I am interested in the cell count, though maybe the total volume would be sufficient as a tracking parameter (effectively the same as a volume weighted count). I don’t know what data type a cell register is stored as with respect to Fluent expressions. (array of centroids?, big 1D list of boolean types for each cell id?, something else?) That information would make it a lot easier to constuct a sum or integral.
Despite the name, using a “limiter” cell register wasn’t the option I wanted. It seems to return the total cell count in the domain. Instead, I created a field variable register:
Type: Cells Outside Range
Derivative and scaling: None
Iso-min and Iso-max set to the same as the solver limiter values.
Then, my expression was as follows (using placeholders for generality):
CountIf(register_name, [list of all cell zones cells in domain])
And I simply get what I wanted; a count of the number of cells being limited by the min/max temperature limit (set in the solver controls).