What is gas detection?
PREVENTIVE SAFETY MEASURES
Gas detection is one of a range of preventive safety measures. It detects abnormal concentrations of gases in a given environment before they can lead to dramatic consequences for property and people (explosions, fire, poisoning, etc.)
Since it is a matter of safety, it is important that gas detection is subject to stringent quality controls both during design and production and during installation and maintenance. These checks are carried out by various official bodies, which guarantees total reliability.
COMPOSITION OF A GAS DETECTION SYSTEM
A gas detection system consists of:
- Detectors for one or more gases, ATEX approved where required (1)
- Control units which collect data from the detectors (2) and respond using various peripherals (closing inlet valves, controlling ventilation, driving warning panels or sirens, etc.) (3).
Most explosive or toxic gases and oxygen can be efficiently detected and measured.
CH4 – C3H8 – C4H10 – CxHy – Cl2 – CO – CO2 – H2 – O2 – LPG – NH3 – NO2 …
Most important applications
The detector sensing element is made up of two platinum filaments electrically heated to around 400ÆC. One of them (1) is covered with an active catalytic layer which heats up strongly in the presence of a combustible gas. This temperature rise causes an increase in the resistance of the filament which is measured in the unit. The other filament (2), passive, serves as a thermal compensator.
The electrochemical cell is made up of a measurement electrode (1), a counter-electrode (2) and a reference electrode (3). These electrodes are bathed in an electrolyte inside the cell casing, which is itself fitted with a gas permeable membrane (4). The gas which is diffused inside the cell causes a chemical reaction on the measurement electrode and on the counter-electrode. The result is an electric current, proportionate to the concentration of the gas present, which circulates between these two electrodes.
This current is measured by the external circuit (5) to which the cell is connected.
The infrared cell functions according to the non-dispersive infrared (NDIR) principle. It is made up of a casing comprising:
- a diffusion membrane (1),
- a measurement chamber (2),
- an IR radiation source (3),
- an active sensor (4) and
- a reference sensor (4’).
The gas that reaches the measurement chamber absorbs – within a very precise range of wavelengths – a part of the radiation emitted by the IR source. The active sensor measures the remaining IR radiation and thereby determines the concentration of the gas present. The reference sensor measures the IR radiation within a range of wavelengths that is not influenced by the incoming gas. Its signal serves to compensate any variation in IR radiation which is not due to absorption caused by the targeted gas, such as a variation in temperature, humidity level, etc.
This enables us to obtain an accurate and reliable measurement in all conditions.
SEMI-CONDUCTOR MEASUREMENT PRINCIPLES
The cell is composed of a non-conductive element, for example silicon (1) to which a semiconductor metal oxide (2) is applied.
Two electrodes connected to a measuring device are connected to the semiconductor material (3).
In the absence of gas, oxygen is ionised onto the surface of the sensor, which then becomes a semiconductor.
When a gas enters the cell, its molecules take the place of the oxygen ions, resulting in a drop in resistance between the two electrodes.This variation is measured electrically (4) and is proportional to the concentration of the gas present.
A heating element (5) allows the temperature in the sensor to be stabilised, which improves the measurement and reduces the effect of external temperature changes.