MEASUREMENT AND INSTRUMENTATION
FIGURE 6 Photoelectric hydrometer. A glass hydyrometer, similar to the hand-held type, is placed in a continuousflow vessel. The instrument stem is opaque, and as the stem rises and falls, so does the amount of light passing through a slit in the photocell. Thus photocell input is made proportional to specific gravity and can be recorded by a potentiometric instrument. The system is useful for most specific gravity recording applications not harmful to glass and is accurate to two or three decimal places.
FIGURE 8 Balanced-flow vessel for measuring specific gravity and fluid density. A fixed-volume vessel is used, through which the measured liquid flows continuously. The vessel is weighed automatically by a scale, spring balance, or pneumatic force-balance transmitter. Since the weight of a definite volume of the liquid is known, the instrument can be calibrated to read directly in specific gravity or density units. Either open or closed vessels with flexible connections can be used. A high-accuracy measurement results, which is especially useful in automatic density control.
FIGURE 7 Inductance bridge hydrometer. The level of the measured liquid is held constant by an overflow tube. A glass hydrometer either rises or falls in the liquid as the specific gravity varies. The lower end of the hydrometer supports an armature in an inductance coil. Any movement of this armature is duplicated by a similar coil in a recording instrument. With this system, the temperature of the liquid usually is recorded along with the value of specific gravity, so that corrections can be made.
FIGURE 9 Displacement meter for measuring specific gravity and density. Liquid flows continuously through the displacer chamber. An upward force acts on the balance beam because of the volume of liquid displaced by the float. A pneumatic system, similar to the one shown, balances this upward force and transmits a signal proportional to the density of the liquid. Liquids with specific gravities 0.5 and higher can be measured with this equipment so long as suitable materials are used to prevent damage from corrosion. If the temperature of the flowing liquid changes, thermostatic heater may be used to maintain a constant temperature.
One of the most important developments in fluid density measurement over the last decade or so has been the Coriolis flowmeter, which measures flows in terms of their mass. See the article, â€œFlow Systemsâ€? in this handbook section. Controlling the density of oil-well drilling mud is described by K. Zanker (Sensors, vol. 40, Oct. 1991). FOR MORE DETAILS VISIT US ON WWW.IMTSINSTITUTE.COM OR CALL ON +9199994621
Mechanical BE (Measurement and Instrumentation)