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As a result, technical divers ascending from moderate depths often carry out a free ascent, hanging on a reel under their red 6-foot-tall sausage-like delayed surface marker buoy (DSMB), which is inflated and sent to the surface as they ascend so that topside know where they are – in an hour of decompression in UK waters, divers will drift perhaps half a mile or more away from the dive boat. When the skipper of the dive boat sees DSMBs coming up, he knows to leave the fixed downline and shadow the DSMBs until the divers break the surface.

The alternative way of doing this sort of free decompression ascent in tidal waters is to deploy a free-drifting decompression station. This can be a decompression trapeze, or at its simplest, a weighted line, both of which get carabinered to the downline at 20–30 metres and have their own big surface buoy(s).

The trapeze is simply three long aluminium tubes that are horizontally secured to vertical ropes at either end of them, the tubes being positioned at depths of 12, 9 and 6 metres. The ropes at either side are tied off to their own large buoys, which suspend the whole contraption.