46 CFR §154.1826

1. (a)The master shall ensure that each cargo tank opening is fully closed at all times.
2. (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.
3. (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.