Disclosed are systems and methods that include providing a vessel having an inlet and an outlet for communicating a carrier gas through the vessel; determining a differential pressure between the inlet and the outlet for a known flow rate at the outlet; and, determining a unit flow per unit pressure based on a ratio of the flow rate and the differential pressure. In certain embodiments, the flow rate at the outlet is established by controlling the flow rate of the gas. In some embodiments, the methods and systems can include a viscosity in the ratio, determining a flow rate at ambient pressure using a factor based on a ratio of a gas pressure at the output and ambient pressure, and/or, determining a flow rate at ambient temperature using a factor based on a ratio of a temperature at the output and ambient temperature.