Flow meters are crucial instruments used across many industries to measure the flow rate of liquids, gases, or vapors. Among the many calibration parameters, the K-factor is the most important. Whether for batch chemical preparation or monitoring process water, the accuracy of flow readings directly impacts system efficiency. The accuracy of flow measurement depends on a key calibration constant: the K-factor. It is the foundation for achieving accurate and reliable flow measurement.
The post introduces the definition, calculation, and importance of the K-factor of flow meters.
What is the K-factor of a Flow Meter?
The K-factor is a coefficient calculated by calibrating the flow meter against other measuring devices that meet accuracy requirements before it is put into use. This K factor is a fixed value. The actual coefficient obtained by the flow meter manufacturer after calibration for each instrument.
The K-factor of a flow meter is one of the key parameters representing its metering characteristics. It represents the number of signal pulses output by the sensor when a unit volume of fluid flows through the flow meter sensor.
In other words, the K-factor establishes the proportional relationship between the actual fluid flow rate and the flow meter’s output signal. It is the foundation for achieving accurate flow measurement.
Read More about: Flow Meter Calibration – A Comprehensive Guide
Pulse Signal K-Factors and Analog Input Signals
K-Factor for Analog Input Signals:
When analog input signals must be used for batch processing, indication, or accumulation, the flow meter first converts 4-20 mA signals to 0 to 10000 Hz signals. The K-factor is then calculated by associating the engineering unit equivalent of 20 mA with 10000 Hz.
K factor represents: the number of pulses output per cubic meter.
Pulse signal input is set to the K1 coefficient in the primary parameters based on the K-factor provided on the pulse flow meter. The K-factor cannot be calculated automatically. If the K-factor on the flow meter is several pulses per liter (P/L), the corresponding mass unit is kilograms per hour (kg/h). And the corresponding volume unit is liters per hour (L/h).
If the K-factor on the flow meter is some pulses per cubic meter (P/m³), the corresponding mass unit is tons per hour (t/h). And the corresponding volume unit is cubic meters per hour (m³/h).
If the instantaneous flow rate needs to be calculated in L/min (liters per minute), first set the time unit (Q-Tn) of the instantaneous flow rate in the secondary parameter to 1. And then set the K coefficient in the primary parameter to the coefficient corresponding to liters per hour (L/h).
The unit of cumulative flow rate is determined by the unit of instantaneous flow rate. For example, if the unit of instantaneous flow rate is liters per hour (L/h), then the unit of cumulative flow rate is liters (L).
Flow Meter Types and K Factor
The correspondence between different flow meters and the K factor is as follows:
K Factor of a Volumetric Flow Meter:
The number of pulses emitted by the metering chamber (composed of a rotating element and a stationary chamber) when a unit volume of fluid flows through the flow meter. The K factor is the ratio of the actual volume flowing through the flow meter to the indicated volume (usually expressed as MF).
Read More about: Volumetric Flow Meter – Guide
K Factor of a Turbine Flow Meter:
The number of pulses output by the magnetoelectric converter when a unit volume of fluid flows through the sensor. The K factor is affected by fluid properties, bearing damping, etc. And it needs to be calibrated after long-term use or under changing operating conditions.
Read More about: An Overview of Turbine Flow Meters
Vortex flow meter K-factor:
The number of pulses output by the vortex sensor when a unit volume of fluid flows through the flow meter. The K-factor is related to the geometry of the vortex generator and the pipe inner diameter. And it is affected by factors such as temperature and Reynolds number.
Read More about: Industrial Steam Vortex Flow Meters-Multivariate Measurement
Electromagnetic flow meter K-factor:
This is a conversion parameter determined through calibration experiments before shipment (users cannot directly calculate it). It is used to convert the induced electromotive force into the actual flow rate value.
When on-site operating conditions change, it can be corrected by the ratio of the actual flow rate to the displayed flow rate. For specific questions regarding the K factor, please consult the manufacturer.
Read More about: Magnetic Flow Meter vs. Ultrasonic Flow Meter-Which One is Better?
How to Calculate the K-factor?
The basic formula for calculating the K-factor of a flow meter is:
K = N / V
Where:
N is the number of pulses output by the flow meter; V is the fluid volume.
The specific expression for the K-factor varies for different types of flow meters:
For vortex flow meters, the instrument coefficient K is a key parameter determining their measured flow rate value. Once the K value is determined, its measurement accuracy is closely related to it.
The formula for calculating the K value is:
K = f/qv,
where:
f represents the output pulse frequency of the turbine flow meter,
qv is the flow rate through the flow meter.
It is worth noting that as the diameter of the vortex flow meter increases, the instrument’s K factor may gradually decrease. This can lead to a decrease in the stability and accuracy of the instrument.
The K factor of a turbine flow meter refers to the ratio between turbine speed and flow rate. It is generally determined experimentally. And its calculation formula is:
K = Q/π·d/4·N,
where
Q is the flow rate,
d is the turbine diameter,
N is the turbine speed.
The floating-point value of the turbine flow meter should automatically correspond to each diameter by default.
Q (instantaneous flow rate, m³/h) = 3600 × F (frequency, Hz) ÷ K factor
After completing the actual flow test, the final K coefficient needs to be set here. The K factor represents the number of pulses output per cubic meter.
The Importance of the K Factor
The K factor is very important for the flow meter, and it mainly has the following three functions.
By introducing the K-factor, we can improve the measurement accuracy of various flow meters.
Secondly, the K-factor can be corrected for different media, temperatures, pressures, and other operating conditions. It ensures that the flow meter can measure accurately under various conditions.
Finally, the K-factor is an important parameter in the flow meter design process, which contributes to improved overall flow meter performance.
What is Multi-Point K-factor?
Multi-point K-factor refers to collecting data at multiple flow points (e.g., 30%, 50%, 80% of the flow range) during flow meter calibration. By calculating and setting the corresponding K-factor for each point, the measurement accuracy of the flow meter in different flow ranges is improved.
The flow meter’s K-factor (instrument constant) is the proportional relationship between the number of output pulses and the actual flow rate. Traditional methods usually use single-point calibration (e.g., 50% of the flow range) to obtain a single K-factor.
But in actual operating conditions, the characteristics of the flow meter may change with the flow rate. And a single-point K-factor can’t guarantee accuracy across the entire flow range.
Multi-point K-factor calibration can be achieved through calibration at multiple flow points, which yields a more accurate flow-pulse correspondence. It is usually suitable for scenarios with a wide range of ratios or large operating condition fluctuations.
Sino-Inst Featured Flow Meter
What Affects the K Factor?
The K-factor is mainly affected by the following factors:
(1) Changes in flow velocity and viscosity:
The Re values of most fluids in industrial pipelines are greater than 2 x 10⁴. However, if the fluid velocity (v) decreases or the kinematic viscosity (y) increases for any reason, Re will decrease. When the Re value is below 2 x 10⁴, the K value changes significantly, resulting in nonlinear errors. This can be addressed by selecting a smaller pipe diameter to increase the flow velocity and thus improve Re.
(2) Accumulation:
If the measured fluid contains viscous particulate matter or a large amount of fibrous material, it may gradually accumulate on the vortex generator. It causes changes in its geometry. Consequently, the instrument coefficient K changes, affecting vortex formation and generating measurement errors. To reduce this error, if conditions permit, the accumulation on the vortex generator can be periodically removed, or the measured fluid can be pretreated.
(3) Wear:
If the two edges of the vortex generator are worn due to the action of the measured fluid, the instrument coefficient K will also change. In some cases, this error can be minimized by selecting a suitable prism material.
(4) Temperature Change:
The K factor is usually calibrated at room temperature (approximately 20°C). If the temperature of the measured fluid is higher or lower than room temperature, the geometric dimensions of the instrument body will change due to thermal expansion and contraction, which causes changes in the flow area and results in additional errors. So, the K factor must be compensated and corrected. In the software, it is sometimes necessary to correct the instrument coefficient according to the formula.
(5) Piping Inner Diameter:
The inner diameter of the piping where the flow meter is installed should generally be equal to or slightly larger than the inner diameter of the flow meter. If the actual inner diameter of the piping is slightly smaller than the inner diameter of the flow meter (within 3%), it will not affect the inherent K factor of the meter itself. It may produce additional errors due to the change in apparent velocity caused by the sudden change in flow area. This can be corrected to improve measurement accuracy.
Tips for Accurate K-Factor Calibration
The k-factor needs adjustment in the following situations:
1. Initial calibration of a new meter, where the measured value does not match the standard value;
2. Older meters, due to long-term use, sensor wear, or changes in operating conditions (e.g., medium viscosity, temperature fluctuations), may result in errors exceeding the acceptable range;
3. Incorrect K-factor input in the initial calibration, leading to completely inaccurate measurement results.
During the K-factor calibration process, we need to pay attention to the following two points:
1. Carefully read the instruction manual. Different manufacturers use different K-factors. During actual K-factor calibration, we can contact the manufacturer and professional technicians promptly.
2. Maintain a standardized calibration log. We should keep a record of each K-factor calibration for easy future calibration.
Is a higher K-factor better?
The K-factor is not necessarily better the higher it is. It is a conversion parameter for flow meter calibration and must be matched to the actual operating conditions. A K-factor that is too high or too low will affect measurement accuracy.
What is the K-factor in a vortex flow meter?
The K-factor of a vortex flow meter represents the volume of fluid passing through the pipe per unit time. The K-factor is a preset value provided by the manufacturer during calibration and testing. However, due to differences in installation and usage conditions, the preset K-factor may deviate. Therefore, adjustment is necessary to improve measurement stability.
What is the normal K-factor?
The K-factor of a conventional flow meter does not have a fixed value. It varies depending on the flow meter type, diameter, structure, and calibration conditions. The K-factor is determined by the manufacturer through actual flow calibration and marked on the instrument nameplate or certificate of conformity.
Common ranges (based on typical flow meters):
Turbine flow meters:
Approximately tens to hundreds of pulses/liter (e.g., diameter DN15-DN80, specific value adjusted according to operating conditions).
Vortex flow meters:
Approximately several to tens of pulses/liter (e.g., K≈1.2 pulses/liter for DN100, ≈2.344 pulses/liter for DN80).
Electromagnetic flow meters:
Typically, several thousand to tens of thousands of pulses/cubic meter (users generally do not calculate this themselves; Manufacturer calibration is required).
In short, the K-factor is a key parameter for confirming the accuracy of a flow meter. Different flow meters have different K-factors. When encountering related issues, we recommend consulting the manufacturer or professional technicians.
Sino-Inst has been engaged in the flow measurement industry for many years. If you need related technical support, we can provide it free of charge. Please feel free to contact us; we are your best flow measurement partner.
