An example printing cartridge includes a sensing die including a plurality of sensing locations in thermal contact with a fluid container, an on-die controller to select a sensing location of the plurality of sensing locations to activate and to provide a variable threshold voltage, and a voltage comparator to compare a sensed voltage generated at a selected sensing location to the variable threshold voltage, and output time-based information based on the comparison, the time-based information representative of whether a fluid is present at a fluid level associated with the selected sensing location.