DEC790 Ultrapure Water Conductivity Probe with 0.01 µS/cm Resolution

Product Overview

The DEC790 ultrapure water conductivity probe is the low range member of the DEC790 graphite sensor family. Its first measuring range starts at 0~20 µS/cm with a display resolution of 0.01 µS/cm, which is fine enough to show the small conductivity changes that matter in rinse water, demineralised water and condensate. Instead of a single wide range instrument that becomes insensitive at the bottom of the scale, the probe offers four selectable ranges: 0~20 µS/cm, 20~200 µS/cm, 200~2000 µS/cm and 2~10 mS/cm. A plant can therefore use one probe type for both the ultrapure loop and the reject stream.

Low conductivity measurement is difficult because the current flowing between the electrodes is extremely small, and any polarization on the electrode surface distorts the reading. The DEC790 uses graphite electrode plates rather than polished stainless steel. Graphite is chemically inert and strongly suppresses polarization, so the zero point stays stable and the reading does not creep upward after a rinse cycle. Together with automatic temperature compensation referenced to 25 °C, this gives an accuracy of ±1.0% FS across the whole working range.

The measuring cell sits inside a sealed PPS housing rated IP68, connected by an M16 cable gland. The 1 inch NPT thread at both ends allows direct installation in a flow-through cell, a pipe tee, or a submersible bracket in a pure water tank. Output is digital RS485 MODBUS-RTU, so the value arrives at the PLC or SCADA system without analog conversion loss, which matters when the interesting change is only a few tenths of a microsiemens.

Why Resolution Matters in Pure Water

In a drinking water application, a change of 20 µS/cm is a minor fluctuation. In a semiconductor rinse tank or a pharmaceutical water loop, the same 20 µS/cm may represent a complete failure of the rinse step. Instruments with a coarse display cannot resolve the trend, and operators end up relying on laboratory samples instead of the online reading. The DEC790 addresses this directly: 0.01 µS/cm resolution, graphite electrodes to hold the zero point, and automatic temperature compensation so that a change in water temperature is not mistaken for a change in purity.

The four ranges can be matched to the process stage. Use 0~20 µS/cm for final rinse and polished water, 20~200 µS/cm for reverse osmosis permeate and soft water, 200~2000 µS/cm for raw water and cooling water, and 2~10 mS/cm for concentrate, reject and cleaning solutions. Because the probe also reports TDS (0~5 g/L), salinity (0~5 ppt) and temperature (0.0~50.0 °C) on the same Modbus interface, a single tapping point can feed a complete water quality dashboard.

Key Advantages

• 0.01 µS/cm resolution for true low range measurement. The probe resolves the small conductivity differences that indicate rinse efficiency and membrane performance, instead of hiding them in display rounding.
• Graphite electrodes hold a stable zero point. Graphite suppresses the polarization effect that pushes metal electrodes into a slow upward drift in pure water, so calibration lasts longer and the trend is trustworthy.
• Four ranges in one sensor, from ultrapure to 10 mS/cm. One probe type covers rinse water, RO permeate, raw water and concentrate, which simplifies spare parts and operator training.
• Automatic temperature compensation to 25 °C. Conductivity changes about 2% per °C, so uncompensated readings are misleading in pure water; the DEC790 compensates internally and outputs the normalised value.
• Digital RS485 MODBUS-RTU output. No 4~20 mA scaling error, no earth loop, and the full resolution reaches the PLC or historian unchanged.
• IP68 PPS body with M16 cable gland. Continuous immersion in pure water tanks and wet benches without ingress, and resistance to the mild chemicals used in clean in place cycles.
• Up to five point calibration with deviation and factor correction. The probe can be matched to a laboratory conductivity meter across the low range, which is essential for audit and compliance work.
• Compact 35 mm body for tight flow cells. The 230 mm length and 1 inch NPT threads fit standard stainless and PVDF flow-through housings used in pure water skids.

Applications

• Semiconductor and electronics ultrapure water rinse stations, final rinse verification and reclaim loops.
• Pharmaceutical purified water and water for injection loops, including clean in place and steam in place rinse checks.
• Power plant condensate, demineraliser outlet and boiler make-up water monitoring.
• Reverse osmosis and electrodeionisation permeate quality monitoring.
• Laboratory and central pure water supply systems in universities, hospitals and research institutes.
• Food and beverage final rinse water, where low conductivity proves the rinse is complete.
• Ultrapure water reuse and polishing loops in photovoltaic and battery manufacturing.

Installation and Calibration

In a flow-through housing, install the probe so that the measuring face is fully wetted and bubbles cannot collect on the electrode. In a tank, orient the front section into the water movement and keep the probe below the minimum level. Never touch the graphite surface with bare hands or oily rags during installation, because contamination is very visible in low conductivity water and may take a long cleaning cycle to remove. Keep the sensor cable separated from power cables; the currents measured in pure water are small and induced noise is a real risk.

For routine work, a two point calibration with low range standards such as 84 µS/cm and 146 µS/cm is sufficient. When the measuring unit is replaced, or when the deviation grows, run a multi point calibration with 84, 146, 1413 and 12880 µS/cm solutions and store up to five calibration points. Calibrate at 25 °C wherever possible, and use function code 06 with value 33 in register 33 to restore factory calibration if a mistake is made. Cleaning is limited to shaking the probe in clean water; for heavy contamination soak the measuring section in a mild detergent for 15 minutes and rinse with distilled water.