Best answer to Rick ! I'd forgotten Popular Mechanics. What a gold mine.
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How about this http://books.google.co.uk/books?id=cNQDAAAAMBAJ&pg=PA69&dq=popular+mechanics+diving&hl=en&sa=X&ei=B45MT_yhB4ax0AWJ2fydBQ&redir_esc=y#v=onepage&q=popular%20mechanics%20diving&f=false Google books is a great maker source.
This was exactly what came to mine when I read the title also.
There are several links here
Rent one and study it to get the ideas of a working unit.A
What kind of scale are these things ?