Step 1. Visual Inspection
Do not clean the electrode until after calibration unless there are obvious signs of debris build-up. Cleaning immediately can make the source of drift more difficult to determine.
While not every problem is visual, knowing what the sensor, electrode, and electrolyte solution are supposed to look like is important to identify immediate problems.
Is the junction supposed to be grey? (Depends on the manufacturer)
Should there be flecks of black particles in the reference electrolyte? (No)
Start by looking for any damage such as cracks or scratches on the bulb. Obviously, a crack affects proper measurements, but even small scratches create an uneven surface for measurement that can cause drifting and slow stabilization. There may also be microscopic scratches caused by large particulates hitting the bulb and we will go into identifying those later.
While inspecting the bulb, look for any trapped air bubbles. These can be introduced by being shaken in shipping, storage, or strong currents. The bubbles affect the ability for the wires to create good connections and can result in drift.
Next, check if the electrolyte level is low (procedure varies by manufacturer. Electrodes in pHionics products must be removed to clearly see the level. Instructions are available here). If the electrolyte is lower than about ½” (1.27 cm), then the inner wires may be making poor contact, causing drift or extreme fluctuations. Check if the electrode is refillable or replaceable and act accordingly.
While looking at the electrolyte, check for any black precipitate floating or accumulating near the junctions. This is created when something in the sample solution reacts with silver present in the reference half-cell and leads to inaccurate measurements. The phenomenon is especially common with silver/silver chloride single junction electrodes but can occur in double junction electrodes if they are exposed to high levels of heavy metals, organics, or proteins that enter through the junction. The electrode may be regenerated by following the procedure in Step 4 – Cleaning, but the electrode likely requires replacement. A double junction or reference other than silver/silver chloride may be required for your application.
Look for any built-up debris or algae around the bulb or junction. If there is build-up, then cleaning should be done so that any scratches or bubbles in the bulb are not blocked from view. Any type of build-up can interfere with connection between the electrode half-cells and cause drifting. In wastewater applications especially, there is a large variety of debris that can latch onto sensors which can be difficult to combat. The sensor may be cleaned just yesterday but have new debris accumulate by the next day, or it might not happen for many days, making maintenance scheduling difficult. In other applications with cleaner water, build-up large enough to cause drift may only happen every few months so a more consistent maintenance schedule can be created.
If there are no clear signs of damage or electrolyte issues, then move onto Step 2.