Humboldt Bunsen Burners are high-quality, American-made burners with nickel-plated, die-cast zinc bases. Burners feature aluminum mixing tubes and serrated gas inlet connections. *Note: CFHs and BTU Outputs are figured on: 5 inch Water Column Pressures for Artificial and Natural Burners. 11 inch Water Column Pressures for LP Burners. Inlet OD for all Burners: 5/16" Serrated