Openprobe: a frugal multiparameter probe for marine and continental waters monitoring

Aquatic ecosystems need accurate observation to understand its trajectory in a context of increasing anthropogenic pressure and climate change. These environments are however undersampled in many areas (Brewin et al. 2017, Kirchner et al. 2004): acquiring essential variables at relevant spatial and temporal resolution is often not compatible with traditional methods of sample collection followed by laboratory analysis, but field-deployable or in-situ systems are either too expensive, too complicated to be used by non-trained users or even non-existent. In an effort to propose an alternative in-situ measurement system for biogeochemical and physicochemical measurement in marine and continental waters, we developed a multiparameter probe that measures 7 parameters for a very modest bill-of-materials cost (around 300 € for a single prototype). The performance of this probe is sufficient for most water bodies, both in terms of range and sensitivity, and its replication is facilitated by the use of rapid-prototyping methods. The Openprobe multiparameter (Fig. 1) is able to measure conductivity, temperature, pressure (depth or water level above), dissolved oxygen, turbidity, chlorophyll a, photosynthetically active radiation (PAR) as well as water color. These parameters were chosen based on a compromise between their relevance and our capability to measure them at a scientifically appropriate standard using low-cost components, an approach which resonates with the definition of the Standard Observations in eLTER (Zacharias et al. 2021). The electronic architecture is based around an Adafruit Feather M0 microcontroller board, an Adafruit Adalogger microSD-card logger with a Real-Time-Clock, and two custom Printed Circuit Boards (PCBs) that hosts the different sensors. Housing of the probe uses a Blue Robotics 2" diameter enclosure tubes. The sensor's PCBs are integrated in end-caps fabricated by stereolithography (SLA), using a desktop Formlabs Form 3 SLA printer with Black V4 resin. Compared to conventional fused-deposition modelling 3D printing (FDM), SLA is an isotropic printing technique which delivers fully dense and watertight parts. The higher resolution of SLA also allows to create o-ring grooves with appropriate tolerance, and has shown to be a valid fabrication technique even for the challenging pressures encountered in deep-sea science (Karp et al. 2023, Phillips et al. 2019). Atop of that, the 3D printed end-cads ensures auto-alignment of the optical elements (LEDs, photodiodes) and optical filters. The assembly/disassembly can be done manually without any tool, for easy access to the electronics, the battery, or the micro SD card on the field. Polydimethylsiloxane (PDMS), a silicone-based material with good transparency in the VIS-IR, is used within the end-caps to create optical ports, and also acts similarly to a potting material to ensure watertightness. The first PCB is integrated at the top of the probe, and contains the CTD (Conductivity, Depth, Temperature) as well as the PAR sensor. The conductivity sensor is based around a two-electrode cell on a ceramic substrate connected to an impedance analyzer integrated circuit (IC). Intercomparison with a Decagon CTD-10 and an Atlas Scientific EZO sensor showed that our sensor offered a better sensitivity, together with a smaller cost and a lower power consumption. Pressure and temperature sensors are based around the same IC used by Blue Robotics pressure and temperature sensors, and as such as been validated in many conditions (Poulsen et al. 2022). We worked on their integration on a custom PCB, with special care taken in minimizing the thermal inertia around the temperature sensor to reduce its response time when performing profiling analysis. The PAR sensor completes this part of the probe. It is based around the AMS AS7341, modified with a diffuser cut precisely with the xurography technique (Bartholomeusz et al. 2005), and intercompares with a+/- 5% confidence interval versus an Apogee SQ-512-SS sensor (Fig. 2), a value commonly exceeded when comparing different commercial sensors (Long et al. 2012). The second PCB, located at the opposite end of the probe, contains a chlorophyll a fluorometer, an oxygen optode, and a turbidity sensor. The chlorophyll a fluorometer uses synchronous detection: this approach allows efficient ambient light rejection, and captures the weak fluorescence signals observed due to the small fluorescence yield of in-vivo chl.a (Morrison 2003). The oxygen otpode uses PreSens Pst3 oxygen sensor spots, paired with a custom optoelectronic readout architecture. A DDS (Direct Digital Synthesizer) generates a sine wave to excite the Pst3 luminophore, and an I/Q demodulator measures the phase shift between excitation and the emitted fluorescence. The fluorescence signal is quenched in the presence of oxygen, which translates to a decrease of the fluorescence decay and in-fine by a phase shift . This is similar to the operating principle of Aanderaa optodes, commonly found in the Biogeochemical Argo floats. Our optode is intercompared with a benchtop PreSens Fibox 4 system and shown good accuracy (Fig. 3). Finally, the turbidity sensor implements 90° nephelometry as well as 180° backscattering measurements to cover a broad range of turbidity. Its architecture is based around an Analog Devices ADPD1080, an IC usually found in smartwatches for the measurement of photoplethysmography (PPG). The Openprobe multiparameter probe could be a very valuable tool in order to increase spatial and temporal resolution in aquatic ecosystem monitoring. Through the use of advanced, yet affordable electronic components initially developed for the Internet Of Things market, we created a frugal tool without compromising on accuracy.