Anhydrous copper sulfate has a molar mass of 159.61 g mol -1 . The most water rich hydrate stable at ambient temperature is the pentahydrate with a molar mass of 249.69 g mol -1 which make the hydrate 1.56 times heavier per mole. So the capacity is 56 %. Furthermore, copper sulfate pentahydrate is not hygroscopic anymore (at ambient conditions). For hygroscopic substances which do not form stable hydrates, the deliquescence humidity can be used for comparing the capacity with other substances. The deliquescence humidity is the relative humidity where a solid salt starts to liquefy in the absorbed water. The theoretical capacity is shown in figure and in table . Phosphorous pentoxide (Siccapent) and Silicagel do not absorb water stochiometrically, so values from were used. The potassium salts are excluded from this diagram since there is only the deliquescent humidity given and not distinct capacity can be calculated. For determining the residual moisture and the speed of absorption of water experiments were performed (see fig. ). The lowest residual moisture shows magnesium perchlorate, phosphorous pentoxide and molecular sieves; the highest amount of residual water shows both sodium salts and magnesium sulfate. The most rapid drying was possible with magnesium perchlorate, silicagel and potassium carbonate. For reducing the waste of chemicals, the desiccant should be regenerated. Normally, it is heated to higher temperature for several hours to remove the water. The energy consumption of the process is lower if a lower temperature can be used. A lower temperature can be achieved by heating under reduced pressure in a dynamic vacuum. So the released water is pumped off the regeneration area. The heating speed has to be small because of possible side reactions for some desiccants (like CaCl 2 ) or problems occurring from rapid 11 180 185 evaporation of water in the pores of molecular sieves. The formed gas bubbles can blow up the structure and destroy the molecular sieve. The lowest regeneration temperature without reducing the pressure have potassium carbonate, sodium carbonate and silicagel at 100 to 130 °C. Copper sulfate can be dehydratisated at even lower temperature but under vacuum conditions.