Industrial
Task Group
Industrial
Task Group
Gas monitoring, detection and analysis equipment has increasing importance in protecting people, property and the environment that they live and work in. CoGDEM play an important part in ensuring that standards are maintained not only for the products they design, manufacture and distribute but also in a wider context, assisting in the preparation of standards, supporting basic research etc. These activities are essential in ensuring that legislation, standards and products are continuously improved.
Although products may be manufactured to National or International Standards, the actual measurements being undertaken are only as good as the calibration of the instrument itself. CoGDEM believes that correct calibration of product and competence of users and maintainers is essential to best monitoring practice.
CoGDEM has written a gas detection and calibration guide instrument calibration. It should be used in conjunction with specific manufacturer’s instructions by trained personnel.
CoGDEM hopes you find the information useful in ensuring that the measurements you take are meaningful and your process is improved by reading it.
There are many physical and chemical properties of gases that are of interest from a safety point of view. These include the density of the gas and whether the gas is heavier or lighter than air. This helps determine whether the hazard in an enclosed space is at the top or the bottom of the room, but even in an open ventilated space it is important since heavier than air gases can remain on the ground or at lower levels. The density is not the only or sometimes even the major factor that determines gas distribution, for example, pressure of escape, air movement and temperature differences all have an impact when gas escapes.
An important feature of flammable gas, particularly in terms of hazardous area equipment, is the self-ignition temperature of the gas. This is the temperature at which the gas-in-air will spontaneously combust with no open ignition source. This is important since hazardous area equipment such as flammable or intrinsically safe gas detectors have a temperature classification, which is the maximum surface temperature that they can attain either under fault or normal operation. It is important that this temperature is less than the self-ignition temperature of the gas the product will be used in. This topic is discussed fully in a number of other publications (for example, BS 5345, IEC–1997, BSEN 50014 and BSEN 60079).
The flammable limits are of vital importance for the gases we measure. These determine the maximum and minimum concentrations of a mixture of the gas in air that will burn under normal conditions of temperature and pressure.
When applying these different gases to a particular instrument other factors may have to be taken into account with respect to instrument performance (for example, temperature, pressure and humidity). With heavier hydrocarbon gases the response time may be increased considerably and this will have to be taken into account particularly when zeroing the instrument after exposure to high gas concentrations. Some gases may absorb into filters, tubing (for example, hydrogen sulphide) and later reabsorb back into the sensor system causing apparent shifts or changes in zero or fault conditions.
The chemical activity of different gases varies considerably; methane is not a particularly reactive gas, it is difficult to break down or burn. It is relatively stable and has a high ignition temperature.
Some gases (for example, butane, propane and hydrogen) are easily combustible and can break down into their chemical constituents readily.
Other gases (for example, chlorine, hydrogen sulphide and sulphur dioxide) are less stable and will react easily with many common materials. These reactive gases require careful handling both from a calibration point of view and safety. They may react with the materials used in the calibration system (for example, tubing, flow meter etc.) as well as be affected by moisture content and other factors in the sampling system. Also, since in general we are dealing with small concentrations of these gases in air, small changes in the system may produce large effects in terms of the calibration results.