A downhole formation test pump includes a progressive cavity pump having a helical rotor received in a helical cavity of a stator. The pump may be disengaged by withdrawing the rotor from the stator to provide a bypass position so that the downhole formation being tested can be allowed to flow freely...http://www.google.es/patents/US5220829?utm_source=gb-gplus-sharePatente US5220829 - Downhole formation pump
