Pure Water pH and ORP Sensors
Ultrapure water (UPW), Water for Injection (WFI), high-purity water, and deionized (DI) water all describe water purified to the highest standards by removing all contaminants including organic and inorganic compounds, dissolved and particulate matter, and dissolved gases. This purified water has very low conductivity, making accurate pH measurement challenging due to:
- Reference Electrode Stability
- Glass Electrode Response
- Electrical Noise
- Special Temperature Compensation Requirements
Process Contamination and Reference Junction Potential
A diffusion gradient potential develops at the reference junction when different solutions contact each other, potentially causing unstable reference potential and anomalous pH measurements. In pure water, the liquid junction can develop potentials as high as 20-40 millivolts (≈0.5 pH), leading to erratic, drifting pH values.
Counter Measures
These issues can be minimized by maintaining steady flow across a "positive pressure" reference sensor like the unique bellow system, which prevents fluid ingress and maintains positive ion flow. The system's built-in bellow ensures pressure equalization, making the sensor insensitive to external pressure/flow variations.
Pure water's low conductivity (0.055 µS at 25°C) creates static charges that generate stray currents, causing erratic pH readings. Using a pH sensor with liquid earth electrode and dual amplifier pH transmitter provides optimal noise immunity for stable measurements.
Temperature Effects
Pure water requires special temperature compensation for both Nernstian electrode correction and equilibrium/dissociation constant correction. The dissociation constant of pure water is 0.172 pH/10°C, meaning pH varies from 6.61 at 50°C to 7.47 at 0°C.
Bellomatic Sensor Technology
Yokogawa's Bellomatic sensor addresses high diffusion potentials with a positive pressure electrode design. Its large refillable reservoir provides constant reference electrolyte flow, offering longer service life and independence from process pressure effects.
FU24 pH/ORP Sensor
The all-in-one FU24 pH/ORP sensor incorporates the patented bellow system in a combination electrode design. Originally developed for harsh chemical applications, it performs exceptionally in pure water with:
- Internal bellow system
- Large reference chamber
- Long-life reference probe
- Estimated 20-year lifespan in demineralized water at 20°C
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