WATER METERS. Instruments used to measure and automatically record the quantity of water or other liquid flowing through pipes. They arc divided into three general classes known as the positive, inferential, and proportional. The positive meter measures the actual volume of water passing through pipes with which it is connected. The inferential meter measures some element or factor of the flow, most generally the velocity. The propor tional meter measures a fractional part of the full flow, thus making it possible to use a rela tively small meter, set on a by-pass or pipe branching from and again joining the main pipe. It is used hut little.
Positive meters are provided with recipro cating or rotary pistons, or else with an oscil lating or gyrating disk piston. In all three classes of positive meters each complete move ment corresponds to the filling and emptying of a chamber or series of chambers of known size. There is no escape for the water until it has actuated the piston. The reeiproonting pis tons may he single or double. If double. as is generally the ease, each piston effeets the re ciprocal action of the other, much the same as in a duplex pump. (See AND PUMPING if single, the return stroke is ef fected by means of a weight or spring. Tho rotary piston meters may also be double or single. The double type is much like the rotary pump, two pistons with interlocking faces revolving in an air-tight chamber. Some of the single rotary pistons have a series of curved pro jections and indentations on their outer sur faces. Corresponding to these are similar va riations in the cylinder in which the piston revolves. The piston is not fixed rigidly at its
centre. The incoming water causes the piston to enter and leave successive indentations in the inclosing cylinder. to other words, a series of chambers are successively tilled from the inlet and discharged toward and finally at the outlet of the meter. In place of a piston of this sort a disk or diaphragm is very commonly employed, so construeted as to giye an oscillating or wob bling motion. In this way the chambers are alternately filled and emptied. Inicrential meters generally have as their primary moving part a series of vanes or buckets, or else employ a screw. The velocity of the water is indicated by this means.
The Venturi meter generally stands in a class by itself. Its usefulness depends wholly upon the increase in velocity and consequent reduction in pressure caused by contracting the diameter of a pipe through which water is flowing. By measuring the pressures before and directly at the point of contraction. and taking into account the diameter of the pipe, the flow may be com puted. There are no working parts in the meter itself. The contracted tube is composed of two conical-shaped pipes. generally of east lion or riveted steel, with their small, truncated ends joined together with a bronze throat piece. The whole tube might be made of masonry or wood. The total length of the meter is from S to times the diameter of the uncontraeted portion. The contraction has from one-half to one-fourth the diameter of the full size of the tube.