Since we are looking at Brute force method here, I would like to suggest the current draw be kept between 0.25A and 0.5A/sq. in of the surface area of the smallest electrode. Meaning, the current draw for 0.5" inner tube, 4" long, should be:
pi*0.5"*4"=6.28 sq. in, divided by 4 = 1.57A min, divided by 2 = 3.14A max. The copper cathode's significantly lower resistance than SS, should produce much more gas at these current levels than using a SS cathode.
Also, the optimum voltage per cell should be 1.8V-2.5V. A series configuration will be best and efficiency goes very high when the voltage across the series cell is more than 24V.
Hope this helps.
