An electrosurgical generator for coagulating tissue comprises a source of radio frequency (RF) energy, and at least a pair of output terminals for connection to a bipolar electrosurgical instrument adapted to contact tissue, the output terminals delivering RF energy from the source to the instrument such that the source delivers RF energy into tissue. The generator also includes boiling detection means for determining when the RF energy starts to cause the boiling of electrolytes within the tissue. A timer is adapted to determine when a predetermined period of time has elapsed since the boiling detection means has indicated the start of the boiling of electrolytes. An indication means gives an indication when the predetermined period of time has elapsed.
