This invention concerns liquid metering, and in particular the invention concerns a method of measuring the volumetric flow rate of liquid flowing along a path, and also concerns a liquid meter to measure the volumetric flow rate of liquid flowing along a path.
The liquid may be water.
Conventional water meters use a variety of technologies. They may use turbines or other rotating elements that are responsive to the volume of water flowing and possessing a known characteristic relating flow rate to the turbine rotational speed. Or they may use ultrasonic velocity measurement, where the "time of flight" between the transmission and reception of an ultrasound pulse is related to the velocity of the flowing water. Or they may use electromagnetic velocity measurement, where the average velocity of passing magnetic particles or ions suspended in the water is related to a bulk flow measurement.
Existing low cost flow meters, particularly those fitted to measure domestic water consumption, suffer from a number of deficiencies, including poor reliability and poor accuracy.
The reliability problem is mainly due to the susceptibility of existing technologies to contaminants being deposited from the water supply over a long period. Turbine meters are particularly susceptible to scaling, affecting bearing/spindle friction, this problem being more likely to affect the calibration for low flow rates.
However the electromagnetic meters can also be affected by the use of very clean, de-ionised water, where the measurement principle does not operate at all.
Poor accuracies can be caused partly by contaminants, partly by poor installation and partly by the particular technology used. scaling will affect the calibration of turbine, ultrasonic and electromagnetic meters, due to the change in the dimensions of flow channels, which in turn affects the velocities of the fluids for a given volumetric flow rate.
The characteristics of the turbine impeller will also be altered by scale build up. Installation errors can affect the accuracy of all water meters that measure velocity. Particular care needs to be exercised to ensure that sufficient lengths of straight pipe are installed both upstream and downstream of the meter.
Limited operational range of flow measurement provided by many conventional water meters can cause large errors, particularly where meters are oversized for the particular installation or where the actual flow rates vary over a wide range.
For acceptance and reliability, high costs must be paid. It has been estimated that for a utility water meter with +or -1% it accuracy, similar to that provided by a gas or electricity meter, the price is between five and ten times as much. Even with such an investment, long term reliability is unlikely to be as good as that provided by either gas or electricity meters. Experience of poor meter reliability is indicated by the requirement in Germany that all water meters must by removed from service every five years for re-calibration and servicing if they are being used for billing purposes.
One object of the invention is to provide a method of measuring volumetric flow rate of a liquid, which if the liquid should be water, avoids the above mentioned disadvantages. Another object is to provide a liquid meter which if used to measure the volumetric flow rate of water avoids the above mentioned disadvantages.