Page 60 - International safety guide for oil tankers and terminals

INTERNATIONAL SAFETY GUIDE FOR OIL TANKERS AND TERMINALS

used for inerting. In this case, the use of soda lime as an absorbent for
carbon dioxide is recommended, provided the reading is corrected
---ใช้เพื่อการศึกษาเท่านั้น---
appropriately.

The refractive index meter is not affected by gas concentrations in excess
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
of its scale range. The instrument reading goes off the scale and remains
in this position as long as the gas chambers are filled with the gas
mixture.

2.4.5.2 Instrument Check Procedures
A mixture of known hydrocarbon, e.g. propane in nitrogen at a known
concentration, should be used to check the instrument. If the hydrocarbon
test gas differs from the original calibration gas, the indicated reading
should be multiplied by the appropriate correction factor before judging
the accuracy and stability of the instrument.

2.4.6 Infra-red (IR) Instruments

2.4.6.1 Operating Principle

The infra-red (IR) sensor is a transducer for the measurement of the
concentration of hydrocarbons in the atmosphere, by the absorption of
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infra-red radiation.
The vapour to be monitored reaches the measuring chamber by diffusion
or by means of a pump. Infra-red light radiation from the light source
shines through a window into the chamber, is reflected and focused by
the spherical mirror, and then passes through another window and hits
the beam splitter. The portion of the radiation that passes through the
beam splitter passes through a broadband interference filter (measuring
filter) into the housing cover of the measuring detector, and is converted
into an electric signal.

The portion of the radiation reflected by the beam splitter passes through
the reference filter to reach the reference detector.

If the gas mixture in the chamber contains hydrocarbons, a part of the
radiation is absorbed in the wavelength range of the measurement filter,
and a reduced electric signal is given. At the same time, the signal of the
reference detector remains unchanged. Gas concentration is determined
by comparing the relative values of the reference detector and the
measuring detector.

Differences in the output of the IR light source, dirt on mirrors and
windows as well as dust of aerosols contained in the air have an identical
effect on both detectors and are therefore compensated.

2.4.6.2 Instrument Check Procedures
This instrument should be checked using a check gas of a known mixture
of hydrocarbons. The IR sensor does not require the presence of air or
inert gas in the gas concentration, as it is reliant solely on the
hydrocarbon molecules. In general, these instruments are very stable and
require little maintenance. Calibration should be checked frequently in
accordance with the manufacturer’s instructions and ship’s Safety
Management System procedures. (See also Section 2.4.4.4.)

26 © ICS/OCIMF/IAPH 2006