We present a comparison in the various tidal bands, between two different spectral analyses of long gravimetric time series. The first one is performed using a long gravity series recorded by superconducting gravimeters at J9 Observatory (Strasbourg) and the second one uses a theoretical series of the same length, almost 28 years, computed for the same location according to the Hartmann and Wenzel tidal potential development. Long term gravity records are of great interest when performing spectral analysis. The length of the data series allows us to retrieve small amplitude waves in the major tidal groups (e.g. tides generated by the third-degree potential, as for example 3MO1 in the diurnal, 3MO2 in the semi-diurnal, and MN3 in the ter-diurnal frequency band, with amplitudes respectively of 2.29, 5.97 and 1.44 nm/s2), to separate waves close in frequency, as the waves NO1 and NO1X which need more than 18 year data length to be separated and finally to detect very low-frequency signals such as the monthly, semiannual or annual waves. Several examples for each of these cases are shown in our series.