A potentially fatal accident occurred during welding repairs to a fuel tanker. A fuel tanker was undergoing repairs to cracks in the tank support structure. The tank had been flushed out two times with water and was checked with an explosion meter. The welder was trained and precautions taken for the hazards identified. One or two minutes after welding repairs were complete the tank over pressured and propelled the hatch through the roof and down the roadway over 150 meters. It was determined during the accident investigation that the gas testing equipment used to verify the safe atmosphere prior to welding was unserviceable.