Ok, thanks for the answers, i'll try to explain better, so
here's the cell
the plates connected to the pwm are the first and the last
concerning the concentration of NaOH, M is for "molar concentration" or molarity, the standard unit for the concentration of anything; it's (1 mol of solute)/(1 liter of solution)
sorry i'm a chemist....
in my case it was about 1M, so, since the molar weight of NaOH is 40,00 grams/mol, there were about 40,00 grams in 1 liter
the water i used is not distilled water, but i think this could not affect the HHO production.
i know that pwm is not a current generator, but it transorm the continous current generated by the battery, into alternated current at the selected frequency, and of sure it limitates the amount of Amps given to the cell.
the gas production was the same at all frequencies, and the gas was produced just on the first and the last plate and only in the hole area, in the center-lower part of the plate. (im' able to see into the cell from the transparent enclosure and from the lover holes)
it is right to have the same production at all frequencies??
thanks