Water Quality Sonde Product

Overview

Water quality sondes are autonomous or continuously-connected sensor platforms that measure dissolved oxygen, pH, temperature, salinity, and turbidity in real time. Deployed in fish farms (net-pens, raceways, tanks), they provide the critical data streams that operators and control systems rely on to manage stock health, detect equipment failure, and respond to environmental crises.

A single sonde can monitor 10–20 parameters simultaneously, replacing five to ten separate hand-held meters that would otherwise require daily manual checks. Modern sondes integrate onboard data logging (SD card) and wireless telemetry (4G LTE, LoRaWAN), automatically transmitting alerts if oxygen drops below safe threshold or pH swings unexpectedly.

How it works

The Sonde Housing & Cage contains all sensors and electronics. Water contacts the probes via sensor ports penetrating the pressure hull.

Dissolved oxygen: The Dissolved Oxygen Sensor uses optical fluorescence quenching. A DO Excitation LED (blue or red, 470–660 nm) excites a DO Optical Window containing a ruthenium dye (analyte). The analyte fluoresces; oxygen quenches (collides with) excited molecules, reducing fluorescence lifetime. A DO Photodiode measures fluorescence intensity; the PLC computes oxygen partial pressure via the Stern-Volmer equation, converting to mg/L using calibration constants. Accuracy: ±0.5 mg/L, no membrane needed (advantage over electrochemical probes, which degrade in weeks).

pH: The pH Sensor Assembly (combination glass bulb + reference cell) generates a voltage proportional to H+ concentration. A glass pH Preamplifier (high-impedance, 10¹¹ Ω) buffers the millivolt signal near the electrode. The sonde's ADC samples at 1 Hz, logging pH to ±0.2 units. Glass electrodes drift 1–2% per month in field conditions; automatic two-point calibration (using buffer standards 4.01, 7.0, 10.01 pH solutions, dosed manually or via calibration chamber) corrects drift.

Temperature: A RTD Thermometer (Pt100 thermometer) changes resistance with temperature. The RTD Signal Conditioner measures resistance via 4-wire connection, outputting voltage proportional to °C. Accuracy ±0.5°C; also used internally to temperature-compensate pH and conductivity readings.

Conductivity/Salinity: The Conductivity Electrodes (graphite or stainless pair, 20–50 mm spacing) are driven at constant current (1 kHz AC, 100 mA). The Conductivity Detector measures voltage, calculating conductivity (mS/cm). Seawater is ~53,000 µS/cm; freshwater ~200 µS/cm. The sonde displays conductivity and auto-converts to practical salinity units (PSU) using TEOS-10 UNESCO formula.

Turbidity: The Turbidity Backscatter LED emits near-infrared (850–950 nm). A Turbidity Photodiode at 45–90° angle detects backscattered photons; intensity correlates to suspended solids (nephelometric turbidity units, NTU). Measured range 0–1000 NTU; particle size, shape, and optical properties affect calibration, but the sonde reports relative trend data useful for detecting algal blooms or sediment disturbance.

The Automated Optical Wiper brush motor runs on a programmable schedule (default 4× daily: 0:00, 6:00, 12:00, 18:00 UTC), sweeping the optical windows and electrode tips to remove algae and biofilm that degrade signal quality within days.

The Onboard Data Logger & Telemetry (ARM MCU with SD card socket) stores readings every 15–60 minutes to CSV files. The Telemetry Modem sends a subset of data (e.g., hourly averages, alarm events) to a cloud server via 4G LTE every 60–120 minutes. The Umbilical Cable & Connections cable carries 12–24 VDC power and serial data (RS-232 or proprietary protocol) to a surface terminal, allowing real-time HMI display and manual download.

Design considerations

Optical window fouling. Despite the wiper, organic biofilm (diatoms, bacteria) grows on sapphire and polymer windows within 1–2 weeks in nutrient-rich farm water. UV-absorbing coatings slow growth. Daily wiper cycles become mandatory; some sites add dilute bleach (0.1% sodium hypochlorite) to the wiper chamber, chemically supplementing mechanical cleaning.

pH electrode maintenance. Glass electrodes are fragile (1–3 mm tip thickness) and age quickly underwater (1–3 months field life vs. 1–2 years in lab). The sonde must include a reference electrode with a wetted KCl bridge; this bridge leaks KCl slowly, requiring refill every 3–6 months. Some sites use gel-filled electrode cartridges (1–2 year lifespan) instead, slightly sacrificing measurement speed but improving durability.

Conductivity cell blockage. Graphite electrodes are soft and can corrode in high-chlorine environments. Stainless electrodes resist corrosion but are prone to scale (calcium, magnesium carbonate) precipitation, increasing measured resistance. Periodic acid rinse (1% HCl, 10 min soak) dissolves scale; automated acid-flush cartridges exist for remote sites but add complexity.

Temporal resolution and aliasing. At 15 min logging interval, changes occurring faster than 30 min (Nyquist frequency) are not captured. A sudden oxygen crash (fish kill event spanning 5–10 min) might be missed if sonde samples between the peak and trough. Some farms run 1 min logging intervals during high-risk periods (summer heat waves, after stocking events), switching to 15 min in stable seasons.

Electromagnetic interference. Sondes on underwater cables near high-voltage power cables or pumps can pick up 50/60 Hz noise. Armored cables and grounded shields mitigate this; some installations use optical isolation between sonde and surface logger, eliminating ground loops entirely.

Integration with RAS control systems

Sondes feed directly into PLC control loops:

• Dissolved oxygen: if <5 mg/L, trigger emergency oxygen injection; if >95% saturation, reduce blower speed (energy save)
• pH: if <6.5 or >8.0, trigger alkalinity dosing or CO₂ degassing
• Temperature: if >25°C (salmon stress threshold), activate chiller; if <10°C, reduce feeding rate
• Turbidity: if >50 NTU, slow water exchange and clean filters (clogging imminent)

A 24-hour sonde data archive enables predictive alerting: if turbidity increases 10 NTU/day for 3 days, alert operator that drum filter needs replacement within 1 day.

Deployment options

Floating platform: Sonde mounted on a buoy or raft 1–2 m below surface, umbilical to shore station. Typical for large ponds and net-pens.

Submerged fixed mooring: Sonde on a weighted frame 2–5 m depth. Requires periodic diver service for battery swap and sensor cleaning.

Automated surface sampler: Sonde in a small drip cell, sampling tank overflow continuously. Minimum fouling, easiest maintenance, but representative of average tank conditions only (not stratification).

Regulatory and compliance use

EU regulations (Control Regulation 710/2014) require daily dissolved oxygen and pH monitoring in farming facilities. Sondes logged data satisfy this requirement with electronic audit trail (timestamp, sensor serial, calibration history). Export to CSV and upload to regulatory databases (e.g., NRW in Norway) within 24–48 hours post-event.