page_title: Gas blending for scuba diving
section_title: Real gas effects
hierarchical_section_title: Gas blending for scuba diving / Quantities and accuracy / Real gas effects
caption_attribution_description: English: Graph of pressure versus compression ratio calculated for typical diving gases using Van der Waals equation of state at constant temperature (293K)
context_page_description: Gas blending for scuba diving is the filling of diving cylinders with non-air breathing gases such as nitrox, trimix and heliox. Use of these gases is generally intended to improve overall safety of the planned dive, by reducing the risk of decompression sickness and/or nitrogen narcosis, and may improve ease of breathing.
  Filling cylinders with a mixture of gases has dangers for both the filler and the diver. During filling there is a risk of fire due to use of oxygen and a risk of explosion due to the use of high-pressure gases. The composition of the mix must be safe for the depth and duration of the planned dive. If the concentration of oxygen is too lean the diver may lose consciousness due to hypoxia and if it is too rich the diver may suffer oxygen toxicity. The concentration of inert gases, such as nitrogen and helium, are planned and checked to avoid nitrogen narcosis and decompression sickness.
  Methods used include batch mixing by partial pressure or by mass fraction, and continuous blending processes. Completed blends are analysed for composition for the safety of the user. Gas blenders may be required by legislation to prove competence if filling for other persons.
context_section_description: At pressures above about 200 bar the compression of gases begins to deviate from the ideal gas laws, and consequently partial pressure blending must take into account that the gases added at higher pressure will provide lower volumetric proportion than gases added at lower pressure, and these deviations from linearity will vary according to the gas. Calculations for high pressure partial pressure blends may require use of the Van der Waals equation. This does not affect premixed gases, which will retain their mix ratio at any pressure, so continuous blending is not affected by this problem.
  Both nitrogen and oxygen compress relatively linearly and will approximate the ideal gas to much higher pressures than helium, which deviates significantly even below 200 bar. Air and nitrox mixtures can be approximated as ideal without significant error up to about 230 bar at normal temperatures.
caption: Comparison of compressibility versus pressure for typical breathing gases for diving at 293K (20°C)
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