfaces, these must fully “wet” to ensure accurate measurements are made. Moulded cells are often used in applications where a high level of contamination may be expected. This can include contaminates which do not contribute directly to the measured conductivity, e.g. organics, rust and suspended solids, but may form deposits on the electrode surface. In general these may be cleaned with the bristle brush provided and a weak detergent solution mixed with scouring powder.

Problems may occur in hard water areas where the gradual formation of scale will reduce the active area of the electrodes. Simple brush cleaning alone will not remove a hard deposit from the electrode surface. If scaling is suspected the cell should be removed from the system and treated with a 10% solution of hydrochloric or formic acid. The presence of bubbles will indicate that the scale is being dissolved. Cleaning is completed when the bubbles cease and usually takes 2-3 minutes. The cell must be thoroughly rinsed to remove all traces of acid before it is replaced in the system.

Note: Follow the supplier’s data sheet when handling acids and dispose of carefully according to your local authority regulations. Cells with stainless steel electrodes are generally used in applications where a low conductivity is combined with a low level of organic contamination and cleaning is rarely necessary. Errors in measurements can often be traced to faulty connections or incorrect setting on the instrument. However if contamination is suspected the cell should be removed from the system and cleaned if necessary. Handling of the cell electrodes will leave residues of oils and greases which will affect the wetting of the surfaces, leading to inaccurate readings. After touching the electrodes wash with a detergent solution and rinse thoroughly. After rinsing check that the surfaces maintain a complete film of water for approximately 10 seconds.