Model 7976 : Multi-parameter Water Quality Monitor

IntroductionThe Model 7976 has been redesigned to meet the ever increasing demands for continuous monitoring of surface waters, reservoirs and intake protection sites. With over 20 years experience of this application the Model 7976 has evolved to give the user an easy-to-operate, easy-to-maintain monitor, providing accurate and reliable measurements. By utilizing standard, well-proven products, the monitor provides a neat and compact solution to continuous on-line water quality monitoring. The design philosophy of the Model 7976 is to minimize and simplify the operational and maintenance aspects for the end user. This has been achieved through the mounting of the electronic transmitters and sampling systems on to a single back plate, which may be conveniently and easily wall-mounted.The main electronic transmitters are from the highly successful 4600 series product range, comprising separate instruments for the measurement of pH, Conductivity, Dissolved Oxygen, Electrolytic Conductivity, Turbidity and Temperature; the latter also controlling the operation of the integral biocide cleaning function.General The Model 7976 is designed for continuous on-line monitoring and engineered for versatility and robustness with a low maintenance and servicing requirement. For each of the five parameters, an easy-to-read, backlit liquid crystal display gives local indication and an isolated current output, customer configurable to 0 to 10mA, 0 to 20mA or 4 to 20mA, is available. These can be individually programmed to suit the particular application requirements and may be expanded to a small window within the overall measuring range. An alarm relay output is supplied as standard, while an optional RS485 serial interface allows the monitor to be easily incorporated into an ABB, or other, supervisory system. The large, easy-to-read display is used in conjunction with four tactile membrane key pads to prompt the user through the programming procedure for each of the five parameters. A five language software package includes options for configuration of the monitor in one of English, French, German, Italian or Spanish languages.All the electronics are housed in IP66 rated enclosures, ensuring that the instrument can be installed in virtually any location without the need for additional protection. All interconnections between the sensors and transmitters are supplied ready wired, ensuring that installation costs and commissioning time are kept to a minimum. The design has been aimed at providing an easy-to-use, easyto- maintain system. Calibration of the pH and dissolved oxygen parameters is a semi-automatic process, whereby introducing the pH sensors to buffer solutions and the oxygen sensor to air enables the measuring instruments to automatically apply any correction that is necessary. Dry calibration standards are an important feature of the calibration procedure for the turbidity unit, obviating the requirement for formazine calibration standards. Electrode status checks are carried out by the measuring instruments and full diagnostics are available on all parameters.CleaningThe Model 7976 includes the facility for a fully automatic biocide cleaning cycle, the frequency of which is user-programmable to suit application requirements from 2 hours to 14 days. This feature maximizes the on-line availability of the monitor through minimizing down time for cleaning that would otherwise be necessary to overcome problems of, for example, algal growth. Air cleaning is also available for the pH measurement system to overcome potential problems of sensor contamination in samples containing matter likely to promote fouling. An air clean valve is controlled by the pH transmitter which allows compressed air (not exceeding 1.4 bar above sample line pressure), from an external source, to be directed at the combination pH electrode mounted in the flow cell. A pulsating air cleaning cycle is used to guarantee optimum effectiveness.