Firstly calorimetry was performed as described by Lyng (1995) using the same probe as used here to determine if the equipment had degraded with time. This involved sonication of 0.05kg of water (thermostated at 17-19 °c) in an insulated 100 ml beaker, with 82.5 W for 3 min. Secondly, calorimetry was performed on the experimental apparatus (fig. 1) to determine the actual intensity achieved.