The pH electrode on the controller. Everything the CO₂ schedule is judged by comes through it.
Why I chose it
A double-junction laboratory electrode to replace an eighteen-month-old probe on the controller. Double junction because the reference is the part that fouls, and pH here drives the CO₂ figure the whole schedule is judged from.
What I've learned
·It arrived with a factory QC certificate against NIST-traceable standards: offset within ±20 mV, slope at least 97%, 98% of response within 30 seconds, QC passed 27 October 2025.
·Calibrated on the controller against pH 7.00 and 4.00 buffers with the BNC extension already fitted, it verified at approximately 7.00 and 4.04 in fresh buffer.
·It disagrees with a separately recalibrated handheld meter in aquarium water by roughly 0.24 to 0.30 pH, while both instruments read conventional buffers reasonably well. That is recorded as a disagreement, not as a fault in either instrument.
Installation
Fitted to the existing Apex BNC input using a 3-foot BNC extension, then calibrated in place.
Configuration
·Installed on the Neptune Apex BNC input through a 3-foot BNC extension
·Calibrated with the extension fitted, so the extension is part of the calibrated path
·pH 7.00 and 4.00 buffers
Maintenance
·Verify against fresh 7.00 buffer rather than recalibrating by habit.
·Keep the junction wet and the connector dry.
Known limitations
·It reads aquarium water differently from a recalibrated handheld meter, and which one is closer to true is not established.
·An electrode is a consumable: slope and response degrade with age whether or not a reading looks plausible.
Controller integration
Read by the Neptune Apex, which supplies the pH the CO₂ schedule is judged from.
Calibration
Two-point, pH 7.00 and 4.00, performed on the controller with the BNC extension fitted. The plan is verification rather than routine recalibration: a spot check in fresh 7.00 buffer on the evening of 23 September 2026, and occasional buffer checks over the first couple of weeks to establish how stable it is. If it holds, checks move toward roughly monthly. It is not recalibrated unless a check shows drift, because recalibrating on a schedule destroys the record of whether it was drifting.
Current observations
·This is the instrument CO₂ trend figures are calculated from, on both sides of the comparison. Readings from it and from the handheld meter are not mixed when a pH drop is worked out.
·The degassed reference measured with it is 6.55, which replaces the 6.22 taken with the previous probe.
·Keep the BNC connection dry, and keep the extension away from power cabling where the routing allows it.
What this item is doing now, as distinct from what has been learned over time.