Gas blending
Gas blending is the process of mixing gases for a specific purpose where the composition of the resulting mixture is defined, and therefore, controlled.
A wide range of applications include scientific and industrial processes, food production and storage and breathing gases.
[2] Modified atmosphere packaging preserves fresh produce to improve delivered quality of the product and extend its life.
Gas blending for surface supplied and saturation diving may include the filling of bulk storage cylinders and bailout cylinders with breathing gases, but it also involves the mixing of breathing gases at lower pressure which are supplied directly to the diver or to the hyperbaric life-support system.
Part of the operation of the life-support system is the replenishment of oxygen used by the occupants, and removal of the carbon dioxide waste product by the gas conditioning unit.
These may include carbon dioxide (CO2), used to stimulate respiration, and helium (He) to reduce resistance to flow or to enhance heat transfer.
[6] Gas mixing systems may be mechanical, using conventional rotameter banks, or electronic, using proportional solenoids or pulsed injectors, and control may be manual or automatic.
The gravimetric method uses sensitive and accurately calibrated scales to weigh the amounts of gases added into the cylinder.
Batch gas blending requires the appropriate amounts of the constituent gases to be measured and mixed together until the mixture is homogeneous.
The accuracy required for this application can be achieved by using a pressure gauge which reads accurately to 0.5 bar, and allowing the temperature to equilibrate after each gas is added.
The method is not suited to moving platforms where the accelerations can cause inaccurate measurement, and therefore is unsuitable for mixing diving gases on vessels.
Oxygen content is relatively simple to monitor using electro-galvanic cells and these are routinely used in the underwater diving industry for this purpose, though other methods may be more accurate and reliable.
