Page 59 - International safety guide for oil tankers and terminals
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HAZARDS OF PETROLEUM
2.4.4.3 Precision of Measurement
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Correct response from these instruments is achieved only when
measuring gas concentrations in mixtures for which the instrument has
been calibrated and which remain gaseous at the temperature of the
instrument.
When selecting an instrument that uses an infra-red sensor for measuring์นาวี
Relatively small deviations from normal atmospheric pressure in the
instrument produce significant differences in the indicated gas
concentration. If a space that is under elevated pressure is sampled, it
may be necessary to detach the sampling line from the instrument and
allow the sample pressure to equalise with the atmosphere pressure.
care should be taken to ensure that the sensor will provide accurateย
readings over the spectrum of gases likely to be present in theช
2.4.4.4 Instruments with Infra-red Sensors
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the percentage by volume of hydrocarbon in an inert gas atmosphere,
atmosphere to be measured. It may be prudent to make comparison
readings with a Tankscope to verify the acceptability of the readings
provided by the instrument under consideration.
2.4.5 Inferometer (Refractive Index Meter)
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2.4.5.1 Operating Principle
An inferometer is an optical device that utilises the difference between the
refractive indices of the gas sample and air.
In this type of instrument, a beam of light is divided into two and these
are then recombined at the eyepiece. The recombined beams exhibit an
interference pattern that appears to the observer as a number of dark
lines in the eyepiece.
One light path is via chambers filled with air. The other path is via
chambers through which the sample gas is pumped. Initially, the latter
chambers are filled with air and the instrument is adjusted so that one of
the dark lines coincides with the zero line on the instrument scale. If a
gas mixture is then pumped into the sample chambers, the dark lines are
displaced across the scale by an amount proportional to the change of
refractive index.
The displacement is measured by noting the new position on the scale of
the line that was used initially to zero the instrument. The scale may be
calibrated in concentration units or it may be an arbitrary scale whose
readings are converted to the required units by a table or graph.
The response of the instrument is linear and a one-point test with a
standard mixture at a known concentration is sufficient for checking
purposes.
The instrument is normally calibrated for a particular hydrocarbon gas
mixture. As long as the use of the instrument is restricted to the
calibration gas mixture, it provides accurate measurements of gas
concentrations.
The measurement of the concentration of hydrocarbon gas in an inerted
atmosphere is affected by the carbon dioxide present when flue gas is
© ICS/OCIMF/IAPH 2006 25