A flow meter is a test device used to measure the flow rate of process fluids and gases in industrial plants and facilities. Flow rate is the rate at which a process fluid passes through a pipe, orifice or vessel at a given time. Control and instrumentation engineers measure this value to monitor and regulate the speed and efficiency of industrial processes and equipment.
Flow meters need to be calibrated at regular intervals, which means that their performance must be compared to a standard, and deviations of the flow meter from that standard must be determined and corrected.
In industrial environments, flow meters are calibrated periodically to ensure accurate measurements so that operations can be performed in a safe and timely manner.
What is flow meter calibration?
Flow meter calibration is the process of comparing a flowmeter’s preset scale or measurement to a standard measurement scale and adjusting its measurement to conform to the standard. Calibration is an important aspect of instrumentation in a wide range of industries such as oil and gas, petrochemical and manufacturing that require high precision measurements with negligible percent error.
Flow meters are calibrated by comparing and adjusting their measurements to meet predefined standards. Flowmeter manufacturers typically calibrate their products in-house after production or send them to an independent calibration facility for adjustment.
Flowmeter calibration and recalibration
Flowmeter calibration involves comparing the measured values of a flowmeter with those of a standard flow measurement device under identical conditions and adjusting the scale of the flowmeter to bring it close to the standard.
Flowmeter recalibration involves calibrating a flowmeter that is already in use. Periodic recalibration is essential because flowmeter readings are often “out of phase” over time due to the variable conditions involved in industrial processes.
The main difference between these two procedures is that flow calibration is performed before the meter leaves the factory, while recalibration is performed after the meter has been in operation for a period of time. Software tools can also be used to verify the accuracy of the measurement after the meter has been calibrated.
How to calibrate the flow meter
Liquid flow meter calibration can be accomplished in a number of ways, but always involves comparing and adjusting the meter under test to meet the standard.
Some of the most widely used flowmeter calibration procedures are:
- Master meter calibration
- Weight calibration and
- Piston calibrator calibration
Master meter calibration procedure
Master meter calibration compares the measured value of the meter under test with the measured value of a calibrated meter or “master” meter operating at the desired flow standard and adjusts its calibration accordingly. A master meter is typically a device whose calibration is set to a national or international standard.
To perform a master meter calibration.
- Connect the master meter in series with the meter under test.
- Compare the readings of the master meter and the meter using the measured volume of liquid.
- Calibrate the meter under test to match the master meter calibration.
Weight calibration procedures
Weight calibration is one of the most accurate and cost effective volumetric and mass flow meter calibration procedures. The weight method is ideal for liquid flowmeter calibration in the petroleum, water purification and petrochemical industries.
To perform a weight calibration.
- Place an aliquot (small portion) of process fluid into the test meter and weigh it for an exact amount of time while it flows for 60 seconds.
- Use a calibrated scale to accurately measure the weight of the test fluid.
- At the end of the test period, transfer the test fluid to the discharge vessel.
- Obtain the flow rate of the aliquot by dividing its volumetric weight by the test duration.
- The calculated flow rate is compared to the flow rate of the flow meter and adjusted to the measured flow rate.
Piston calibrator calibration procedure
During the piston calibrator flow meter calibration process, a known volume of fluid is forced through the flow meter under test. The piston verifier is a cylindrical device with a known internal diameter.
The piston tester contains a piston that produces a volumetric flow rate through a positive displacement. The piston calibration method is well suited for high-precision ultrasonic flow meter calibration, fuel flow meter calibration, and turbine flow meter calibration.
To perform a piston calibrator calibration.
- Place an aliquot of process fluid into the piston calibrator and flowmeter under test.
- Obtain the volume of fluid displaced in the piston checker by multiplying its inner diameter by the length of the piston travel.
- Compare this value to the measured value obtained from the flowmeter and adjust the flowmeter calibration accordingly.
Calibration is not absolute
When a meter is used in its intended application, any deviation from the laboratory conditions of the calibrated device may result in inaccuracy. For example, using a flow meter calibrated on clean water to measure wastewater containing high concentrations of suspended solids can significantly affect accuracy.
Unexpected turbulence in the piping system introduced upstream of the meter can result in performance that is very different from its calibrated performance. The length of the piping upstream and downstream of the device, the piping material and even the roughness of the inner surface of the piping can all affect accuracy. The angle of incidence of the installed equipment can also affect accuracy and function. In fact, a large number of systematic, random and spurious errors can lead to inaccuracies under real-world conditions. To make matters worse, these errors are often not easily observed or measured in closed piping systems, and therefore not easily detected.
Flow meters without moving parts, such as ultrasonic flowmeter and electromagnetic flow meters, remove the variables surrounding mechanical degradation. Because these flowmeters have no fluid intrusion components, the technology used to sense and measure flow is not affected by erosion or sediment, allowing the device to maintain its accuracy and reliability. Electromagnetic flowmeters are also less susceptible to inaccuracies caused by piping configurations. For example, the Apure AXT Series pumping and gravity condensate flow meter is capable of providing accurate measurements in virtually any location where it can be installed, even near elbows and other areas where there are no significant straight runs. Combined with its other benefits, this electromagnetic flowmeter simply makes all mechanical flowmeter and calibration needs obsolete