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The provision and use of vapour detection equipment is required by the IMO Codes for a number of functions, including:

• detecting cargo vapour in air, inert gas or the vapour of another cargo; • measuring concentrations of gas in or near the flammable range;

• measuring concentrations of oxygen in inert gas or cargo vapour, or in enclosed spaces.

Personnel should fully understand the purpose and limitations of vapour detection equipment, whether fixed or portable.

A6.7.2 Infra-red Detectors

Gases absorb infra-red radiation, and this property is used in fixed equipment to detect gas in concentrations over the full range 0-100%.

Infra-red radiation is passed through two tubes, one containing a known concentration of gas, the other containing the sample to be measured. The extent of absorption is in proportion to the gas concentration, and the output from the two tubes is compared electronically. The electronic signal can be used to drive an indicating meter, a pen recorder or other equipment. Calibration of the instrument is set for each gas to be measured.

A6.7.3 Thermal Conductivity Meters

Some gas detectors work by measuring thermal conductivity of samples. They are called thermal conductivity meters or 'catharometers', and can be used to detect concentrations of gas from 0-100%.

When power is applied to a heater filament used as the sensing element its temperature stabilises at a value depending on the thermal conductivity of the gas around it. Any variation in the

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concentration of the gas affects the filament temperature, resulting in a change in electrical resistance which is indicated by a meter.

The filament may be mounted so that the sampled gas flows directly over it or diffuses into it. The direct-flow type responds more quickly to concentration changes but is dependent on flow rates; the diffusion type gives a slower response but is not flow sensitive. It is important to note that almost any change in operating conditions (e.g. filament voltage or gas flow rate) will alter the filament temperature.

A thermal conductivity meter can be set to detect cargo vapour mixed with inert gas. The meter must be calibrated to suit the gas being tested for, and therefore separate instruments arc usually held on board for use with different gases.

Reference must be made to manufacturers instructions before every occasion of use.

Note: The roles of thermal conductivity meter and combustible gas detector (see below) can be combined into one instrument, although the two functions - measuring concentration of vapour and percentage of LFL respectively - remain distinct.

A6.7.4 Combustible Gas Detectors

This type of equipment can be used to detect certain combustible hydrocarbon gases. The principle is similar to that of the thermal conductivity meter with a head filament of a special metal to oxidise the gas catalytically, but the filament temperatures are higher and the concentration range lower (usually 0-100% LFL). The equipment works by resistance measurement and can be fixed or portable.

The filament can easily be de-activated by materials such as silicones, halogenated gases, acids, water, oil and lead. Filters may be required in the sample lines.

Calibration may be affected by the vapour sampled, and if different from that used for calibration, an appropriate conversion factor should be used. The equipment needs oxygen to operate, and can only detect combustible gas in air atmospheres, not inerted atmospheres. (If an inert gas/cargo vapour mixture has to be tested, either an infra-red or thermal conductivity meter must be used, or a sample must be mixed with air before a combustible gas detector can be used.)

Portable combustible gas detectors are frequently used to check atmospheres. When used for this purpose, the following precautions should be observed:

• readings should be taken by or under the supervision of a responsible officer who should be satisfied that the instrument readings are correct, and are accurately interpreted, before allowing the safety of personnel to depend upon them;

• the responsible officer should ensure that the calibration is correct; # readings will be inaccurate if inert gas is present in the sample;

# readings will not be sufficiently accurate to indicate a safe atmosphere if the vapour concerned is toxic: in such cases a different type of instrument should be used;

• readings should be taken from the top or bottom of a space depending on the vapour density of the cargo; # in using the instrument every reaction of the meter is important, and not just the final resting position: the first movement indicates the presence of combustible vapour, the final rest position gives a calibrated reading of the vapour concentration expressed as percentage LFL:

a final rest position within the scale indicates a gas concentration below LFL, expressed as a percentage of LFL; a final rest position beyond 100% LFL indicates a concentration within the flammable range; a needle movement first above 100% LFL and then to a final rest position of zero indicates a concentration above UFL.

It is therefore strongly recommended that when a space is being checked the responsible officer should not be satisfied that an atmosphere is safe until consistent zero readings are obtained.

Fixed gas detectors working on this principle have the same limitations as portable ones. In some ships the fixed gas detection equipment may use combustible gas indicators for reading from 0-100% LFL and thermal conductivity or infra-red detectors for LFL-100% gas concentration.

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A6.7.5 Chemical Absorption Indicators

These instruments normally function by drawing a sample of vapour through a chemical reagent in a glass tube. The detecting reagent becomes progressively discoloured if vapour is present and the length of discoloration - which can be read from the tube or from a graduated scale - gives a measure of vapour concentration.

Chemical absorption indicators give an accurate indication of vapour concentration whatever the oxygen content of the mixture.

It is important that the correct volume of sample, according to the manufacturer's instructions, is passed through the indicator, otherwise the measurement will not be accurate. Too small a sample volume will give a low value. With some instruments the length of hose is a critical factor in obtaining a correct reading.

The presence of a 'second' gas may affect readings and cause inaccuracies. Chemical detector tubes are specific for particular gases or vapours, which need not have flammable or combustible properties. The tubes are designed to measure low vapour concentrations accurately. They should always be used when the cargo vapour presents a serious inhalation hazard, e.g. ammonia or vinyl chloride.

A6.7.6 Oxygen Analysers

There are several types of oxygen analysers; some are chemical absorption indicators using special tubes, others use chemicals to dissolve oxygen from a sample and yet others rely on the

paramagnetic property of oxygen.

If detector tubes are used the readings may be affected by the presence of chemical vapours. An indicator which may be reliable for measuring the oxygen content of a space after thorough ventilation may not be suitable for

checking the oxygen content of an air/inert gas/cargo vapour mixture. Manufacturers' instructions should be observed.

Paramagnetic instruments measure the deflection of a magnet pivoted in a symmetrical non-uniform magnetic field; the magnet is in a sealed chamber into which the gas sample is introduced. The deflection is directly proportional to oxygen concentration. Some other gases, notably oxides of nitrogen, have comparable

paramagnetic properties; therefore this technique cannot be used if such gases may be present in other than trace amounts. These instruments can be used for detecting oxygen in mixtures of other vapours.

In liquid absorption instruments a sample of known volume is passed through the absorbing liquid and the final volume is measured on a scale which indicates the oxygen content of the original sample.

The use of oxygen detectors for checking the atmosphere in enclosed spaces is discussed in Chapter 6.

A6.7.7 General Precautions

Vapour detection is a means of measuring vapour concentrations, and great care is necessary to ensure that the readings are accurate, especially when the lives of personnel depend upon them. The following precautions should be observed:

• the maker's handbook should be studied before use or for calibration purposes;

• zero points should be checked regularly and reset if necessary before the instrument is calibrated: great care should be taken to ensure that the sample is free from any gas that will give a reading when the zero is set: pure nitrogen should be used if possible.

• the instrument should be calibrated frequently throughout its operating range: the concentration and composition of the gas used for calibration (span gas) should be accurately known: calibration may 'drift' in as little as one hour in some cases: re-calibration should be recorded on or near the instrument;

• when oxygen detectors are calibrated it is essential to use clean and uncontaminated air;

• the same precautions must be observed when handling span gas which is toxic or flammable as would apply if the gas were carried as cargo;

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• tubes or liquids for equipment using the chemical absorption or reaction principles have a limited life with an expiry date: they should be replaced before expiry, otherwise readings will be inaccurate;

• all sample lines should be clean, unobstructed, leak-tight and connected to the correct point;

• if upper or lower sample points are provided (for lighter- or heavier-than-air cargoes respectively) the correct position should be used for the cargo carried and care should be taken to change sample points when changing cargoes if this is necessary;

• care should be taken when using portable equipment: even on an open deck personnel have been overcome because of oxygen deficiency caused by inert gas from a sampling point;

• portable sensing equipment should not be used in a flammable atmosphere, other than in a certified safe enclosure;

• pumps, filters, flame screens and other components of the system should be well maintained to ensure accurate readings;

• catalytic filament elements may be de-activated if exposed to water or oil vapour (see paragraph A6.7.4); • remote and local read-outs should be compared to ensure accuracy;

• calibration of most fixed instruments depends on flow rate, and fluctuations can cause inaccuracy; flows should be kept steady and flows from each points should be balanced;

• the battery voltage of portable instruments should be checked frequently to ensure an instrument will provide accurate readings.