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Residual stress measurements with portable X-ray devices using diffraction techniques were first demonstrated in the 1960s. Since then, the measurement precision and portability of these devices have improved significantly because of advances in high-resolution detectors and analysis methods. However, questions regarding accuracy have persisted because of the intrinsic limitations of such small devices and the unknown true stress states in materials of interest. Previous studies of precision and accuracy have suggested that precision is mainly a function of the instrument and measurement environments and that accuracy depends not only on instrumental parameters, but also involves sample conditions such as anisotropy, heterogeneity, and processing history. Even though sample-dependent accuracy characterization is not trivial, it is worthwhile to quantify measurement errors from a nonideal sample when general assumptions (isotropic and homogeneous ensembles under a biaxial stress state) are not satisfied. Therefore, we carried out in situ uniaxial tension and torsion experiments with a round 1010 carbon steel tube, and then compared the measured normal and shear stresses with the applied (true) values for accuracy assessment. Based on the obtained uncertainties, we propose practical measurement and analysis protocols for precise and accurate stress measurements. [ABSTRACT FROM AUTHOR]