Graphite Two Electrode Conductivity Sensor with RS485 Modbus RTU and IP68 Protection for Online Water Analysis

The DEC790 is a digital, online conductivity sensor built around a graphite two-electrode measuring cell. Two parallel graphite plates are immersed in the measured solution and a stable driving voltage is applied across them. The conductivity of the water is then calculated from the current flowing between the plates. Because graphite offers excellent chemical inertness and a very low polarization tendency, the DEC790 keeps its reading stable in wastewater, surface water and industrial process water where ordinary metal electrodes drift quickly.

All signal processing, temperature compensation and Modbus communication are handled inside the stainless-free PPS sensor body. The probe outputs a clean RS485 MODBUS-RTU signal that can be read directly by a PLC, a data logger, a SCADA system or any compatible conductivity controller. There is no fragile analog transmitter to mount, no long cable run picking up electrical noise, and no separate temperature probe to wire in parallel.

Four conductivity ranges are available in one instrument family: 0~20 µS/cm, 20~200 µS/cm, 200~2000 µS/cm and 2~10 mS/cm. The same sensor platform also reports TDS up to 5 g/L, salinity up to 5 ppt and water temperature from 0.0 to 50.0 °C, which makes it a practical single-point solution for plants that need more than one water quality figure from the same tapping point.

The DEC790 is designed for continuous duty. The sealed PPS housing is rated IP68, so the probe can stay submerged for months without water ingress. The 1 inch NPT thread at both ends allows direct mounting into a pipe tee, a flow-through cell, a floating ballast or a side-wall bracket. A standard 5 metre cable is supplied and longer lengths can be ordered for deep tanks and remote sumps.

Conductivity is the ability of water to carry an electrical current, and it is directly related to the concentration of dissolved ions. In the DEC790, the two graphite electrode plates form a fixed cell constant. When the driving voltage is applied, the current between the plates rises with the ionic strength of the solution. The built-in electronics convert this current into a conductivity value, apply automatic temperature compensation to a 25 °C reference, and publish the result over Modbus.

Graphite was chosen for the electrode plates because it suppresses the polarization effect that builds up on polished metal surfaces. Polarization is the main cause of slow response and reading drift in low conductivity water. With graphite plates, the DEC790 holds a resolution of 0.01 µS/cm and an accuracy of ±1.0% FS across the working range, which is sufficient for both compliance monitoring and day-to-day process control.

• Graphite two-electrode cell resists polarization and fouling. The graphite plates minimise the polarization effect that causes drift on stainless steel electrodes, so the reading stays trustworthy in dirty, oily or high-solids water.
• Four conductivity ranges plus TDS, salinity and temperature. One sensor covers 0~20 µS/cm up to 2~10 mS/cm and simultaneously reports TDS to 5 g/L, salinity to 5 ppt and temperature to 50 °C, reducing the number of instruments on site.
• Native RS485 MODBUS-RTU output. Digital communication delivers the measured value without analog signal loss, so the probe can sit 100 metres or more from the control cabinet and still report accurately to a PLC or SCADA platform.
• IP68 sealed PPS body for long-term immersion. The polyphenylene sulfide housing and M16 cable gland withstand continuous submersion, mild acids, alkalis and typical plant washdown without corrosion.
• Multi-point calibration for demanding duty. Two-point, three-point and up to five-point calibration are supported, together with deviation and factor correction, so the sensor can be matched precisely to the plant reference meter.
• Very low power consumption. The probe draws only about 0.2 W from a 9~24 VDC supply, which makes solar and battery-backed remote stations practical.
• Simple mechanical installation. The 1 inch NPT thread at both ends accepts standard pipe fittings, flow-through housings and submersible brackets, and the 35 mm diameter body fits compact tapping points.

The DEC790 is used wherever dissolved ion concentration has to be tracked continuously and cost-effectively:

• Municipal and industrial sewage treatment plants, for inlet, aeration tank, membrane tank and final effluent monitoring.
• Drinking water plants, booster stations and distribution network water quality monitoring points.
• Surface water, river, lake and reservoir monitoring buoys and remote stations.
• Environmental protection water treatment projects and river basin supervision platforms.
• Metallurgical, electronics and semiconductor rinse water, where low conductivity rinse water must be verified.
• Mining process water, tailings water and thickener overflow conductivity control.
• Pulp and paper mills, for white water and process water conductivity logging.
• Cooling towers, boiler feed water and circulating water systems, as a low-cost blowdown indicator.

For pipeline installation, mount the sensor at a low point of the run where the flow is steady and air bubbles are unlikely to collect. For submerged installation, point the front section of the probe into the direction of the water flow and sink it deep enough to stay inside the active water body. Always keep the measuring surface clean during installation, and never touch the electrode surface with bare hands or oily rags, because a contaminated electrode can read incorrectly for a long period. Never run the sensor cable in the same conduit as power cables; induced noise will disturb the measurement and damaged insulation can break down the measuring unit.

Routine work is limited to a monthly visual inspection and a monthly calibration check, adjusted to the actual water quality. Normal cleaning only requires shaking the probe in clean water. If the measuring surface is heavily fouled, soak it in a mild detergent or weak acid for 15 minutes and then rinse with distilled water. Two-point calibration uses two water samples of known conductivity, writes a deviation value and a factor value into the sensor registers, and is normally enough for daily maintenance. When the deviation becomes large, or when the measuring unit has been replaced, a multi-point calibration with 84, 146, 1413 and 12880 µS/cm standard solutions restores full linearity. Calibrate at 25 °C where possible to avoid extra temperature compensation error, and use function code 06 with value 33 in register 33 to reset to factory calibration if needed.