The specification relates to a meter proving apparatus which has a calibration conduit containing a spherical plug for passing along the conduit with fluid flow and two plug detector devices spaced apart along the conduit for signalling the passing of the plug. The conduit has enlarged end stations in which the plug is located prior to a test operation and there being valve controlled by-passes for the end stations so that fluid flow can be established through the calibration conduit around the end station containing the spherical plug prior to a test operation.