Nevertheless, the cell could be used successfully if experiments were carried out quickly and carefully. A set-up similar to this, with the reference electrode holes excluded, was used for all acoustic emission testing. In accordance with the voltage fluctuation technique described previously, 10 the standard deviation, , of the voltage data at each of the anode and cathode was calculated over the last 10 s of each 20 s step period and plotted against the respective current density. Low values of  were typically associated with bubble-free gas generation (e.g. less than  ~0.2 mV), while higher values (e.g. more than  ~0.2 mV) were consistent with bubble formation.