Autonomous Sensor Solutions for Ocean pH and Oxygen Observations

Marine environments are complex and dynamic ecosystems that are subject to both natural processes and human activities that can significantly impact their balance. Determining essential ocean variables such as pH and oxygen levels is crucial for understanding the health of marine ecosystems and detecting potential risks to marine life, such as ocean acidification and deoxygenation. However, the vastness of the world's oceans makes traditional water sampling and analytical methods difficult and impractical, resulting in limitations in comprehending temporal and spatial variations in seawater.

In situ monitoring of pH and oxygen levels in marine waters is becoming more prevalent with the development of several types of autonomous sensors. Some examples include optical sensors that utilize light, electrochemical sensors that detect changes in water chemistry using electrodes, and spectrophotometric sensors that employ indicator dyes to detect the parameter of interest.

Sensors as Solutions

Among these, PyroScience GmbH's easily integrable optical underwater OEM modules, PICO- pH-SUB and PICO-O2-SUB, stand out for their ultra-compact size (17x 59 mm), speed (10 samples per second), robustness, cost-effectiveness, and performance efficiency. These modules utilize the innovative REDFLASH technology to detect pH and oxygen levels and are equipped with a standard UART interface, which allows for seamless integration into custom electronics. For analog output or MODBUS interface, the PICO-EXT extension module is available. The PICO-pH-SUB and PICO-O2-SUB OEM modules have a pressure-stable optical feed-through for connecting optical oxygen or pH sensor heads. The PICO-SUBs are easy to mount and can also be used to retrofit existing underwater equipment since its thread is compatible with some of the most popular deep-sea connectors. The titanium connector is open face pressure stable up to 4,000 m water depth (400 bars). They are specifically designed to be integrated into custom underwater housings (requiring pressure tests after integration) and are typically mounted in the lid of pressure cylinders (Fig. 1).

Fig. 1 PICO SUB with sensor caps

Figure. 1. PICO-SUB OEM Modules for oxygen and total scale pH monitoring down to 4000 m after custom integration into custom-made underwater housings. (Image credit: PyroScience)

The PICO-SUB OEM modules have one optical pressure-stable sensor port that is compatible with a variety of optical pH and oxygen sensor heads with underwater connectors, ranging from fiber-based mini probes to sensor caps easily screwed onto the sensor port (Fig 1). The oxygen and pH sensor caps are particularly suitable for deep-sea applications. An additional protective cap makes them highly robust against mechanical impact. The total scale pH sensor cap PHCAP-PK8T-SUB can be applied for the pH range of 7.0 - 9.0 on the total scale. Based on its optical detection principle, no reference electrode is required, leading to a minimized effect of high ionic strength. This minimal cross-sensitivity to high ionic strength makes it a suitable sensor solution for long-term measurements in seawater.

The oxygen sensor cap OXCAP-SUB can be used in general oceanographic applications and long-term underwater deployments. It is also available as a high-speed version of OXCAP-HS-SUB with a t90 response time of less than one second for fast oxygen monitoring applications, e.g., during water column profiling.

Precise Measurements for Ocean Observing

The new ultra-trace oxygen sensor cap, UTROXCAP-SUB, features an outstanding low detection limit (0.04 μg/L, 0.0013 μmol/L) and a x10 higher resolution compared to standard range oxygen sensors. It comes pre-calibrated and can be easily calibrated at 0% oxygen with our calibration capsule OXCAL. After mounting on a PyroScience underwater meter (AquapHOx and PICO-SUB series), it is perfectly suited for high-precision deep-sea measurements close to 0% oxygen, e.g., in oxygen minimum zones or during ocean deoxygenation events (Fig. 2).


Figure 2. New ultra-trace oxygen sensor cap UTROXCAP-SUB for PyroScience underwater meters (AquapHOx and PICO-SUB series). (Image credit: PyroScience)

Plug and play underwater meters with pressure-stable housing ready for deployment in the Deepsea are available from PyroScience as AquapHOx long-term loggers (AquapHOx-LX) and real-time transmitters (AquapHOx-TX) (Fig. 3). These devices have a titanium housing (63 x 300 mm), making them capable of withstanding the harsh conditions of deep-sea environments, down to 4,000-meter water depth. The loggers and transmitters are single-channel devices but can be used with either pH or oxygen sensor heads from PyroScience. They both come with an integrated fast-responding NTC temperature sensor for automatic temperature compensation of the oxygen or pH sensor readings.


Figure 3. Deep sea long-term logger AquapHOx-LX with oxygen sensor cap and real-time transmitter AquapHOx-TX with total scale pH sensor cap for underwater application down to 4000-meter water depth. (Image credit: PyroScience)

The AquapHOx logger has a long-endurance rechargeable battery and data logger with huge 4 GB data memory integrated, making them ideally suitable for stand-alone and long-term underwater logging and monitoring in the open ocean, Deepsea, and coastal habitats. The AquapHOx-TX is a real-time data transmitter for underwater operation down to 4000-meter water depth. The digital interface is an RS485-Modbus interface, whereas the electrical interface offers four additional high-resolution analog outputs. Combined with our ultra-high-speed sensors (e.g., OXCAPG-UHS- SUB), the AquapHOx transmitters are ideal tools for fast oxygen water column profiling or aqueous eddy covariance measurements.

The ultra-compact PICO-SUB underwater OEM modules and flexible AquapHOx logger and transmitter can be easily integrated into various autonomous observation platforms such as acoustic doppler current profilers (ADCPs), buoys, gliders, autonomous underwater vehicles (AUVs) and uncrewed surface vehicles (USVs), making them a versatile option for continuous and real-time ocean monitoring of pH and oxygen levels in various oceanographic studies. These state-of-the-art devices are essential tools for researchers, providing critical data for understanding the complex interactions between the ocean and the environment and thus facilitating the implementation of appropriate precautionary measures to ensure the sustainability of our oceans. The high precision and robustness of these devices make them ideal for monitoring changes in ocean water chemistry, even in deep-sea environments.

To learn more about PyroScience GmbH and their ocean observation sensors, visit: https://www.pyroscience.com/en/.

This feature appeared in Environment, Coastal & Offshore (ECO) Magazine's 2023 Deep Dive I special edition Ocean Observation, to read more access the magazine here.

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