Keeping OCEMS accurate: calibration and upkeep
Clean, check, calibrate and record
An online analyser drifts as its optics foul and its electrodes age. Readings stay defensible when the instruments are cleaned, checked and calibrated on a schedule, and each step is recorded. CPCB's guidelines for OCEMS set out the approach; your consent conditions may add to it.
What needs attention
- Optical probe
- Clean the measuring window regularly; fouling makes COD, BOD and TSS read wrongly. Automatic cleaning helps but does not replace visits.
- pH electrode
- Calibrate with buffer solutions, and replace the electrode when it responds slowly or will not calibrate.
- Check against the laboratory
- Test a grab sample in the laboratory and compare it with the analyser's reading for the same time; a lasting difference means recalibration.
- BOD correlation
- Continuous BOD is derived from the spectrum with a correlation set against laboratory results; check it when the effluent changes.
- Flow meter
- Check the zero and the totaliser, and for a flume the level probe against the staff gauge.
- Sample line and pump
- Keep the sample line clear and the pump running where the panel is away from the channel.
Keep these records
- The date, instrument and values of every calibration
- The laboratory results used for comparison
- Faults, repairs and parts replaced
- When the station was offline, and why
Questions
How often should OCEMS be calibrated?
At commissioning, then at the intervals in CPCB's guidelines and your consent conditions, and whenever a check against the laboratory shows drift.
Can calibration be done remotely?
Settings and calibration values can be checked and corrected remotely, but standards and buffer solutions need someone on site.
See it working
Want us to take over calibration and upkeep? Ask about an AMC.