DEC790 RS485 Modbus Conductivity Transmitter for System Integration and Remote Stations

Product Overview

The DEC790 is a digital conductivity transmitter built for system integrators, OEM skid builders and remote monitoring projects. There is no local display to configure and no analog loop to trim: the probe is polled over RS485 MODBUS-RTU, returns conductivity, TDS, salinity and temperature as floating point registers, and can be commissioned from the PLC or the gateway in a few minutes. Factory defaults are Modbus address 17, 9600 bps, no parity and one stop bit, and every parameter can be changed through the register map.

Power consumption is the detail integrators notice first: about 0.2 W from a 9~24 VDC supply. That is low enough to run the probe from a small solar panel and battery, or from the 24 VDC rail of a telemetry cabinet, without a dedicated power supply. Several probes can share one bus and one power rail, provided each is given a unique Modbus address before installation.

The measurement cell is a graphite two-electrode design with four ranges from 0~20 µS/cm to 2~10 mS/cm, ±1.0% FS accuracy and automatic temperature compensation. Everything is packaged in a 35 mm PPS body rated IP68 with 1 inch NPT threads at both ends, so the same probe can be used in a skid, a pipe tee, a float assembly or a stainless flow-through housing. Measuring data is transmitted as 4 byte floating point values in little endian byte swapped order, which is the format most PLC Modbus libraries expect once the word order is set.

Designed for Integrators

Most conductivity projects fail in the integration phase rather than the sensor phase: analog scaling is misconfigured, the cable picks up noise from a variable frequency drive, or the transmitter cannot be addressed on an existing bus. The DEC790 removes those failure modes. The value is already linearised and temperature compensated inside the probe, the communication is digital, and the register map is documented in the user manual with the exact addresses for conductivity, TDS, salinity and temperature.

Commissioning steps are short: wire red to V+, black to V-, white to RS485 A and green to RS485 B, set the master to 9600 8N1, read the register block, and confirm the float word order. If the value looks implausible, the word order is the first thing to check. For multi probe installations, assign addresses one at a time so that no two sensors share address 17.

Key Advantages

• 0.2 W low power consumption. Ideal for solar and battery backed remote stations, telemetry cabinets and unmanned outfalls, where every milliamp of standby current counts.
• RS485 MODBUS-RTU with a documented register map. Direct polling from PLC, RTU, datalogger or gateway, with conductivity, TDS, salinity and temperature in one register block.
• No analog scaling or trimming. The digital value is already linearised and temperature compensated, removing 4~20 mA scaling errors and earth loop problems.
• Long cable runs without accuracy loss. Digital transmission allows installation hundreds of metres from the cabinet, with standard 5 m cable and 10, 20 or 30 m options.
• Multi drop on one bus. Multiple probes can share a single RS485 line and a single 24 VDC supply, reducing wiring cost in multi point stations.
• Compact IP68 PPS probe for OEM skids. 35 mm diameter, 230 mm long, 1 inch NPT both ends, 6 bar rating, easy to integrate into a packaged plant or a flow-through manifold.
• Graphite two electrode cell with four ranges. 0~20 µS/cm up to 2~10 mS/cm with 0.01 µS/cm resolution and ±1.0% FS accuracy, so one part number serves several markets.
• Field replaceable measuring unit. Integrators can offer a serviceable product: swap the cell module, keep the installed body, cable and addressing, and avoid a site visit to re-commission.

Typical Integration Scenarios

• Water and wastewater telemetry cabinets with RTU or gateway based data acquisition.
• Solar powered surface water monitoring buoys, river stations and reservoir platforms.
• Packaged reverse osmosis and softening skids with PLC control and remote diagnostics.
• Cooling water and boiler control panels that already run a Modbus RS485 network.
• Aquaculture controllers combining salinity, temperature and dissolved oxygen on one bus.
• Mining and industrial plant SCADA upgrades where analog transmitters are being replaced by digital sensors.
• OEM equipment builders needing a private label conductivity probe with a stable register interface.
• Building management and district energy systems monitoring make-up and circulating water quality.

Wiring, Addressing and Data Format

Four wires are required: red is V+ power positive, black is V- power negative, white is RS485 data A(+) and green is RS485 data B(-). The cable enters the PPS body through an M16 gland. Default communication parameters are address 17, baud rate 9600, no parity, one stop bit; all can be modified through the register map described in the user manual. Measurement values are 4 byte floating point numbers in little endian byte swapped order, so the Modbus master must be configured with the matching word order. Calibration values, deviation, factor and temperature correction are also written through standard Modbus function codes, which means the whole commissioning sequence can be scripted from the control system instead of being performed with a handheld configurator.

Maintenance

Maintenance is limited to a monthly visual inspection and a monthly calibration check, with the interval extended once the drift trend is known. Cleaning requires only shaking the probe in clean water; for heavy deposits, soak the measuring section in a mild detergent or weak acid for 15 minutes and rinse with distilled water. Do not wipe or scrape the graphite surface. If a replacement is needed, the measuring module is changed in the field and the Modbus address, cable and mounting stay untouched, so no re-addressing work is required at the gateway.