All water used to generate steam contains impurities. After removing as many of these impurities as possible through reverse osmosis, ion exchange, or other processes, there are still some impurities even if only in minute quantities.
As water is heated, the cleaner part of the water turns to steam leaving slightly less pure water in the boiler. As the steam passes through its processes, some of the steam is lost before it returns to the boiler. This lost water must be replaced with new makeup water. In time, the solids content of the boiler water increases to a point where the carryover of the dissolved solids into steam reach unacceptable levels.
In less critical applications, the build-up of dissolved solids can be controlled by boiler blowdown. In more critical applications, this can be monitored through the use of return condensate monitoring systems and the Model CH16D or Model CH16RB Larson-Lane condensate monitors. When the steam is used to operate a turbine, the requirements of the steam purity are usually very high, and the measurement of the steam itself may be used in addition to return condensate measurements.