Chemical Tanker Guide Online
Home || Chemical hazards || Various chemicals || Cargo Stowage || Care ||Tank cleaning ||Handling equipments ||



Combustible gas detectors, thermal conductivity meters, infrared detectors & Chemical detector tubes

Ships carrying toxic or flammable products (or both) should be equipped with at least two instruments that are designed and calibrated for testing the gases of the products carried. If the instruments are not capable of testing for both toxic concentrations and flammable concentrations, then separate sets of instruments should be provided.

Vapour-detection instruments may be portable or fixed. If a fixed system is installed at least one additional portable instrument should be provided.

When toxic-vapour-detection equipment is not available for certain products that require such detection the Administration may exempt the ship from the requirement, provided an appropriate entry is made on the International Certificate of Fitness for the Carriage of Dangerous Chemicals in Bulk. When granting such an exemption, the Administration should recognise the necessity for additional breathing-air supply and a further entry must be made on the International Certificate of Fitness for the Carriage of Dangerous Chemicals in Bulk.

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

measuring concentrations of gas in or near the flammable range;

detecting low concentraflons of cargo vapour in air and in inert gas, or in the vapour of another cargo;

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

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


Combustible gas detectors

Combustible gas detectors are very common and are used to detect and measure combustible gases, usually within the concentration range of 0-100% LFL; that is, up to the point of flammability. Equipment can be fixed or portable.

A sensor containing a filament of a special metal is heated electrically and a sample of gas is passed over it. Any combustible gas in the sample is oxidised catalytically. The heat given out alters the electrical resistance of the filament in proportion to the gas concentration, and this effect is displayed on a suitably marked meter. The filament can easily be de-activated by materials such as silicones, halogenated gases, acids, water, oil and lead. Filters may therefore be required in the sample lines.

The equipment needs oxygen to operate, and can only be relied upon to detect combustible gas in air atmospheres, not in inerted atmospheres. If a mixture of inert gas and cargo vapour has to be tested, either an infrared or thermal conductivity meter must be used, or a sample must be mixed with air before a combustible gas detector can be used. A combustible gas detector will not indicate a safe atmosphere if a toxic vapour is involved: in such a case a different type of instrument should be used.




The instruments are calibrated against a known gas, called a span gas. Performance in use may be affected if the gas sampled is different from that used for calibration, and an appropriate conversion factor may to be applied to the readings.

Portable combustible gas detectors are frequently used to confirm the state of atmospheres believed to be free of cargo vapours, such as prior to tank entry or hot work. When used for this purpose, 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 calibration should be confirmed, and readings should be taken from the top or bottom of a space depending on the vapour density of the cargo. Readings will be inaccurate if inert gas is present in the sample.

When 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, while the final rest position indicates the concentration, as follows:

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.

Personal combustible gas detectors, capable of continuously sampling an atmosphere to detect the presence of small amounts of combustible gas, are also available. They should automatically provide an audible and visual alarm when the level of combustible gas reaches a set level, to give the wearer adequate warning of unsafe conditions.


Thermal conductivity meters

These instruments work by measuring thermal conductivity of samples of gas. They are sometimes called catharometers. Electrical power is applied to a heater filament which is used as the sensing element: the filament temperature stabilises at a value depending on the thermal conductivity of the gas around it. Any variation in the concentration of the gas affects the filament temperature, resulting in a change in electrical resistance which is in turn indicated by a meter.

The principle is similar to that of the combustible gas detector, but the filament temperatures are lower and the instruments can be used to detect concentrations of gas from 0-100% by volume (compared to 0-100% LFL).

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 less flow sensitive. It is important to note that changes in operating conditions (e.g. filament voltage or gas flow rate) may alter the filament temperature. The maker's handbook for the instrument should be checked.

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, or manufacturer's correction curves used. Reference should be made to the manufacturer's instructions before every occasion of use.

Note: The roles of combustible gas detector and thermal conductivity meter can be combined into one instrument, although the two functions - measuring percentage of LFL and concentration of vapour by volume respectively - remain distinct. In some ships, fixed gas detection equipment uses this combination technique.


Infrared detectors

Organic gases such as butane, methane and petroleum absorb infrared radiation. This property is used in fixed or portable equipment to detect such gases in concentrations over the range 0-100% LFL or 0-100% volume. Infrared 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 or a pen recorder, or to trigger other equipment such as an alarm. Calibration of the instrument is set for each gas to be measured.

Infrared detectors will not reliably detect chemical gases, and are not commonly used on chemical tankers.


Chemical detector tubes

These instruments, often referred to as Draeger tubes, normally function by drawing a sample of the atmosphere to be tested through a proprietary chemical reagent in a glass tube.

The detecting reagent becomes progressively discoloured if a contaminant vapour is present in the sample. The length of the discoloration stain gives a measure of the concentration of the chemical vapour which can be read from the graduated scale printed on the tube. Detector tubes give an accurate indication of chemical vapour concentration, whatever the oxygen content of the mixture.

It is important that the correct volume of atmosphere sample, according to the manufacturer's instructions, is passed through the tube, 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 - for example, oxygen or water vapour (to establish dewpoint).

The tubes are designed to measure low vapour concentrations accurately, and are probably the most convenient and suitable equipment to use. They should always be used when the cargo vapour presents a serious inhalation hazard, e.g. acrylonitrile. The storage life of these tubes is usually limited, and it is necessary to ensure that out of date tubes do not remain available for use.


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 calibration or use;

zero points should be checked regularly and reset if necessary before an instrument is calibrated. Great care should be taken when the zero is being set to ensure that the sample is free from any gas that would otherwise give a reading: pure nitrogen should be used if necessary;

the instrument should be calibrated as often as recommended by the makers. The concentration and composition of the gas used for calibration (known as span gas) should be accurately known. Re-calibration should be recorded on or near the instrument;

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

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 may be inaccurate;

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

all sample lines should be made of the correct material as specified by the maker. Incorrect tubing may absorb gas from the sample and cause misleading readings;

if upper or lower sample points are provided (for lighter than air or heavier than air vapours respectively) the correct position should be used for the cargo;

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

for fixed instruments, remote and local read-outs should be compared to detect discrepancies;

performance of most fixed instruments depends on flow rate, and fluctuations can cause inaccuracy. Flows should be kept steady, and flows from separate points should be balanced;

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



Other info pages

Draegar Chemical detector tubes use and reading correction guideline
These instruments, often referred to as Draeger tubes, normally function by drawing a sample of the atmosphere to be tested through a proprietary chemical reagent in a glass tube. The detecting reagent becomes progressively discoloured if a contaminant vapour is present in the sample. The length of the discoloration stain gives a measure of the concentration of the chemical vapour which can be read from the graduated scale printed on the tube. Detector tubes give an accurate indication of chemical vapour concentration, whatever the oxygen content of the mixture

Requirements of various grade chemical cargo heating
: The voyage orders will contain heating information, if heating is required. As a rule the final heating instructions are given by the Shipper in writing to the Master / Chief Officer in the port of loading. If those written instructions are not given, the master should request them and issue a Letter of Protest if they are not received at departure. In the latter case the management office should be immediately informed.


Recommended temperature monitoring equipments onboard
:Temperature sensors are fitted so that the temperature of the cargo can be monitored, especially where required by the IBC Code. It is important to know the cargo temperature in order to be able to calculate the weight of cargo on board, and because tanks or their coatings often have a maximum temperature limit. Many cargoes are temperature sensitive, and can be damaged by overheating or if permitted to solidify. Sensors may also be fitted to monitor the temperatures of the structure around the cargo system.

Cargo instruments
:In order to maintain a proper control of the tank atmosphere and to check the effectiveness of gas freeing, especially prior to tank entry, several different gas measuring instruments need to be available for use. Which one to use will depend upon the type of atmosphere being measured.

Liquid level gauges
:The accuracy required of chemical carrier level gauges is high because of the nature and value of the cargo. To limit personnel exposure to chemicals or their vapours while cargo is being handled, or during carriage at sea, the IBC Code specifies three methods of gauging the level of a liquid in a tank - open, restricted or closed

Overflow control
:Certain cargoes require the designated tank to be fitted with a separate high level alarm to give warning before the tank becomes full. The alarm may be activated by either a float operating a switch device, a capacitive pressure transmitter, or an ultrasonic or radioactive source. The activation point is usually pre-set at 95% of tank capacity.

Oxygen analysers
:Oxygen analysers are normally used to determine the oxygen level in the atmosphere of an enclosed space: for instance, to check that a cargo tank can be considered fully inerted, or whether a compartment is safe for entry.

Vapour detection
:Ships carrying toxic or flammable products (or both) should be equipped with at least two instruments that are designed and calibrated for testing the gases of the products carried. If the instruments are not capable of testing for both toxic concentrations and flammable concentrations, then separate sets of instruments should be provided.

Alarm circuit
:An important feature of many modern measurement and control instruments is the ability to signal a particular situation. This can be a main operational alarm that gives an indication of a pre-set situation such as liquid level in a tank, or a malfunction alarm indicating a failure within a sensor's own operating mechanism. The designs and purposes of alarm and shutdown circuits vary widely, and their operating system may be pneumatic, hydraulic, electrical or electronic. Safe operation of plant and systems depends on the correct operation of these circuits and a knowledgeable reaction to them.

Gas freeing
:Gas freeing onboard chemical tankers is required for entry into cargo tanks, for hot works or washing for clean ballast tanks. Gas Freeing is one of the most hazardous operations routinely undertaken onboard a Chemical Tanker and the additional risk created by cargo gases expelled from the tanks, which may be toxic, flammable and corrosive, cannot be over-emphasised.

Cargo tank damage during pigging operations
:Blowing and pigging of pipelines at terminals poses inherent risks for the terminal and a chemical tanker. Frequent damages to tanks have occurred. If there are doubts about the shore operation or signs of problems ashore the OOW must immediately request clarification.


Reference publications



Main Info pages!

Home page ||| Chemical hazards ||| Cargo planning & Stowage ||| Cargo loading ||| Cargo documents ||| Safe stability ||| Cargo care ||| Preparation for unloading ||| Inert gas systems |||Gas freeing ||| Nitrogen handling ||| Chemical handling Safe practice |||Handling equipments ||| Cargo & Ballast pumps ||| Cargo tanks |||Tank cleaning |||Special cargoes |||Spills emergencies |||Fire protection








Chemicaltankerguide.com is merely an informational site about various aspects of chemical tankers and safety tips that may be particular value to those working in: Chemical Handling, Chemical Storage, Liquefied Chemical Suppliers, Chemical Shipping, Chemical Transportation, Chemical Terminals, Bulk Chemical Services and Chemical Processing. If you are interested in finding out more about chemical tanker guideline please visit IMO official website. For any comment please Contact us

Copyright © 2011 Chemical Tanker Guide.com All rights reserved.