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Yep...
An interesting idea that didn't work very well. Here's one variant (not the ones I've seen, which were just released and followed; just dawned on me that I don't remember them being yellow, but then I don't remember 'em being any other color in particular either - if you'd asked me I'd have probably said white)
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United States Patent # 5308272
A non-mechanical ascent rate indicator for use in saltwater scuba diving. A high density sphere attached by means of a ball chain tether to the gauge, or hose, of a scuba diver has a free rate of ascent of less than 60 feet/minute, the difference in this rate and that of the diver will be shown by the amount to free loop in the tether as it is observed on a horizontal plane even to the diver's eyes. A safe ascent rate will provide a loop of approximately 1 to 11/2 inch.
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Rick