NOVELTY - A signal (6) representing flow volume of stream (S1) containing propane, and signal (18) representing targeted concentration of ethane in stream (S3) are generated. A signal representing concentration of propane in stream (S1) is generated. A signal representing concentration of propane in stream (S2) having ethane is generated. Signals are adjusted to generate another signal representing flow rate of stream (S2) necessary to produce composition comprising ethane and propane by blending streams (S1,S2). Stream (S3) has vapor pressure of 198-208 psig at 100 degrees F.    USE - Blending of stream containing propane and stream containing ethane for producing stream containing ethane and propane.    ADVANTAGE - The stream having desired compositional characteristics is produced by precise blending of hydrocarbon streams.    DETAILED DESCRIPTION - A signal (6) representative of flow volume of stream (S1) containing propane (90 volume% or more), and a signal (18) representative of targeted concentration of ethane in stream (S3) are generated. A signal representative of the concentration of propane in stream (S1) is generated. A signal representative of concentration of propane in stream (S2) containing ethane (60 volume% or more) is generated. The signals are adjusted to generate another signal representative of flow rate of stream (S2) necessary to produce a desired composition comprising ethane and propane in stream (S3) produced by blending the streams (S1,S2). The stream (S3) has vapor pressure of 198 psig at 100 degrees F to 208 psig at 100 degrees F. Thus, stream (S3) containing ethane and propane is produced by blending streams (S1,S2). An INDEPENDENT CLAIM is included for blending system for producing blended product stream. The system has a line (2) transporting stream (S1), a line (14) transporting stream (S2), unit (34) for blending streams (S1,S2) to produce stream (S3) comprising less than 95 volume% of propane and 4 volume% or more of ethane.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic view explaining monitoring and blending of liquid or gaseous streams.    Lines (2,14,24)    Meters for measuring flow of streams (4,16)    Signals (6,12,18,32)    Controller (8)    Analysis units (10,12)    Mixer (34)