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Dissolved / In Solution
These terms refer to a homogenous mixture of two fluids – in this case oil and water – implying that the individual water molecules are discrete and mixed with the oil molecules. The water is in solution. The sample cannot be separated by allowing the solution to stand at a given temperature. The fluid is clear.

Free Water
This describes the condition in which a fluid is saturated and is past the point where water is the solution. If more water is added to the oil, the water sinks to the bottom and the oil rises to the top. The visible horizontal line at the boundary between the two elements is called the interface.

Another example of free water is emulsions. They form when enough mechanical agitation acts on the fluid so that the free water forms a cloudy mixture of water and hydrocarbons. The mechanical shearing action creates very small water droplets which have too much surface tension to join and form an interface. This is still free water as it is not in solution, but it does not create an interface boundary, causing a visible cloud or haze instead.

Saturation / Saturation Point
At this point the fluid carries as much water in the dissolved state as it possibly can at a given temperature. At this point the saturation level is 100%. If any more water were to be added, a free water condition would result and that would be the beginning of an emulsion or interface. When the saturation point is given, a corresponding temperature is also given because saturation varies according to temperature.

Saturation Level / Percent Saturation
This is the degree of saturation which indicates what percent of maximum possible water in a dissolved state is in the oil. A reading of 0% would indicate oil free of water, while a reading of 100% would indicate oil that is saturated with water.

Water Vapor Pressure
This is the pressure exerted by water vapor. Water gives off vapor, consisting of molecules that have evaporated and are in a gaseous state. The presence of water in oil results in a water vapor pressure on the surface of the oil. This water vapor pressure depends on the water content, the type of oil (including additives and particles), and temperature. If the ambient water vapor pressure is higher than that of the oil, water moves into the oil. By contrast, if the ambient water vapor is lower, water evaporates out of the oil.

Saturated Water Vapor Pressure
When adding water to oil, the water vapor pressure increases until a maximum value. The vapor is then said to be saturated vapor and the pressure it exerts saturated water vapor pressure. In oil this is the case when a maximum amount of water is dissolved.