46 CFR § 153.404 - Standards for containment systems having required closed gauges.
When Table 1 requires a cargo's containment system to have a closed gauge, the containment system must have the following:
(a) A permanently installed closed gauging system.
(b) A vapor return connection.
(c) The high level alarm described in § 153.409.
(d) Either a closed cargo sampling system or a cargo sampling arrangement allowing the retrieval of a sample through an orifice not exceeding:
(1) 0.635 cm (approx. 0.25 in.) diameter when the cargo's vapor pressure is 28 kPa gauge (approx. 4 psig) or less; or
(2) 0.140 cm (approx. 0.055 in.) diameter when the cargo's vapor pressure exceeds 28 kPa (approx. 4 psig).