In this article we report research into the computer representation and automatic assessment of
expressions which are dimensional numerical quantities. We consider how machines represent and
manipulate dimensional numerical data. Through action research we examine how students enter
dimensional numerical data into a machine for online assessment, and examine the properties of such
answers which practical teachers seek to establish. We consider the extent to which these properties
can be established automatically, and report the outcomes of our action research: extensions to the
STACK online assessment system which implement both syntactic and semantic layers to represent
dimensional numerical quantities in a form suitable for online assessment.