Although gaseous at normal temperatures and pressures, the gases making up the mixture that is natural gas are variable in form and may be found either as gases or as liquids under suitable conditions of temperature and pressure. Net volume : the total volume of liquid in a tank after adjustments have been included for S&W content, temperature, and density. Compare gross volume. NIST abbr: the National Institute of Standards and Technology . observed volumes : those volumes of liquid or gas that are indicated by the register on the meter. Compare true volume . octane level : a measure of the antiknock properties of a fuel. oil-water interface : the boundary between the oil on top and the water below when both are contained in the same vessel. opening gauge : the measurement of liquid in a tank before a delivery or receipt. Compare closing gauge. outage measurement : a measure of the space between the top of the liquid in a tank and the reference point on the gauge hatch. outage tape-and-bob procedure : a method of measuring the liquid level in a tank by measuring the space between the top of the liquid in a tank and the reference point on the gauge hatch. Compare innage tape-and-bob procedure.
pressure gauge : an instrument that measures fluid pressure and usually registers the difference between atmospheric pressure and the pressure of the fluid by indicating the effect of such pressures on a measuring element (e.g., a column of liquid, pressure in a Bourdon tube, a weighted piston, or a diaphragm). pressure-reducing valve : a type of valve that changes the upstream pressure to a lower downstream pressure. pressure relief valve : a valve that opens at a preset pressure to relieve excessive pressures within a vessel or line. Also called a pop-off valve or safety relief valve. pressure surge : a sudden increase in pressure in fluid flowing in a pipe or in liquid flowing into a storage tank. probe : any small device that, when brought into contact with or inserted into a system, can make measurements on that system. In sampling, probes are inserted into pipelines so that small samples of liquid can be removed and then stored for future testing. psia abbr: poundsper square inch absolute. Rankine temperature scale : a temperature scale with the zero point at absolute zero. On the Rankine scale, water freezes at 491.60º and boils at 671.69º.
manifold valve : a valve placed in a series of piping that is called a manifold. master meter : a meter that is proved using a certified prover and then used to calibrate other provers or to prove other meters. liquid displacement : the crude oil or product that is forced into the hatch or up around the edges of a floating roof by the weight of the roof. Meniscus : the curved upper surface of a liquid column, concave when the containing walls are wet by the liquid (negative meniscus) and convex when not (positive meniscus). Liquid-in-glass thermometer : a type of thermometer in which the sensitive part of the instrument consists of liquid contained in an envelope of glass. Meter : a device used to measure and often record volumes, quantities, or flow rates of gases, liquids, or electric currents. Naphtha : a volatile, flammable liquid hydrocarbon distilled from petroleum and used as a solvent or a fuel. Natural gas : a highly compressible, highly expansible mixture of hydrocarbons with a low specific gravity and occurring naturally in a gaseous form. Besides hydrocarbon gases, natural gas may contain appreciable quantities of nitrogen, helium, carbon dioxide, hydrogen sulfide, and water vapor.
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