MEASUREMENT AND INSTRUMENTATION
PRESSURE GAGE INDICATING WEIGHT OR CONTROLLER
APPLIED WEIGHT OR FORCE TARE CHAMBER
FLAPPER AIR BLEED NOZZLE TARE SUPPLY REGULATOR AIR
AIR SET CONSTANT DIFF. PRESS REG. NET WEIGHT CHAMBER
FIGURE 1 For some applications, a pneumatic load cell may be preferred over the widely used strain-gage cell.
midtwentieth century continued to rely on mechanical levers, pivots, bearings, and counterweights. Even without the benefits of advanced computing, electronics, and microprocessing techniques, a large truck or railroad platform scale could weigh 1 pound of butter or 1 part in 10,000. The principal design disadvantages of the former very rugged mechanical scales were their bulk, weight, and manual requirement to achieve balance. Self-balancing scales involving springs or counterbalancing pendulums essentially eliminated the need for manual operation. Invention of the strain gage and associated load cells revolutionized scale design. It should be pointed out that mechanical scales still are found in numerous isolated installations, although replacement parts are difficult to find. Concurrent with the development of the strain gage, pneumatic and hydraulic load cells were developed and continue to be found in some installations (Fig. 1). However, the strain gage, by far, is the principal weight sensor used today. Refer to the article, “Fluid Pressure Systems” in this handbook section. Industrial scales may be classified by several criteria. 1. Physical configurations, influenced by application needs, include (1) ordinary platform scales as found in warehousing, receiving, or shipping departments, (2) very large highway or railroad track scales, which today frequently are designed to weigh “on the move,” (3) tank scales, in which entire tanks of liquids can be weighed periodically or continuously—for inventory control (actually used as flow controllers for some difficult-to-handle materials) and for feeding batch processes, (4) bulk-weighing or hopper scales, and (5) counting scales used in inventory and production control. 2. Scales also fall into a number of capacity categories, ranging from the most sensitive laboratorytype balances (not covered in this article) to platform scales from as low as 50 lb (23 kg) through 1000, 50,000, 200,000 up to 1 million lb (453,597 kg). The upper limit on platform scales usually is about 30,000 lb (13,607 kg). Scales also are available for weighing materials which travel along a monorail, such as in a meat-packing plant. Another specialized application for weight indication is that of a crane or derrick for lifting and lowering heavy loads. A number of installations are shown in Figs. 2 through 5.
FLUID DENSITY Density may be defined as the mass per unit volume and usually is expressed in units of grams per cubic centimeter, pounds per cubic foot, or pounds per gallon. Specific gravity is the ratio of the FOR MORE DETAILS VISIT US ON WWW.IMTSINSTITUTE.COM OR CALL ON +9199994621
Mechanical BE (Measurement and Instrumentation)