Abstract: A low flow calibrator in which a closed manifold type system is provided that includes a differential pressure transducer with null indication with valving means for opening the system to the atmosphere and for initially nulling the differential pressure transducer and then closing the system, the system includes means for connecting a source with a standard leak therein to the system and the system has an adjustable bellows for varying the volume of the system and for causing the differential pressure transducer to be adjusted to a null position after the system has been closed off by the valve means, and the system has a bourdon tube that can be mechanically adjusted to increase the volume of the system with indicator means being connected relative to the bourdon tube to indicate the amount of volume increase by actuating a closed end of the bourdon tube to enable the system to be used to calibrate the amount of leak from the source.