Fluoride is widely added to drinking water systems to help prevent tooth decay. It is normally added in the form of liquid hydrofluorosilicic acid, which can be easily handled and applied using standard metering pumps. While control of the fluoridation process is easily handled by simple flow proportional feed, water quality monitoring of final fluoride concentrations is useful in providing an alarm in the case of overfeed problems. Optimum fluoride levels of around 1ppm are safe, but control system problems resulting in concentrations above 2ppm are considered excessive and need to be detected as early as possible. Loss of chemical feed can also be detected quickly and reliably with on-line fluoride monitoring.
The Q46F provides continuous water quality monitoring of free fluoride concentration in potable water. The system employs a fluoride sensitive ion selective electrode (ISE) which provides reliable measurement down to 0.1ppm and as high as 1000ppm. A chemistry module provides sample conditioning for the sensor and the measured fluoride concentration is displayed on a separate electronics module that also provides alarm and analogue output functions.
The Auto-Cal system will periodically stop normal sample flow and open the inlet for calibration standards, normally 1 and 10 PPM fluoride. After the first standard has been read, and the mV value stored, the calibration standard valve switches to the second standard solution. Once the two standards have been read, the monitor automatically adjusts the zero offset and slope in the monitor and normal measurement resumes. During the
calibration cycle of about 5 minutes, the 4-20 mA output is held at the value measured just before starting the cycle. At the end of the cycle, the output will hold for a short period to allow sample to stabilize again and then the output is released to update.
