The precise determination of the 3He/3H mass ratio, and hence the tritium β-decay endpoint energy E0, is of relevance for the measurement of the electron anti-neutrino mass performed by the Karlsruhe Tritium Neutrino experiment (KATRIN). By determining this ratio to an uncertainty of 1 part in 1011, systematic errors of E0 can be checked in the data analysis of KATRIN. To reach this precision, a Penning Trap Mass Spectrometer was constructed at the University of Washington and has been transferred to the Max Planck Institute for Nuclear Physics in Heidelberg at the end of 2008. Since then it is called MPIK/UW-PTMS. Special design features are the utilization of an external ion source and a double trap configuration. The external Penning ion source efficiently ionizes the helium and tritium gas and can give superior elimination of unwanted ion species compared to the previously utilized in-trap-ionization by electrons from a field-emission point. The design as a double Penning trap allows a faster measurement procedure. This should help to avoid problems resulting from long-term drifts in the experimental conditions. Additionally, the laboratory in Heidelberg was carefully prepared to have very stable environmental conditions. Experimental challenges and the first Heidelberg results with the new spectrometer are presented.