In this work, a geometric observer (GO) is formulated and tested using experimental data from the Automatic Continuous Online Monitoring of Polymerization reactions (ACOMP) system for a semi-batch freer-dical polymerization reactor that synthetizes polyacrylamide. The available measurements include the weight-average molecular weight, the concentration of monomer, and the volume of internal contents. Different combinations between innovated states and measurements offer a number of possible structures of the GO. The computation of the minimum singular values and condition numbers permit to select the most promising structures. To overcome inadequacies observed in full-order architectures, low order GOs with passive structures are explored. The best observer is selected, compared with a standard estimation strategy and tested under various experimental operating conditions. The observer performance is evaluated qualitatively and quantitatively as a tradeoff between dynamic property estimation and signal processing. (C) 2019 Elsevier Ltd. All rights reserved.