Intended primarily as a simple and rapid quality-control test during the manufacture and conversion of PVC compounds. Suitable for coloured compounds. Is not recommended for compounds in the form of dry blends. The determination is carried out on a sample of the PVC compound which is maintained at an agreed temperature in still air until the colour of a Congo red paper held above it changes from red to blue. If universal indicator paper is used, the colour change is that corresponding to pH 3. The time required for the colour change is taken to be the stability time, ts. Together with ISO 182-2, replaces and cancels ISO Recommendation R 182:1970.