Abstract: A real-time cross-correlation between concentration, temperature and electrical conductivity of KDP aqueous solution was determined by using an transformer-type
conductivity sensor. This correlation can be expressed by L(T_s,t)=L₀(t)+b₁(t)T_s+b₂(t)T_s². On the base of the correlation, a computer system was made to realize the in-line
measurement and control of the supersaturation of the growth solution. The system can control the supersaturation within ±0.04g KDP/100g water (±0.12% relative
supersaturation). The system was used to study the relationship between the supersaturation of the growing solution and the properties of the crystals grown from different
raw materials. It was found that growth rate rise but the homogeneity of KDP crystal goes down as the supersaturation increases. The system makes KDP crystal growing
under a stable supersaturation and the growth band and scattering particles inside the crystals suppressed, so the optical transmittance and laser damage threshold
of the crystal improved.

Key words: supersaturation, KDP crystal, automatic control, growth, homogeneity, electrical conductivity