13.2.1 Each cargo tank should
be fitted with at least one liquid level gauging device, designed
to operate at pressures not less than the MARVS of the cargo tank
and at temperatures within the cargo operating temperature range.
Where only one liquid level gauge is fitted it should be so arranged
that any necessary maintenance can be carried out while the cargo
tank is in service.
13.2.2 Cargo tank liquid
level gauges may be of the following types subject to any special
requirement for particular cargoes shown in column g in the table
of chapter 19:
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.1 indirect devices, which determine the amount of
cargo by means such as weighing or pipe flow meters;
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.2 closed devices, which do not penetrate the cargo
tank, such as devices using radioisotopes or ultrasonic devices;
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.3 closed devices, which penetrate the cargo tank,
but which form part of a closed system and keep the cargo from being released,
such as float type systems, electronic probes, magnetic probes and bubble tube
indicators. If a closed gauging device is not mounted directly on the tank it
should be provided with a shutoff valve located as close as possible to the tank;
and
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.4 restricted devices, which penetrate the tank and
when in use permit a small quantity of cargo vapour or liquid to escape to the
atmosphere, such as fixed tube and slip tube gauges. When not in use, the devices
should be kept completely closed. The design and installation should ensure that
no dangerous escape of cargo can take place when opening the device. Such gauging
devices should be so designed that the maximum opening does not exceed 1.5 mm
diameter or equivalent area unless the device is provided with an excess flow
valve.
13.2.3 Sighting ports with
a suitable protective cover and situated above the liquid level with
an internal scale may be allowed by the Administration as a secondary
means of gauging for cargo tanks having a design vapour pressure not
higher than 0.7 bar.
13.2.4 Tubular gauge glasses
should not be fitted. Gauge glasses of the robust type as fitted on
high-pressure boilers and fitted with excess flow valves may be allowed
by the Administration for deck tanks, subject to any provisions of chapter 17.