Thorough study of the equipment is essential before any measurements are made. Relative tidal gravimeters are complex technical devices; they enable measuring gravity time variations with high accuracy (1 μGal). The main source of systematic errors is their instrumental drift. Value and nonlinearity of the drift for each unit is individual. There are plenty of considering methods; they depend on material and technical opportunities. The simplest and less resource-consuming algorithm is to take the drift into account using the piecewise linear approximation technique. This research deals with assessing the accuracy of accounting the instrumental drift of the gPhoneX#117 (Micro-g LaCoste, USA) tidal gravimeter using the specified means. For this purpose, the drift was obtained through approximation by polynomials of the first and second degree was estimated in comparison with the results of reference (absolute) observations. It is concluded that the method of piecewise linear approximation can only be used for rejecting poor quality measurements. It is recommended to identify, control and accounting the instrumental drift by comparing measurements with the data of regular absolute monitoring.