A mixed bed deioniser produces very pure water quickly, economically and efficiently. It contains an intimate mixture of Cation and anion resins. Thus, in effect, water passing through a mixed bed is subjected to an infinite series of two stage demineralization. The treated water produced will have residual dissolved solids below 1.0 PPM, conductivity below 1.0 PPM and a near neutral pH. In general it conforms to I.P (Indian Pharmacopoeia) standards for deionised water. The mixed bed unit has a very sharp exhaustion point, marked by a sharp rise in the conductivity, which is at a constant, low value till then. At this point the unit is regenerated.