The accuracy of a flow meter is pivotal in choosing the right flow meter for the application in the field. The accuracy is how close the measured value is to the true value. In flow meters, that means how close the output of the flow meter is to its calibration curve. This is expressed as a percentage, e.g. ±2%. It means that any given reading can be in error 2% above or below the calibration curve. In general, it can be said; that the lower the percentage, the more accurate the flow meter. However, this is not true all the time. This also depends on how the accuracy of the flow meter is stated. The accuracy of the flow meter can be stated in two ways: either as a percentage of full scale (FS) or as a percentage of reading (Rd). It is important to understand both so that the flow meter performs as executed with the desired accuracy.
Percentage of full scale (FS) accuracy:
The definition of Full Scale is “Closeness to the actual value expressed as a percentage of the maximum scale value.” The percentage varies with the flow within the range as error remains the same with more percentage error at less flow and less percentage error at high flow. For example, if the accuracy is calibrated to 1% of range 50-200 litres/min then the error, if the flow is 200 litres/min, is 0.01 x 200 litres/min = 2 litres/min. If the flow is 50 litres/min, the error is still 2 litres/min or 4%, a much bigger percentage, as shown in graph below.
Percentage of reading (RD) accuracy:
The definition of Reading (RD) is “Closeness to the actual value expressed as a percentage of the actual value. The percentage stays the same, no matter where the flow is in the flow range. If it is 1% at 200 litres/min it would be 1% at 50 litres/min. So the error for a 200 litres/min flow would be 2 litres/min but for 50 litres/min it would be 0.5 litres/min rather than the 2 litres/min of Full Scale, better by 4 times.
Rd flow meters are desirable if the flow is fluctuating and accuracy throughout the flow fluctuation range is important, especially at low flows, whereas the Full scale (FS) flow meters are desirable if the flow is full scale.
Field methods to validate the accuracy of the flow meter:
Flow rate is the volume of fluid that passes in a unit of time. In water resources, the flow rate is often measured in units of litres per minute. Measurement of flow rate in water resources is important for applications such as system control, billing, design, and many other applications. It is also important to validate the accuracy of the flow meter installed in the field as it is pivotal in choosing the right flow meter. For this, often a field test that can validate the accuracy of the flow meter is required. Various field test to measure flow meter exists that are described below.
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1. Volumetric Measurements aka Bucket fill test:
Flow rate is generally measured in litres per minute. In the bucket fill test, the flow rate is measured made by measuring the time required for the flow to fill a container of known volume. Therefore Volume divided by time is equal to the rate of flow. This method is very simple and requires little equipment. It can be done with the help of a household bucket of known volume and a watch. In order to get accurate results where the flow rate is not uniform, frequent measurements are to be taken to get accurate data. It is to be kept in mind that the pipe flow should be full to avoid inaccuracies. It can be calculated using the formula below.
Flow rate (L/min) = [Bucket Size (L)] ÷ [Fill Time (sec)] × 60 × 0.8
Ø 60 is to convert the time unit from seconds to minutes.
Ø 0.8 refers to a 20% reduction in flow rate to allow for pressure loss in your system. If the bucket test is taken off the main water feed at the correct pressure then this 20% allowance is not necessary.
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2. Gravimetric Measurements aka weighing method:
A weighing method to measure flow rate can be easily used where the volume of the container is not known or to avoid the human errors in the bucket fill test during the filling of the bucket. This is a simple field method and is based on the mass of the fluid which can be measured more accurately. In this method, the mass of the water filled into a container in a given time is measured and then the value divided by density shall give the flow rate. In addition to container and watch, a weighing balance is required in this method. The formula is described below.
Flow Rate (Litres/min) = [W2 - W1] ÷ [T × D] × 60 × 1000
Where,
W2 - Weight of the container filled with fluid in kg
W1 - Weight of the empty container in kg
T - Time in seconds
D - Density of fluid in kg/m3
60 is to convert the time unit from seconds to minutes
1000 is to convert volume m3 to litres
3. Doppler and transit time ultrasonic clamp-on flow meter:
The flow rate can be calculated using the continuity equation:
Flow rate = A x Velocity x 60000
Area, A = π/4 x D2
Where,
Flow rate is in litres/min
Area, A is in m2
D - diameter of the pipe in metre
Velocity is in m/s
60000 is to convert the flow rate from m3/s to litres/min
The accuracy of both Doppler and transit time ultrasonic clamp-on flow meter is better compared to the volumetric and gravimetric measurements
Doppler ultrasonic clamp-on flow meter:
Doppler clamp-on flow meters are based on the Doppler Effect to measure fluid flow rate. In this, the ultrasonic waves are transmitted into the fluid, which is then reflected back to the sensor by the particles or bubbles flowing in the liquid. The change in the frequency between the transmitted and received waves is used to calculate the flow velocity.
Transit time ultrasonic clamp-on flow meter:
Transit ultrasonic clamp-on flow meters measure flow rate using the travel time of the ultrasonic wave. One wave is sent from an upstream to the downstream transducer, and another wave is sent from the downstream to the upstream transducer. The difference between the two travel times and
the distance is used to calculate flow velocity as velocity is distance divided by time.
