This section provides guidelines on the appropriate selection of water meters to help improve water management, as well as high-level information on the various considerations involved in determining what type of meter to select. These include:
- Metering technology options
- Installation considerations
- Relative costs to own and operate.
Managing water-efficient facilities and operations requires the application of water meters and timely data analysis. These actions provide critical data that allow facility managers to implement follow-on actions and develop a water balance that can help target water efficiency measures.
A key component of this approach is selecting water meters that best fit the specific application needs.
The Federal Energy Management Program (FEMP) developed the Federal Building Metering Guidance to provide federal agencies with direction on how to meet metering requirements in Section 103 of the Energy Policy Act of 2005 (EPAct 2005), as amended (and codified at 42 U.S.C. § 8253(e)). This document focuses on helping federal agencies prioritize metering for specific applications but does not provide information on how to select the appropriate meter. This section serves as a supplement to the guidance document by providing agencies with a selection guide.
Common Water Metering Technologies and Key Criteria for Selection
The following table provides a comparative summary of water flow meter types. Descriptions of the meters are found in the Water Metering Technology Options section below. Keep in mind that this table is a composite summary and that meter specifications and prices vary by manufacturer, model, and options/configuration.
|Water Flow Meter Types|
|Meter Operating Design||Positive Displacement||Differential Pressure||Velocity|
|Criteria*||Nutating Disk||Orifice||Venturi||Turbine||Vortex Shedding||Ultrasonic|
|Accuracy||Good||Moderate to Good||Good||Good||Good||Moderate to Good|
|Installation Ease||Easy||Easy||Moderate||Challenging||Moderate||Very Easy|
|Capital Cost**||$||$||$$||$$||$$||$$ - $$$|
*Hover over the blue items to see definitions.
**Capital, installed, and maintenance costs respectively are relative to each meter type.
Water Metering Technology Options
The following is an introductory summary of each type of meter presented in this guideline.
Positive Displacement Meters
Positive displacement meters mechanically displace water to measure the flow.
The nutating disk meter is a positive displacement meter that consists of a disk mounted on a spherically shaped head and housed in a measuring chamber. As the fluid flows through the meter passing on either side of the disk, it imparts a rocking or nutating motion to the disk. The motion is then transferred to a shaft mounted perpendicular to the disk. This shaft traces out a circular motion—transferring this action to a register that records flow.
Nutating disks are the most common meter technology used by water utilities to measure potable water consumption for service connections up to 3 inches. The meters typically have an accuracy range of 0.5% to 1.0%.
Key Points for Selection
Nutating disks are known to be easy to install and to have a relatively low cost.
Differential Pressure Meters
Differential pressure meters measure flow by measuring the pressure drop across the upstream and downstream portion of the meter.
The orifice meter is a differential pressure meter that uses an orifice element that is typically a thin, circular metal disk held between two flanges in the fluid stream. The center of the disk is drilled with a specific-size hole, depending on the expected fluid flow parameters (e.g., pressure and flow range). As the fluid flows through the orifice, the restriction creates a pressure differential upstream and downstream of the orifice proportional to the fluid flow rate. This differential is measured, and the flow rate is mathematically calculated based on the differential pressure and fluid temperature.
Orifice meters are commonly used on line sizes from 0.25 to 4 inches, and typically have accuracies ranging from 0.25% to 2%.
Key Points for Selection
Orifice meters, by design, develop significant pressure drop within the system. The benefits of this technology are its relative compactness, accuracy, and simple function, which should be considered against the potential pressure drop at the end use.
The venturi meter is a differential pressure meter that relies on the velocity-pressure relationship of flowing fluids where change in pressure is proportional to the square of the velocity. In this case, the device causing the change in pressure is a section of pipe that gently converges to a small-diameter area (called a throat) before diverging back to the full diameter.
Venturi meters are used mostly in specialty potable water applications where size, space, and/or accuracy dictate their use. They can be used on connection sizes ranging from 0.25 to 4 inches and have accuracies in the range of 0.25% to 2%, depending on the installation.
Key Points for Selection
The benefit of the venturi meter over the orifice meter (also a differential pressure meter) lies in the reduced pressure loss experienced by the fluid. In situations where the cost to pump water is high, this benefit can represent significant savings over the life of the system. These meters are also durable, so they do not suffer from potential erosion issues.
Velocity meters directly measure the velocity of the fluid.
The turbine meter is a velocity meter that has a multi-blade impellor-like device located in, and horizontal to, the fluid stream. As fluid passes through the turbine blades, the impeller rotates at a speed related to the fluid’s velocity. Blade speed can be sensed by several techniques, including magnetic pick-up, mechanical gears, and photocell. The pulses are generated by blade rotations directly proportional to fluid velocity, hence flow rate. There are a variety of turbine designs and mounting configurations.
Potable water applications for turbine meters are typically for larger industrial metering functions. Turbine meters are used on connection sizes ranging from 2 to 20 inches and have accuracies in the range of 0.5% to 1%.
Key Points for Selection
These meters can be susceptible to wear and resulting inaccuracies. For example, bearings can quickly wear and result in inaccuracy if they are exposed to corrosive or contaminated fluids.
Vortex Shedding Meter
The vortex shedding meter is a velocity meter that senses flow disturbances around a stationary body (called a shedder bar) positioned in the middle of the fluid stream. As fluid flows around the shedder bar, eddies or vortices are created downstream; the frequencies of these vortices are directly proportional to the fluid velocity.
Vortex shedding meters can be used on connection sizes of 1 to 12 inches and have accuracies in the 1% to 2% range.
Key Points for Selection
Because the vortex shedding meter has no moving parts, it is a very reliable method of potable water measurement.
Ultrasonic meters are completely non-intrusive. That is, this technology does not require any permanent modifications or penetrations to piping or disruption of service for installation. There are two main types of ultrasonic meters—Doppler and transit time.
In a Doppler ultrasonic meter, a transducer emits an ultrasonic beam into the fluid stream. Particles in the moving fluid shift the frequency of the beam and reflect it back to a second transducer. The frequency shift caused by the particles or bubbles in the fluid stream (known as the Doppler Effect) is read by the second transducer and is proportional to the flow rate. This technology works best when the fluid has suspended solids, bubbles, or other particles to reflect the ultrasonic signal.
In a transit time ultrasonic meter, transceivers send an ultrasonic pulse through the pipe and fluid. One pulse moves in the direction of the fluid and another pulse moves against the fluid. The difference in upstream and downstream time measurements to reach the opposing transceiver is used to calculate the fluid velocity and consequently the flow rate. Transit time meters need clean and viscous liquids for best accuracy.
These meters can be used on connection sizes up to 20 inches or larger and have accuracies ranging from 1% to 5%, although accuracy is highly dependent on calibration and proper maintenance.
Key Points for Selection
Ultrasonic meters mount on the outside of the piping and can be used as both temporary and permanent metering. Ultrasonic flow meters are very reliable because they have no moving parts. Ultrasonic meters can have a high purchase cost, but installation is usually easy.
Selecting the right water meter requires identifying and addressing the considerations unique to each application. Several of the primary considerations are called out below.
Each Meter Needs to Address a Known Objective
- Objectives can focus on operational improvements or support water management initiatives and goals
- Determining the objective for applying meters up front helps ensure that the proper meter performance requirements are identified. For example, water data recording in hourly or even 15-minute intervals is important if the objective is to identify operational improvement opportunities but is not important for tenant billing or goal tracking
- Operational improvements that reduce water use can be identified using analyzed metered data. Examples of resulting opportunities include understanding when and where water is used, and locating leaks, unnecessary off-hours consumption, or even cooling tower operating issues
- Water conservation programs and goals require metered data to monitor consumption and costs and review and report progress. Examples of metered water data applications supporting management purposes are tenant billing, goal tracking and benchmarking, and water bill verification.
Identify and Address the Physical Considerations for Each Meter’s Application
- Determine the expected range of water flow and pipe sizes
- Determine the accuracy requirements over the flow range
- Identify any physical installation requirements for meter location, straight lengths of pipe, available communications, and any other applicable requirements
- Communication interoperability—consider standardization on communication between meters and other data acquisition systems
- Determine how the data will be collected and processed. Does the metering equipment vendor offer this function or service? What effort is needed to create a process to collect, store, and archive the data?
Cost Considerations over the Meter Lifetime
- Capital costs to purchase the meters are based largely on the type of meter selected and its capabilities. Do not over-specify the meter’s performance requirements such as accuracy, resolution (the smallest increment in which a meter can detect a change, which can be useful when recording higher-frequency data such as five-minute time intervals), and data storage
- Installation costs not only include labor costs, but also may include significant materials costs and even limits on service interruptions that may require off-hour installations
- All meter types require monthly and annual inspections. Monthly inspections address operational observations such as leakage, noise, and vibration. Annual inspections address recalibration in general, and meter-type-specific items such as wear on orifice edges, wear and buildup on orifice meters, or wear and damage to the impellor blades on velocity meters
- Data analysis and data storage costs are components of the lifecycle cost for a water meter. For meters to be effective, the data must be analyzed in support of the original meter objective.
Ultimately, data security will be a function of the selected meter and site metering system’s communications system. Identify and involve an interdisciplinary team including operational technology and site information technology staff for issues of data security and system cybersecurity to ensure any solution is appropriate, relevant, and compatible with existing IT security systems.