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By Richard Lowrie
In large water distribution networks, flowmeters of more than 48 inches in diameter measure tremendous water volumes. Yet at these volumes, the smallest errors are magnified. Large diameter meters are a calibration challenge because of a paucity of suitable facilities. Different calibration and accreditation methods achieve and certify volumetric flow measuring accuracies in large-diameter flowmeters.
One of the best ways to ensure measurement accuracy is by calibrating the primary measuring device on a calibration rig that has been accredited by a national organization. This type accreditation constitutes an independent evaluation of conformity against recognized industry standards for that calibration rig. This should only be done by a recognized entity.
In the United States, the National Institute of Standards and Technology (NIST) oversees the National Voluntary Laboratory Accreditation Program (NVLAP) that provides third-party accreditation to testing and calibration laboratories. NVLAP accredited laboratories are assessed against the management and technical requirements published in the International Standard, ISO/IEC 17025:2005.
Accreditation to ISO/IEC 17025:2005 for calibration rigs is claimed by different parties, but it only certifies laboratory and personnel competence. It doesn’t actually accredit or certify the calibration rig itself.
Depending on location, actual calibration-rig accreditation comes from a country’s metrological organization, subject to oversight by an authoritative body. The International Laboratory Accreditation Cooperation (ILAC) was formed more than 30 years ago. In the United States, the relevant body is the American Association for Laboratory Accreditation (A2LA).
So how do you know if a measuring device is calibrated to the stated accuracy by an accredited calibration facility? To do so, verify that the flow meter manufacturer’s calibration facility accreditation is registered as a member of ILAC. Check that all available meter sizes have been calibrated.
Actual calibration methods
Most manufacturers use one of the two available calibration methods, either “master-meter” or “direct-volume comparison.” One additional method is “tower, or tank, calibration.”
A master meter is one that has had its measurement performance proven by a recognized standard for the purpose of use as a calibration device. These meters are highly accurate and stable, but must be periodically recalibrated to ensure performance remains valid. Calibration rigs using the master meter method can achieve accurate calibrations.
However, any uncertainties with the calibration rig are passed to the meters being calibrated. Also, to be truly effective, the master meter should be comparable in size to the meter under test. Calibrating an 84-inch meter with a master meter of 42 inches won’t provide the volumes or velocities needed for accurate calibration.
Direct-volume comparison is done using a prover, a tower, or a tank. A known volume of liquid is passed though a meter, recording meter output — usually a pulse-per-volume coun t— and comparing it to the known volume of the chamber used for calibration.
On a prover, flow is timed using high-accuracy switches. The first is activated upon the piston or ball passing, and the second is activated when the piston passes it. Meter flow is compared to the known volume of the prover chamber. A meter factor, or calibration factor, is developed. This calibration method is widely used and accepted. Provers must be calibrated (water drawn) each year. Uncertainty is less with a prover than a master meter because chamber volume is verified directly.
A final direct-volume method sometimes used is tower or tank calibration. Larger liquid volumes must be available for this method; scales may be used with lower liquid volumes with small-diameter meters. Like the piston prover, high-accuracy switches identify a precise volume and flow time between two tank or tower level points, which is compared to the total reported on the flow meter being calibrated. A meter, or calibration, factor is determined. Tank calibration involves the same methods but measurements are taken while filling the tank rather than by emptying it.
One flow meter supplier boasts towers as large as 144 feet tall and 13 feet in diameter for direct-volume calibration of large-diameter meters. These towers can flow over 130,000 gallons per minute of water for several minutes during calibration.
Large diameter challenges
Why is calibration important to a design engineer or an end user? In a nutshell, accuracy equals money, and especially so for large-diameter flowmeters.
A magnetic flowmeter is actually a velocity device, as it measures conductive fluid velocity passing through a magnetic field. As the conductive fluid travels through the travels through this field, a voltage proportional to the fluid velocity is created and measured at the electrodes located in the meter. Volumetric flow rate is determined by multiplying the fluid velocity by the cross-sectional area of the measured section. Most times, with large diameters, the meter’s size is selected to operate at about 10 to 12 feet per second for the maximum volumetric flow rate.
Magnetic flowmeter accuracy is normally stated as a function of measured value or rate of flow. For example, a small meter, with a stated accuracy of 0.5% and measuring a flow rate of 200 gpm, is expected to deliver accuracy within one gallon per minute. However, a large-diameter meter with that same stated accuracy, measuring a flow rate of 100 million gallons per day (mgd) is expected to be within 0.5 million gallons per day, or 34,700 gallons.
Calibration of a smaller diameter meter can be done at the expected maximum velocity. Confidence is therefore high through the full performance range.
However, for a large-diameter meter selected to measure flowing velocities between 7 to 12 feet per second, one could assume a flow rig with the ability to reach those velocities should be used for its calibration. However, if that meter can only be calibrated at best at 10% of that velocity range, would you have the same high degree of confidence in its performance?
Let’s consider a 72-inch flow meter with a maximum flow rate of 86,800 gallons per minute. This equates to a flowing velocity of 6.8 feet per second. This doesn’t sound like an unreasonable velocity for calibration, but you may be surprised to learn that most magnetic flow meter manufactures cannot achieve near this velocity on this size of meter.
As mentioned earlier, to produce a recognized calibration certificate, each flowmeter manufacturer’s calibration rig should be accredited by a recognized agency. The accreditation certificate will list the total volume and the sizes of the calibration rigs. The certificate also lists the verified uncertainty of the calibration rig.
A brief summary of the published accreditation certificates of several manufacturers of large diameter flowmeters is found in Table 1. They show the flowing velocities available to calibrate large-diameter meters. These figures are based on a 72-inch diameter meter.
Table 2 shows the largest diameter meter each manufacturer has calibration accreditation calibration for, along with the velocity which can be achieved. The rightmost column in this table shows flow rates for the largest common diameter from each manufacturer, and the velocities they have available for calibration.
When reviewing calibration-rig accreditation certificates, additional factors to consider are combined-measurement capability (CMC) or best-measurement capability (BMC) for each calibration rig. CMC or BMC are the percentage of uncertainty in which there is 95% confidence in the measurement. This statistical measure states that in more than 95% of occurrences, calibration will be within the stated certified accuracy.
Interestingly, Table 3 shows a wide variability in these stated factors.
The accreditation of the calibration rig on which the meter is calibrated should not be overlooked. If the calibration rig is accredited to be five or ten times better than the calibrated meter’s target accuracy, there can be higher confidence in that meter’s performance.
When selecting a large-diameter flowmeter, one should also consider the calibration rig. Rig accreditation should demonstrate that the meter is calibrated to address the complete flow measurement range, with a high degree of confidence, when installed, in line with recommended upstream and downstream piping straight runs.
Richard Lowrie is the Water and Wastewater industry manager at KROHNE, Inc.