CondX 100 Online conductivity monitor

CondX 100 Series

Online Ultrapure Water Conductivity / Resistivity Monitor

A 24-bit delta-sigma ADC with AC synchronous demodulation brings online ultrapure water monitoring to 0.001 µS/cm display resolution, with 0.34% repeatability measured on 18.2 MΩ·cm water.

What resolution hides is the trend

The theoretical limit of ultrapure water at 25℃ is 18.2 MΩ·cm, which is 0.055 µS/cm of conductivity. Yet the inline monitoring built into most ultrapure water systems stops at 0.2 µS/cm resolution — a single step nearly four times the quantity being measured.

In that range such a meter can express only two things: there, or not there. When polishing resin approaches breakthrough, outlet conductivity does not jump into alarm; it first rises very slowly. If that rise is visible, there are days to schedule a cartridge change. If it is not, it becomes an alarm one morning with the line waiting for water.

Reading signal, not noise

Conductivity cannot be measured with DC: direct current polarises the cell and the reading drifts downward over tens of seconds, looking like improving water while charge accumulates on the electrode surface. The entire front end was rebuilt around that fact.

• 24-bit delta-sigma differential ADC at 1000 SPS. One-second block averaging pushes noise down by the square root of N, so the third decimal place carries information rather than jitter.
• 47.17 Hz AC square-wave excitation, deliberately offset from mains. If the excitation frequency sat at 50 Hz, any mains pickup would be integrated as valid signal by the synchronous demodulator. Moving off it closes that path.
• Capacitively coupled AC excitation. No net DC across the cell, eliminating polarisation drift at its source.
• Synchronous demodulation across positive and negative half-cycles. Subtracting the two half-cycles cancels amplifier offset, thermal drift and slow baseline movement.
• Five-step automatic PGA ranging: 1× / 4× / 16× / 64× / 128×. Gain steps up automatically on weak signals, and can be locked manually for diagnostics.
• The temperature channel shares the same 24-bit ADC. Two-wire ratiometric measurement cancels the absolute error of the excitation current; temperature compensation is automatic and its coefficient is configurable from field calibration.

Calibration handed back to the user

Most inline conductivity meters are calibrated once at the factory, so a new cell or a new water source means a trip back to the manufacturer. CondX 100 puts calibration entirely in the field: as many points as you have standard solutions.

• Least-squares calibration on 2 to 5 standard solutions. Corrects gain and zero offset together instead of forcing the fit through the origin. With a single solution it degrades gracefully to a through-origin single-point fit.
• Four independent probe calibration slots. Change the cell, switch the slot — no recalibration needed.
• CRC32-protected storage in a dedicated flash page. The linker script keeps that page physically outside the firmware image, so calibration survives power loss and firmware re-flashing — verified in practice.
• A fully transparent fit. Every calibration point can be listed with its raw reading and nominal value, and the fitted slope and intercept read back. A slope far from 1 means the cell or the loop needs attention.

Out-of-range readings are flagged, not disguised

A K=0.01 or K=0.02 ultrapure cell is only physically trustworthy between 0.055 and 1 µS/cm. Most meters keep returning a number past that boundary, and the number looks perfectly ordinary.

• Reading out of range: the status word sets a flag; the value is still output but explicitly marked as outside the verified range of that cell.
• Calibration out of range: a standard solution beyond the rated window is rejected outright, so one unsuitable solution cannot skew the whole fit.

The boundary is enforced in firmware, not buried in a disclaimer at the back of the manual.

Three interfaces, ready for your system

The three protocol stacks are independent and read the same set of measurements. Conductivity, resistivity, temperature, active gain and the status word are readable; cell constant, calibration constant and temperature coefficient are writable, with a remote save-to-flash command.

• UART service console (115200 8N1): a plain-text command line where one ALL command returns every value; calibration, diagnostics and range locking all happen here.
• RS485 Modbus RTU slave: standard function codes 0x03 / 0x06 / 0x10, CRC16, IDLE-based frame detection; floating-point values occupy two registers as big-endian IEEE754.
• CAN 2.0A at 500 kbit/s: one frame each for request and response, completing the exchange in a single frame — suited to multi-device bus networks.

Built to run continuously

• FreeRTOS task isolation: measurement, service console and fieldbus run as separate tasks, so one blocked path does not stall the rest.
• Independent watchdog (IWDG): a hardware backstop for software faults.
• ADC-chain stall auto-recovery: a halted data stream is restarted automatically and counted, so the event stays traceable afterwards.
• Full diagnostics on demand: interrupt counters, SPI errors, UART overruns and per-task stack headroom, all returned by a single command.

Key Specifications

• Measured values: Conductivity / Resistivity / Temperature
• Conductivity range: 0.055–1 µS/cm (rated range of a K=0.01 or 0.02 cell)
• Resistivity range: 1–18.2 MΩ·cm
• Display resolution: 0.001 µS/cm
• Repeatability: Better than 0.4% RSD (0.34% measured at 18.3 MΩ·cm, 30 samples over ~40 s)
• Cell constant: 0.01–20.0 cm⁻¹, configurable (two-electrode cells)
• ADC: 24-bit delta-sigma, differential, 1000 SPS
• Excitation: 47.17 Hz AC square wave, capacitively coupled
• Auto-ranging: Five-step PGA: 1× / 4× / 16× / 64× / 128×
• Temperature measurement: Two-wire ratiometric measurement on the shared 24-bit ADC, automatic compensation with configurable coefficient
• Calibration: 2–5 point least squares, 4 independent slots, CRC32-protected flash storage
• Interfaces: UART service console · RS485 Modbus RTU · CAN 2.0A
• Power supply: 24 V DC
• Controller: STM32F103C8T6, ARM Cortex-M3 @72 MHz

Applications

• Semiconductor and photovoltaic water plants: 18.2 MΩ·cm trend monitoring with out-of-range flagging.
• Pharmaceutical production: purified water and water-for-injection monitoring with traceable calibration records.
• Laboratories: outlet quality logging for ultrapure water systems, resin and membrane life assessment.
• Power generation: boiler feedwater and condensate conductivity monitoring, connectable to DCS over fieldbus.
• Electronics cleaning and PCB rinse lines: rinse endpoint detection.
• OEM integration: measurement front end for ultrapure water system manufacturers.

Developed in-house, open to customisation

The embedded control software of CondX 100 is developed entirely by STON Technologies — ADC driver, synchronous demodulation, multi-point calibration fitting and all three communication stacks are our own code. Software copyright registration for Embedded Control Software for Ultrapure Water Conductivity Meter V1.0 has been filed and accepted under reference 2026R11L2907592.

System integrators and instrument manufacturers needing custom protocols, register maps or calibration strategies are welcome to contact us directly.