What is a water meter? What are the types and what is the working principle of water meter? Information on water meter
WATER METER, a device installed in a pipe under pressure for measuring and registering the quantity of water passing through it.
Sextus Julius Frontinus (c. 30-104), a curator aquarum in charge of the many aqueducts delivering upland water to Rome, had bronze apertures installed where the quantities of water diverted to large consumers could be observed and estimated; for smaller users, orifices were installed. The original United States patent for measuring flow of water under pressure was granted in 1850 to William Sewell, and in 1852, Joseph Maudsley took out a British patent on a similar principle. In 1875, a patent providing legible dials above water greatly facilitated meter reading. General interest in meters increased during the next decade; 39 patents were issued in 1886. The Venturi meter, based on principles laid down by Giovanni Battista Venturi (1746-1822), was given practical reality by Clemens Herschel in 1887. Standard specifications for cold-water meters have been issued by the American Water Works Association, and designated C-700 (displacement type); C-701 (current type); C-702 (compound type); C-703 (fire-service type); and C-704 (current type, propeller driven).
There are in use six principal types of meters for pressure pipes: disk (or displacement), velocity, compound, fire-service, orifice, and Venturi. Domestic meters generally are of the disk, velocity, compound, or fire-service types; for larger quantity measurements the orifice and Venturi meters are used.
The disk (or displacement) rheter is provided with a measuring chamber of a known size. As this chamber fills and empties, the mechanism of the meter records the quantity of water displaced each time the water leaves the chamber. The mechanism is operated by various means, including nutating disk, oscillating piston, and rotating gear. The disk type is costlier than the velocity type; it is made in 5/8-inch to 6-inch sizes. While relatively accurate at low flows, the disk meter has a limited capacity range for a given size and high friction loss at high flow. If improperly installed, it tends to be noisy. For high capacities, a battery of disk meters, a velocity meter, or a compound meter will prove more practical.
A velocity meter (also termed current, turbine, or torrent) is provided with a piston or impeller, the speed of the mechanism being calibrated to the quantity of water passing through it. This type is low in initial cost, inexpensive in repairs, and measures relatively large quantities with little friction loss. It comes in sizes 1,5 to 16 inches. Inasmuch as there is no obstruction to the free flow of water through this meter, it meets the requirements of the National Board of Fire Underwriters for use on fire-service lines. However, it is difficult to maintain at a high degree of accuracy, and to repair.
A compound meter comprises a combined disk and velocity meter. The low flow is measured through the disk mechanism, after the flow reaches a predetermined rate, measurement is picked up by the velocity mechanism. It is available in 1,5-to 10-inch sizes. Compound meters have a wide capacity range, measuring both large and small flows accurately; at the “change point” from displacement to velocity mechanisms, there is usually a sharp drop in accuracy. Because of its size, it ordinarily must be tested in place, a hindrance to effective repair. A battery of two or more small disk meters is frequently used in place of a compound.
In the fire-service meter, a certain proportion of the total flow passing through the meter is bypassed through a measuring device which is usually of the disk type, although sometimes of the velocity type.
Where there is little or no frost or snow, it is advantageous to set the meter in a box in the sidewalk area, affording easy access for reading or changing the meter. In climates where the winters are severe, however, it is practically a necessity to install a domestic meter in the basement to prevent damage from freezing.
The working parts of domestic meters are made light and durable. Hard rubber generally is used for disks, and also for the rotary pistons of some meters. Where the liquids are hot, or consist of chemical solutions, brass generally is employed. Possible damage due to clogging by the entry of fish or gravel is prevented by various forms of strainers and sieves, while the effect of frost is abated by frost cases.
Orifice meters have no moving parts in the pipe, merely an orifice in a thin plate. Their merits are low initial cost, short length, ease of installation, adaptability to large capacity, and their freedom from clogging which promotes continuous accuracy; however, the head loss is high.
The Venturi meter is based upon the hydraulic principle that when water flows at an increased velocity, there will be a corresponding reduction in head, or pressure. The meter usually consists of an upstream reducer whereby the area is diminished to approximately one fourth to one ninth of the pipe area. Downstream from the reducer is a short throat piece and a relatively long section to increase from the throat diameter back to the normal pipe diameter. The upstream pressure is measured just above the point at which the pipe begins to taper down to the throat; pressure is also measured in the throat, where the high velocity occurs. The relatively long downstream section permits a gradual change, with little loss of head, from the high velocity to the normal pipe velocity; this loss is estimated generally to be about one tenth of the velocity head in the throat.
Venturi meters are relatively inexpensive. For large flows they have a high degree of accuracy over a broad range, with a very small friction loss; but they do not measure accurately at low flows. They require a substantial space for installation, and once installed cannot readily be removed to a new position. With reasonable care they may be operated for years without adjustment; the clear waterway through the meter prevents clogging.