46 C.F.R. § 154.1826 – Opening of cargo tanks and cargo sampling.

(a) The master shall ensure that each cargo tank opening is fully closed at all times.
(b) The master may authorize the opening of a cargo tank:
1. (1) During tank cleaning; and
2. (2) To sample a cargo that Table 4 allows to be carried in a containment system having a restricted gauging system if:
  1. (i) The cargo tank is not being filled during sampling;
  2. (ii) The vent system has relieved any pressure in the tank; and
  3. (iii) The person sampling the cargo wears protective clothing.
(c) The master shall ensure that cargoes requiring closed gauging as listed in Table 4 are sampled only through the controlled sampling arrangement of the cargo tank.