This paper develops an experimental protocol for the characterisation of a concentrated cellulose acetate solution using a viscoelastic integral constitutive equation. Rheological parameters are obtained from both a Rheometrics RDSII spectrometer and a newly developed Multipass Rheometer (MPR). Reasonable self-consistency is found between machines and model. A numerical simulation of the MPR processing flow is carried out using Polyflow and the integral constitutive equation and the results are compared with the experimental data. In general, the fit is found to be good and we conclude that the modelling approach adopted here is capable of predicting the viscoelastic processing flow of concentrated polymer solutions with reasonable accuracy. The MPR is also shown to be a useful machine for the study of systems containing volatile fluids.