California Code of Regulations
Title 8 - Industrial Relations
Division 1 - Department of Industrial Relations
Chapter 4 - Division of Industrial Safety
Subchapter 1 - Unfired Pressure Vessel Safety Orders
Article 7 - Compressed and Liquefied Natural Gas System
Section 539 - Pressure Gages

Every bulk storage tank, dispensing tank, transportation tank and bulk delivery tank shall be provided with a suitable pressure gage. The pressure gage shall be installed in a special gage outlet installed permanently on the tank or tank system and having an opening not to exceed No. 54 drill size opening at the tank connection. Pressure gages for LNG vessels shall have their connection above the maximum permissible liquid level. The pressure gage shall have a dial graduated to read approximately double the operating pressure, but in no case less than 1.2 times the pressure at which the pressure relief valve is set to function.