It is vital that the dampening solution's pH and conductivity levels be consistent and optimum. Experts recommend that these two must be measured with every fresh dampening solution batch. It must be tested every four hours while the press runs. The readings must be noted in water pan located at the recirculation and every printing unit. Probable print problems can be detected early through water pan readings.
Water, impurities and conductivity
Conductivity is the measure of any material's ability to conduct electrical currents. Pure water is not a good conductor of electricity, having 0 micromhos conductivity. Water conductivity is directly proportional to quantity of undissolved solids like sodium and potassium. Water becomes much more conductive when more quantities of solid are present. It follows that conductivity can be utilized as a rough measure of the quality of water. Ion free materials are bad electrical conductors. They lower dampening solutions' conductivity.
Impurities can cause unusual changes in dampening solution conductivity. The source of these impurities can be water, ink particles, paper fibers or paper fillers among many others. It is sensible to recheck the water conductivity and also the concentrate of the dampening solution prior to making the assumption that there was improper mixing of dampening solution. It is perfectly normal for conductivity to jump when the press runs. This is as the dampening solution is contaminated by materials leaking from paper and ink.
Measuring conductivity
A thumb rule to be followed in this respect is that of the conductivity rises in excess of 500 micromhos, the dampening solution must be replaced. If you allow the conductivity to reach that high, there can be scumming and a number of other printing problems.
You must measure pH while measuring conductivity. The pH is described as the measure of concentration of the hydrogen ions in the water. A neutral pH is 7.0. An acidic solution is less then 7 and an alkaline solution more than 7. Dampening solutions should be acidic in sheeted offset. In web offset, the solution should be alkaline.
The pH of the dampening solution must be as instructed by the manufacturer. If this is done, the non-image parts of the plate will be kept clean. This is as gum Arabic used in a majority of solutions is redeposited on non-image areas. This is important as the gum Arabic’s initial coating during the longer runs get worn away. Plates could blind if pH gets too low. Drying times will increase as well.
