When an online chlorine reading differs from a reference test, first check that both methods measure the same disinfectant and compare water from the same point and time. Verify sample flow and the reference method before investigating calibration.
Check that both methods measure the same residual
Confirm the analyser channel, units and test method in use. A free-chlorine reading and a total-chlorine result can differ even when both instruments are working as intended. For chloraminated water, confirm whether the question calls for total chlorine or a species-specific measurement, and choose a reference method suited to that question.
Establish where the sample came from and when
Record the sampling point and time alongside the online trend. If the analyser is fed through a sample line, a change at the take-off reaches the instrument after a delay. A grab sample taken elsewhere or at a different time may represent a different parcel of water. Use the sampling point and procedure in the site plan.
Look for changes in the sample supply
Check the recorded valve position, flow indication and any change to the water path since the last satisfactory check. Follow the site’s safe access and isolation procedures. PMA’s published MP5 field account describes a low reading investigated by checking sample supply and confirming the restored residual with a reference measurement.
Check the reference test and operating conditions
Use the operator’s approved reference procedure and check the equipment and reagents are suitable for the expected residual. Record conditions such as pH and temperature where they matter to the selected instrument. Capture any alarm or diagnostic state and the reported channel at the time of the comparison.
Make the decision using the approved procedure
Repeat the comparison under suitable conditions if the initial checks reveal a timing or sampling problem. If a repeatable difference remains, use the applicable instrument manual and site criteria to decide whether calibration, servicing or an investigation is required. Escalate unresolved readings under the site’s operating procedures.
Keep a record the next operator can use
Keep the sample point, residual species, test method, timestamps, both readings, operating conditions and any action together. Record the follow-up result so that others can distinguish a restored sample supply from a change made to the instrument.
