A pH sensor that works wet or dry

Wet or dry pH monitoring at a glance

• The problem: in boreholes, drains, outfalls and tidal areas the sensor is regularly out of the water, sometimes for hours, sometimes for weeks.
• Why glass fails: a glass electrode needs a hydrated glass surface and a wet reference junction. Let it dry out and the reference electrolyte can crystallise and block the junction. The probe may need long reconditioning and is often permanently damaged.
• How ANB differs: no glass and no hydrated glass layer. The sensor calibrates itself in situ, can be stored wet or dry, and its immersion detection pauses measurement while it is dry and resumes it automatically once it is immersed again.
• Field evidence: at a drainage outfall that repeatedly dried out, the sensor resumed normal operation after each rainfall event without manual intervention. In storm drains, two sensors kept providing useful measurements through a month of fluctuating water levels.
• Applications: boreholes, surface water and storm drains, tidal areas, mine and quarry stormwater outfalls, and many other places where exposure is intermittent.

Water levels rise and fall

Most pH sensors are designed on the assumption that they will stay under water. Many of the places where water quality matters most do not work like that:

• Boreholes drop with abstraction pumping and with the seasons, so a sensor hung at a fixed depth can end up above the water.
• Surface water and storm drains carry flow only when it rains, and can sit dry for days or weeks between events.
• Tidal areas are covered and uncovered on every tide, and water depth and salinity change over hours.
• Outfalls and ephemeral channels flow in response to rainfall and dry out in between.

For a glass electrode, each of these dry spells is a risk to the probe itself, not just a gap in the data. The usual workarounds are to install the probe deeper than the lowest expected water level, to keep it in a pumped or flooded chamber, or to visit the site to retrieve, store and recalibrate it. All of them add cost, complexity or site visits, and some locations simply can’t be monitored that way.

Why a glass pH probe dies when it dries out

A glass pH electrode is really two half-cells in one body, and both depend on water. For the background, see what is pH? and our glass vs solid-state comparison.

What happens inside a glass electrode when it dries out

The measuring half is a thin membrane of pH-sensitive glass. It only responds to pH because its surface carries a thin, hydrated gel layer that exchanges hydrogen ions with the sample. The reference half is typically a silver/silver chloride (Ag/AgCl) element in a concentrated potassium chloride (KCl) electrolyte, which connects to the sample through a porous liquid junction. When the probe is left in air, the electrolyte in and around the junction evaporates and KCl can crystallise in the pores, blocking the junction. The electrical path between the reference and the sample becomes high-resistance or is lost altogether, so the reference potential becomes unstable or the circuit is effectively open. At the same time the hydrated layer on the glass dehydrates, and the membrane responds slowly, erratically or not at all. That is why manufacturers tell users to keep glass electrodes in storage solution. A probe that has dried out may need long soaking and reconditioning before it reads correctly again, and even then it may not return to its original response. Often, particularly once the reference electrolyte has dried out or the junction is blocked, the damage is permanent and the probe has to be replaced.

In a laboratory, drying out is an occasional mishap. In a borehole, a storm drain or the intertidal zone it is a routine condition, and it can happen between site visits with nobody there to notice. As ANB’s Illegal Connections white paper puts it, storm drains frequently experience intermittent wetting and drying, conditions that can permanently damage conventional glass pH probes or require them to be recalibrated before further use.

How the ANB sensor differs

ANB’s patented solid-state sensor measures pH electrochemically, but without glass. Its sensing element is a solid material rather than a glass membrane with a liquid-filled reference. A pH-sensitive redox-active molecule on a working electrode shifts its peak potential as hydrogen ion concentration changes, and that shift gives the pH reading. A second working electrode carries a pH-independent molecule that tracks the reference electrode. The sensor checks this reference tracker before each measurement cycle and compensates for any reference drift in real time, which is how it calibrates itself in situ, with no buffers or operator intervention.

Because there is no glass membrane and no hydrated glass layer to keep wet, the sensor can be stored wet or dry and deployed wet or dry, with no loss of performance. It can sit dry on a shelf, travel dry, and go straight into the water when needed.

Immersion detection

An integrated immersion sensor checks whether the sensor is submerged. If it is not, the sensor stops measuring, sets its transducer health status to 4 and enters a low-power state. In continuous mode it checks again every five minutes; in interval mode it rechecks at the configured interval. Measurement resumes automatically once the sensor is immersed again, without recalibration or maintenance. This avoids pointless measurements in air, protects the sensing surface and saves battery power on intermittently submerged or splash-exposed deployments.

Glass and ANB side by side

For the wider comparison, including drift, biofouling and maintenance, see the table on our what is pH? page.

Storm drain monitoring and illegal connections

Storm drains should carry only rainwater run-off. If anything else turns up in them, something is wrong: an illegal or unintended connection from the foul sewer, which can happen on new housing developments, or someone putting something down the drain that shouldn’t be there. Wastewater in the surface water network has environmental impacts and creates regulatory compliance issues, and water utilities are under increasing pressure to find these cross-connections. Traditional inspection is labour-intensive and intermittent, and often can’t provide continuous evidence of abnormal discharges. Continuous monitoring can reveal the recurring patterns and anomalies that happen over daily or weekly cycles.

The catch is that a storm drain is one of the harshest places to leave a pH sensor. Flow comes and goes with the rain, and the sensor is often partly or fully out of the water.

The trial

For a major water utility, ANB deployed two AQ² sensors in two different storm drain outlets on the same street in South East England. Despite periods when the sensors were not fully immersed, both sensors continued to provide useful measurements throughout the monitoring period. Unlike a glass probe, the sensor tells you when it is not fully immersed, so you can see which readings were taken in partial immersion.

Read the full Illegal Connections white paper (PDF).

Surface water drains and outfalls

Surface water drains, outfalls and the small channels they discharge into are temporal waterways: they flow after rain and dry out in between. They are also where pollution from misconnections, run-off and discharges first shows up. A sensor that can be left in place through the dry spells lets you record what happens when the flow returns, which is often exactly when you need the data.

Field deployment at a drainage outfall

ANB installed a sensor at a drainage outfall discharging into a small field channel in the south of England, a temporal waterway subject to variable flow, periods of drying and rainfall-driven recharge. During the month-long second deployment, rainfall records from a nearby station showed prolonged dry periods that matched the drying and re-wetting of the channel. The sensor stayed operational throughout, with no cleaning, recalibration or other maintenance.

Read the Drainage Outfall white paper (PDF).

Borehole monitoring

Groundwater levels in a borehole move with abstraction pumping, recovery and the seasons. A pH sensor installed at a fixed depth can spend part of the year above the water, and every retrieval for sampling or inspection takes it out of the water too. With a glass probe, that means either installing it deep enough to stay submerged at the lowest level, or accepting that it may dry out and need reconditioning or replacement.

A sensor that tolerates dry periods removes that constraint. If the water level falls below the sensor, immersion detection suspends measurement and the sensor waits in a low-power state, then resumes automatically when the water returns. It needs no buffer calibration, which matters in a borehole where every calibration means pulling the sensor out.

Borehole monitoring is listed among our applications. We have not published a borehole case study yet, so if you are planning a deployment, talk to us about your borehole diameter, depth and water-level range.

Tidal areas and estuaries

In tidal rivers, estuaries and the intertidal zone, water chemistry changes over hours. Freshwater and seawater mix on every tide, so pH, salinity and temperature can all shift significantly within a single tidal cycle. Sensors in the intertidal zone are also uncovered at low water. Glass electrodes struggle on both counts: they are sensitive to changes in ionic strength and often need frequent recalibration where salinity changes quickly, and they cannot be left to dry out. ANB’s sensor measures from 0.1 to 40 ppt salinity without recalibration, and immersion detection handles the periods when it is out of the water.

River Dart, Totnes

ANB deployed a calibration-free sensor in the River Dart at Totnes in South West England, a location strongly influenced by tidal exchange from the South Devon coast. It measured pH, temperature and salinity every 10 minutes for several weeks, covering multiple spring and neap tides.

• Periodic spot measurements with conventional glass electrode equipment agreed well with the ANB data, confirming both the overall pH range and the trends.

Read the Estuarine Monitoring white paper (PDF).

Mine and quarry stormwater outfalls

Stormwater outfalls on mine and quarry sites only run during and after storms, so flow is intermittent and dry cycles can last weeks to months. Take, as an illustration, a remote mining-site stormwater outfall where pH, turbidity and flow are monitored for internal purposes, and a water-level sensor switches the water quality sensors on during larger storm events. The sensors sit in a forebay upstream of a flume, which holds a continuous pool for a few days after each event before the level drops below the sensors and a dry cycle begins. The site is remote, running on solar power and a battery, with telemetry back to the site control system.

This is a difficult place for a glass electrode. It would either dry out in every dry cycle, or need a site visit to remove it, store it wet and recalibrate it before the next storm. The ANB sensor suits this kind of site because:

• it can sit dry in the forebay between events, with no need to keep it wet and no storage solution;
• it needs no calibration, so there is no buffer calibration visit before or after each wet season;
• measurements can be event-triggered: the sensor can be started on a programmable timer or automatically when it is immersed, or powered up by the site’s own level switch;
• immersion detection and the health number show which readings were taken fully immersed, so readings from a falling, partly drained pool can be identified;
• routine maintenance is a brief abrasion of the transducer surface when the health number calls for it, so service visits can be planned around the sensor’s own diagnostics rather than a calibration schedule.

More places where intermittent exposure matters

The same capability, measuring when wet, tolerating dry, needing no calibration, makes the sensor suitable for many other sites where water comes and goes. These are applications where wet or dry operation is relevant, not a list of completed trials:

• Ephemeral streams and seasonal watercourses that flow only after rain or in winter.
• SuDS features: attenuation ponds, swales and detention basins that fill during storms and drain down afterwards.
• Combined sewer overflow and outfall chambers, where flow is event-driven.
• Flood plains and flood events, where you may want a sensor in place before the water arrives.
• Mine and quarry dewatering, tailings and acid mine drainage, where pumping and storms change water levels.
• Landfill leachate and groundwater monitoring wells, where levels vary and access is limited.
• Agricultural drains, irrigation channels and ditches, which the Drainage Outfall white paper also highlights.
• Tank and pond drain-down, and intermittent flow-through systems in aquaculture, where the sensor can stay in place while a tank is emptied. See pH in aquaculture.
• Intertidal rock pools, estuary mudflats and saltmarsh creeks, which are uncovered at low water.
• Coastal splash zones and structures in the tidal range.
• Portable and spot-check use, where a sensor is carried between sites and stored dry between uses.
• Vehicle and ROV launch and recovery, and FerryBox systems that are drained or flushed between runs.

Where dry storage has already helped

• Uncrewed surface vessel, Hawaii. On a Blue Robotics BlueBoat, AQ² needed no calibration immediately before deployment, no specialised wet storage and no adjustment as the vehicle moved from a sheltered harbour into open coastal water (Vehicle Deployment white paper, PDF).
• FerryBox, Antarctic. An ANB AQ² 5m sensor (previously the AQ5) in a FerryBox on a research vessel: its ability to be stored wet or dry made it convenient on remote missions (FerryBox case study, PDF).
• ROV, Norwegian Continental Shelf. Stinger Technology AS fitted an ANB AQ² 300m sensor (previously the OC300) to an ROV to detect cement returns during drilling, and noted that it “doesn’t require careful storage, which is a real bonus in the field” (Stinger case study, PDF).

All of our papers are on the white papers page.

Practical notes for wet or dry deployments

• Immersion detection. When the sensor is not immersed it stops measuring and sets transducer health status 4. In continuous mode it checks again every 5 minutes; in interval mode, at the configured interval. Measurement resumes automatically when it is immersed again.
• Starting measurements. Measurement can start on a user-programmable timer or automatically when the sensor is immersed. You can run continuously or at an interval between 10 and 240 minutes.
• Stabilisation. Allow approximately 30 minutes after power-up for readings to stabilise. If you are updating or redeploying a legacy sensor, stabilisation may take longer than 30 minutes.
• First reading. In continuous mode the first measurement takes 4 minutes while the sensor performs its reference checks; after that, readings are output every 5 seconds.
• Health number. The sensor checks itself continuously and outputs a health number that shows when the transducer needs abrading or replacing. Wear depends on the number of measurements, not elapsed time, and the transducer is replaced in the field after approximately 300,000 measurements.
• Storage. No special storage conditions: store the sensor wet or dry, with no buffers or storage solution.
• Data. The integral SD card stores over 15 million measurements of pH, temperature and conductivity, downloadable on retrieval or available live over the communications link.