Water. Desalination + reuse
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TECHNOLOGY | 34 | Desalination & Water Reuse | May-June 2015 Table 3. Published calibration rig accreditation CMC factors. reach those velocities should be used for that meter's calibration. However, if that meter can only be calibrated at 10-20% of its maximum velocity range, confdence in the calibration cannot be high. For example: a 72 inch (1.85 m) fow meter with a maximum fow rate of 625 Ml/d equates to a fowing velocity of 6.8 fps (about 2 m/s). But most magnetic fow meters cannot achieve near this velocity on this size of meter. DEmONsTraTED mEasurEmENT uNCErTaiNTY When reviewing calibration rig accreditation certifcates, it is not just the mean percentage accuracy that is important – the tightness of the spread of measurements too is a vital consideration. The Combined Measurement Capability (CMC) – formerly known as the Best Measurement Capability – of each calibration rig is defned by UKAS as expressing "the lowest uncertainty of measurement that could be achieved during a calibration". Alternatively it can be viewed as showing the percentage of uncertainty in there being 95% confdence in the measurement. Table 3 shows a wide variability in CMC factors. Clearly the accreditation of the rig on which the meter is to be calibrated should not be overlooked. If the calibration rig is accredited to be fve or ten times better than the target accuracy of the calibrated meter, confdence in that meter's performance can be high. Manufacturer A B C* D** E F CMC (%) 0.17 Unknown 0.02 0.02 0.05 0.15 Calibration methods There are chiefy two calibration methods used by most manufacturers. They are the master meter method and the direct volume comparison method. masTEr mETEr mETHOD A master meter is a meter which has had its measurement performance proven by a recognized standard for the purpose of being used as a calibration device. Normally, these meters are highly accurate and stable but must be verifed and periodically recalibrated to ensure their performance remains valid. Calibration rigs using the master meter method can achieve accurate calibration but there are uncertainties with this method. Clearly any uncertainties with the calibration rig will be passed along to the meters they are calibrating. Also, to be truly effective the master meter should be comparable in size to the meter under test. For example, calibrating an 84 inch (210 cm) meter with a master meter of 42 inches (105 cm) cannot achieve ample volumes or velocities for calibration. DirECT vOLumE COmparisON Direct volume comparison involves passing a known volume of liquid though a meter, recording the meter's output (usually a pulse per volume count), and comparing it to the known volume of a chamber used for the calibration. The chamber can be a prover, a tower, or a tank. With a prover, the fow is timed by use of high-accuracy switches. The frst switch is activated upon the piston or ball passing and the second switch is activated when the piston passes it. Measurement of the meter's fow is compared to the known volume of the prover chamber and a meter factor, or a calibration factor is developed. This calibration method is widely used and accepted. Provers must be calibrated each year by the water they draw. Uncertainties in the measurements taken using a prover are normally lower than those in master meters because the volume of the chamber is verifed directly. The tower or tank calibration version of direct volume comparison is usually used for calibration of high volumes of liquid although it can also be scaled down for use with very low liquid volumes for small diameter meters. Like in the piston prover, high accuracy switches are used to identify the precisely known volume and fowing time between two points of level in the tank or tower. This volume and fow rate is compared to the total reported on the fow meter being calibrated and a meter factor, or calibration factor is determined. Tank calibrations utilize the same methods but measurements between the switches are recorded while flling the tank rather than by emptying it. Some manufacturers including Krohne have towers as large as 44 m tall and 4 m in diameter for direct volume calibration of large diameter meters. These towers can fow over 500,000 l of water over several minutes during calibration. Each Krohne fowmeter is wet-calibrated in a direct comparison of volumes, which is by far the most accurate calibration method. Such calibration stations – like Krohne's facilities in the Netherlands and China – are typically fve times more accurate than the fowmeters to be tested. CONfiDENCE iN CaLibraTiON The rig used to calibrate a large diameter fow meter is a vital consideration when choosing such a meter. Failing to take due account of the accreditation of the rig to ensure it demonstrates that the meter is verifed and calibrated to address the complete fow measurement range with a high degree of confdence could result in signifcant revenue losses that are likely only to grow in signifcance as water stress becomes more prevalent. l No comparison: Krohne fowmeters are calibrated by direct comparison of volumes – by far the most accurate method.