Flow, level & control
Factors that determine flow meter performance
Here, Titan Enterprises discusses the factors involved in determining the overall performance of pulse output turbine flow meters and how each factor influences the accuracy of flow measurement.
F
low meters have become essential measuring devices used in a vast array of different industries. From commercial
applications measuring the fuel feeding industrial boilers, for flow monitoring of lubricant supply lines for wind turbines, to dosing chemicals for agricultural sprays. Domestically, flow meters have also become
intrinsic to our lives. Installed to manage the dispensing of beer or coffee, flow meters are an integral component in bar taps, and are built into washing machines and dishwashers to meter efficient water usage.
TurbinE Flow METErs Turbine flow meters are invaluable measurement tools and their repeatability and linearity are the key elements which determine the flowmeter’s performance and overall accuracy. The linearity, repeatability and ultimately accuracy, of the turbine flow meter depend on several variables including pressure, temperature, density, friction and compressibility of the fluid. As such, both the mechanical properties of the flow meter and the physical properties of the fluid combine to influence the performance of the flow measurement device. Turbine flow meters can be either radial or
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axial where liquid passing through the flow device causes the turbine to rotate. As the blades of the turbine rotate, they pass a sensor and are detected either optically or, more frequently, magnetically, and converted into electronic pulses. The pulse frequency (Hz) produced is proportional to the flow velocity. In the industry, a number of terms are used
when looking at the operation of a flowmeter. The following descriptions aim to put the terms into an accessible format and should not be read as the strict definitions as stated in published standards (ISO 11631:1998(en) Measurement of fluid flow — Methods of specifying flowmeter performance).
THE K-FacTor The K-factor is the number of pulses per volume (by litres) calculated by dividing the frequency of electronic pulses by the flow rate. The K-factor of a turbine flowmeter is unique to that individual device, although batch produced meters are likely to have very similar, although not identical, K-factors. When the flow meter is calibrated at specific
flow rate and flow range values (as determined by its design), the required repeatability and linearity are measured to give the K-factor for each point of measurement. The calibration
certificate for that specific flow meter records the actual performance expected under ideal installation conditions. To illustrate, a turbine flow meter may have a
flow rate of two litres per second (120 l/min) and give an output frequency of 2,000 Hz. This means that 1,000 pulses are generated for each litre of fluid that is measured. Titan will typically calibrate at different flows across the meters entire flow range to determine the best overall K-factor for that particular flow device. This K- factor is used for the monitoring instrumentation, such as Titan’s Pulsite Solo display, and the repeatability and linearity are calculated by this calibration for the flow range. Electronic flow meters such as ultrasonic, electromagnetic or Coriolis flow devices are usually programmable with a preferred K-factor to suit the application. As an example, Titan’s Atrato and Metraflow range of ultrasonic flow sensors are able to accurately measure to one per cent of reading with operator configurable K-factors to 10KHz.
calibraTion uncErTainTy Calibration uncertainty is the total uncertainty of the reference measuring equipment used to calibrate a device, such as a flow meter. How
October 2021 Instrumentation Monthly
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