Gas measuring Instruments

SOLAS Requirement:

SOLAS Regulation XI/1-7 states that:

• Every ship to which SOLAS Chapter 1(General provisions) applies shall carry an appropriate portable atmosphere testing instrument or instruments.
• As a minimum, these shall be capable of measuring concentrations of 1)oxygen, 2)flammable gases or vapours, 3)hydrogen sulphide, 4)carbon monoxide.

The SOLAS Convention requires that ships carrying cargoes that are likely to emit a toxic or flammable gas or to cause oxygen depletion in a cargo space are provided with an appropriate instrument for measuring the concentration of gas or oxygen in the air, together with detailed instructions for its use.

Explosimeter or combustible gas indicators:

Working principle:

• Explosimeter works on the principle of Wheatstone bridge.
• In this circuit, one element is coated with a chemical, which is a sensing element.
• The coated filament is also called as catalytic hot filament (400℃-600℃)
• The sample is drawn over a hot filament.
• When the HC gas enters the circuit, it burns and more heat will produce in the element.
• Thus temperature is raised and its resistance increases in proportional to the amount of gas burnt.
• If the resistance is changing it will cause deflection in the meter.
• The scale is graduated in the percentage of LEL 0% to 100%.

Limitations of Explosimeter:

• The sample will burn only when the oxygen is more than 11%
• Cannot be used when the tank is in inert condition.
• It indicates only flammable gas concentration in LEL (no O2, no toxic gas).

Uses of Explosimeter:

• Used to determine the flammable atmosphere in % LEL
• Mainly used during gas freeing operations.

Tankscope or HC gas meter:

Working principle:

• Tankscope works on the principle of Wheatstone bridge.
• In this circuit, one element is a sensing element.
• The sensing element is a non-catalytic hot filament, which is not coated with a chemical.
• When the HC gas enters the circuit, the sensing filament starts cooling.
• The cooling of the filament causes the change in temperature and hence the resistance.
• The HC concentration is determined by the rate of loss of heat from the filament, without combustion.
• If the resistance is changing it will cause deflection in the meter.
• The scale is graduated in the % volume.

Limitations of Tankscope:

• Sometimes, the sample itself has high temperature may affect the rate of loss of heat in the filament.

Advantages of Tankscope:

• Since there is no burning takes place in the filament, it can work in inert condition, where the O2 is less than 8%.

Uses of Tankscope:

• Used to determine the flammable atmosphere in % volume.
• Mainly used during inerting or purging.

Oxygen analyser:

    This is an instrument used to measure O2 content in the atmosphere. The sample gas will enter the instrument and concentration of oxygen is measured. The sensors used in the instrument can be different types:

• Paramagnetic sensor.
• Electrochemical sensor.

Paramagnetic sensor:

• Oxygen is paramagnetic in nature (i.e. attracted by the poles of the magnet).
• The sample cell suspended in a magnetic field.
• The sample of gas is drawn through the cell and the torque that experiences by the cell is proportional to the amount of the oxygen (magnetism of the oxygen).
• The electric current passing through the coil produces an equal and opposing torque.4
• The equalising current is proportional to the measure of magnetic force which is related to its oxygen content.

Electrochemical sensor:

• This sensor determines the oxygen content of a gas mixture by measuring the output of an electrochemical cell.
• The Sample is drawn through the cell, the oxygen content in the sample causing the current to flow between two electrodes separated by a liquid electrolyte.
• The current flow between the electrodes is directly proportional to the Oxygen concentration in the sample.
• The scale arranged to give a direct indication of the oxygen content.
• Certain gases may affect the sensor and give rise to false readings.

Toxic gas detector:

    Toxic gases are generally measured onboard using

• Chemical tubes.
• Electrochemical sensors.

Chemical tubes:

• Chemical indicator tubes are used to measure the variety of toxic gases onboard tankers. Eg. H2s, Benzene, Mercaptans.
• Chemical indicator tubes consist of a sealed glass tube containing a proprietary filling, Which is designed to react with specific gas and to give a visible indication of the concentration of that gas.
• The colour changes along the tube and the length of discolouration is a measure of toxic gas concentration, is read off an internal scale.

Electrochemical sensors:

There are numerous electrochemical sensors available covering a number of gases which may be present in the shipboard environment, such as ammonia, hydrogen sulphide, carbon monoxide, carbon dioxide and sulphur dioxide.

• This sensor determines the Toxic content of a gas mixture by measuring the output of an electrochemical cell.
• The Sample is drawn through the cell, the Toxic content in the sample causing the current to flow between two electrodes separated by a liquid electrolyte.
• The current flow between the electrodes is directly proportional to the Toxic gas concentration in the sample.
• The scale arranged to give a direct indication of the Toxic content of particular gas. Eg. H2S
• Certain gases may affect the sensor and give rise to false readings.
• This sensor measures gases in ppm.
• This is generally used in portable, personal and fixed instruments.

Multi-gas detector:

• Multi-gas instruments are now widely used nowadays and are usually capable of housing two or more different sensors.
• The sensor output will be changed electronically into a numerical display showing a concentration of gas.
• There are some basic types of portable gas sensors:
• Catalytic
• Electrochemical
• Infrared
• Photoionization detectors (PID)
• Hydrocarbon vapour as a %LFL (Explosimeter function using a Catalytic sensor).
• Hydrocarbon vapour in inert gas as a %Vol (Tankscope function using an infrared sensor).
• Oxygen (using an electrochemical sensor).
• Hydrogen Sulphide (using an electrochemical sensor).
• Carbon Mono Oxide (using an electrochemical sensor)
• PID sensors are designed to detect special gases or low concentration gas.