It was just getting light on one of the most miserable Saturday mornings of the year. Howling wind, driving rain, conditions under which most sensible divers would call the trip off. But we were happy and going to Plymouth anyway. We had a tour and chamber dive booked at the Diving Diseases Research Centre (DDRC).
Air shrieked into the chamber. The descent was fast and hot. As the chamber pressurised, the temperature increased to that of a sauna, then rapidly cooled back to normal room temperature as our depth stabilised at 42 metres. We all peeled down to T-shirts. A balloon filled with air at the surface had shrunk to a limp rubber blob.
Something which I had not anticipated was that we all had squeaky Donald Duck voices, an effect I had always associated with helium, but which is actually the due to an increase in the velocity of sound. In our case this was a result of increased air density at a chamber pressure of over 5 atmospheres.
Amidst much giggling we inflated our balloons ready for the ascent. Cans of Pepsi were opened and to no-ones surprise there was no fizz, but it didn't taste flat. As we ascended it regained its fizz to behave more normally.
Sweaters and fleeces were pulled back on as the chamber cooled during the ascent. Balloons filled at 42 metres grew larger and became hot property. A game of pass the parcel developed with the looser being the diver holding the balloon when it went pop!