Walchem WEL / WDS Series pH/ORP Sensors and Preamplifiers

Walchem’s pH/ORP line is built around differential measurement: the WEL series with replaceable measuring cartridges, the WDS series with a replaceable salt bridge, preamplifiers that connect standard electrodes to Walchem controllers, and special electrodes for high temperature and pressure. The differential technique makes the signal resistant to stray voltages and currents and to ground loop problems. As the Walchem distributor in Turkey, Mikron Proses (Üsküdar, Istanbul) supplies WEL and WDS electrodes, replacement cartridges and preamplifiers and provides application consulting for electrode selection, controller connection, commissioning and calibration support.

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Comparison

Product group	Measuring principle and cartridge / reference	Limits	Typical use
WEL series (modular cartridge)	Differential, titanium ground rod in the housing; cartridges PHF, PHB, PHH, PHLI (pH) and MVF, MVR (ORP)	CPVC housing, 6.9 bar	Industrial pH/ORP measurement, inline or submersion
WDS series (full differential)	Glass reference in pH 7 buffer with replaceable porous salt bridge, third metal ground electrode; 0.001 pH / 0.1 mV sensitivity	-5 to 95 °C, 0.7 MPa at 65 °C	Industrial and municipal applications
102029 (pH) / 102963 (ORP)	Double-junction Na₂SO₄ + KCl / Ag/AgCl reference, 58.7 ± 0.3 mV per pH	34.5 bar at 25 °C; -5 to 135 °C at 13.8 bar	High temperature and high pressure lines
Preamplifier	Converts the high-impedance signal of a standard pH/ORP/ISE electrode to a differential signal	Up to 305 m (22 AWG) after amplification	Standard BNC electrodes on Walchem controllers

WEL series: modular cartridge electrodes

The WEL has a rugged CPVC housing that contains the electronics. The pH and ORP cartridges are connected or replaced in minutes without tools, with a threaded interlock connection and a double FKM o-ring seal. The preamplifier (or an optional 4-20 mA transmitter) and the temperature compensation element stay in the housing when a cartridge is replaced, which lowers operating cost. The titanium solution ground rod in the housing enables the differential technique, which extends electrode life. The preamplifier is powered by the controller, so the signal is always amplified and there is no battery. Cartridge options are flat surface (PHF), bulb (PHB) and HF resistant (PHH) for pH, flat surface (MVF) and rod style (MVR) for ORP, and a flat pH cartridge (PHLI) for samples between 10 and 100 µS/cm. The same housing and cartridge can be used for submersion and inline mounting, which reduces spare parts stock.

WDS series: differential sensors with a salt bridge

The WDS series uses two electrodes, one for the process measurement and one for the reference measurement, each measured against a third metal electrode. The reference is a glass electrode embedded in pH 7 buffer behind a porous, replaceable salt bridge; if the bridge clogs or the solution is contaminated it is easily replaced, and the glass reference is not prone to chemical attack in the way a silver/silver chloride reference can be. The range is 0-14 pH and -2000 to 2000 mV (ORP), with a stability of 0.03 pH and 2 mV per day.

Preamplifiers and high-temperature electrodes

If you already have a standard, non-amplified pH/ORP/ISE electrode, a Walchem preamplifier makes it usable with Walchem controllers. For total cable runs under 6 m the internal preamplifier is mounted in the controller enclosure; for longer runs the external NEMA 4X wall-mount enclosure is used, and the amplified signal is carried up to 305 m. For conditions that standard CPVC electrodes cannot take, the 102029 (pH) and 102963 (ORP) electrodes are available.

Related pages

The electrodes connect to the W100P panel-mount controller, the Intuition-6 and Intuition-9 controllers; the WEL and WDS series are the sensors recommended in the Intuition brochures. For background see the pH electrode selection guide and the Walchem brand page.