Parts per Million by Weight in Water The concentration at ppm of gas that is present in water is typically expressed in weight. To measure this concentration in metric units, it is necessary to determine the density. will be needed. In pure waters, the density will be 1000.0000 kg/m 3. at a temperature of 3.98degC and the norm atmospheric pressure that was used up until 1969. In the past, this was the norm for the kilogram. In modern times, the kilo may be defined as equal to the mass that is the model for the world of the kilogram. High purity water (VSMOW) at temperatures of 4 degrees Celsius (IPTS-68) or typical atmospheric pressure could be defined as having an average densities that is over 999.9750 kg/m 3. . [5] The density of water is affected by temperature, pressure as well as the existence of contaminants i.e. gasses that disintegrate, as well as the salinity of water. Furthermore, the high concentration of gasses that dissolve in water alters its density. This ...