3rd March 2020
How to Calibrate a Flowmeter?
A flowmeter is an instrument that is used to measure the flow rate of a gas or fluid and then display the flow reading to the instrument user via an indicator, analogue or digital output. In general flowmeters provide crucial measurement data to industry types, from pharmaceutical to Oil & Gas. The flowmeters types discussed here refer to closed pipe type flowmeters and not open channel systems. The flow rate refers to the average velocity at which a gas or fluid moves through the pipe. To ensure the flow meter indicates or outputs a reliable and accurate reading, the flowmeter must be calibrated on a regular basis.
What is Flow Meter Calibration?
Calibration is the process of checking an instrument to check it is accurate. Flowmeter calibration is the same process where the measurement is flow. Calibration of a flowmeter can be achieved in one of several ways, but generally involves the comparison of the flowmeter against a reference standard of higher accuracy. Calibration is an essential part of an instrumentation system to ensure a line of traceability of the measurement system is maintained. To understand the flowmeter performance over a period of time, flowmeter calibration should always be conducted on “as Found” basis so meter drift analysis can be assessed. With further customer agreement, a further adjustment of the flow meter reading maybe performed to minimise the flowmeter errors.
Flowmeter Calibration Procedures?
In this series of discussions, we will attempt the describe the various flowmeter methods and procedures associated with liquid flowmeters. Later topics will discuss the calibration methods for air and gas flowmeters. Liquid flow meter calibration can be done in several ways, but always involves the comparison against a standard, this maybe a primary or secondary reference standard, depending upon the accuracy required. The three main calibration principles for fluid flowmeter calibration use Gravimetric, Volume or transfer standard calibration methods.
Gravimetric Calibration
Gravimetric calibration is one of the most accurate methods for flowmeter calibration both for volumetric and mass based flow calibration and is classed as a primary standard calibration. The gravimetric method is ideal for liquid flowmeter calibrations on oil and water based flowmeters where they can be sent to an external calibration laboratory. The flowmeter calibration procedure involves the diversion of the fluid onto a mass comparator over a given timescale, where the basic mass flow rate measurement is derived by mass over time calculation. This is a simple process often referred to as the bucket and stopwatch method. Even though this is a simple procedural method, to achieve measurement accuracies suitable for the calibration for high accuracy and transfer standard reference meters a significant effort is required to understand and minimise the measurement uncertainty contributors. To attain a volume based flow measurement the fluid density needs to be measured.
Piston Prover Calibration
Piston prover flow meter calibration can also achieve high accuracy results accurate results and is available to a wider customer base due to the easier portability of the piston provers. The calibration method is to drive a known volume of fluid through the flowmeter at a given rate. The volume flow rate is derived from the volume of fluid displaced by the piston multiplied by the piston travel length over the measured time period.
Transfer Standard Master Meter Calibration
A transfer standard calibration uses a reference grade or master flowmeter and compares the reference flow against the flowmeter under test whist operating at the same conditions. It is important to understand any differences in the operating conditions due to the placement of another meter in the test line and any changes to temperature or pressure within the fluid. Reference meters should also be of highest quality and performance and have a detailed history of performance. Master flowmeters are almost often calibrated to an accredited ISO 17025 standard via an approved body such as NIST, UKAS, DaKKs, COFRAC, etc.
Young Calibration Ltd, UK
Contact: ay@youngcalibration.co.uk
Useful Flow Calibration References
Water Meter Calibration: https://www.youngcalibration.co.uk/calibration/water-flow-calibration
Oil Flow Calibration: https://www.youngcalibration.co.uk/calibration/oil-flow-calibration
Air Flow Calibration: https://www.youngcalibration.co.uk/calibration/airflow-calibration
Air Velocity Calibration: https://www.youngcalibration.co.uk/calibration/air-velocity-calibration
