Fluoride is widely added to drinking water systems to help prevent tooth decay. It is normally added in the form of liquid hydrofluorosalicic acid, which can be easily handled and applied using standard metering pumps. While the fluoridation process is often controlled by simple flow proportional feed, monitoring of final fluoride concentrations is useful in providing an alarm in the case of overfeed problems. Optimum fluoride levels of around 1 PPM are safe, but control system problems resulting in concentrations above 2 PPM 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.

ATI’s Model A15/82 Fluoride Monitor provides continuous measurement of free fluoride concentration in potable water. The system employs a fluoride sensitive ion selective electrode (ISE), which provides reliable measurements down to 0.1 PPM and as high as 1000 PPM. 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 analog output functions.

Fluoride ISE sensors measure F-ion in solution the same way that a pH sensor measures hydrogen ions. A lanthanum fluoride crystal on the tip of the sensor develops a voltage that is proportional to fluoride ion activity. An integral reference electrode is used as a comparator to measure the developed voltage, with the measurement made at a differential input amplifier. Since the activity of fluoride ions in solution is a function of pH and ionic strength, a small amount of buffer solution is added to the measured sample.  This creates a stable condition in which the concentration of fluoride ion and the activity of fluoride ion are directly proportional.

The A15/82 Fluoride Monitor requires very little maintenance. One gallon of buffer will operate the system for approximately 45 days. The system also performs an automatic calibration routine to correct for electrode “zero” drift. The fluoride standard is automatically pumped into the chemistry module and the analyzer zero is automatically adjusted. The frequency of this calibration is user-selectable from every 1 to 999 hours. Calibration standards are easily mixed from a stock solution provided with the unit, and the buffer is prepared using common vinegar.

 ________ (Quantity) Fluoride Monitors shall be supplied for continuous monitoring of the fluoride ion in solution at the ________ (Specify Location or Application).  The fluoride monitoring system shall consist of an electronic monitor, chemistry module containing a fluoride sensor with integral reference element, chemical pump and associated accessories listed below. The Fluoride Monitor shall be ATI Model A15/82 or equal.

Fluoride Monitor

The Fluoride Monitor shall be a compact ¼ DIN size instrument suitable for panel mounting. For outdoor applications, the monitor shall be supplied in a NEMA 4X enclosure with a clear hinged door to allow tool-less access to controls and for viewing the LCD display. The display shall be alphanumeric LCD and capable of indicating fluorine up to 1000 PPM. Additionally the display shall indicate control and alarm condition and setpoints, temperature and all configuration information programmed through the front keypad. The keypad shall be protected with a software lock and all configuration programming information shall be protected by access code.

The monitor shall provide three independently programmable alarm/control relays selectable over the entire range of the instrument as either low or high alarms and to be programmable for variable deadband and variable time delay.

An isolated 4-20 mA analog signal shall be provided, adjustable to any range within the display scale and suitable for loads up to 600 ohms.

Chemistry Module

The chemistry module shall be housed in a separate NEMA 4X enclosure with clear hinged cover and containing an ion selective fluoride electrode with integral silver/silver chloride reference element, sample and buffer pump and cast acrylic measuring chamber.

The sample shall be drawn from a sample overflow chamber at a fixed rate of 6 cc/minute with overflow discharged to a drain line.

The system shall perform and automatic calibration routine to correct for sensor zero drift.  The frequency of this automatic calibration shall be user-selectable from 1 to 999 hours.