GASES IN WATER There is a basic relationship at equilibrium, between the amount of gas dissolved in water and the amount of the gas in the atmosphere surrounding the water. Under normal conditions, in air, the composition is about 20% oxygen, 79% nitrogen and 1% argon. Equilibrium occurs when the transfer of gas into is equal to that out of the water. Men this occurs the gas pressure in the water is the same as in the atmosphere. The amount of gas in the water at saturation varies with changes in temperature and atmospheric pressure. Standard conditions are referred to as 58 degrees F and pressure at spa level which is 14.7 lbs per square inch, or in metric, 20 degrees C and 760 mm of mercury. The amount of dissolved gas in water can be referred to as parts per million (ppm) or percent saturation. That is the number of parts gas in one million parts water by weight. Using metric measurements, 3 milligrams (mg) in a liter of water is equal to 3 PPM One milligram is 1/1000 of a gram and a liter (L) of water contains 1000 milliliters (mL); at standard temperature and pressure 1 ml of water weighs approximately 1 gram. Water, saturated in air, at standard temperature and pressure, has 9.1 PPM oxygen and 14.9 PPM nitrogen. This is 100% saturation with both gases. If there is less gas than saturation (equilibrium) dissolved in the water, the oxygen will diffuse into the water as will the nitrogen. If the gas level in the water is higher than equilibrium (saturation) the gas will diffuse into the atmosphere. This last condition (gas level higher than equilibrium) is called supersaturation. Diffusion is a very slow process in still water. It can be speeded up by agitating the water or by the having the water flow over rocks as in a stream or from the agitation from wind and waves over a lake or pond. Water thus treated is probably near saturation (equilibrium).