tanker familiarisation - gas detecting instruments - 2006

Tanker Familiarization

Gas detecting Instruments

Gas Indicators Measurement of Hydrocarbon

concentration Catalytic Filament Combustible Gas

indicator (CFCG) is used to measure HC gas in air at concentration below LFL. ( % LEL)

Non catalytic heated filament gas indicator and Refractive Index meter – used for measuring HC gas in excess of LEL or oxygen deficient (inerted) atmosphere.

Catalytic Filament Combustible Gas indicator (CFCG)

Sensing element is catalytic metal filament heated by an electric current.

When a mixture of HC in air is drawn over the filament, gas oxidises on the hot filament and makes it hotter.

This increases the resistance and change of resistance provides a measure of concentration of HC gas in the mixture.


Works on Wheatstone bridge with sensor element forming one arm of the bridge.

Deflection is shown on a scale calibrated to read 0 – 100 % LEL.

Some instrument may have extended range of 1-10% LEL.

Follow Manufacturer’s detailed instructions for the correct use and calibration.


If concentration of HC is more than about twice the LEL, there is insufficient oxygen in the mixture to burn the HC gas completely.

Due to above, needle will point to max reading on the scale and falls back to reading near zero.

Continuous observation of the needle is necessary to avoid overlooking this kind of response.

For the same reason, this instrument does not give reliable reading in inerted or oxygen deficient atmosphere.

Catalytic Filament Combustible Gas indicator (CFCG) - Checks

Check instrument prior use in fresh air. Also check after renewing filament. Calibrate as per manufacturer instructions Check sensing lines for any leakages Leak test by pinching aspirator if fitted. Only instruments fitted with flashback

arrestors in inlet and oulet of detector filament should be used

No filters other than cotton filters to be used in inlet side.

Water trap or water absorbent material may be used if gas is very wet.

Non Catalytic heated filament Gas Indicator

Sensing element is non catalytic hot filament.

Composition of surrounding gas determines the rate of loss of heat from the filament, and hence its temperature and resistance.

Sensor filament forms one arm of Wheatstone bridge.


Reading is shown as % HC. Not affected by gas concentration in

excess of working range Follow Manufacture’s detailed

instructions Note reading under no flow condition

and atmospheric pressure.


Checks Calibrate regularly prior use and

follow manufacturer’s instructions Reading taken with gas at high

pressure will vary significantly from actual reading

Refractive Index Meter Optical device depending upon the

refractive index of the gas sample and air. A beam of light is divided into two Recombined at eye piece Recombined beam exhibit an interference

pattern which appears to the observer as a number of dark lines to the observer.

One light path is through chamber filled with air and other filled with sample gas.

Refractive Index Meter

Initially zeroed with both chambers in air.

After passing the sample in one chamber, displacement of dark line is measured which gives the concentration of the gas.

Calibrated for a particular HC gas and used for same mixture for measurement

Fixed flammable gas detection

For pumprooms, accommodation, db tanks, pipe tunnels etc

Sensing devices at multiple locations and central control station for monitoring

Use of vacuum pump Use of infra red sensors

Toxic gases Chemical indicator tubes Sealed gas tube filled with proprietary

filling designed to react with specific gas and to give visible indication of concentration of that gas

Bellow type fixed volume displacement hand pump

Color change along the length is the measure of the toxic gas

Toxic gases

Must follow manufacturer’s detailed instruction for correct use

Pumps and tubes of same manufacture to be used

Cross sensitivity of one gas with other – to consult the manufacturer for detailed guidance

Measurement of Oxygen

Required to know the atmosphere in an enclosed space or cargo tanks, whether inerted or gas free

Most common type of oxygen analyzer in use are:

1. Paramagnetic sensor2. Electrolytic sensor3. Selective chemical absorption iquids

Paramagnetic sensors Substances having a magnetic

susceptibility greater than 0 are paramagnetic. They are drawn into a magnetic field.( Substances having a negative magnetic susceptibility are diamagnetic. They are repelled out of a magnetic field ) Magnetic susceptibility is the degree of magnetization of a material in response to a magnetic field.

Paramagnetic sensors

Oxygen is strongly paramagnetic whereas most other gases are not.

Sample cell has a light weight body suspended in a magnetic field.

When sample gas is drawn through the cell, the suspended body experiences a torque proportional to the magnetic susceptibility of the gas.

Paramagnetic sensors An equal and opposing torque is produced

by an electric current passing through the coil wound round the suspended body.

Equalizing current is a measure of magnetic force and is measure of magnetic susceptibility of the sample ie related to its oxygen content.

Before use, calibrate with nitrogen or co2 to purge the sample cell for zero check and with air for 21% oxygen for span.

Paramagnetic sensors Analyzer readings are directly proportional

to the pressure in measuring cell. Calibrated for specific atmospheric pressure Reading errors can be significant for

pressure variation Can avoid by reducing the sample air

pressure to atmospheric pressure Filter should be clean and dry for smooth

gas flow.

Electrolytic sensors

Oxygen content measured by output of an electrolytic cell

Oxygen diffuses through a membrane into the cell causing current to flow between two special electrodes separated by a liquid or gel electrolyte.

Current flow is related to the oxygen concentration in the sample

Electrolytic sensors Analyzer readings are directly

proportional to the pressure in the measuring cell but only small errors are caused by normal variations in atmospheric pressure.

Certain gases may affect the sensor and give rise to false readings.

SO2 and oxides of nitrogen if concentration more than 0.25% by volume.

Electrolytic sensors

Mercaptans and H2S can poison the sensor if levels are greater than 1% by volume. Poisoning does not occur immediately but over a period of time.

For such cases, reference to be made to manufacturer’s instruction.

Chemical absorption liquids A known volume of sample gas is brought

in contact with a liquid which absorbs oxygen, causing a volume change in liquid.

The relationship between these volumes is a measure of oxygen content in the sample.

Not recommended for checking oxygen level in ullage space due to high concentration of hydrocarbon gases.

Personal oxygen monitors Capable of continuously measuring the

oxygen content of the atmosphere Employ an electrolytic sensor Automatically provide an audible and

visual alarm when atmosphere becomes deficient in oxygen so as to give the wearer adequate warning of unsafe conditions

These should be tested at regular interval

Precaution for use of gas measuring instruments

Due to their vital importance, need to maintain and test carefully as per Manufacturer’s instructions.

Each time an instrument is to be used, a check is made of batteries, zero setting and calibration.

Monitor the function and response closely to get accurate reading


Material and condition of sample lines affect the accuracy of gas measurement.

Use only recommended hose types Must be resistant to water Cracked or blocked tubes,

contaminated tubes with cargo residues – all affect readings, so need to regularly inspect, check and replace sampling tubes if necessary.


‘Dead spots’ in any enclosed space opr cargo tanks – where ventilation or purging is less than the average in the bulk of the tank. eg bottom of tank, corners as per structural members etc

Sample lines to be long enough to reach these spots

Proper use of filters in the sampling line for filtering moisture – follow makers advise closely as per supplied instruction booklet.

tanker familiarisation - gas detecting instruments - 2006

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