Anniina Kittilä, ETH Zürich, Institute of Geophysics, Geothermische Energie u. Geofluide. Sonneggstrasse 5, 8092 Zürich, Switzerland e-mail: anniina.kittila(at)erdw.ethz.ch
Water meters are common especially in residential and commercial water supply networks, but industrial usage is also possible due to the wide variety of different water meters. The types of the water meters can be divided into four groups: i) displacement water meters, ii) velocity water meters, iii) electromagnetic meters, and iv) ultrasonic meters. Displacement meters are only usable when the flow of extremely good quality water is measured, for example in potable water supply (Delta Flowtech 2015), so they are not discussed here. There are many different devices that are velocity water meters, but function based on slightly different technique. The most common ones are single jet and multi jet meters, but the latter is more recommended as it has longer life and it retains accuracy better (Delta Flowtech 2015). The mechanism of multi jet meter is briefly described. Another velocity based meter is current meter, explained in another section of this site. Separate from these are the electromagnetic and ultrasonic flow meters. The electromagnetic meters are based on a production of a voltage when a conductor moves through a magnetic field, and the theory behind the ultrasonic meters is that the frequency of a reflected signal is modified by the velocity and direction of the fluid flow (Mustonen 1986, Anonymous 2001).
Description of the method
The principle of multi jet water meter is to force inflowing water through many ports in a capsule to a turbine. The flow of water through these ports generates multiple symmetrical jets that impact the turbine. The rotation of the turbine is proportional to the velocity of the water flow, and can be monitored through a reading mechanism (B Meters 2015).
Electromagnetic meters require a nonmagnetic and nonelectrical pipe through which the water flows. As conductive liquid flows through the electromagnetic meter, it induces a voltage in the magnetic field of the meter. The magnetic field is created perpendicular to the direction of the flow with magnetic coils on each side of the pipe, and the voltage is measured with two electrodes in each side of the pipe. An imaginary line connecting the electrodes would be perpendicular both to the direction of the flow and the magnetic field. The magnitude of the induced voltage is proportional to the magnetic flux density and velocity of the flowing water (Mustonen 1986, Anonymous 2001).
The function of an ultrasonic water meter is usually based on transit-time method. The principle of this method is that acoustic (ultrasonic) waves in flowing water move faster in the direction of the flow and slower against the flow. If acoustic waves are sent diagonally both upstream and downstream, the transit-time between these waves is proportional to the average flow velocity along their paths. The average velocity can then be calculated, knowing the path angle with respect to the direction of the flow (Mustonen 1986, Anonymous 2001).
Appropriate applications and performance
Suspended solids and deposition can significantly affect water meters, particularly mechanical ones, like multi jet water meters. For example, deposition of the turbine may cause over registration at medium and high flows, but under registration at low flows. Deposition can also grow over time to an extent that it prevents the turbine from rotating. At low flow conditions the friction can also overcome the energy of the flow to move the turbine, which can be seen in typical accuracy curves for different meters. The sizing of a multi jet water meter is thus very important, also because a water meter often functioning at high flows will degrade faster than one that works below nominal flowrate (Arregui et al. 2005). A recommendation is that water meters should be sized between transitional flow (the point at which the meter is accurate) and nominal flow (everyday flow rate). The minimum flow values given for each device only refer to the minimum flow required to operate the meter, and with that flow rate the meters will not be accurate (Delta Flowtech 2015).
The activation of the magnetic field in electromagnetic water meters requires a source of electrical power, and the major disadvantage is the high initial costs of constructing a measurement station, particularly for open channels (Mustonen 1986). Usage in pipe structures, however, is very popular because of its high accuracy and competitive prices (Mustonen 1986, Anonymous 2001). The accuracy can typically be of 0.2-3% (Wolkersdorfer 2008). The method needs sufficiently conductive water, but other properties such as temperature, viscosity, density or solid particles do not affect the accuracy, so it is also suitable in measuring difficult and corrosive liquids. The surface of the electrodes, however, should be kept clean because precipitation on them can cause errors to the measurements (Anonymous 2001, Wolkersdorfer 2008).
The ultrasonic flow measurements are very accurate, although the initial cost might be relatively high. Maintenance also requires electronic technician and the meter must be programmed for each pipeline material, diameter and wall thickness (Anonymous 2001). Additionally, inhomogeneity and impurities, such as solid content and air bubbles in flowing water affect the acoustic signal strength. The advantages of the method include: good repeatability, bidirectional flow measurement capability, possibility for continuous measurements, nonintrusive (no head loss), usually no field calibrations needed, and the absence of moving parts (Mustonen 1986, Anonymous 2001).
Anonymous 2001. Water measurement manual. United States Department of the Interior, Bureau of Raclamation, Water Resources Research Laboratory, 317 p.
Arregui, F., Cabrera, E. Jr., Cobacho, R. & Garcia, J. 2005. Key factors affecting water meter accuracy. Leakage 2005, Specialised Conference of the IWA, 10 p.
B Meters 2015. Residential water meters, Multi jet. Site visited 17.4.2015. http://www.bmeters.com/en/multi-jet-water-meter.htm
Delta Flowtech 2015. Water meters FAQ. Site visited 17.4.2015. http://www.deltaflowtech.com/categories/water-meters-faq.aspx
Mustonen, S. 1986. Sovellettu hydrologia. Vesiyhdistys r.y., Helsinki, 436 p. (In Finnish)
Wolkersdorfer, C. 2008. Water Management at Abandoned Flooded Underground Mines. Springer, 465 p.