Originally, I was talking about RH's cylinders, excluding the top and bottom caps. After I looked at yours it dawned on me that I could measure the piston rods.
It's the one from Ebay (thanks for the link). It has no handle and says
14.5". It would be more accurate to expand it to fill my screen but more foolproof to lat a cm = an inch.I can see why. The estimates I calculated were for 1 cubic foot a minute. For the 1.5 that's recommended these days, the pumps would have been worked 50% faster.
No wonder they went to two-man pumps. Maybe I can analyze one. Two men using four arms on two levers could apply more force. If the bore were 40% bigger and the stroke twice as long, they could go 75% slower.
The gage on RH's pump was made for the expansion tank in a heating system. The red hand was set for normal conditions. On a diver's pump I think it was a depth redline. That would mean the safe limit was 15 feet (for the diver's head).
Suppose a man could pump 60 cycles (120 strokes) a minute. If the pump put its whole displacement into the supply line, that would be 1.25 cubic feet per minute at the surface and 0.87 cubic feet entering the helmet 15 feet down. That's 58% of what's recommended.
I think it's feasible. The problem is not lacking oxygen but reacting to CO2. In grade school I'd be so cold at night that I'd sleep curled up with the sheet and blankets over my head. The elevated CO2 didn't bother me because I was used to it.
To supply a diver with 0.87 cubic feet a minute with 120 strokes a minute, the Ebay pump would have been good for a helmet 35 feet down. The Cleveland pump you found would have been good to 42 feet. It would depend on the diver's tolerance for CO2, his muscle mass, his exertion, and perhaps the design of his helmet.