How to Calibrate a Flow Meter & Calibration Procedures
March 18, 2019 Welcome
A flow meter is a test device used to measure the flow rate of process fluids and gases in industrial plants and facilities. The flow rate refers to the speed at which a process fluid is moving through pipelines, orifices, or vessels at a given time. Control and instrumentation engineers measure this value to monitor and regulate the speed and efficiency of industrial flow processes and devices.
Ideally, test devices must be ‘reset’ from time to time to prevent having inaccurate readings. For example, a bathroom scale that indicates 10lbs when nobody is standing on it needs to be recalibrated to indicate an initial zero value.
In industrial settings, flow meters are periodically calibrated to ensure that measurements are accurate allowing operations to proceed in a safe and timely manner.
What is Flow Meter Calibration?
Flow meter calibration is the process of comparing the pre-set scale or metering of a flow meter to a standard scale of measurement and adjusting its metering to conform to the standard. Calibration is an essential aspect of instrumentation in a broad range of industries that require high-accuracy measurements with a negligible percentage of error e.g. in Oil & Gas, Petrochemical, and manufacturing.
Flow meters are calibrated by comparing and adjusting their metering to correspond with a predefined standard. Flow meter manufacturers typically calibrate their products in-house after production or send them to an independent calibration facility for adjustment.
Flow Meter Calibration vs. Recalibration
Flow meter calibration involves comparing the measurements of a flow meter in operation to that of a standard flow measurement device under the same conditions and adjusting the scale of the flow meter to be in close agreement with the standard.
Flow meter recalibration involves calibration of a flow meter which is already in use. Periodic recalibration is essential as flow meter readings will often go ‘out of phase’ over time due to variable conditions involved in industrial processes.
The main difference between the two procedures is that flow calibration is done before the meter is sent out for use while recalibration is done after the flow meter has been in operation for a while. Software tools can also be used to verify the accuracy of measurements after a flow meter has been calibrated.
How to Calibrate a Flow Meter
Liquid flow meter calibration can be done in several ways, but always involves comparison and adjustment of the flow meter under test to conform to a standard. In the U.S., calibration obtained from the National Institute of Standards and Technology (NIST) is the standard, while for most of Europe, the calibration standard originates from the Van Swinden Laboratory in the Netherlands.
Some of the most widely-used flow meter calibration procedures are:
- Master Meter Calibration
- Gravimetric Calibration &
- Piston Prover Calibration
Master Meter Calibration Procedures
Master meter calibration compares the measurements of a flow meter under test to that of a calibrated flow meter or ‘master’ flow meter operating at the desired flow standard and adjusts its calibration accordingly. The master flow meter is usually a device whose calibration is set to a national or international standard.
To perform Master meter calibration:
- Place the master meter in series with the flow meter under test.
- Compare the readings of the master flow meter and flow meter using a measured volume of liquid.
- Calibrate the flowmeter under test to conform with the master flow meter calibration.
Gravimetric Calibration Procedures
Gravimetric calibration is one of the most accurate and cost-effective volumetric and mass flow meter calibration procedures. The gravimetric method is ideal for liquid flowmeter calibration in oil, water purification, and petrochemical industries.
To perform a gravimetric calibration:
- Place an aliquot (small portion) of process fluid in a test meter and weigh it for a precise amount of time while it flows 60 seconds.
- Use a calibrated scale to obtain an accurate measurement of the weight of test fluid.
- After the test period is completed, divert the test fluid into a discharge container.
- Obtain the flow rate of the aliquot by dividing its volumetric weight by the test duration.
- Compare the calculated flow rate to the flow rate of the flow meter and adjust it to the measured flow rate.
Piston Prover Calibration Procedures
In the piston prover flow meter calibration procedure, a known volume of fluid is forced through a flow meter under test. The piston prover is a cylindrical device with a known internal diameter.
The piston prover contains a piston which produces volumetric flow by positive displacement. The piston prover method is ideal for ultrasonic flow meter calibration, fuel flow meter calibration, and turbine flow meter calibration to a high degree of accuracy.
To perform a piston prover calibration:
- Place an aliquot of process fluid in the piston prover and flow meter under test.
- Obtain the volume of fluid that was displaced in the piston prover by multiplying its internal diameter by the length traveled by the piston.
- Compare this value to the measurement obtained from your flow meter and adjust your flow meter’s calibration accordingly.
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