The Ultimate Guide To Flow Meter Calibration

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A flow meter measures the rate of a fluid or gas in various applications where it’s critical to obtain accurate flow measurements. Therefore, calibration is a must to assure the flow meter measurements are accurate. Flow meters are calibrated in one or several ways, but the general guide involves the comparison of the flow meter against a reference standard that has higher accuracy.

Flow meters are devices that measure the flow rate of fluids and gasses and regulate the speed and efficiency of industrial flow processes in a wide range of applications. It refers to how fast the fluid or gas moves through a system, such as a pipeline or a vessel, at a certain time. 

A flow meter must be calibrated every six months or so, to obtain accurate flow meter measurements. For example, flow meters are calibrated periodically in industrial settings to ensure operations are safe and work in a timely manner.

Even the most expensive or reliable flow meter can fall out of calibration, as technical equipment degrades over time. For example, some parts may break or wear out from high usage or corrosion. Flow meters can also be affected by improper installation, so understanding how to correctly calibrate a flow meter is essential

Do Flow Meters Require Calibration?

Flow meter calibration compares a flow scale (which is pre-set) of a specific type of flow meter to a standard measurement scale. Calibration is a key part of measuring liquids and gasses in a broad range of applications and industries that depend on measurements that are highly accurate and have little to no error. 

Common Flow Meter Applications:

  • Industrial Plants
  • Oil Industries 
  • Gas Industries 
  • Pharmaceutical
  • Building Automation
  • Metals & Mining
  • Water and Wastewater Treatment Plants
  • Marine Flow Meters
  • Animal Waste Digesters
  • Landfill Digesters

The Difference Between Flow Meter Calibration And Recalibration

In short, the main difference between the two is that flow re-calibration is when the flow meter is sent back to the manufacturer’s factory for a periodic calibration check. So, they are essentially the same, and the calibration principle doesn’t involve any adjustments to the flow meter, it is re-checked and compared to a fixed reference. 

Flow Meter Calibration:

Calibration compares the measurements of an operative flow meter to a standard flow measurement device, exposed to the same conditions. Newly purchased flow meters will, usually, already be calibrated for use when it arrives. 

Flow Meter Recalibration:

Recalibration is necessary when the flow meter has already been in use. Over time, flow meters degrade or become damaged, particularly in industrial processes where they are in constant use and work with vigorous liquids and gasses. 

Why Is It Necessary To Recalibrate Flow Meters? 

If a flow meter has not been in operation for a while, the flow meter is sent back to the manufacturer for recalibration.

Recalibration is necessary to confirm the accuracy of the flow meter and reduce uncertainty. It is also important to re-test the flow meter to reduce financial and technical risks, avoid systematic bias in the flow meter, avoid costly accidents, and ensure compliance with local and international regulations.

How Often Should A Flow Meter Be Calibrated?

Calibration is a key part of successful process operations. Without frequent flow meter calibration, there would be little confidence in measured values. The frequency of calibration depends on three approaches:

1. Calender-Based Approach

This approach relies on how long it has been since the last flow meter calibration. 

2. Risk-Based Approach

Using the previous calibration data results, how far the flow meter has drifted from the standard will determine if recalibration is needed. 

Risk-based approaches give the user a realistic and evidence-based interval between flow meter calibrations. 

3. Condition-Based Approach

Condition-based approaches look at the flow meter’s health; it tells the user if the flow meter has been damaged or needs replacing. 

Whichever approach, it is always good practice to recalibrate a flow meter every two to four years. However, if the manufacturer’s specification recommends a minimum flow meter calibration, always follow that frequency, even if it is more often than every two years. 

Flow Meter Calibration

Wet calibrations use a fluid (liquid) flow. This method of flow meter calibration is the highest calibration accuracy for a flow meter. There are three main calibration principles for flow meter calibration.

1. Reference Master Meter Calibration

While operating at the same conditions, a transfer standard calibration uses a reference grade and compares the reference flow against the flow meter. The master flow meter is typically used when the calibration follows a national or international standard.

Master meter calibration is used to regularly check measure systems, as they are easy to operate. They can be used to measure the flow of liquids with different viscosities without losing accuracy for continuous testing. 

Master meter calibration is typically less accurate than other flow meter calibration methods, yet they are more compact, so they can easily be transferred between different sites.

A Guide To Master Meter Calibration

  1. Put the reference master meter in place.
  2. Compare the flow meter readings of the flow meter and the master flow meter.
  3. Calibrate the flow meter to follow the master flow calibration.

2. Gravimetric Calibration

One of the most cost-effective methods for high-accuracy flow meter calibration is gravimetric calibration. It can be used for volumetric or mass-based flow, and therefore it is seen as the primary standard calibration method. For volume-based flow measurements, the fluid density must be measured. 

Gravimetric calibration is a simple process, and sometimes it is referred to as “the bucket and stopwatch method”. Although this type of flow meter calibration method is simple, significant effort is still required for high-accuracy calibration.

Gravimetric calibrations are commonly used for oil and water-based flow meter applications, where the measurements can be transferred to an external calibration laboratory. Gravimetric calibration is also used in water purification and industries that work with hydrocarbons. 

Its principle involves the diversion of the fluid, which we will explain more below.

A Guide To Gravimetric Calibration

  1. Put a small amount of fluid into the test meter and weigh it while it flows for one minute using a scale that has been correctly calibrated. The flow is started and stopped using a valve. 
  2. Following the test period (one minute), redirect the test fluid into a separate container (discharge container). 
  3. To secure the flow rate, divide the volumetric weight by the test duration.
  4. Next, compare the flow rate from step 3, against the flow rate of the flow meter. 
  5. Finally, adjust the flow meter to the measured flow rate.

3. Primary Air By Bell/Piston Prover Calibration

To achieve high accuracy and reliable results, piston prover flow meter calibration is used. The piston prover is a cylindrical flow meter with a known internal diameter. 

This calibration method drives the fluid through the flow meter at a specific rate. The piston prover consists of a dedicated flow stream with a volume between the prover loop detectors. The timer is started when the fluid passes the first detector, and then stopped at detector two.

The volume flow rate is calculated from the volume of fluid displaced by the piston prover flow meter and then multiplied by the length the piston travels over the same period. 

Piston provers calibrate to a high level of accuracy and are most commonly used for ultrasonic flow meter calibration. But they are also used to calibrate flow meters in fuel and turbine applications. Piston provers are very useful for calibrating in situ, given there is a dedicated stream.

A Guide To Piston Prover Calibration

  1. Put a sample of fluid into the piston prover under test.
  2. Next, multiply the piston prover’s internal diameter by the distance traveled by the piston to obtain the volume of the fluid displaced. 
  3. Finally, compare the displacement volume obtained from the flow meter and adjust the flow meter accordingly.

Gas Flow Meter Calibration

Gas flow meter calibration is similar to wet/fluid flow meter calibration systems, in that a comparison is made to make adjustments to the flow meter.

Sonic Nozzle/Critical Flow Venturi (CFV) Calibration

Critical flow venturi (CFV) or sonic nozzle calibration is used when there are different flow rate capacities. They are considered the best reference standard for flow meter calibration in the oil and gas industry for custody transfer. 

The gas flow accelerates until it reaches a steady flow state. This is known as sonic velocity. The sonic velocity is used as a reference velocity when calibrating the flow meter.

Sonic nozzle calibration can be used to calibrate the following fixed-type flow meters:

A Guide To Sonic Nozzle/CFV Calibration

  1. Fix the flow meter.
  2. Turn on the air compressor in the calibration testing system.
  3. When the sonic velocity gets to the measuring section, the flow meter being calibrated needs the reading adjusted and set. 
  4. If you are working with different sonic velocity points, repeat the above steps for specific flow rates. 
  5. Finally, the flow rate from the sonic nozzle and the flow meter are compared. 

Wind Tunnel Calibration 

When calibrating flow meters that measure air velocity and air turbulence, wind tunnel calibration is used. 

Wind tunnel installations have different designs, but they all have an open test chamber for calibration. These calibration procedures are used for various flow sensor calibrations, including, but not limited to, the insertion of thermal mass flow meters, pilot tubes, measuring probes, and hot-wire/vane anemometers.

A Guide To Wind Tunnel Calibration

  1. Install the flow meter/sensor probe inside the wind tunnel test chamber – you can change the insertion depth and orientation depending on what application you are working with. 
  2. Gradually increase the reference velocity and measure the flow sensor under test. 
  3. Record the measurements. 
  4. On a graph, plot a curve based on the reference velocity measured. 
  5. Upload the reference velocity curve into the device under test.

Onsite Validation Calibration 

Not as commonly used because it is not as accurate as the above flow meter calibration methods, but onsite validation of gas flow is also possible in some industrial settings. 

Onsite validation can validate when gas flow meters have high errors or non-repeatability issues. There are two types of onsite validation processes.

Pump-Up Test Using A Pressure Vessel

  1. Install the pressure vessel with the flow meter under validation.
  2. Initiate the gas flow and write down the gas flow meter reading and timer inside the filling pressure vessel. 
  3. Validation is completed by comparing the pressure vessel pressure to volume capacity by the gas flow meter totalizer*. 

*A flow meter totalizer measures how much fluid (either a gas, liquid, or steam) has passed the flow meter sensor within a given time.

Calibrated Reference Gas Flow Meter

  1. Install the calibrated reference gas flow meter with the flow meter being tested. 
  2. Compare the flow meter result and the computed error. 

Flow Meter Calibration Tips

When calibrating a flow meter, ensure the standard used for calibration is accurate. The standard used should be 4x more accurate than the DUT (device under test). Also, as the standard calibration and DUT are compared in real-time during flow meter calibration, check the rate of flow between the standard calibration and DUT is in a steady state.

The standard used should be identifiable as a recognized flow meter standard and always match the calibration with the flow meter’s application/actual operation. 

Finally, when calibrating a flow meter, ensure there are no leaks or significant temperature changes that could affect the flow meter calibration process.

Summary

Calibration is a comparison operation to ensure that a flow meter is operating accurately. A flow meter is evaluated against a reference standard to recalibrate the meter.

There are three main flow meter calibration processes: master meter calibration, gravimetric calibration, and piston prover calibration. Which one you use will depend on the application you are working with. 

If you have any questions on how to re-calibrate a flow meter or what flow meter will suit your application needs, do not hesitate to contact the world-class team at Atlas Scientific

Flow Meters

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