46 CFR § 153.406 - Standards for containment systems having required restricted gauges.
When Table 1 requires a cargo's containment system to have a restricted gauge, the containment system must have:
(a) A closed gauging system; or
(b) A system that has:
(1) A restricted gauge (e.g., a sounding tube) with an orifice diameter not exceeding 20 cm (approx. 7.8 in.);
(2) A permanently attached gauge cover that is vapor tight when in place; and
(3) A venting system that has either:
(i) Lock open PV valves; or
(ii) Valved bypasses around the PV valves.