Fixed-roof tanks should have a quick opening gauge hatch in the roof, which allows the operator access to the tank to "gauge" the tank, determine if water is present, measure the height of the oil/water interface, and take samples of the crude oil. Standards for manual gauging of petroleum and petroleum products are given in the API Manual of Petroleum Measurement Standards, Chap. When a volatile product is stored in a freely ventilated fixed-roof tank, the concentration of volatile vapors in the vapor space can vary depending on the tank operating conditions. During holding periods, when no liquid is added or removed from the tank, the vapor space comes to equilibrium conditions based on product temperature and vapor pressure. Emissions during holding are generated by the vapor space breathing process.