The purpose of this study was to develop and apply a scheme for the analysis of flow diagrams. The flow diagrams in question are a schematic representation of written instructions that require students to process the text of their practical manual. It was hoped that an analysis of the flow diagrams would provide insight into students' understanding of the manual. To this end, we describe the development of a rubric for the analysis of flow diagrams. The analysis showed that most students were able to draw flow diagrams that showed evidence of deep processing, many of which could be used in place of the practical manual. Most students had some concept of how the apparatus worked, providing evidence of their initiation into the social practice of laboratory work. The analysis also showed that flow diagrams could be used to gauge the students' level of understanding of the practical manual. Flow diagrams could be considered to be learning tools as well as part of the assessment for a practical session.