The implementation of digital substations is becoming increasingly common and one of the principal elements is the stand-alone merging unit (SAMU). Therefore, the calibration of the SAMU is essential to ensure the accuracy and reliability of measurements, as the operation of the protection equipment depends on them. This study focuses on evaluating testbeds used in different laboratories to calibrate the same SAMU: CIRCE, RISE, VSL, and VTT. Following IEC 61869-13 standard, magnitude and phase displacement were calibrated on all channels for current and voltage. Additionally, the compatibility among laboratories was evaluated using a compatibility index, confirming their mutual compatibility in the calibration process