46 CFR §197.458

1. (a)Each depth gage and timekeeping device is tested or calibrated against a master reference gage or time-keeping device every 6 months;
2. (b)A depth gage is tested when a discrepancy exists in a depth gage reading greater than 2 percent of full scale between any two gages of similar range and calibration;
3. (c)A timekeeping device is tested when a discrepancy exists in a timekeeping device reading greater than one-quarter of a minute in a 4-hour period between any two timekeeping devices; and
4. (d)Each depth gage and timekeeping device is inspected before diving operations are begun.