“Volatile flammable liquids” as used in this part means any petroleum or liquid product of petroleum or natural gas having a flash point below 100 degrees Fahrenheit, and includes any petroleum or liquid product of petroleum or natural gas while at a temperature above its flash point. Flash points shall be as determined by means of the Tag Closed Tester, Designation D56-36 American Society for Testing Materials, or the Pensky-Martens Closed Tester, Designation D93-42 American Society for Testing Materials.