Method of controlling urea system

In a urea synthesis process system with ammonium carbamate solution recycle, wherein fluid ammonia, fluid carbon dioxide and a solution comprising ammonium carbamate are reacted at elevated pressure and temperature to synthesize urea in a urea reactor, wherein a fluid product stream containing urea, water, unconverted ammonium carbamate and ammonia is subjected to the separation of an aqueous urea solution from unconverted reactants ammonium carbamate and ammonia, said separation being accomplished by heating said product stream in a decomposer system at reduced pressure for the purpose of decomposing said unconverted ammonium carbamate to gaseous NH.sub.3 and CO.sub.2, of vaporizing said ammonia and of expelling the total mixture of gaseous NH.sub.3 and CO.sub.2 from the aqueous urea solution together with some water vapor, and wherein said mixture of gaseous NH.sub.3 and CO.sub.2 containing some water vapor, separated from the residual aqueous urea solution, is partially or totally condensed in a condensing system to a liquid phase containing ammonium carbamate, ammonia and water, and said liquid phase is recycled to said urea reactor;

• the improvement which comprises:
• 1. sensing a refractive index value of said liquid phase recycled to said reactor, said refractive index value being a function of the weight ratio of CO.sub.2 as carbamate to water in said liquid phase;
• 2.