Calibration columns are used to measure flow and help calibrate metering/dosing pumps.
The column is usually connected at the inlet of a pump with a shut-off valve between the tank and the column and one at the bottom of the column. This allows for isolation between the tank and pump and the column and the pump & tank.
With the calibration column full, the valve leading to the tank will be closed and allow the pump to start drawing down the fluid. The fluid is measured at the start, and then again from 30 seconds to a few minutes depending on the size, flow, and type of pump/operation. Based on the delta between the two readings over a fixed amount of time will provide the use a flow rate (lph, gph, lpm, gpm, etc.).