Water metering

A typical residential water meter

Water metering is the process of measuring water use.

In many developed countries water meters are used to measure the volume of water used by residential and commercial building that are supplied with water by a public water supply system. Water meters can also be used at the water source, well, or throughout a water system to determine flow through that portion of the system. In most of the world water meters measure flow in Cubic meters (m³) or litres but in the USA and some other countries water meters are calibrated in cubic feet (ft.³), or US gallons on a mechanical or electronic register. Some electronic meter registers can display rate-of-flow in addition to total usage.

There are several types of water meters in common use. Selection is based on different flow measurement methods, the type of end user, the required flow rates, and accuracy requirements.

In North America, standards for manufacturing of water meters are made by the American Water Works Association.

Types of meters

There are two major methods of flow measurement in use, displacement and velocity, with sub-technologies within each of them. Common displacement designs include oscillating piston and nutating disk meters. Velocity-based designs include single- and multi-jet meters and turbine meters. There are also non-mechanical designs, for example electromagnetic and ultrasonic meters.

In addition to the more common types of meter, there are meters designed for special uses. Most meters in a typical water distribution system are designed for cold potable water only. There are specialty water meters manufactured for specific other uses. Hot water meters are designed with special materials that can withstand higher temperatures. Meters for reclaimed water have special lavender register covers to signify that the water is non-potable and should not be used for drinking.

Water meters are generally owned, read, and maintained by a public water provider such as a city, rural water association, or private water company. In some cases an owner of a mobile home park, apartment complex or commercial building may be billed by a utility on one meter, and want to share the cost of the bill among the tenants. In this case, private meters may be purchased to separately track usage of each unit in what is called submetering.

Displacement water meters

Water meter in Belo Horizonte.

This type of water meter is most often used in residential and small commercial applications. Displacement meters are commonly referred to as Positive Displacement, or "PD" meters. Two common methods of positive displacement measuring are Oscillating Piston meters and Nutating Disk meters. Either method relies on the water to physically displace the moving measuring element in direct relation to the amount of water that passes through the meter. The piston or disk moves a magnet that drives the register.

PD meters are generally very accurate at low to moderate flow rates typical of residential and small commercial users, and are common in sizes from 5/8" to 2". Because displacement meters rely on all water flowing through the meter to "push" the measuring element, they generally are not practical in large commercial applications requiring high flow rates or low pressure loss. PD meters normally have a built-in strainer to protect the measuring element from rocks or other debris that could stop or break the measuring element. PD meters normally have bronze, brass or plastic bodies with internal measuring chambers made from molded plastics and stainless steel.

Velocity water meters

A velocity-type meter measures the velocity of flow through a meter of a known internal capacity. The speed of the flow can then be converted into volume of flow for usage. There are several types of meters that measure water flow velocity to determine totality usage. They include jet meters (single-jet and multi-jet), turbine meters, propeller meters, and mag meters. Most velocity-based meters have an adjustment vane for calibration of the meter to required accuracy.

Multi-jet meters

Multi-jet meters are very accurate in small sizes and are commonly used in 5/8" to 2" sizes for residential and smaller commercial uses. Multi-jet meters use multiple ports surrounding an internal chamber, to create multiple jets of water against an impeller. The impeller rotation speed is in relation to the velocity of water flow. Multi-jets are very accurate at low flow rates, but are not used in larger sizes, since they don't have the straight-through flow path needed for the high flow rates used in large pipe diameters. Multi-jet meters generally have an internal strainer element that can protect the jet ports from getting clogged. Multi-jet meters normally have bronze alloy bodies or outer casings, with internal measuring parts made from modern thermoplastics and stainless steel.

Turbine meters

Turbine meters are less accurate than displacement and jet meters at low flow rates, but the measuring element does not occupy or severely restrict the entire path of flow. The flow direction is generally straight through the meter, allowing for higher flow rates and less pressure loss than displacement-type meters. They are the meter of choice for large commercial users, fire protection, and as master meters for the water distribution system. Strainers are generally required to be installed in front of the meter to protect the measuring element from gravel or other debris that could enter the water distribution system. Turbine meters are generally available for 1-1/2" to 12" or higher pipe sizes. Turbine meter bodies are commonly made of bronze, cast Iron, or ductile iron. Internal turbine elements can be plastic or non-corrosive metal alloys. they are accurate in normal working conditions to 0.2l/s however are affect greatly with dog mix interference.

• Fire meters are a specialized type of turbine meter with approvals for the high flow rates required in fire protection. They are often approved by Underwriters Laboratories (UL) or Factory Mutual (FM) for use in fire protection.

• Fire hydrant meters are a specialized type of portable turbine meter that are attached to a fire hydrant to measure water out of the hydrant. The meters are normally made of aluminum to be light weight, and are usually 3" capacity. Utilities often require them for measurement of water used in construction, pool filling, or where a permanent meter is not yet installed.

Compound meters

A compound meter is used where high flow rates are necessary, but at times there are smaller rates of flow that still need to be accurately measured. Compound meters have two measuring elements and a check valve to regulate flow between them. At high flow rates, water is normally diverted primarily or completely to the turbine part of the meter. When flow rates drop to where the turbine meter cannot measure accurately, a check valve closes to divert water to a smaller meter that can measure the lower flow rates accurately. The low flow meter is typically a multi-jet or PD meter. By adding the registration of the high and low meter registers, the utility has the total consumption through the meter.

Electromagnetic meters

Electromagnetic flow meter

Magnetic flow meters, commonly referred to as "mag meters", are technically a velocity-type water meter, except that they use electromagnetic properties to determine the water flow velocity, rather than mechanical means which jet and turbine meters use. Mag meters use the physics principle of Faraday's law of induction for measurement, and require AC or DC electricity from line or battery to operate the electromagnets. Since mag meters have no mechanical measuring element, they normally have the advantage of being able to measure flow in either direction, and use electronics for measuring and totalizing the flow. Mag meters can also be useful for measuring untreated water, raw (untreated/unfiltered) water, and waste-water, since there is no mechanical measuring element to get clogged or damaged by debris flowing through the meter. Strainers are not required with mag meters, since there is no measuring element in the stream of flow that could be damaged. Stray electrical energy flowing through the flow tube can cause inaccurate readings, therefore most mag meters are installed with either grounding rings or grounding electrodes to divert stray electricity away from the electrodes inside the flow tube which are used to measure the flow.

Ultrasonic meters

Ultrasonic water meters use an ultrasonic transducer to send ultrasonic sound waves through the fluid to determine the velocity and translate the velocity into measurement of the water volume.

Registers

A typical water meter register showing a meter reading of 8.3 gallons. Notice the black "1" on the odometer has not yet fully turned over, so the red hand is read in its place.

There are several types of registers on water meters. A standard register normally has a dial similar to a clock, with gradations around the perimeter to indicate water usage measured by the meter, as well as a set of odometer wheels similar to that in a car. Modern registers are normally driven by a magnetic coupling between a magnet in the measuring chamber attached to the measuring element, and another attached to the bottom of the register. Gears in the register convert the motion of the measuring element to the proper usage increment for display on the sweep hand and the odometer. Many registers also have a leak detector. This is a small visible disk or hand that is geared closer to the rotation speed of the drive magnet, so that very small flows that would be visually undetectable on the regular sweep hand can be seen.

With Automatic Meter Reading, manufacturers have developed pulse or encoder registers to produce electronic output for radio transmitters, reading storage devices, and data logging devices. Pulse meters send a digital or analog electronic pulse to a recording device. Encoder registers have an electronic means for an external device to interrogate the register for either the position of the odometer wheels or a stored electronic reading. Frequent transmissions of consumption can be used to give smart meter functionality.

There are also some specialized types of registers such as meters with an LCD display instead of mechanical odometers, and registers to output data or pulses to a variety of recording and controller devices. For industrial applications, output is often 4-20 mA analog, for the recording or control of different flow rates in addition to totalization.

Water meter reading

Different size meters may indicate a reading in different resolutions. One rotation of the sweep hand may be 10 gallons or 1000 gallons (1 to 100 ft.³, 0.1 to 10 m³). If one rotation of the hand represents 10 gallons, the meter has a 10 gallon sweep. Sometimes the last number(s) of the odometer are non-rotating or printed on the dial face. The fixed zero number(s) are represented by the position of the rotating sweep hand. If one rotation of the hand is 10 gallons, the sweep hand is on 7, and the odometer shows 123456 plus a fixed zero, the actual total usage would be 1,234,567 gallons.

In the United States most utilities bill only to the nearest 100 or 1000 gallons (10 to 100 ft.³, 1 to 10 m³), and often only read the leftmost 4 or 5 numbers on the odometer. They would only read and bill using 1234 and rounding to 1,234,000 gallons based on a 1000 gallon billing resolution and the example above. The most common rounding for a particular size meter will often have different colored number wheels, the ones ignored being black, and the ones used for billing being white.

Prevalence

Water metering is common for residential and commercial drinking water supply in many countries, as well as for industrial self-supply with water. However, it is less common in irrigated agriculture, which is the major water user worldwide. Water metering is also uncommon for piped drinking water supply in rural areas and small towns, although there are examples of successful metering in rural areas in developing countries, such as in El Salvador.

Metering of water supplied by utilities to residential, commercial and industrial users is common in most developed countries, except for the United Kingdom where only about 30% of users are metered. In some developing countries metering is very high, such as in Chile where it stands at 96%, while in others it still remains low, such as in Argentina.

The share of residential water metering in selected cities in developing countries is as follows:

• 99% in Santiago de Chile (1998)
• 96% in Abidjan, Côte d'Ivoire (1987)
• 62% in cities in Guatemala (2000)
• 30% in Lima, Peru (1991)
• 28% in Kathmandu, Nepal (2001)
• 2% in Buenos Aires, Argentina (1992)

Nearly two-thirds of OECD countries meter more than 90% of single-family houses. A few are also expanding their metering of apartments (e.g. France and Germany).

Benefits

The benefits of metering are that:

• in conjunction with volumetric pricing it provides an incentive for water conservation,
• it helps to detect water leaks in the distribution network, thus providing a basis for the reduction of Non-revenue water;
• it is a precondition for quantity-targeting of water subsidies to the poor.

Costs

The costs of metering include:

• the investment costs to purchase and to install meters, as well as
• the recurrent costs to read meters and to issue bills based on consumption instead of bills based on monthly flat fees.

While the cost of purchasing residential meters is low (in the range of US$20 per meter), the total life cycle costs of metering are higher. For example, retrofitting apartments in large buildings with meters for every apartment can involve major plumbing work and can be very costly.

Problems

Problems associated with metering arise particularly in the case of intermittent supply, which is common in many developing countries. Sudden changes in pressure can damage meters, so that many meters in cities in developing countries are not functional. Also, some types of meters become less accurate as they age and under-register consumption thus leading to lower revenues, unless they are being replaced regularly. Many types of meters also register air flows, which can lead to over-registration of consumption, especially in systems with intermittent supply, when water supply is re-established and the incoming water pushes air through the meters.

Effect on consumption

There is disagreement as to the effect of metering and water pricing on water consumption. The price elasticity of metered water demand varies greatly depending on local conditions. The effect of volumetric water pricing on consumption tends to be higher if the water bill represents a significant share in household expenditures. There is evidence from the UK that there is an instant drop in consumption of some 10% when meters are being installed. In Hamburg, Germany, domestic water consumption for metered apartments (112 liter/capita/day) was 18% lower than for unmetered apartments (137 liter/capita/day) in 1992. The municipal utility Hamburger Wasserwerke GmbH had installed more than 40,000 water meters in individual apartments of older houses since 1985. All new apartments had to be metered by law. Previously there had been only a single meter for the entire house in multi-apartment houses.

See also

• Advanced Metering Infrastructure
• American Water Works Association
• Automated meter reading
• Drinking water
• Electricity Meter
• Flow measurement
• Gas meter
• Meter Data Management
• Public utility
• Utility submeter
• Water conservation

Further reading

• American Water Works Association Manual of Water Supply Practices Manual M6, Water Meters — Selection, Installation, Testing, and Maintenance, ISBN 1-58321-017-2
• American Water Works Association standards C700-02: Cold-Water Meters—Displacement Type, Bronze Main Case
• American Water Works Association standards C701-02: Cold-Water Meters—Turbine Type
• American Water Works Association standards C702-01: Cold-Water Meters—Compound Type
• American Water Works Association standards C703-96: Cold-Water Meters—Fire Service Type
• American Water Works Association standards C707-05: Encoder-Type Remote-Registration Systems for Cold-Water Meters
• American Water Works Association standards C708-05: Cold-Water Meters Multijet Type

External links

• Water Measurement Manual of the United States Bureau of Reclamation
• Utilimetrics - Non-profit industry trade association (Formerly known as the Automatic Meter Reading Association AMRA)
• How to read different size water meters (PDF file)
• How Meter Keeps Tab On The Water You Use, Popular Science, July 1950 very detailed article with good illustrations