Abstract: The pump comprises a measuring enclosure, a plunger mounted within the enclosure, a second enclosure which is contiguous with the first, a pneumatic jack rigidly fixed to the plunger for sliding motion within the second enclosure and defining therein a first and a second chamber, a stop rod which establishes the range of travel of the jack and a linear distributor which communicates at each end with a source of driving fluid. The distributor has three grooves such that in one end position, the measuring chamber is connected to a storage tank and one face of the jack is put into communication with one of the two sources of driving fluid and in the other end position, the measuring enclosure is connected to a collecting tank and the other face of the jack is put into communication with the other source of driving fluid.