FLOW METERS

Mechanical flow meters are classified as rate meter because they measure and record or locates the rate of flow when measuring liquid or gases, the fluid are in direct contact with the actuating elements. when steam is measured, it condenses to water in the connecting pipe at atmospheric temperature  and is in contact with the actuating elements. the rate of flow is determined by measuring the drop in pressure of the fluid flowing through a restriction in the flow line. The restriction is shown in the primary elements. The flow meter has several designed of secondary actuating elements which are available and are in used in the industries. A early design is a mercury manometer , with flow resting on the surface of the mercury  manometer , with the flow resting on the surface in one leg .of the manometer .T he flow condition changes as flow varies and by a linkage mechanism. The pen or pointer is moved to correspond with the new rate of flow.for several reasons, such lower cost and easier installation and maintenance the use of  mercury less meters is  increasing rapidly, There were two general types ,i.e  (i) differential meter for measuring  pressure drop and its motion, (ii) the other type measures the pressure drop with a capsular assembly and transmit a standard of about 3 to 5 psig pneumatic signal. Most manufacturer published an accuracy of plus minus 0.5 percent for full scale displacement, for bellow meters. If an installation require better accuracy, it could be obtained by recalibration.
The general published for pneumatic transmitter is plus minus 0.5 percent for full scale displacement. Both types could be operated up to temperature  of 232 degree centigrade of the fluid in meter body.