De-ionisation
De-ionised water or DI water, which can also be known as de-mineralised water is water that has had mineral ions removed. For example, cations from sodium, calcium, iron and copper and anions such as chloride and bromide.

De-ionisation involves the removal of electrically charged or ionised dissolved substances by binding them to positively or negatively charged sites on specially-manufactured ion-exchange resins which also filter out the mineral salts from water. Because the majority of water impurities are dissolved salts, de-ionisation produces high purity water.

In a twin bed de-ioniser, synthetic ion-exchange resins absorb cations and anions in the water as they pass through the cation and anion beds. A conductivity sensor on the outlet of the cation bed can indicate resin depletion by a drop in conductivity while a conductivity sensor on the outlet of the anion bed will indicate the depletion of resin in either bed by a rise in conductivity.

In large de-ionisers, additional conductivity measurement systems are frequently used to monitor the concentration of acid and caustic solutions used for regeneration of the beds.

An efficient properly maintained operating de-ioniser will remove between 98-99.9% of all ionic impurities.

In a mixed-bed de-ioniser, the anion and cation resins are mixed. This produces the same effect as that of a number of two-bed de-ionisers in series and produces very pure water.

In this system a conductivity sensor on the outlet is used and when it detects a rise in conductivity this indicates the depletion of either of the resins.  Again, additional conductivity measurement systems are frequently used to monitor the concentration of acid and caustic solutions used for regeneration of the beds.

The final product water will be ultra-pure water having a conductivity of 0.055 μS/cm (@ 25°C). This may also be displayed in resistivity as 18.2 MΩ/cm (@25°C)

The temperature coefficient of a solution is normally between 1-3% per °C but it should be noted that the conductivity of pure water – below 0.1µS/cm there is a large and non-linear temperature dependence.

In order to ensure the conductivity measurement is accurate it is essential that the conductivity instrument used has the correct temperature compensation algorithms for pure water measurement.

LTH Electronics offer a range of on-line instruments and sensors for conductivity measurement in de-ionisation applications and the AquaCal 2000 pure water measurement kit for the measurement and validation of pure water systems.

Call us on +44 (0)1582 593693 or email sales@lth.co.uk to find out how we can help you.