The employer should determine the frequency of calibration based on a number of factors including work practice and procedures, frequency of use, work conditions, care, maintenance, and condition of the equipment, etc. CHANCE voltage detectors should be checked often on a known voltage source to verify accuracy of the readings and have additional testers to check for proper functionality of the detector. Please check the instruction manual of the voltage detector for the correct tester. Many customers send their voltage detectors for annual calibration. For calibration, please contact Bevins.