June 2025 | Oceanography
67
Open-source/DIY fluorometers exist in the ocean sciences
space, with Chl-a fluorometers and “fluorometry-like” turbid
ity (Matos et al., 2020) and backscattering sensors (Downing,
2006) being popular. Costs are low in most instances, though
two trends are noticeable. Either fluorometers tend to exhibit
detection limits of 0.1 μg L–1 (or 0.1 ppb) or higher (Leeuw
et al., 2013; Attivissimo et al., 2015; Park et al., 2023), which is
at least one order of magnitude worse than industrial sensors
(Park et al., 2023), or higher performing devices are configured
as benchtop units (Truter, 2015) and have not made the sacri
fices necessary to package the technology into a form capable
of in situ deployment. The task of maintaining optical and elec
trical performance in a small, water-tight, pressure-safe hous
ing is not trivial, and making concessions on size/mass rules out
some of the most attractive use-cases of low-cost in situ fluo
rometers (Dever et al., 2020; Park et al., 2023). Thus, there is a
gap in extant sensors between the advantages provided by open-
source in situ sensors and the performance provided by indus
trial in situ fluorometers.
With this gap in mind, we introduce the PIXIE, a low-cost,
open-source, four-channel in situ fluorometer. In lab testing,
the PIXIE performs fluorometry with precision and accuracy
comparable to the sensors available on the market. The default-
configuration PIXIE can be assembled for $1,392.75 USD with
one equipped channel. Each addition channel costs $525.25 USD,
for an average of $742.13 USD per channel when the instrument
is fully equipped.
For our work, a PIXIE unit was calibrated to demonstrate a
limit of detection (Arar and Collins, 1997; Sieben et al., 2010)
of 0.01 ppb RWT over a range 0 to ~60 ppb. The same unit was
calibrated to demonstrate a limit of detection of 0.02 ppb Chl-a
over a range of 0 to ~80 ppb. Deployed as part of a dye tracer
experiment in Halifax Harbor, Canada, (see Figure 1) to study
the near-field dispersion of RWT added to the cooling outfall of
the Tufts Cove Power Generation Station, the PIXIE was config
ured to capture both RWT and Chl-a profiles, demonstrating its
multichannel functionality. The in situ data were checked against
discrete water samples collected in conjunction with the profiling
to assure quality, demonstrating how this low-cost, open-source
technology could assist in solving complex oceanographic tasks.
MATERIALS AND METHODS
Open-Source Fluorometer
The materials needed to assemble a PIXIE are available on
GitHub (https://github.com/KylePark0/PIXIE/tree/main), and
fall into one of three categories: documentation, firmware, or
hardware. The documentation includes a comprehensive user
guide that details the design, assembly, calibration, and opera
tion of the device. Bills of materials (BOMs) are provided for the
mechanical and optical hardware, including vendors, and the
electrical BOM comes pre-packaged to fabricate with PCBWay
(PCBWay, Hangzhou, China). The listed optics include the com
ponents needed to assemble any of five presets: PC, phycoeri
thrin (PE), RWT, Chl-a, and crude oil. CAD models for every
component, including machined and 3D-printed parts, are
included. A rendering of the PIXIE with some dimensions (see
Figure 2) is provided, in both normal and exploded views.
The PIXIE can be powered using a range of 5–20 V. It com
municates with an external terminal or datalogger via RS-232,
while drawing an average of 45 mA during active measurement
(225 mW). Using a dedicated 12 V lithium-ion cell with a nom
inal capacity of two ampere-hours, the PIXIE can be expected to
measure for 40 hours in 4°C waters. Its off-the-shelf components
are rated for depths of at least 500 m. The housing is composed
of anodized aluminum and borosilicate glass, allowing it to with
stand a range of solvents used in laboratory calibration of fluoro
meters. Acetone is used to prepare standards of Chl-a (Arar and
Collins, 1997), a nearly-neutral phosphate buffer solution (PBS)
FIGURE 1. Drone photograph
of the August 2023 Halifax
Harbor tracer release experi
ment conducted from the div
ing vessel Eastcom. Insets: The
PIXIE open-source fluorometer
is shown mounted to the side of
a Niskin bottle (top) and during
rhodamine water tracer (RWT)
calibration (bottom) in the lab.