One aspect of the present disclosure includes a transfer standard apparatus for in situ calibration of measuring devices including a manifold having a first header in fluid communication with a second header via two or more flow lines, the first header and the second header each having ports at each end structured to accept external flow lines, two or more flow meters, each disposed in one of the flow lines such that the flow meters are connected in parallel relative to the first header and second header, and valves disposed in the flow lines adjacent the first header and the second header and operable to selectively isolate each flow meter from the first header and the second header. The transfer standard apparatus may include a mobile platform on which the manifold is mounted, including access to the ports of the first header and second header.