Abstract: On September 2, 2011, Employee #1 of Membrane Technology & Research filled up a Matheson 1F gas cylinder approximately 15 inches in diameter and 45 inches tall with 1% Butane, 2% Helium, 20% Nitrogen and 78% Methane. With this concentration, the cylinder's pressure was 700 to 800 psi. The maximum capacity allowable working pressure for this originally propane cylinder is 300 psi and the pressure relief valve would only release pressure at or above 3360 psi. At approximately 3:40 pm, Employee #1 proceeded to fill up this Matheson 1F gas cylinder with more Methane from a 1700 psi cylinder tank. The employer's goal was to increase the Methane concentration in the Matheson 1F to be at 90% Methane and at an approximate pressure greater than 1200 psi. The pressure relief valve did not release pressure above the maximum capacity of this cylinder. The Matheson 1F gas cylinder accumulated pressure to approximately 800 to 900 psi and it exploded to pieces due to over-pressurization. Employee #1 that was filling this tank was blown 15 feet away to the adjoining laboratory and suffered fatal injuries.