Page 58 - International safety guide for oil tankers and terminals

INTERNATIONAL SAFETY GUIDE FOR OIL TANKERS AND TERMINALS

properly. Modern pellistor type instruments have sintered filters usually
built into the pellistor body.
---ใช้เพื่อการศึกษาเท่านั้น---
Some authorities require, as a condition of their approval, that PVC
covers be fitted around meters with aluminium cases to avoid the risk of
across the filament, thus ensuring the correct operating temperature.ชย์นาวี
incendive sparking if the case strikes rusty steel.

2.4.4 Non-Catalytic Heated Filament Gas Indicators (Tankscopes)

2.4.4.1 Operating Principle

The sensing element of this instrument is usually a non-catalytic hot
filament. The composition of the surrounding gas determines the rate of
loss of heat from the filament, and hence its temperature and resistance.
งานห้องสมุด ศูนย์ฝกพาณิ
The sensor filament forms one arm of a Wheatstone Bridge. The initial
zeroing operation balances the bridge and establishes the correct voltage

During zeroing, the sensor filament is purged with air or inert gas that is
free from hydrocarbons. As in the Explosimeter, there is a second
identical filament in another arm of the bridge which is kept permanently
in contact with air and which acts as a compensator filament.

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The presence of hydrocarbon changes the resistance of the sensor
filament and this is shown by a deflection on the bridge meter. The rate of
heat loss from the filament is a non-linear function of hydrocarbon
concentration and the meter scale reflects this non-linearity. The meter
gives a direct reading of % volume hydrocarbons.

When using the instrument, the manufacturer’s detailed instructions
should always be followed. After the instrument has been initially set at
zero with fresh air in contact with the sensor filament, a sample is drawn
into the meter by means of a rubber aspirator bulb. The bulb should be
operated until the meter pointer comes to rest on the scale (usually within
15-20 squeezes) then aspirating should be stopped and the final reading
taken. It is important that the reading should be taken with no flow
through the instrument and with the gas at normal atmospheric pressure.

The non-catalytic filament is not affected by gas concentrations in excess
of its working scale. The instrument reading goes off the scale and
remains in this position as long as the filament is exposed to the rich gas
mixture.

2.4.4.2 Instrument Check Procedures

The checking of a non-catalytic heated filament instrument requires the
provision of gas mixtures of a known total hydrocarbon concentration.

The carrier gas may be air, nitrogen or carbon dioxide or a mixture of
these. Since this type of instrument may be required to measure
accurately either low concentrations (1%-3% by volume) or high
concentrations (greater than 10% by volume) it is desirable to have either
two test mixtures, say 2% and 15% by volume, or one mixture between
these two numbers, say 8% by volume. Test gas mixtures may be
obtained in small aerosol type dispensers or small pressurised gas
cylinders, or may be prepared in a special test kit.

24 © ICS/OCIMF/IAPH 2006