Gas blending means the mixing of gases for a purpose where the final gas mixture is specified. A gas mixture is defined in a molar gas fraction, by percentage, parts per thousand or parts per million. The applications are scientific processes, industrial processes, food production, food storage and breathing gases.

Gas blending for welding (Shielding gas)

Shielding gases for welding are (semi) inert gases used in gas metal arc welding (GMAW) and in gas tungsten arc welding (GTAW). This types of gases protect the weld area from oxygen and water vapour, which could negatively influence the quality of the weld or make the welding process more difficult.

What is gas metal arc welding? Gas metal arc welding is a process in which a continuous wire feed as a consumable electrode is used and a (semi) inert gas mixture to protect the weld. A variation on this is gas tungsten arc welding. This is a manual welding process in which a non-consumable tungsten electrode is used, a (semi) inert gas mixture and a separate filler material.

Gas blending for the food and beverage industry (Modified Atmosphere Packaging)

Modified atmosphere packaging is used to preserve the quality of fresh products, and it could be sold to markets far away from where it is grown and to extend the marketing period. The gas composition used to pack food products depends on each product. For example, a high oxygen content helps to retain the red colour of meat. In comparison to high oxygen, low oxygen reduces mould growth in bread and vegetables.

Gas mixtures for brewing

Sparging

Inert gas (nitrogen) is bubbled through the wine, which removes oxygen and carbon dioxide. To ensure a certain amount of carbon dioxide remains, a mixture of nitrogen and carbon dioxide may be used for the sparging gas.

Purging and blanketing

Purging is the removal of oxygen from the headspace above the wine in a container by flushing with a similar gas mixture used for sparging. Blanketing or inserting it is called if the oxygen is left there.

Breathing gas mixtures for diving

A breathing gas is a mixture of chemical gases and compounds for respiration. The essential component of any breathing gas is a partial pressure of oxygen of between approximately 0.16 and 1.60 bar at the ambient pressure. The oxygen is usually the only active component in the breathing gas unless the gas is an anaesthetic mixture. Some of the oxygen in the breathing gas is consumed by the metabolic processes. The inert components stay unchanged and serve mainly to dilute the oxygen to an appropriate concentration. Therefore inert gas components are also known as diluent gases.

Gas blending for scuba diving is the filling of a scuba tank with non-air breathing gases such as nitrox, trimix and heliox. Use of these gases is to reduce the risk of decompression sickness or nitrogen narcosis. It may improve ease of breathing.

Gas blending for surface supplied and saturation diving may include the filling of bulk storage cylinders with breathing gases. It may also involve the mixing of breathing gases at a lower pressure which are supplied directly to the scuba diver or the life-support system. Part of the operation of the life-support system is the replenishment of oxygen used by the divers and the removal of the carbon dioxide waste product by the gas conditioning unit.

“Gas blending means the mixing of gases for a purpose where the final gas mixture is specified”

Gas mixtures for medical purpose

For patients, under anaesthesia surgery, the anaesthetic machine is used to blend breathing gas. The gas mixing and delivery system let the anaesthetist control oxygen fraction, nitrous oxide concentration and the concentration of volatile anaesthetic agents. The machine is usually supplied with oxygen (O2) and nitrous oxide (N2O) from low-pressure lines and high-pressure reserve cylinders. The gas is mixed at ambient pressure. After which additional anaesthetic agents may be added to a vaporiser, and the gas may be humidified. Air is used as a diluent to decrease oxygen concentration. In exceptional cases, other gases may also be added to the mixture. These may include carbon dioxide (CO2 gas) used to stimulate respiration and helium (He) to reduce resistance to flow or to enhance heat transfer. Gas mixing systems could be mechanical or electronic, and control may be manual or automatic.

Analytical customised gas mixtures

Calibration gases mixtures

Span gases are used for testing and calibrating gas detection equipment by exposing the sensor of a gas detector to a known concentration of span gas. These span gases are a reference point to ensure correct readings after calibration. Span gases have a very exact composition with a content of the gas to be detected close to the setting(s) of the gas detector.

Calibration gas mixtures are produced in batches by gravimetric or volumetric methods. The gravimetric method uses sensitive and accurately calibrated scales to weigh the amounts of gases added into the cylinder. Precise measurement is important because inaccuracy or impurities could result in incorrect calibration. The calibration gas cylinder must be perfectly clean as possible. The cylinders may be cleaned by purging with high purity nitrogen and vacuumed. For specific critical mixtures, the cylinder may be heated while being vacuumed to facilitate removal of any impurities adhering to the walls of the cylinder.

After filling the gas mixture should be thoroughly mixed to ensure all components are evenly distributed throughout the cylinder. The reason for this is to prevent possible variations on composition within the cylinder. This cleaning process is usually done by rolling the container horizontally for 2 to 4 hours.