Test Measurements

discharge The choice of which method of discharge measurement to use should be made by agreement between all parties concerned. Some test codes and procedures in regular use permit or even recommend certain methods for model or shop testing, but restrict their use in field or index testing. Some methods are more adaptable to the site conditions than others, and so the test engineers and interested parties should be completely familiar with the several methods applicable before settling on the one to be used.

The most commonly used methods of discharge measurement are those that use quantity meters and those that use rate-of-flow meters. Both meters are usually classified as liquid meters, and their functions are listed in Table 3.

quantity meters The term quantity is here used to designate those meters in which the fluid passes through the primary element in successive and more or less completely isolated quantities, either weights or volumes, by alternately filling and emptying containers of known capacities. The secondary element of a quantity meter consists of a counter with suitably graduated dials for registering the total quantity that has passed through. Quantity meters are classified into two groups: weighing meters and volumetric meters.

weighing meters There are two types of weighing meters: weighing tank and tilting trap. In the tilting trap meter, the equilibrium of a container is upset by a rise of the center of gravity as the container is filled. Weighing tank meters employ a container suspended from a counterbalanced scale beam. The weighing tank and the tilting trap are affected slightly by the temperature of the liquid but not enough to cause concern in normal testing.

volumetric meters Volumetric meters measure volumes instead of weights. There are four types: tank, reciprocating piston, rotary piston, and nutating disk.

Tank meters are a very elementary form of meter of limited commercial importance. As the name implies, they consist of one or more tanks that are alternately filled and emptied. The height to which they are filled can be regulated manually or automatically. In some cases, the rising liquid operates a float that controls the inflow and outflow; in others, it

TABLE 3 Liquid meters and their functions

Quantity meters

Rate-of-flow meters

Weighing meters

Differential pressure meters

Weighing tank

Venturi

Tilting trap

Nozzle

Volumetric meters

orifice plate

Tank

Pitot tube

Reciprocating piston

Head area meters

Rotary piston

Weir

Nutating disk

Flume

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