While simple 2-electrode sensors are inexpensive and can provide accurate data, continuous monitoring of even relatively clean water can foul the electrodes and degrade the measurement. Maintaining accuracy is made more difficult when the amount of solids dissolved in the process varies over a wide concentration range. In this case, a simple 2-electrode sensor is limited by its cell constant, which means it cannot measure the process accurately.

ATI’s Q45C4 4-Electrode Conductivity Monitor is the answer for monitoring almost any water-based process. Drinking water, plating bath solutions, cooling water, process wash water, or virtually any other aqueous system can be monitored accurately and reliably. The unique drive/control scheme used in the 4-electrode system allows a single sensor configuration to be used in conductivity ranges from 0-200 μS to as high as 0-2,000 mS (0-2 S).  The “auto-ranging” feature enables the monitor to display the actual conductivity value during “overshoot” conditions. For chemical mixing applications, a concentration display can be selected for a limited number of acids and bases.

In a traditional 2-electrode sensor, each electrode performs two functions. First, the electrodes are used to generate a “drive potential” through the aqueous solution. Second, the electrodes are used to measure the level of current flow as the conductivity of the solution varies. It is this dual role that limits a 2-electrode sensor to a narrow functional conductivity range.

A 4-electrode sensor splits the two functions between two sets of electrodes. One set of electrodes is used to generate the drive potential through the aqueous solution. The other set of electrodes is used to measure the current that flows between the two drive electrodes. Because these two functions are separate, the 4-electrode sensor does not suffer the polarization effects that limit a 2-electrode sensor to a narrow functional range. This allows a single 4-electrode sensor to operate reliably over the entire 0-2 S/cm conductivity range.

Conductivity Monitors shall be supplied for continuous monitoring of conductivity in ___________ (Specify Application and Location). The conductivity monitoring system shall consist of an electronic monitor housed in a NEMA 4X enclosure suitable for wall, pipe, or panel mounting, a 4-electrode style conductivity sensor, and accessories listed below. The Conductivity Monitoring System shall be ATI Series Q45C4/Q25C4 as described below.

The conductivity sensor shall be a 1" NPT, four-electrode design capable of a measuring range of 0 to 2 Siemens with the same cell. The sensor electrodes shall be constructed of titanium for optimum chemical resistance. O-ring seals shall be composed of Viton.

The sensor shall be constructed of molded PEEK (poly-ether-ether-ketone) components. These components include the body, electrode holder, all rear seal components, and all internal components. The body shall be hex shaped to facilitate quick connection to standard 1" NPT fittings. The sensor shall be encapsulated and mechanically sealed by dual, high performance, o-rings at every solution interface.

The sensor shall include a Pt1000 RTD for high accuracy temperature measurements.

The sensor shall include a highly chemical resistant, cross-linked polyethylene jacketed cable. The sensor cable shall contain two foil shields for optimum electrical performance.

The sensor shall be available in mounting styles that include convertible, insertion, and submersion configurations.

The conductivity Monitor electronic assembly shall be: (select one version below)

A. A loop-powered 2-wire instrument providing an isolated 4-20 mA output proportional to conductivity into a maximum load of 500 ohms.

B. An AC powered instrument for operation on (specify either 115 VAC or 230 VAC) single-phase line power. The monitor shall provide two isolated 4-20 mA outputs configurable for conductivity or temperature. Analog outputs shall be both ground isolated and isolated from each other. AC powered conductivity monitors shall also contain two SPDT relays.

C. A battery operated data logging monitor capable of operating from an internal 9 VDC battery. The monitor shall provide two 0-2.5 VDC outputs suitable for use by the internal data logger. The monitor shall operate for up to 4 days continuously on an alkaline battery and up to 10 days on a lithium battery.

The conductivity monitor electronic assembly shall provide a variety of functions as follows.

1. Provide user selectable display of conductivity or process temperature on the main display. Main display variable shall be indicated with a minimum character height of 0.75” to allow easy readability up to 20 feet away. The transmitter shall include functions for displaying concentration values from built-in tables.

2. When operated on AC power, provide two isolated 4-20 mA outputs, with output spans programmable by the user for any segment of a display range.

3. The transmitter shall allow the 4-20 mA output to be set to any two points within the measuring span of 0-2,000,000 μS, as long as the points are at least 20 μS away from each other. The points may also be reversed. The transmitter shall allow the user to place a delay on the reaction time of the output and display.

4. Provide output hold and output simulate functions to allow for testing or remote receiving devices or to allow maintenance without disturbing control systems.

5. When operated on AC power, provide two SPDT relays. Relays shall be programmable for either control or alarm function, or relays may be assigned to diagnostic functions for use in indicating trouble conditions at a remote location.

6. The transmitter shall contain calibration functions for 1-point calibration for conductivity. An air-zero calibration routine shall be provided for calibrating the sensor zero point at initial installation. Calibration stability monitors shall be provided to hold calibration status until stable calibration conditions have occurred. In addition, the transmitter shall allow a sensor cell constant value to be entered directly for calibration without solutions.

7. Diagnostic functions shall be incorporated into the transmitter. The 4-20 mA output shall be capable of being assigned to safely rise to 20 mA, fall to 4 mA, or be left alone, during diagnostic failures. Diagnostic error messages shall be displayed in clear language; no confusing error codes shall be displayed.

8. The transmitter shall provide a function for automatically compensating for sensor fouling. If the sensor fouling reaches a level to which it can no longer be compensated for, the transmitter shall indicate a fouling warning.

9. When operated from a 9 VDC battery, provide data logging of conductivity and temperature over programmable time intervals. Provide software with the data logger for easy data download to standard computers, with graphical and tabular viewing of data. Software shall allow export of data to standard spreadsheet programs.

The complete Conductivity Monitor shall be an Analytical Technology Inc. Model Q45C4/Q25C4, or approved equivalent.