Designing a home-made floating case for GPS trackers for sargassum mat deployment

by Emma L. Tompkins, Robert Marsh, Sien van der Plank, Jadu Dash, Nikolaos Skliris, Yanna Fidai

Our first UK-based fieldwork involved use of: a bathtub, chicken-wire, Play-Doh and a dip in the sea.

Our objective – to make a home-made floating case for GPS trackers for sargassum mat deployment

GPS trackers need to be housed in a water-proof case. However, most of these cases are expensive, and possibly desirable to any passing interested party, and bulky, which means they can be blown off the rafts by wind. To avoid reliance on bulky and expensive casings for GPS trackers, the University of Southampton team met in July 2021 to experiment with different options to create an affordable, home-made casing for GPS trackers that will be deployed on sargassum rafts in the tropical Atlantic. Our day of fieldwork involved two discrete activities. First, we experimented with the components of the casings in an indoor bathtub to check for buoyancy and stability (and to avoid getting soaked in very heavy rain). Second, we took the casing to the sea, to see how it performed in the surf on a seaweed raft.

Our objective was to create casings that were water-tight, and hold themselves upright in the water with the tracker at the top of the bottle (for the best signal). In the tropical Atlantic, seawater density is around 1022 kg m^-3, slightly higher than the density of freshwater (1000 kg m^-3). For the encased trackers to remain buoyant and to sit upright, the combined density (casing plus tracker) must be close to seawater density, but with a low centre of gravity.

The process of making the casing

Yanna Fidai managed to find old laboratory bottles, old packing foam and packing nuggets, chicken wire, and silica as the basic ingredients for the casing (Fig. 1). However, over the course of the day we trialled other components that can be found in most locations (e.g. homemade Play-Doh).

*Fig. 1: the basic ingredients for the home-made GPS tracker casing*

The first four experiments led to failure as the bottles kept tipping on their sides. The failed designs included (i) Silica (in various quantities) + foam + tracker; and (ii) Stones + silica + foam + tracker See one of the failed experimental designs being trialled here: (add in working link to https://sotonac.sharepoint.com/teams/MONISARG/Shared%20Documents/General/GPS%20housing/Bath-trial-2-little-Play-Doh-fail.mov). We quickly established that combined density needed to be around 1000 kg m^-3, taking into account our bathtub testing was with fresh water and accounting for the higher density of seawater. With 1 litre bottles, allowing for additional attachments, this density translates to a combined weight of around 1000 g.

A good analogy is with an iceberg drifting through a polar ocean. Being frozen fresh water, 90% of an iceberg is below the surface; icebergs are typically stable for long periods, only rolling over as they melt into a modified shape. We want our trackers to drift upright like icebergs!

The successful design required a heavy stable base at the bottom of the bottle – we used Play-Doh, but any heavy malleable dough (that does not rapidly decompose) would be good.

Fig. 2: Chicken wire and cable ties used to secure the ‘skirt’ to the casing

See the successful design description here: https://sotonac.sharepoint.com/teams/MONISARG/Shared%20Documents/General/GPS%20housing/Bath-trial-4-900g-success.mov

Once we had designed a stable casing, we then endeavoured to create a ‘mesh skirt’ that would allow the GPS tracker casing to become tangled in the seaweed so that it does not separate from the surface of the sargassum raft. We made a mesh skirt out of chicken wire, initially cutting a square of chicken wire, approximately 40cm x 40cm. (see Fig. 2).

A hole was cut in the centre of the chicken wire (using gardening secateurs), and it was then attached to the top of the bottle with a cable tie, see Bob Marsh explain this here: https://sotonac.sharepoint.com/teams/MONISARG/Shared%20Documents/General/GPS%20housing/Bob-adding-chicken-wire-skirt.mov

We trialled this version in the bath again (to check for stability and buoyancy), before feeling confident that we could venture to the beach to trial the final product in the sea.

Trialling the casing in the sea

We waited for the rain to stop and then dashed down to Bournemouth beach (Fig. 3) before the predicted hailstorm arrived (Fig. 4).

*Fig. 3: UoS team carrying the two experimental designs to Bournemouth beach*

The sea trial was as successful as the bath-tub experiments, with the casings retaining their upright positions in the water, even in waves, and remaining entangled in the seaweed. However, when the casing was in nearshore surf, the chicken wire became loose (possibly due to being bashed into the sand). Our conclusion from this was to wrap two pieces of chicken wire around the entire casing. The contents of the casing remained dry throughout the short test, although for operational deployment, we plan to watertight the seal.

Lessons learned in making water-tight, stable and buoyant GPS tracker casings from the process

Best GPS housing structure can be achieved using the following process:
- use a heavy plastic water bottle (1 litre is a good size) with a wide rim,
- put in a solid heavy base without air gaps or air bubbles e.g. Play-Doh or modelling clay – approx. half of a 1 litre water bottle,
- put foam on top of the Play-Doh,
- put silica or sand on top of the foam,
- put another piece of foam on top of the silica or sand
- add the tracker
- add in a note for anyone who finds it to email you with the location of where they found it (see Fig. 5 for possible signage to add as a laminated sheet)
- screw the lid on tight
- seal with silicone sealant
- securely attach a ‘skirt’
- throw into a sargassum raft – if possible throwing handfuls of sargassum onto the chicken wire, or ideally wrapping some sargassum around the casing
Home-made casings are relatively easy to make, low cost and require low skill and effort.
If silica is not available – use sand.
A heavy base is required – long lasting home-made Play-Doh works very well (see recipe here: https://researchparent.com/long-lasting-homemade-play-dough-recipe/). Squeeze the plat-doh into the bottom of the container to avoid any air bubbles. Try and flatten it as much as possible, to ensure stability.
The container with all attachments should weigh around 1000g (for density a few % less than that of seawater)
The chicken wire should be wrapped on in two layers, and held in place with cable ties and sealed around the neck with silicone sealant.