Comparing Laboratory and Bench Calibration for HVT Venturi Meters
A measurement system is designated valid if it is both accurate and precise – and when it comes to the importance of metering water/wastewater facilities, these critical requirements cannot be understated. Venturi meters are most arguably one of the more precise measurement systems in existence; and therefore, the equipment and methods used to calibrate Venturi meters must be more precise than the metering element that is being tested.
Venturi meters are known for their longevity, high accuracy, and dependability in a wide variety of measurement options in piped systems for liquids, gas, steam, and mixed media. Since Venturi meters have no moving parts they can function reliably and accurately for decades, and when appropriately calibrated, can provide accurate, traceable, and defensible measurements for more than 100 years of service time.
Importance of Calibration for Accuracy and Precision
Measurement calibration must be executed by trained engineers and technicians, who follow written procedures, often called a calibration plan, and utilize calibration equipment that must be more precise than the metering element that is being tested. For Venturi meters, there are two options available: bench calibration and laboratory calibration. Although each method has its benefits, every meter should be bench calibrated; whereas laboratory calibration ensures an extra level of traceable accuracy for even the most precise metering requirements. Note that the PFS-HVT modified short form venturi meter accuracy is documented to be +/-0.5% of actual rate of flow for “bench calibrated (non-wet flow calibrated) accuracy while the laboratory flow calibrated accuracy is +/-0.25% of actual rate of flow. The bench calibrated accuracy is based on a minimum pipe Reynolds number condition of 75,000.
Bench Calibration
Venturis, if engineered and manufactured correctly, will produce the same differential pressure at the same flow rate for decades; however, to determine the meter’s accuracy, it must be geometrically correct. The Venturi meter shape guides the flow within its boundaries so that the geometry of the flow becomes the same as the metering shape of the tube. To this end, the metering shape can be accurately measured and inspected on the bench and will undergo strict calibration testing to ensure every angle, cross section, and plane is up to standards.
Venturi meters do not require laboratory flow calibration to verify accuracy, as long as the manufacturer has a well-documented quality control procedure, supported by independent calibration results of previously tested Venturis of similar design that establishes the value and stability of the discharge coefficient. This is why bench calibrations are vital, as they ensure the accuracy and the integrity of each meter. The essence of a documented “bench calibration” process is built on the foundation of prior venturi meter laboratory flow calibrations, of the same design, which result in what is called a “twice standard deviation” mathematical analysis which supports the assignment of the meter factor (discharge coefficient) that is assigned to a new meter which is not laboratory flow calibrated. In the case of the PFS-HVT venturi meter, the standard discharge coefficient has been proven to be 0.9900 +/-0.41%. This means that a new HVT that is “bench calibrated” will be assigned a “C” value of 0.9900 as long as it was built according to the same geometry, surface finish, tap locations and quality, as the previously lab calibrated meter’s that form the “twice standard deviation” analysis.
Once the new HVT has been built, critical measurements with a variety of certified measurement devices are recorded, the correct shape and geometry has been thoroughly documented, and its surface finish has been properly verified, the Venturi is considered “bench calibrated” and will carry an assigned discharge coefficient of 0.9900 and a certified accuracy of +/-0.5% of actual rate of flow down to 75,000 pipe Reynolds number.
However, for applications that require a greater level of documented accuracy, the meter can be sent to a certified flow laboratory, which will ensure that a higher standard of accuracy is documented which also takes into account specialized flow calibration plans that may be requested by the customer or end user.
Laboratory Calibration
Laboratory calibration is used to obtain the accuracy of a flow meter by comparing its measurement performance against a known value, which is provided by a reference device, such as a weight tank or master flow meter.
There are several methods for the laboratory to utilize to calibrate a venturi meter. One choice would be: The meter that requires calibration is placed in a test section and is supplied with a constant water flow from a steady head source. Over a timed period, this flow is collected in one (or multiple) weighting tanks which measures the mass increase with calibrated “load cells”. A special diverter with a very short switchover time directs and stops the flow into the calibration tank intermittently. The flow rate is determined from the measured mass increase, filling time of the calibration tank, and water density. This procedure is carried out for different flow rates to cover the full operating range of the tested meter, and is repeated once for every flow rate requested in the calibration plan.
It’s important to note that laboratory performance can vary significantly from one lab to the next, depending on the configuration of the laboratory and the experience of those who operate it. The laboratory method itself must also be held to certification standards, such as NIST and ISO. Meter accuracy or measurement uncertainty is almost always of paramount importance when a meter is to be flow calibrated. Most laboratory operators define their facility measurement performance in terms of total measurement uncertainty – which is the upper limit of measurement error.
Conclusions:
The modified short form Venturi meter remains as the most reliable and arguably one of the most accurate flow metering devices in existence. They are versatile in that their laying length can be changed to fit a defined space; they can be modified to provide rate of flow control or measure sewage; they can be used reliably for billing or custody transfer; and they can be used for rectangular or circular metering. Built to specific standards, the Venturi meter can deliver a standard and traceable accuracy by meeting the qualifications of a thorough bench calibration test that measures and confirms the internal geometry of the meter. However, if deemed necessary independent laboratory flow calibrations are also recommended when it’s required to: define and quantify accuracy of an existing installation; develop flow calibration plan that will be useful to the end-user; provide a new operation and maintenance manual for the system and the new calibrated certified meter; and/or provide flow calculations for flow elements that are no longer supported by their original manufacturers. Finally, for billing meter applications, custody transfer or compliance applications, a laboratory flow calibrated venturi meter will provide maximum defendability should its readings be challenged.