Wash Water Monitoring: pH and PAH Sensor Calibration for Marine ScrubbersWash Water Monitoring: pH and PAH Sensor Calibration for Marine Scrubbers
A washwater sensor can look fine and still be wrong
Originally published on Alright Engineering Solutions
A pH sensor can give a perfectly believable reading and still be off. A PAH monitor can report a steady, reassuring number through a window with a film nobody has cleaned. Neither fault announces itself, and a scrubber discharging through a drifted sensor can end up outside its limits, or unable to prove it stayed inside them.
Under the IMO’s EGCS guidelines, pH, PAH, turbidity, and temperature have to be monitored and recorded continuously whenever a scrubber operates in ports, harbours, or estuaries. This guide covers what each sensor is proving, how the pH and PAH calibrations differ, why the sensors drift at sea, and what a record needs to contain to satisfy an inspector.
What the Monitoring System Measures
Four parameters make up the washwater dataset, and each has a different job.
Turbidity and temperature matter more than they look. A turbidity sensor that’s drifted can quietly undermine a PAH reading, and a temperature error feeds straight into pH compensation.
The Limits the Sensors Have to Prove
The pH of the discharge has to be at least 6.5 at the overboard discharge. During manoeuvring and transit, the guidelines allow a maximum difference of 2 pH units between inlet and outlet instead. For PAH, the continuous concentration can’t exceed 50 µg/L PAHphe above the inlet water’s PAH, normalised to a washwater flow rate of 45 tonnes per MWh.
PAH isn’t a direct measure of oil. It’s a proxy: the instrument responds to fluorescence and reports the result as if the sample were phenanthrene. That’s why the calibration standard matters so much, because the number only means something relative to the standard behind it.
The guidelines also set a recording rate. pH, PAH, and turbidity have to be recorded at no less than 0.0111 Hz, which works out to roughly one reading every 90 seconds. A sensor that’s calibrated but logging with gaps still leaves a hole in the record.
pH Calibration: Buffers, Slope, and Temperature
The guidelines call for a pH electrode and meter with 0.1 pH unit resolution and temperature compensation. MPA Singapore’s circular on EGCS requirements also names standards for the electrode (BS 2586 or ASTM D1293–18) and the meter (IEC 60746–2:2003 or an equivalent), so it’s worth checking that the fitted equipment matches.
Calibration starts before any buffer comes out. Check the electrode for fouling, cracking, depleted electrolyte, and air bubbles, and clean it using the manufacturer’s method. Then calibrate against certified pH buffers, normally at least two points that span the range the sensor works in, following the manufacturer’s procedure. Verify the slope, the offset, the response time, and the temperature compensation, and record the as-found and as-left values.
The slope and offset checks are what separate a calibration from a quick look. A sensor can read plausibly in the middle of its range while its slope has drifted enough to be wrong at the 6.5 line, which is exactly where it needs to be right.
PAH Calibration: Phenanthrene and the 5% Rule
PAH monitors typically work by UV fluorescence, and the guidelines require them to be calibrated with phenanthrene, with results reported as PAHphe. The equipment has to measure up to at least twice the applicable discharge limit, and it has to stay accurate within 5% in discharge water whose turbidity is within the working range of the application.
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Manufacturers set their own triggers for recalibration. One manufacturer’s manual says calibration is needed when validation deviates by more than 5% either way from the nominal standard concentration, where that concentration is at least 80% of the sensor range. Your sensor’s manual may say something different, so treat that as an example and not a universal rule.
The sensible order of work is simple. Clean and inspect first, then validate against the phenanthrene-equivalent standard, then recalibrate or replace if the reading falls outside tolerance, and finally write down the as-found and as-left results. Cleaning comes first for a reason: a fouled optical window can look exactly like a calibration problem, and calibrating over a dirty window just bakes the error in.
Why pH and PAH Sensors Fail at Sea
Washwater is rough on instruments. Deposits build up on pH electrodes. Biofouling, silt, and scale coat the optical windows of PAH sensors, and high turbidity interferes with fluorescence readings. Sample flow can be blocked, air bubbles can form, and pumps degrade. Temperature compensation can go wrong, standards can pass their expiry or get stored badly, and the data logger can have gaps or the wrong range configured.
Several of these are sample-path problems, not sensor problems, which is why the whole system needs checking and not just the probe. A different instrument, but the habits carry over: our walkthrough of an annual OWS 15ppm calibration on a Fellow Kogyo Focas 1800 shows how an oil-in-water monitor is cleaned, checked, and documented, and the same discipline applies here.
What an Audit-Ready Calibration Record Contains
A certificate that says “calibrated” isn’t enough. A useful record names the vessel, the EGCS unit, the sensor serial numbers, and the monitoring system model. It shows the date, time, and place, and who did the work. For pH, it gives the buffer lot numbers, values, and expiry dates, plus the slope, offset, and temperature compensation results. For PAH, it gives the standard’s concentration, its phenanthrene-equivalent value, and the certificate number and expiry. It records the as-found and as-left readings, the pass or fail decision against the tolerance, and any cleaning, repair, or replacement carried out. Finally, it points to the Record Book entry and the data logger confirmation.
Calibration and drift data have to end up either in the data-recording system or in the EGCS Record Book, as set out in the vessel’s Onboard Monitoring Manual. If an outside lab or service company does the work, check its accreditation scope as well. SAC-SINGLAS accredits calibration laboratories to ISO/IEC 17025, and only for the specific calibrations in a lab’s stated scope.
How Often: The Manual Sets the Interval
There isn’t a single interval that applies everywhere. Calibration frequency should follow the vessel’s Onboard Monitoring Manual and the sensor manufacturer’s instructions. Some manufacturers specify multi-year intervals for certain sensors, but a vessel shouldn’t wait that long after fouling, maintenance, an alarm event, or a reading that doesn’t look right. A structured calibration interval schedule is a good way to keep the manual’s intervals visible and the extra checks on the calendar.
Getting It Done in Singapore
The practical approach is to settle the details before the ship arrives. Confirm which sensors and monitoring system are fitted, that the buffers and phenanthrene standards are in date, and when the last validation and calibration were done. A good visit cleans the sensors, checks the flow and sample path, validates pH and PAH against fresh standards, records the as-found and as-left readings, and leaves a record the Chief Engineer can file straight into the Record Book.
Our fixed and portable measurement equipment calibration service and fixed measurement and automation service cover instrument work for vessels calling at Singapore.
Before the Next Scrubber Operation
- Check the Onboard Monitoring Manual and the sensor manuals for the calibration interval and validation tolerance on each sensor.
- Inspect the pH electrode and the PAH optical window for fouling, deposits, and bubbles, and clean them per the manufacturer.
- Check buffer and phenanthrene standards against their expiry dates and storage conditions.
- Confirm the as-found and as-left readings are recorded, and that the entry is in the data system or the EGCS Record Book.
- Check the data logger for gaps, and confirm turbidity and temperature are being recorded alongside pH and PAH.
Read the original article on Alright Engineering Solutions.
