A diver in front of the underwater window in Transfo Duiktank in Zwevegem

© Peter Ryngaert

Underwater windows for extreme conditions

An industrial ruin in Zwevegem, Belgium has been transformed into an impressive diving tank. Panels made of PLEXIGLAS® offer exciting views into the tank and withstand the enormous water pressure.

If buildings or old tanks are standing empty and unused, why not turn them into an oversized swimming pool? This was the thinking of a business man in Belgium when he heard of the demolition of an old industrial ruin in Zwevegem. It did not stay a thought for long. He applied for a grant and looked for professionals who could help him in making his idea come to life. The old substation was to be developed into a diving paradise and the visitors were to be given an incredible view into the underwater world through the many large and small windows made of PLEXIGLAS®.

From substation to giant pool

Moritz Meinesz, Managing Director of Hydrosight, supported the plan. “We have been successfully building underwater windows made of PLEXIGLAS® for 13 years. We are totally impressed by the material,” said the Emmerich-based Director. Meinesz can still recall the Belgian’s exact plan, “since the industrial ruin, an old substation, had been standing empty for years, it was, of course, no longer completely waterproof.” First off, a concrete tank had to be built into the old 17-meter-deep industrial tank made of steel. After all, the structure was to be filled with 4.7 million liters of water.

Dive in the Transfo Duiktank in Zwevegem // Copyright: Youtube/Arthur Daveloose

Installation of the underwater windows

As soon as the tank-in-the-tank was installed, Meinesz started putting in several underwater windows. At the foot of the “Duiktanks” (in English: diving tank) was, for example, a maintenance access, which was used as an entrance into the tank during the construction work and which had to be closed. But it would have been a shame to overlay it with concrete, rather it should be put to better use. “From this we have made an oversized window inside the tank,” Meinesz says. A challenge – even for the underwater window specialist from Emmerich: the 2.5-meter-high and 80-cm-wide panel made of PLEXIGLAS® must withstand enormous water pressure at a depth of 16 meters. “At a depth of ten meters, there is twice the atmospheric pressure. Every ten meters, the pressure on the PLEXIGLAS® panel doubles,” says Meinesz. “There is a total of about two bar of pressure on the PLEXIGLAS® and the silicone with which we sealed the panel,” says Meinesz.

“To make this extraordinary project as safe as possible, I cut extra thick panels of PLEXIGLAS® with a thickness of up to 100 mm,” he says. “Of course this is much thicker than for a normal swimming pool. In addition, we had Evonik issue a GL certificate and a test report. The experimental results show that the block of PLEXIGLAS® meets all requirements. The stability is therefore guaranteed.”

“PLEXIGLAS® is particularly suitable for underwater windows because it can withstand the pressure of the water mass better than glass and does not darken the view”

Moritz Meinesz
Managing Director, Hydrosight

Peepholes made from PLEXIGLAS®

Divers can now go underwater in the Transfo Duiktank and search for the sunken city of Atlantis. For all those who prefer dry ground under their feet, the concept includes a cafeteria with Belgian waffles, pancakes and coffee. “For this, we produced four round panels of PLEXIGLAS®, so that you get a good view of the divers in the tank,” says Meinesz. “PLEXIGLAS® is particularly suitable for underwater windows because it can withstand the pressure of the water mass better than glass and does not darken the view into the oversized swimming pool. In general, PLEXIGLAS® has a higher level of transparency than glass.” Evonik’s brand acrylic glass also offers a distortion-free view: Colors and proportions inside the diving tank can be seen from the outside as they really are, without any cloudiness or green tint. “Guests are astonished when they look through the underwater windows into this huge water mass,” Meinesz reports.