Precision Meters: Water Meter Selection Options.

Water Meters:

An important factor in quality water treatment is the use of the correct water meter.

Before the introduction of water meters, water users had little or no understanding of the implications of water usage effect to both upstream resources and downstream wastage.

• Approximately two thirds of the world is covered with water.

• There is no more water on earth now than there was millions of years ago.

• Of the two thirds of water, only about 3% is fresh water fit for consumption.

• Population expansion is placing an ever increasing burden on the 3% of fresh water.

• Water quality improvement measures have to be expanded to reduce fresh water loss.

• Conservation requirements of fresh water sources is at critical levels.

Water management is now being addressed in a more meaningful manner with the advent of a range of meters that, if correctly selected and installed, increase user understanding of their water consumption in meaningful terms.

Although there are many different ways to measure water volumes used, three meter types have become the dominant metering method for many years.

• Volumetric – using positive displacement.

• Inferential – using turbine/propeller rotation.

• Electronic – using magnetic differentiation.

The bulk of water metering is generally done by smaller volumetric – positive displacement – meters ranging in pipe bore sizes from 13mm to 25mm with some being as large as 50mm.

For larger sized pipe bores the Inferential – turbine/propeller – meters are used with sizes up 30cm.

Electronic cold water meters range in pipe bore sizes from 13mm to 60cm and cover a wide range of applications such as bulk potable water mains, water vending/treatment by filtration, chemical, Ultraviolet (UV) or ozone (O3).

Volumetric – Positive Displacement (PD) – meter:

The components and meter body of Volumetric (PD) meters may be manufactured from Plastic, Bronze or Brass and are suited to applications that require direct reading of the total amount of water that passes through them.

The main operating principal is an oscillating piston with each piston revolution being equivalent to a known volume of water. Volume is measured by counting the number of rotations of the piston as it discharges and fills the fixed volume chamber. The volume flow rate is found by counting the number of revolutions of the piston over a given time span. e.g. 1 minute.

An advantage is that Volumetric meters are able to be installed at any inclination except upside down.

The piston rotations are transferred via a magnetic coupling to a geared direct read register or counter with each rotation resulting in a count being displayed numerically.

Thus by taking a note of the numerical sequence on a given day and then comparing this against a second reading in a given time period such as 7 days, the difference between the two readings will be equal to the amount of water units that has passed through the meter during that time.

Positive displacement meters can be installed in any inclination except upside down.

The meters are also available with an electronic pulser that is either low speed for remote counters or batch controllers or high speed for more accuracy.

The pulser transmits each piston rotation either via cable or wireless to remote data loggers or computers thus enabling remote metering of either individual or clusters of meters and thereby centralize the metering process.

The electronic pulsers are also used in applications such as water vending where a push button or coin actuates the vending sequence which then dispenses a measured amount of water to the purchaser/activator.

Inferential – Horizontal Woltman type turbine – meter:

These meters tend to be used where larger volumes of water needs to be measured and are thus made in larger diameter bore sizes, typically 40mm to 300mm and are usually manufactured with a cast bronze or cast iron body with coupling flanges.

Water flowing through the meter drives a rotor. The blades of the rotor rotate according to the amount of water passing over them, with each rotation representing a given amount of water. The rotor shaft is coupled to a register that records the number of revolutions with each being equal to a given volume of water.

As with other water meters, by comparing the difference in the registered values over a given time period, the amount of water that has moved during that period is determined.

Unlike the Volumetric (PD) meters these meters need to be installed horizontally and to have a minimum of 10 x their bore diameter of straight pipe before the meter and a minimum of 5 x their bore diameter after the meter so as to minimize turbulence that would affect the laminar flow over the rotor blades and thus induce inaccuracy.

These meters are also available with an electronic pulser that is either low speed for remote counters or batch controllers or high speed for remote data loggers or computers.

Electronic – magnetic – meter:

These meters operate according to magnetic induction – Faraday’s principal – and typically range in sizes from 50mm to 600mm.

They have low head loss, no noise, no moving parts and maintain stable calibration over extended service periods.

Faraday’s principle:

A conductor – in this instance water – moving through a magnetic field – induced by the electronics of the meter – will induce an electric current proportional to the velocity of the conductor – water.

By measuring the value of the induced electric current, the water velocity is derived. A given velocity in a known pipe bore size will result in the volume of water that is flowing through the pipe being calculated.

These meters are also available with an electronic pulser that is either low speed for remote counters or batch controllers or high speed for remote data loggers or computers.

Electronic meters need to have a minimum of 3 x their bore diameter of straight pipe before the meter and a minimum of 2 x their bore diameter after for optimum accuracy and are NOT to be used in Reverse Osmosis or De-ionization installations.

Precision Meters: Water Meter Selection Options.

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