An apparatus capable of characterizing the fluid content of chemicals in a reaction line comprises a rheometer. The apparatus can be connected to or included in a reaction flow stream, and has a retractable wall 26 that opens to provide an opening 28 that permits a portion of the reaction flow steam to enter into sampling chamber 12. The sample is isolated from the flow stream and rheometric data is obtained from the sample. In the method of the invention, data on the sample is collected and transferred to a computer. The computer is used to analyze the data, and optionally can be used to control reaction parameters in the reaction flow stream.