Void/Cavity Monitoring For tanks containing hazardous material, double skin formats often require the addition of void monitoring as a leak identification back up. This is supplied by means of the SPEL void monitoring alarm which can detect the smallest build up of fluid in the cavity at the base of the tank. This SPEL system allows access to the void at the base of the tank (once the alarm probe is removed) enabling a sample to be taken of the fluid in the cavity. Checking this sample enables the operator to confirm if this is simply condensation, ground water or a leak from the tank itself.