In an apparatus for testing pneumatic flow resistance of a test component, such as an air filter useful in a system for admixing parenteral solutions, a pneumatic bridge has two branches connected, via a filter, a flow restrictor, and a flowmeter, to a source of gas under regulated pressure. Each branch comprises two arms with a centerpoint. The bridge has a pressure transducer connected between the centerpoints and arranged to detect pressure imbalances in the bridge. Each arm comprises a flow restrictor, namely a needle valve, which is calibrated via a vernier head. The valves are adjustable to balance the bridge. In one arm, in which the needle valve can be fully opened, the test component is connectible to constitute a flow restrictor. The transducer produces an output displayed via a digital voltmeter, which may be calibrated.