pH Analyzer Drift and Agitator-Induced Offset
A pH loop controls caustic addition to a neutralization tank. The analyzer is a two-wire transmitter with a glass electrode and a separate reference, connected through a junction box 30 m from the tank. The DCS reads 8.6 pH, but a lab sample taken at the same time reads 7.1 pH. The analyzer was calibrated yesterday with fresh pH 7 and pH 4 buffers: pH 7 gave -12 mV and pH 4 gave -150 mV at 25°C. The temperature compensator is a Pt100 in the electrode and reads 25°C, matching the lab. The reading shifts by about 0.8 pH when the agitator starts, and the junction box contains visible moisture. The area is not classified, but caustic is present. The operator wants to adjust the analyzer's trim to match the lab and continue.
1. Calculate the electrode slope and offset from the calibration data. Decide whether the electrode is acceptable.
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The measured change is from -12 mV at pH 7 to -150 mV at pH 4, a change of 138 mV over 3 pH units: Slope = 138 / 3 = 46 mV/pH
Theoretical slope at 25°C is about 59.16 mV/pH, so the electrode is operating at: 46 / 59.16 × 100 = 77.8% of theoretical
The offset at pH 7 is -12 mV, which is within a typical ±30 mV acceptance window. The slope, however, is low; many plants require at least 85–90% of theoretical for reliable control. A 78% slope points to an aged or fouled glass bulb, a contaminated reference, or cable/insulation leakage. Clean the electrode and repeat calibration with fresh buffers before condemning it; if the slope remains below the acceptance limit, replace the electrode.
2. The reading is 1.5 pH higher than the lab and shifts when the agitator starts. What does that pattern suggest, and how would you isolate the cause?
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Do not adjust the trim to match the lab. That would mask a measurement problem and could cause incorrect caustic dosing. The low slope already shows the measuring circuit is not performing correctly.
The agitator-related shift plus moisture in the junction box suggests electrical leakage or a ground loop in the high-impedance pH circuit. Possible causes include a wet junction box, damaged cable insulation, a fouled reference junction, a poor shield connection, or motor/agitator ground current. The 1.5 pH discrepancy could also come from a stale lab sample, but the agitator shift makes an electrical cause more likely.
Isolate by checking the electrode in fresh buffers away from the agitator, inspecting and drying the junction box, measuring insulation resistance of the cable, and verifying single-point grounding. Compare the analyzer with a portable high-impedance pH meter on a grab sample taken at the same tap. If the portable meter agrees with the lab and the analyzer does not, the analyzer circuit is at fault.
3. Describe the corrective maintenance and safety sequence.
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Lock out the agitator and caustic feed, put the control loop in manual, and follow the plant's isolation procedure. Wear chemical PPE, including face and eye protection, and use the eyewash and spill kit as required. Remove the electrode, clean the glass bulb with an approved cleaner, rinse thoroughly, and refill or replace the reference electrolyte. Inspect the reference junction for clogging, dry the junction box, and repair or replace damaged cable. Verify shield grounding at the analyzer end only, according to the manufacturer's instructions.
Calibrate with fresh pH 7 and pH 4 buffers, then check a third buffer if available. Accept only if slope and offset meet the plant limits. Reinstall, restore agitator and caustic feed, return the controller to auto, and compare the reading with a lab sample. Document as-found/as-left data, buffer lot numbers, and any parts replaced.