So, the decompression dive! A fun day out, which began by trying on the Siebe-Gorman deep-sea commercial diving gear. This is the big brass helmet that everyone pictures when you say "deep sea diving", and was used by the Royal Navy until around 1980. There is a excellent reason they don't use it any more; it's incredibly heavy. The whole deep-sea rig, including weighed boots and ballast, weighs around two and a half stone more than I do. (For comparison, my usual scuba gear weighs about a third what I do.)
Commercial diving helmets are known as "hats", and it was quite fascinating being given a tour around the equipment, until the time came to get into it. I'm sure regular commercial gear is also pretty heavy, but with this you basically put it on and go in the water, just to get the weight off your shoulders. It was fun, and I'm glad I did it, but my dive log entry reads "Give me scuba any day!" At the end of the dive, the instructors gave us instructions for the decompression "dive"; interrupt the instructor a lot, and take a balloon in. (A balloon blown up at the "deepest" point of a chamber dive will explode as the pressure decreases on the way back up again, making a bang and getting bits of rubber all over the inside of the camber.)
Onwards to the decompression chamber. I don't know what you may be picturing, but try picturing a big metal cyclinder on legs in someone's basement, and you've pretty much got it. (Surrounded, for some reasons, by pictures of pin-up girls. Apparently Scubapro used to think "naked tits" were what they needed to use to sell dive gear back in the day; thank God we've moved on - a bit.) The chamber looks so small, you cannot imagine one person fitting in, but five of us managed to sit next to each other.
A "chamber dive" involves sitting in a decompression chamber whilst the air pressure inside is increased to the equivalent of a 50m deep dive in water (beyond the recreational limit). This allows you to experience the narcosis you get at this level; nitrogen breathed under increased pressure makes you feel drunk. The chamber attendent then decreases the air pressure, including two decompression "stops" on the way "up", and at some point, the balloon goes bang. This is a useful experience to have for two reasons; 1) it gives you the experience of the narcosis you get when diving to 50m, and the sensation of being unable to "surface" (or leave the chamber) until the dive concludes, and 2) you get to experience what chamber treatment is like, which is good since it's the standard treatment for decompression injuries sustained when diving.
I was expecting to be nervous, but the dive was, actually, very good fun. Probably it's the narcosis, but we had a whale of a time. Except for the attendent, who spoke through the chamber's radio in tones of deep resignation "Can you pick up all the pieces of the balloon on the way out, please."