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PYCNOMETERS:  A thermos-type pycnometer is a precision sphere of known mass and volume that is capable of withstanding pipeline pressures.  The pycnometer is recommended for use by API (American Petroleum Institute) for densitometer proving.  During the proving of a densitometer, the pycnometer is connected in series or parallel with the densitometer.  Once temperature stability is maintained in the flow loop, the pycnometer is removed and weighed via a certified scale.  Once the empty calibrated vacuum weight of the pycnometer has been subtracted, the liquid mass per unit volume or density is calculated. This test is normally repeated until the readings are repeatable. Once this occurs, the density value is used to calculate the "density correction factor", DCF, and is used to calculate the actual density in the flow computer or signal converter.

VIBRATING TUBE DENSITOMETERS:  A vibrating tube densitometer is basically a spring-mass system where the frequency of vibration of the tubing is measured and related to the fluid density. The tube assembly is supported at each end and is mechanically "excited" or "displaced" using electro-mechanical devices so that the assembly will vibrate at the natural frequency.  The frequency of vibration of the tube assembly will vary as the density of the fluid in the tubing changes. The tube assembly must have appropriate mechanical properties to resist corrosive attack from the fluid, contain the pipeline pressure, and have desired vibration characteristics. The actual arrangement of the tubes will vary with manufacturer, with parallel tubes, U-tube, and in-line tubing being the most common. The tube material is usually Ni-Span C, Stainless Steel, or Hastelloy although other materials have been used. An efficient densitometer installation design will ensure that a representative liquid pipeline sample is located inside the tube(s) at all times.

 

HYDROMETER:  A hydrometer is an instrument used to measure the specific gravity (or relative density) of liquids; that is, the ratio of the density of the liquid to the density of water.  A hydrometer is usually made of glass and consists of a cylindrical stem and a bulb weighted with mercury or lead shot to make it float upright. The fluid to be measured is poured into a tall container, often a graduated cylinder, and the hydrometer is gently lowered into the liquid until it floats freely. The point at which the surface of the liquid touches the stem of the hydrometer is noted. Hydrometers usually contain a scale inside the stem, so that the specific gravity can be read directly. Varieties of scales exist, and are used depending on the context.  Hydrometers may be calibrated for different uses, such as a Thermohydrometer.  A thermohydrometer is a hydrometer that has a thermometer enclosed in the float section. For measuring the density of petroleum products, like fuel oils.