Section 35.61 - Calibration of survey instruments(a) A licensee shall calibrate the survey instruments used to show compliance with this part and 10 CFR Part 20 before first use, annually, and following a repair that affects the calibration. A licensee shall-(1) Calibrate all scales with readings up to 10 mSv (1000 mrem) per hour with a radiation source;(2) Calibrate two separated readings on each scale or decade that will be used to show compliance; and(3) Conspicuously note on the instrument the date of calibration.(b) A licensee may not use survey instruments if the difference between the indicated exposure rate and the calculated exposure rate is more than 20 percent.(c) A licensee shall retain a record of each survey instrument calibration in accordance with § 35.2061 .