Water is probably the easiest of all liquids to meter and consequently many flow technologies are available to accomplish the task. Usually the application will limit the choice; working pressure, temperature, flow range, required accuracy, price, how clean is the water, is power available etc.
Variable Area Flowmeters: A simple ‘float in a tube’. Probably the least expensive and least accurate of all water flow meters but eminently suitable where a visual indication of flow is all that is required. Care must be taken with regard to not exceeding working pressure and temperature limits … particularly in the case of plastic tube units. V/A flowmeters are available for flows as low as 1 cc/min up to 1000 l/min with accuracy 2-3 percent at best.
Mechanical Water Meters: Available from ¼” up to 6” bore to handle flow rates from a few litres per hour up to 600 m3/hr. Usually used where high accuracy is not required and price is a significant issue. Smaller water meters are manufactured in brass and the larger in epoxy coated cast iron. Versions are available for cold and hot water. Whilst usually just offered with a mechanical non-reset totaliser it is also possible to supply with a low speed pulse transmitter for remote totalising purposes. Water meters 2” and below have BSP union process connections whilst the larger bore sizes have flanged connections. Maximum pressure rating is 16 barg with cold water meters rated to 30/50 deg°C and hot water to 90/130 deg°C.
Turbine Flowmeters: Can be used in cases of higher working pressures up to 30000 psig, where higher accuracy is required, or hygienic approval is mandatory. Turbines don’t have the rangeability of mechanical water flowmeters, usually operating over a 10: turndown, but offer far higher accuracies in the region of 0.2 to 0.5%. They can also be coupled with secondary instrumentation to indicate flow rate and total as well as provide a current and/or pulse output.
Electromagnetic Flowmeters: Are the perfect choice where the metered water contains suspended particles which would prevent other flowmeters with rotating internals from operating satisfactorily. This type of flowmeter can even be used for handling slurries or water with significant solids content. Accuracy is as good as that offered by turbine flowmeters whilst the operating flow range is greater ( 20:1 or better). The price is higher than mechanical water meters or turbines but the added bonus of no internal moving parts often makes the ‘mag’ meter the water flowmeter of choice for clean or dirty water duties.
Ultrasonic Flowmeters: Probably provide the most expensive solution but in situations where the process pipe must not be broken into or where measurements are required at many points in the process the portable ultrasonic flowmeter is the best choice. Whilst the initial outlay may be high it could be far less that purchasing many in-line flowmeters. Ultrasonic flowmeters, whilst not in contact with the water, have no moving parts and so offer the bonus of being maintenance free. Whilst working pressure is not a factor care must be taken not to exceed the max temperature limit (usually +130 deg°C). Measurement accuracy is in the region of 1 to 2%.
Whilst operating constraints may push other flow technologies such as Coriolis mass, vortex shedding and positive displacement flowmeters to the fore it is not so common in the task of water measurement.