How to Calibrate a Flow Meter？Calibration Flow MetersProfessional Partner for Measure & Control

A flow meter is an instrument used to measure and indicate the volume or mass of a gas or liquid. Simply put, it is an instrument used to measure the flow of fluid in pipes or open channels. Also known as flow indicator, liquid flow meter, flow sensor, etc.

Flowmeters are further divided into differential pressure flowmeters, rotameters, throttling flowmeters, slit flowmeters, volumetric flowmeters, electromagnetic flowmeters, ultrasonic flowmeters, etc. According to the medium, it can be divided into: liquid flowmeter and gas flowmeter.

RFQ Flowmeters

What is Flow Meter Calibration?

Flow meter calibration is carried out according to the verification regulations of various flow meters promulgated by the National Metrology Bureau.

For the flowmeters used, except for the standard throttling device, no experimental verification is required. The rest of the flowmeters are almost all verified when they leave the factory. During the use of the flowmeter, it should also be calibrated frequently.

The calibration methods of liquid flowmeter mainly include volumetric method, mass method, standard volume tube method and standard flowmeter comparison method.

The calibration methods of gas flowmeter mainly include sonic nozzle method, servo standard flowmeter comparison method and bell jar method.

Why is it important to calibrate a flow meter?

Most people recognize that a flow meter should be calibrated before it is put into service, but why is flow meter calibration so important?

The flow meter calibration process involves comparing the flow meter to a reference device used as a calibration standard. Calibration of reference equipment should be traceable to national or international standards. The next step involves adjusting the flow meter (if necessary) to counteract any errors.

If, like thousands of other facilities, flow meters are used in your process, it makes good business sense to ensure that the flow meters you use are giving you accurate and consistent information.

No matter what fluid you are metering, the liquid, gas or steam used in the process is most likely a valuable commodity. Accurate flow meter performance is therefore an invaluable part of business planning and costing.

At the same time, Calibration Flow Meters are also very important for the control of material consumption in our industrial production process. Can effectively improve production efficiency. reduce manufacturing cost.

Flow Meter Calibration vs. Recalibration

Flow Meter Verification (Verification): Refers to the process of identifying and confirming whether measuring instruments meet the statutory requirements. It includes inspection, marking and/or issuing a certificate.

Flow Meter Calibration (Calibration): refers to the determination of the value indicated by a measuring instrument or measuring system, or the value represented by a physical measuring tool or reference material, and the corresponding value reproduced by a standard under specified conditions. A set of operations on relationships.

After reading the definition, there must be some confusion, right?
We usually summarize the main differences between verification and calibration into nine aspects:

Different purposes;
Objects are different;
According to different;
Different in nature;
The cycle is different;
In different ways;
The content of the certificate is different;
Different conclusions;
The authorities are different.

For quick understanding, we simply organize the following table:

Verification and Calibration	Verification	Calibration
Definition of Terms	Refers to the process of identifying and confirming whether measuring instruments meet the statutory requirements, which includes inspection, marking (or) issuing a verification certificate.	Refers to a set of operations to determine the relationship between the value indicated by a measuring instrument or measuring system, or the value represented by a physical measuring tool or reference material, and the corresponding value reproduced by the standard under specified conditions .
Character	It belongs to mandatory verification and has legality;	Users can voluntarily trace the source, which is not legal;
Certificate form	If the test result is qualified, a “Certificate of Test” shall be issued; If it is unqualified, a “Notification of Verification Result” shall be issued	Usually a “Calibration Certificate” or “Calibration Report” is issued; calibration results can also be expressed in the form of calibration curves or calibration factors
Specific method	According to the verification regulations, each point is verified 3 times at 100%, 50%, 20% and 100% of the full scale. If the original coefficient is qualified, the coefficient is not adjusted in principle.	According to the calibration procedure, at 100%, 50%, 20% and 100% of the full scale each point 3 times, adjust the coefficient to obtain the best performance near the user’s actual flow. The nameplate remains the same, but the certificate reflects the change.

How to Calibrate a Flow Meter？

The volumetric method is the most commonly used. This is a method of measuring the volume of fluid flowing into a constant volume container within the measurement time to obtain the flow rate.

The figure below is a schematic diagram of the typical structure of the static volume method water flow verification device. Verify the flow meter with pulse output, and replace 21 for verification of other output flow meters.

The commutator 13 is used to change the flow direction of the fluid so that water flows into the standard container (the standard container 15 or 16 can be selected according to the flow rate). The pulse counting controller 21 is triggered when the commutator 13 starts. To ensure the simultaneous measurement of water and pulse signal counts.

When calibrating, use the pump to draw the test fluid from the liquid storage container into the high level water tank. Then through the school flow meter. If standard container 15 is selected, then close drain valve 17, open drain valve 18, and commutator 13 is placed in the position that makes water flow to standard container 16.

After the flow is stabilized, the commutator 13 is started. Water flow is changed into standard container 15 by standard container 16. At the same time, the trigger pulse counter accumulates the pulse number of the calibrated flowmeter. When the predetermined water volume or preset pulse number is reached, the commutator will automatically change direction. Water flow is changed into standard container 16 by standard container 15. Read off the volume V of the fluid that enters the standard container during this period of time from the scale on the reading glass tube of the container. Record the number of pulses N displayed by the pulse count control 21 for the flow meter being calibrated.

Compare the pulse number N of the calibrated flowmeter with the obtained standard volume V to determine the instrument constant and accuracy of the calibrated flowmeter. The instantaneous flow rate of the flowmeter is indicated by the frequency indicator 20 .

The system accuracy of this method can reach 0.2%.

The mass method is a method of weighing the mass of the fluid flowing into the container within the measurement time to obtain the flow rate.

When calibrating, use a pump to draw the test fluid from the liquid storage container through the flowmeter to be calibrated and then enter the container containing the liquid. Measure the temperature of the fluid while weighing it out. It is used to determine the density value of the measured fluid at this temperature.

The volume of the fluid can be obtained by dividing the weight of the measured fluid by the density of the fluid at the measured temperature. Compare it with the volume indication (accumulated pulse number) of the meter. The meter constant and accuracy of the flowmeter to be calibrated can be determined.

The accuracy of this method system can reach 0.1%

The working principle of the standard volumetric tube method is based on the volumetric method (standard volumetric method), but it is a dynamic measurement.

The standard flowmeter comparison method is to connect the flowmeter to be calibrated and the standard flowmeter in series on the pipeline flowing through the test fluid. The error is calculated by comparing the two measured values. The accuracy of the standard flowmeter is 2 to 3 times higher than that of the schooled flowmeter.

The sonic nozzle gas flow standard device is a standard instrument based on the sonic nozzle. It is used to verify mass flowmeters, velocity flowmeters, volume flowmeters, rotameters, differential pressure flowmeters or other types of flowmeters. It is a standard device for measurement and testing units and large enterprises to measure and calibrate gas flowmeters.

According to the principle of aerodynamics, when the pressure ratio between the upstream and downstream of the factor nozzle reaches a critical state, the throat airflow reaches the peak velocity, and the mass flow through the nozzle also reaches the peak value. This value is only related to the parameter at the nozzle inlet and is not affected by changes in downstream conditions. Based on the above principles, this system uses the sonic nozzle as a standard meter to calibrate the measured flowmeter.

Read more about: 5 Factors Affecting Pressure Drop

Field Calibration Method You Must Know

Many users search How to Calibrate a Flow Meter? In order to understand how to adjust the accuracy of the flowmeter in your own working conditions.

On-site “flow comparison” means that the flowmeter is compared with other “reference flows” on site.

For example, the measured value of the temporarily clamped ultrasonic flowmeter, the volume of the liquid that has been measured over the flow in the piping system, etc. can be used as the “reference flow”.

For the meaning of terms such as on-site calibration, comparison and verification, please refer to the standard definitions in relevant documents, and will not repeat them here.

In practice, many professionals have explored various on-site comparison methods and indirect inspection methods. There are mainly 3 types:

(1) Use the container weighing method comparison of storage containers, working containers, pools or weighing instruments in the process. The pool volume comparison method is often used by water supply companies. Water plants can obtain high comparison accuracy by taking advantage of the favorable conditions of large-volume clear water pools.

(2) Reserve a position in the pipeline of the flowmeter to be tested for the flowmeter comparison method of connecting the reference flowmeter.

(3) Ultrasonic flowmeter comparison method of Clamp-on ultrasonic transducer. Clamp-on ultrasonic flowmeter is one of the three most promising methods of flow measurement. Especially the measurement accuracy error of existing foreign products is < ± 0.5%. This is conducive to the establishment of high-precision comparative detection and on-site online calibration system. It is convenient for the metrology and verification department to go to the site for calibration and testing.