Did you mean the bell's descent or ascent? I thought the bell descended at about 66'/20M per minute.
Sadly the tragedy started long before the Atlantis hatch was closed... when you view it with the benefit of hindsight. I know we lost a LOT more Helium just during leak tests on the Mark II deep dive system than Keller had available for the whole dive. Piping and valve quality on the Mark II was the same as on US nuclear submarines so the Atlantis didn't stand a chance. Granted you don't need $1,000 valves, but you need to know the few that hold Helium from the great majority that leak an old faucet.
Some additional information from the book, in case it's of interest:
On hearing of the deaths at Catalina Island, the Italian company Micoperi had approached Shell International Petroleum in The Hague. The Italians said that much as they regretted the loss of life, Keller had at least shown that it was possible to go to 1,000'/305M and survive, and they urged Shell to fund Keller and Bühlmann to do further research. The persuasion worked.
In 1964, Keller and Bühlmann signed a contract with Shell. A new chamber facility, large enough to allow prolonged experiments, was installed at the University Hospital. Work began with saturation dives to 100’/30M to determine the longest half-time values for helium and nitrogen, then progressed in 1965–66 to bounce dives to 720’/220M. At the same time, Shell set up a field-testing program in the Mediterranean with Micoperi, using a large specially designed combination habitat-diving bell,
Capshell.
In August and September 1966 professional divers from Micoperi and sport divers from Switzerland made a 100’/30M saturation dive, followed by three bounce dives and one saturation dive to 720’/220M. After the dives, Shell and Micoperi formed a 60/40 joint-venture company, Sub Sea Oil Services.
The Shell research contract ended in February 1981. Between 1965 and 1981, Bühlmann conducted some 40 dives to 650’/198M and deeper, the last and deepest to 1,650’/503M, with an excursion to 1,900’/579M. By then, seeing the Swiss and Italian research as too academic, Shell had turned their attention to Norway, to more practical research in support of projects such as the laying of a pipeline across the Norwegian trench.
Edited by request to delete duplicate quotes and posts